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Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
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2
The Nature and Value of Geohistorical Information

THE RATIONALE OF GEOHISTORICAL ANALYSIS

The study of ecological dynamics would benefit greatly from a tripartite research effort similar to that used so successfully in climate and ocean dynamics (see Figure 2.1), where information about past systems—their variability in rates, states, and composition; apparent boundary conditions (including those without modern analogs); spatial scaling effects; correlated relationships evident only over periods longer than modern monitoring programs—has proven to be an essential partner to modeling and studies of present day conditions. In fact, it is difficult to imagine understanding climate and ocean systems as well as we do today without the insights gained from geohistorical data. In the study of climate and ocean dynamics, geohistorical records—including ice cores, sediment cores, tree rings, and coral skeletons—have provided the first evidence for the existence of many important phenomena, including climate flickers and other abrupt climate change, the reorganization of deepwater circulation patterns, and Milankovitch oscillations in temperature and atmospheric composition. Geohistorical records also have provided the impetus and an essential empirical framework for modeling and experiments (see Box 2.1). The intellectual impact of individuals and projects that cross methodological interfaces—using some combination of geohistorical data, modeling, and present day observation and experimentation—has been at least as important as original research within subdisciplines in propelling our understanding of these complex environmental systems, including

Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
×

FIGURE 2.1 Schematic illustration showing how interactions among geohistorical data, observations and experiments, and modeling serve to increase understanding of the biotic response to environmental change. Until recently, geohistorical data have not been used extensively by biologists.

BOX 2.1
CO2 Through Geologic Time: Observations, Models, and Geohistorical Data

Royer et al. (2004) illustrated the strength of model-data comparisons in their analysis of the role of CO2 in driving climate during the past 600 Ma. How CO2 functions as a greenhouse gas is now well established through observations, experiments, and modeling of the present day atmosphere as well as the analysis of climatic proxies and CO2 in the Quaternary geohistorical record. However, substantiating CO2’s role in the regulation of pre-Quaternary climate is difficult because of the lack of direct information on the composition of the atmosphere through most of geologic time. Geochemical models, based primarily on sedimentary weathering rates, have been devised to predict the evolution of CO2 through geologic time (e.g., Berner, 2004; Berner and Kothavala, 2001). These predictions can be evaluated by comparing the model results to estimates of past CO2 based on geochemical and paleobotanical indicators in the rock record. Such indicators are said to be “proxy indicators,” in that they substitute for direct measurements. The development and calibration of these and other proxy indicators depends on observations and experiments with modern systems.

Royer et al.’s (2004) review of proxy records indicates a consistent relationship between various CO2 indicators, geologic evidence of glaciations, and CO2 predictions from geochemical models. Accordingly, model-proxy comparisons support the hypothesis that CO2 has acted as the primary driver of climate during the past 600 Ma, identifies key time intervals in which proxy indicators and model predictions do not agree, and evaluates the extent to which proxy indicators differ.

Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
×

Comparison of atmospheric CO2 composition predicted by GEOCARB III model calculations (range shown in pink) and proxy indicators. A. Individual proxy indicators (paleosols = fossil soil δ13C; phytoplankton = δ13C of phytoplankton remains; Stomata = density of stomatal pores in leaves of plants; Boron = δ11B in remains of planktonic foraminifera. B. Mean (black line) and ±1σ (gray bars) of proxy estimates in 10 Ma time steps. C. Frequency distribution of proxy data through time. SOURCE: Royer et al. (2004); used with permission.

Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
×

the especially critical issue of the relative roles of anthropogenic and non-anthropogenic processes in causing biotic change.

The biosphere is an extremely complex system—analogous to climate and ocean systems—with great diversity and variability in the behavior of its components, an enormous variety of interactions and feedbacks that are context specific and probabilistic, and a long memory and residence time for many of its components. Despite this complexity, improved techniques for extracting information on the environments, biotas, and ages of geohistorical records now combine to ensure that geohistorical analysis is well positioned to become a full and essential partner in the analysis of ecological dynamics.

Here, following a brief discussion of strategies for working with incomplete records, we review some of the techniques used to acquire geohistorical data on ecological dynamics, organizing the discussion around questions that are commonly raised by workers unfamiliar with this topic:

  • With what detail and confidence can environmental conditions be inferred for the past?

  • What kinds of biological information are obtainable from the geologic record?

  • How precisely and accurately can the ages of past records be determined?

COMPLETENESS OF THE GEOLOGIC RECORD

Taken as a whole, the geological record is incomplete—not every individual or species that ever lived is preserved, all the environmental conditions for every part of the globe are not recorded in the sediments, and deposition of rocks has not been continuous everywhere on the face of the earth and throughout geologic time. This heterogeneity in the quality of geohistorical data is a fundamental property of the geologic record, just as completeness and bias in written documents and instrumental records are properties that must be considered in the analysis of human cultural history and instrument-based environmental histories. Research over the last ~20 years (e.g., Kidwell and Flessa, 1995; Foote and Raup, 1996; Koch, 1998; Foote et al., 1999; Foote and Sepkoski, 1999; Behrensmeyer et al., 2000; Briggs et al., 2000; Alroy et al., 2001; Kidwell and Holland, 2002) has demonstrated that uniquely valuable data can be extracted from geohistorical records of diverse quality, thus increasing the effective availability of geohistorical information for biosphere analysis. Some of the key elements of this modern strategy, and the stereotypes that are being overturned, are summarized below.

Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
×

Valuable insights can be gained from sparsely sampled time series and from isolated samples that are well positioned before and after significant biotic or environmental events or transitions. Although long, closely sampled time series may provide important geohistorical information, they are not always required. For example, ecological monitoring programs for modern environments may not have begun before human impact. Under these circumstances, geohistorical records of known age—but not directly connected to the present by a continuous series of comparable samples—are unique and valuable sources of information on local pre-monitoring conditions (e.g., the species composition or dominance structure of a benthic or forest community before an environmental change or an extinction bottleneck, or the typical body sizes of animals before commercial extraction or exploitation). This strategy of opportunism—using such records for their unique insights and aggressively seeking out geohistorical records from previously little-explored areas—has proved remarkably productive in bringing valuable geohistorical perspective on modern management issues in both terrestrial (e.g., Steadman, 1995; Swetnam et al., 1999) and marine settings (Kowalewski et al., 2000; J.B.C. Jackson et al., 2001; Pandolfi et al., 2003; Aronson et al., 2003).

Similarly, biotic response to events in the deeper past can be analyzed in the absence of complete time series by using geohistorical records that are appropriately positioned in time or space, for example by comparing samples from before and after mass extinctions (e.g., Raup and Jablonski, 1993; Jablonski and Raup, 1995; Jablonski, 1995; Erwin, 1996; Foote, 2003). In fact, widely spaced samples over longer periods can be superior to closely spaced samples over shorter time series for the analysis of phenomena that occur over periods greater than a few years, and indeed many important biological processes unfold on timescales of millennia or even millions of years. Paleontologic samples of relatively coarse scale (i.e., millions of years) have revealed striking patterns in the loss and subsequent recovery of morphological richness, a measure of biodiversity that may capture functional variety far more effectively than simple counts of higher taxa (e.g., Foote, 1997, 1999; Lockwood, 2003).

For many purposes, time-averaged (time-exposure) information—as is typically captured by sedimentary records—is adequate to the questions being addressed and may be better than instantaneous snapshot samples. Few geohistorical samples represent instants in time (snapshots) comparable to ecological samples but instead represent accumulations of individuals and species over some period (time exposures, or time averaged). Such time-averaged samples tend to filter out short-term (seasons to centuries) variation in composition, relative abundance, and diversity. If such samples are large enough to encompass the full range of variability (e.g., Olszewski [1999] indicates that samples of as few as 29

Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
×

individuals may be sufficient), they can be employed to characterize long-term, average conditions (Kowalewski et al., 1998; Olszewski, 1999; Hadly, 1999). In addition, if the time-averaged specimens can be directly dated (e.g., by radiocarbon dating or amino acid racemization), then a continuous temporal record of specimens can be reconstructed (Kowalewski et al., 1998).

When the goal is an understanding of larger-scale trends and general principles relevant to developing realistic conservation and management policies, time-averaged geohistorical data provide essential biological data. For example, the richness and dominance structure of most communities are inherited in part from periods predating scientific study. Dominant species in some communities may be relict populations whose dominance reflects incumbency and lifespan rather than present day conditions. In such situations, a one-time or very short-term sample could reflect ephemeral conditions, whereas a time-averaged sample is more likely to reflect average conditions of local diversity and structure. Because standing diversity may be the product of long-term as well as short-term processes, and because local populations are highly volatile, combining data from both living communities and geohistorical records allows tests of hypotheses, provides strength to the interpretations, and supplies new insights into ecological processes.

The need for truly long-term geohistorical perspectives on species is especially critical in understanding extinction risks and the potential for speciation and other evolutionary innovation. Raw rates of speciation and extinction (numbers of taxa per unit time), as well as variability in raw rates and the frequency distributions of evolutionary durations, can all only be determined using the fossil record of evolutionary first and last appearances. Without such empirical information, rates of speciation, for example, would have to be assumed to be correlated with present day standing diversity, whereas high standing diversity could reflect either high speciation rates in that region, low extinction rates, high immigration of species that originated elsewhere, or some combination of these variables. Similarly, in the absence of geohistorical information on species ranges, extinction risk would have to be evaluated only in light of the population dynamics measurable today. These are examples in which the unique qualities of time-averaged paleobiological information permit questions to be answered that—if only modern-day biological data were available—could only be addressed indirectly.

High-quality geohistorical records are present throughout the ~4 billion year record of Earth, although the total quantity of records decreases with increasing age. Geohistorical records from the relatively recent past contain a wider variety of extractable information (e.g., ancient DNA is more likely to be preserved, and isotopic signatures are less likely

Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
×

to be overprinted), than more ancient records. The taxonomic and functional characteristics of biological remains are more readily inferred by direct comparison to living forms, and dating resolution is better. Because interpretations based on agreement of multiple lines of evidence (whether direct or proxy) are judged more robust than inferences based on single lines of evidence (Mann, 2002), younger records are more likely to yield high-quality information about both environmental change and its biotic response.

Variation in the quantity and quality of data through time occurs over all timescales. For example, archived materials are subject to loss and deterioration even under the best of conditions, and—due simply to scientific and technological progress—workers during the last decades generally have used more precise and diverse means of characterizing and analyzing objects and systems than workers of 50 to 100 years ago. In Earth history—as in other areas—such variation does not invalidate historical analysis but only requires that analysis be done thoughtfully and with standard scientific rigor.

Both biologically and geohistorically, some regions, habitats, and kinds of organisms are better known than others at any moment in time. Information quality is not randomly distributed with respect to geographic region, environment, or taxonomic group; for example, biotic systems and geohistorical records are better known in regions with long-standing academic programs or field stations (e.g., Western Europe and North America); some groups are better known because of their economic importance (e.g., insects as pests and vectors of disease, or benthic mollusks as food for commercial finfish); and some habitats and systems are better understood because of the greater ease of surveys and experiments (e.g., rocky intertidal settings, rodents and insect population dynamics). This unevenness can persist into more mature stages of data acquisition, because additional work on an already well-studied group or area benefits from past insights and investments. As a consequence, some systems are not as well explored as others despite the likely existence of materials (living communities, geohistorical records) that could yield valuable insights.

In any empirical scientific analysis, the ideal is for sampling to be at regular intervals along a gradient, or through time over the course of an experiment, at the same time holding other conditions constant. The greater the density of sample points along the gradient or during time, the greater the confidence in recognizing the effects of processes operating at fine spatial or temporal scales, discovering correlated relationships among the variables and the greater the likelihood of understanding their underlying significance. Such regular sampling is not always possible in natural systems; instead, it is often necessary to interpolate among irregularly

Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
×

spaced points or forced to compare the end points of a suspected spatial gradient or temporal trend. Geological analysis of ecological dynamics is as subject to these challenges as is field-based biology, where for example, the availability of rocky shores would determine the spacing of samples in a latitudinal analysis between Baja California and Oregon. Whether such limitations on continuous sampling constitute a significant impediment depends on the question at hand, but clearly continuous sampling is not required to answer all questions, nor is it always possible.

What is most important in comparative analyses, whether biological or geohistorical, is to (1) have a good match between the question and the quality and quantity of samples that can be acquired; (2) identify the specific variables to be explored (e.g., gradients in environment, variation over geologic age or geographic region); (3) hold other variables constant (for example, sampling protocol—in geohistorical analysis this would also include the preservation quality of the local records sampled); and (4) have a reasonable number of replicate samples per treatment (i.e., per site, per region, per geologic period). With appropriate diligence, together with adjustment of questions as necessary, these challenges can be met in geohistorical analyses with a success rate that is similar to that achieved in field biology experiments.

INFERRING ENVIRONMENTAL CONDITIONS—RESOLUTION AND CONFIDENCE

Various elements of the geologic record capture environmental information in indirect, or “proxy,” form. For example, variations in air temperature and rainfall affect the width of tree rings and, accordingly, variations in the widths of tree rings are proxy indicators of variation in temperature and moisture. Similarly, variation in sea-surface temperature and ice cap volume are reflected in fluctuations of oxygen isotope ratios in skeletal remains preserved in marine sediments.

In many cases, proxy indicators can be calibrated based on experiments or on their observed variation in known environments, allowing their use for the determination of precise quantitative measures of environmental conditions. The composition of biotic assemblages has long been used as a basis for quantitative inference of climate, salinity, redox, pH, water depth, productivity, and a variety of other environmental factors (e.g., Imbrie and Kipp, 1971; Bartlein et al., 1986; Fritz et al., 1991; Charles et al., 1994; Charman, 2001; Smol et al., 2002a,b). The discrimination of paleotemperatures with a standard error of only ± 2°C using shape and size characteristics of leaves is a good example of a proxy with relatively high precision based on modern calibration (Wilf, 1997), and the Stomatal Index (SI) of fossil leaves can provide historical records of past

Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
×

levels of CO2 (Wagner et al., 1999). Similarly, stable isotopes incorporated into fossils and other geohistorical records are providing extraordinarily detailed insights into population size (e.g., Finney et al., 2000), home ranges (Hoppe et al., 1999), diet (e.g., Burton et al., 2001), photosynthetic pathways (e.g., Fox and Koch, 2003), river discharge (e.g., Dettman et al., 2004), salinity (e.g., Rodriguez et al., 2001), climate (e.g., Edwards et al., 1996; Huang et al., 2002), ocean pH (Lemarchand et al., 2000), and elevation (Morrill and Koch, 2002), and effects of CO2 and climate on vegetation (Huang et al., 2001; Street-Perrott et al., 2004).

Some proxies provide only semi-quantitative or qualitative measures of environmental conditions. For example, sediment grain size, bedforms, and associated paleocurrent evidence for the frequency and magnitude of fairweather and storm reworking permit the confident—but qualitative and comparatively coarse—determination of shallow-, intermediate-, and deep-water paleodepth estimates for continental shelf deposits. Because water depth is rarely an environmental parameter per se (compared, for example, with sediment grain size, frequency of disturbance, or organic content), the recognition of semi-quantitative depth categories in the geologic record is sufficient in most instances. In the terrestrial fossil record, geologists can similarly discriminate among the fluvial sub-habitats of main channel, channel bar, levee, and flood plain by using sedimentary features (e.g., grain size distribution, sediment color, presence and type of root traces) characteristic of water content and soil development. States of preservation of fossil material provide additional information on environmental conditions at and immediately below the depositional interface, including the frequency and agent of burial events. Preservational condition also provides insights as to whether the fossil material is indigenous or exotic to the depositional site and thus whether the inferred ecological tolerances of fossil taxa can be used as proxy environmental information (see next section).

Environmental proxies thus may be based on physical, chemical, or biological features of the geologic record (see Table 2.1). The fossilized remains of organisms—shells, bones, non-mineralized organic tissues—typically carry several kinds of proxy information. For example, the abundance of fossils can reflect original population size, predatory damage can indicate trophic role, and the isotopic composition of their hard parts can capture information on growth temperatures, and ambient salinities (see Box 2.2). Proxy indicators can, of course, be affected by post-depositional alteration and destruction. Some have proven utility through much of the geologic record (e.g., carbon isotopes), some are limited to the Mesozoic and Cenozoic (e.g., stomatal density), and others can be used only in very young deposits under special circumstances (e.g., fossil DNA).

Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
×

TABLE 2.1 Examples of Proxy Indicators and Their Interpretation

Source

Proxy

Type of Information

Individual fossils

Number and variation of growth rings/bands, size

Seasonality, growth rates, growth conditions, generation times, biomass, age, and season of death

Fire scars in tree rings

Fire intensity and frequency

Stomatal density in leaves

Atmospheric CO2

Herbivore or predator damage

Intensity of herbivory or predation, herbivore or predator preferences, evolution of herbivore feeding strategies and plant responses

Leaf shape and size

Mean annual temperature, elevation

Overgrowth relationships in encrusters

Competitive relationships

Larval shell type

Mode of dispersal, mode of development, evolutionary history

Body size

Body size, biomass, size frequency distribution

Density

Individuals per unit area, biomass

Location

Habitat, dispersal history, geographic distribution

Tooth wear

Diet in mammals

Preservational condition

Exposure, hydraulic energy, sedimentation rate

Fossil DNA (generally limited to deposits <130 Ka)

Presence of species not otherwise preserved, evolutionary relationships and rates

Composition and structure of fossil assemblages

Taxonomic composition

Taxa present, guild structure, environmental tolerances, diverse climatological, hydrological, and other environmental variables

Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
×

Source

Proxy

Type of Information

Composition and structure of fossil assemblages

Presence/absence and relative abundance of plant functional types

Physical structure of vegetation—albedo, surface roughness, evapo-transpiration

Species richness, diversity, and evenness

Species richness, diversity, and evenness

Relative abundance of taxa

Relative abundance of taxa, dominance, shape of abundance frequency distribution

Geographic distribution

Geographic distribution, climate

Presence/absence and relative abundance of animal guilds

Sediment consistency, suspended food supply, presence and structure of vegetation

Geochemical and isotopic composition of skeletal hard parts

Strontium isotopes

Source and amount of freshwater, migration histories, home range

Oxygen isotopes

Temperature range and variability, salinity, ice volume, source of moisture

Carbon isotopes

Source of food, vegetation type, productivity, atmospheric CO2, humidity, temperature

Nitrogen isotopes

Source of food, trophic level

Boron isotopes

pCO2 concentrations

Hydrogen isotopes

Temperature, source of moisture

Multi-isotope systematics

Climate, hydrologic conditions, source of organic compounds, biogeochemical cycling

Ba/Ca ratios

Productivity

Mg/Ca ratios

Temperature

Sr/N ratios

Trophic level in mammals

Sr/Ca ratios

Temperature range and variability

Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
×

Source

Proxy

Type of Information

Biogeochemical and geochemical evidence in sediments

Biomarkers and other molecular fossils

Presence of species, trophic groups, or guilds not otherwise preserved

Chlorophyll levels in sediments

Productivity, sedimentation rate

Nitrogen and carbon isotopes in organic compounds

Source and level of productivity

Anthropogenic heavy metals and toxins

Human disturbance

Evaporite minerals

Evaporation rates

Authigenic minerals such as phosphorites and glauconites

Productivity

Mn, Fe, Mo, U, V, and Cr trace elements

Redox conditions

Sulphur isotopes

Redox conditions, atmospheric oxygen, microbial activity

Lithologic evidence

Sediment grain size and sedimentary structures

Hydraulic energy, storm/bottom disturbance, current direction, substrate character

Paleosols

Soil development and climate

Peats and coals

Wetlands hydrology

Trace fossils

Behavior, surface stability, sediment reworking rates, presence of biota not otherwise preserved, water table position

The abundance of fossil species whose living representatives are restricted to particular environmental conditions often has been used as a proxy for those same conditions in the geologic past. For example, the relative abundance in sedimentary cores of warm- versus cool-water species of planktic foraminifera was used through the 1970s to infer water temperatures using factor analysis, integrating knowledge of these

Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
×

BOX 2.2
Historical Ecology of the Colorado River Delta

Shell accumulations on the Colorado River delta provide evidence of environmental and ecological conditions before the construction of upstream dams. Inset shows differences in the relative abundance and density of the shelly fauna. SOURCE: Photograph and data by Karl W. Flessa; used with permission.

As a result of the construction of the Hoover Dam and the Glen Canyon Dam, the now-controlled flow of the lower Colorado River permits diversion of its water for the farms and cities of the Southwest. In most years since 1960, the river no longer reaches the sea. In the northern Gulf of California, the Colorado Delta and estuary have been transformed by

species’ present day ecological and environmental tolerances, until the problems inherent with non-analog associations were appreciated. This information permitted the history of Quaternary climate change to be reconstructed at a then-unprecedented level of temporal detail and environmental precision (e.g., CLIMAP, 1976, 1984), fueling a generation of climate and ocean modeling.

The diversity and sophistication of non-biotic, primarily geochemical proxy indicators that are now available for use is increasingly liberating

Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
×

human activity. But directly measuring that impact was thought impossible, because no surveys of the delta and estuary had been made before upstream dams and diversions.

With analyses of the age, abundance, species composition, and geochemistry of shells and other remains at the mouth of the river, it has proven possible to construct a baseline by which to measure the ecological impact of the river’s diversion (Flessa, 2002). Radiocarbon-calibrated, amino acid dating of shells provides 50-year resolution, indicating that the shells at the river’s mouth date from before the river’s diversion and back to approximately 1,000 years ago. Comparisons of shelly mollusks in the era before the dams to those alive today document a decrease of as much as 90 percent in population density and the near disappearance of Mulinia coloradoensis, a once dominant bivalve (Kowalewski et al., 2000). Analyses of predatory damage on shells of M. coloradoensis indicate that it was an important source of food in the trophic web of the estuary before the dam (Cintra-Buenrostro et al., in press). The oxygen isotope composition of shells of M. coloradoensis show that it inhabited brackish water and that the freshwater plume of the river extended at least 70 km from the river’s mouth (Rodriguez et al., 2001). Oxygen isotope values in the otoliths of two species of endangered fish reveal that they used the brackish waters of the estuary as nursery grounds (Rowell et al., 2004). The analyses of shelly remains have now become a powerful forensic tool in the study of environmental and ecological change.

The case of the Colorado River delta is not unique. In most circumstances, habitats have been modified by human activity long before they could be studied in their pristine condition. The historical record provided by the observations of early explorers, museum collections, or the natural accumulations of resistant remains can now be used to provide a benchmark to measure the effects of human activity.

paleoenvironmental analysis from the potential circular reasoning of using fossils both to indicate environmental change and to measure biotic response to that change. In fact, it is now often possible to apply more than one proxy method to reconstruct the record of many environmental variables (see Box 2.3). Local histories of environmental change that can be reconstructed using such cross-checks—that is, that are robust to several independent proxy methods—are especially compelling (e.g., Mann, 2002).

Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
×

BOX 2.3
Multiple Proxies at Minden Bog: Peatland Paleohydrology, Forest Composition, and Disturbance Regimes

Sediments of peatlands contain paleohydrological and paleoecological records that can be used in tandem to investigate climate variability and its ecological consequences. Ombrotrophic bogs, which receive all their water and minerals directly from the atmosphere, are widespread in the Northern Hemisphere between 45°N and 55°N. Because their water tables are perched above the regional groundwater table, they respond rapidly to variations in precipitation and evaporation. European scientists have exploited archives from ombrotrophic bogs to obtain high-resolution paleohydrological records over much of northwestern Europe (e.g., Charman, 2002; Mauquoy et al., 2002).

A recent study indicates the potential for using ombrotrophic bog archives in North America. Minden Bog, 150 km north of Detroit, Michigan, contains a 3,300-year record of hydrological variation with subcentennial resolution (Booth and Jackson, 2003). The record indicates dry but fluctuating conditions from 3,300 to 1,800 years ago, a wet period 1,800 to 1,000 years ago, and uniformly dry conditions for the past 1,000 years (see figure below). These patterns, inferred from fossil assemblages of testate amoebae, are corroborated by other paleohydrological indicators from the peats (humification, plant macrofossils) and other records from the region (lake levels).

Pollen and charcoal data, representing vegetation and fire history from the regional uplands surrounding the bog, indicate a major change in forest composition and fire regime 1,000 years ago. The shift towards drier climate led to a decline in regional beech (Fagus) populations, an increase in pine (Pinus) populations, and an increase in regional fire incidence (microscopic charcoal). Beech trees are more sensitive to drought and to surface fires than pine trees, and surface fires in humid, forested regions are more frequent during dry periods. This study illustrates the influence of climate change on forest composition and disturbance regime, and the power of using independent paleoclimate proxies together with paleoecological data.

BIOLOGICAL INSIGHTS CONTAINED IN THE GEOLOGIC RECORD

The geologic record contains information on past biotas in many forms, including fossils (the remains of individual bodies), tracks and burrows (trace fossils), and various isotopic and biomolecular (biomarker) proxies for the former existence of species or functional groups. The

Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
×

Upper diagram shows paleohydrological history of Minden Bog, Michigan, during the past 3,300 years, as inferred from testate amoeba assemblages in sediments. Black-shaded portions represent wet periods (water levels above the 3,300-year mean), while gray portions represent dry periods. Lower diagrams show pollen percentages for beech (Fagus) and pine (Pinus), and microscopic charcoal deposition. Black-shaded portions of pollen plots represent values exceeding the 3,300-year mean. SOURCE: Modified from Booth and Jackson (2003); copyright Hodder Arnold, used with permission.

combined information from these diverse sources is known as the fossil record of life on Earth.

The fossil record is widely appreciated as a source of valuable, and in many ways unique, information on the evolutionary relationships and dynamics of taxa. Paleontological data are the most robust basis—in readily fossilized groups—for quantifying key evolutionary variables

Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
×

such as species durations, speciation rates, and extinction rates, and how those relate to external factors such as climate change and such internal factors as body size or feeding strategy (e.g., Stanley, 1979; Jablonski, 1995; McKinney, 1997; Sepkoski, 1998; Kidwell and Holland, 2002). Such paleontological analyses have had, and continue to have, major impacts on evolutionary biology, for example in quantifying stasis and punctuation in species histories (e.g., Gould and Eldredge, 1993, 1997; J.B.C. Jackson and Cheetham, 1999; Gould 2002), large-scale evolutionary trends and their underlying dynamics (e.g., Stanley, 1979; Jablonski, 2000; Alroy, 2001a; Gould, 2002), the nature of evolutionary radiations and rebounds in terms of both taxonomic richness and morphological disparity (e.g., Foote, 1997; Lupia, 1999; Eble, 2000), and even the kinds of changes in embryological development that have fueled major evolutionary transitions and diversifications (e.g., Knoll and Carroll, 1999; Shubin and Marshall, 2000; Valentine, 2004). Paleontology is now an integral part of evolutionary biology (Maynard Smith, 1984; Ruse and Sepkoski, 2005), and macroevolution as conceived from the geohistorical record is becoming a part of the educational and research canon (e.g., inclusion in such general works as Raff, 1996; Ridley, 2003; Freeman and Herron, 2003; Coyne and Orr, 2004).

Less well appreciated—especially outside the specialty—is the enormous value of the fossil record for ecological and biogeographic analysis across a range of temporal and spatial scales, including those directly comparable to modern-day studies. These are the biological insights that are the primary focus of this report on the nature and value of geohistorical records.

Many of the biotic variables that are widely analyzed in biology are commonly captured by body fossils in the geologic record, especially for the higher (multicellular) organisms that constitute an important component of lower trophic levels (photosynthetic primary producers, detritivores, deposit feeders) and the entire middle and upper portions of food webs (herbivores; omnivores, including filter feeders; multiple tiers of carnivores). This readily acquired information includes the taxonomic affinity, body size, sexual dimorphism, habitat preferences, ecophenotypic variability, geographic range, functional group (feeding mode, pollination syndrome, spatial tier), and in many instances, the ontogenetic age and rank abundance of fossil species. Such information can yield information on various aspects of the species’ biology (e.g., growth rates and likely generation times) and ecological roles (e.g., dominance, trophic position). Multispecies assemblages of fossils can provide information on community composition, species richness, diversity, evenness, predator-prey relationships, food-web structure, and species body-size distributions. Indeed, much of the data needed to determine ecological scaling relation-

Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
×

ships (e.g., Brown and West, 2000) is obtainable from geohistorical records.

In addition, some kinds of data used in biological analysis are at present rarely extracted from geologic records, with DNA being perhaps the most notable example (see Box 2.4). DNA in living species is extremely valuable in testing genetic divergence among populations and species under conditions of habitat fragmentation and environmental change, and is an important partner to morphologic data in phylogenetic analysis. Unfortunately, DNA degrades rapidly even before burial, and has been extracted from sedimentary records in an intact (readable) form only from specimens younger than ~150 Ka (Poinar et al., 1996; Poinar and Stankiewicz, 1999). This window for DNA preservation is very narrow compared to the evolutionary duration of most species (e.g., ~5 Ma average duration for metazoans). However, within the window of geologic time where it can be extracted successfully, fossil DNA has great, largely untapped potential for detecting the presence of species that did not contribute body fossils or trace fossils (e.g., Kuch et al., 2002; Willerslev et al., 2003) and for studying among-species variation in population-level responses to climate changes at the centennial to millennial scale (e.g., Orlando et al., 2002; Hadly et al., 2003).

BOX 2.4
Ancient DNA from Arid South America

Outside the microenvironment of a living cell, DNA is an unstable molecule and its high nitrogen and phosphorus content make it subject to rapid degradation and utilization by microbes. Accordingly, ancient DNA is preserved only in a restricted set of environments. Some of the greatest successes in ancient DNA analysis have come from desiccated or mummified organic remains preserved in caves and rock shelters of arid and semiarid regions. In an international collaborative effort, scientists from the United States, Germany, Argentina, and Chile have extracted, amplified, and sequenced ancient DNA from rodent middens and herbivore dung from sites on the arid western slope of the central Andes and the semiarid eastern slope of the southern Andes (Kuch et al., 2002; Hofreiter et al., 2003).

Mitochondrial DNA extracted from late Pleistocene herbivore dung at Cuchillo Curá, southwestern Argentina, represents a previously undescribed, extinct species of ground sloth (Hofreiter et al., 2003). Chloroplast DNA revealed several different plant species in the ground sloth’s diet; many of the same plant taxa were identified in morphological analyses of plant cuticle from the dung.

Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
×

The photo on the left shows the Cuchillo Curá site. Ground-sloth dung is from the base of the large crack, just left of center. The photo on the right shows the interior of the Cuchillo Curá rock shelter; the rock hammer rests on the partly eroded dung layer and rodent midden. Dung from which DNA was extracted was radiocarbon dated at 17,300 years BP. SOURCE: Photographs by J.L. Betancourt, USGS; used with permission.

An 11,700-year-old rodent midden from the central Atacama Desert of northeastern Chile yielded mitochondrial DNA of a midden-forming rodent (leaf-eared mouse) and chloroplast DNA representing at least eight plant taxa (Kuch et al., 2002). The mitochondrial DNA not only permitted identification of the rodent species responsible for building the midden but also provided clues to its biogeographic and evolutionary history.

These and other recent studies clearly show that fossil DNA, far from being a mere curiosity, can provide important information on past biotic communities, environments, animal diets, and species occurrences, and can be used to develop and test specific hypotheses on the evolutionary responses of plant and animal populations to environmental change (see also Poinar et al., 2003; Hadly et al., 2004; Willerslev et al., 2003).

Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
×

Some important paleobiologic information that at present is largely unknown but extremely important is quantitative information on species’ metabolic rates and reproductive success. Information about reproductive rates, dispersal capability, source-sink budgets, and absolute population size or density is essential for energetic analyses, including evaluations of competitive success and adaptive strategies. This information is possibly knowable, but at present can be estimated only by extrapolation from living analogs, and deserves much more vigorous investigation.

The application of new technologies from outside paleobiology has already greatly enhanced the power and sophistication of biological information that can be extracted from geohistorical records. For example, accelerator mass spectrometry now permits 14C dating of very small specimens, microsampling of growth increments and improvements in sample processing now allow stable isotope analyses at daily resolution, computerized tomography of fossil specimens reveals previously hidden morphological details, and database and geographic information system technologies allow new analyses of the temporal and geographic distribution of fossils.

Information on Multicellular Life

The multicellular fossil record has a long history of study, and the kinds of biological information sought from the record continues to increase (as noted above, ranging from information on growth rates and dispersal modes to trends in community structure and diversity over geologic time). Intensifying research over the last two decades is transforming our understanding of the quality and power of these many kinds of paleobiological data, especially actualistic studies in modern environments, rapidly developing chemical proxies (summarized in the section on microbial life below, and illustrated in Box 2.5), meta-analysis and other statistical syntheses of existing data, and model simulations. Research programs in this general area—determining how to extract more and better biological information from geologic records—routinely integrate approaches and information from both the biological and geological sciences, and frequently involve collaborations between biologists and geoscientists. Consequently, they not only provide vital information into the fidelity (faithfulness) and acuity (fineness of resolution) of paleontologic data but also provide models for collaborations between the disciplines.

An improved understanding of the controls and selectivity of fossilization does not in itself alter the quality of the record, but quantitative assessments of bias and acuity provide a more rigorous basis for designing paleobiologic research, including improving protocols for sampling and analysis, and for judging which biological questions are tractable for

Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
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BOX 2.5
Natural and Human-Caused Variation in Salmon Populations

Salmon are a valuable resource in the Pacific Northwest, and fluctuations in their abundance have both economic and cultural impact. How much of this fluctuation is driven directly by human activity and how much is linked to variations in climate?

The ability to look back in time to estimate both the size of salmon populations from δ15N values and the productivity of lake systems from indicator species of plankton, reveals not only important ecological feedbacks but also serves to differentiate between human-caused and natural variation in these ecosystems. Variations in salmon populations in several Alaskan lakes over the past 2,000 years were documented by Finney et al. (2000, 2002) by examining variations in δ15N in lake sediments. Sedimentary δ15N acts as a proxy indicator of salmon population size, because in contrast to the 0‰ value of terrestrial nitrogen sources, sockeye salmon tissues have δ15N values of +12‰, reflecting their marine food diet during growth. Adult salmon migrate from the marine environment to their natal lake to spawn and die, adding their distinctive marine δ15N isotopic signature in lake sediments.

Salmon carcasses are an important source of nutrients to these Alaskan lakes, and a natural positive feedback exists between salmon abundance and lake productivity. Analysis of lake sediments indicates that the abundance of diatom and zooplankton remains—proxy indicators of lake productivity—fluctuate with δ15N. Productive lakes support larger salmon populations, because zooplankton are essential for juvenile salmon growth and, in turn, larger salmon populations result in more productive lakes as a result of greater nutrient loadings.

Fishing activity can disrupt this natural feedback. Finney et al. (2000) showed that the δ15N of lake sediments has decreased since the 1880s, when commercial fishing began to lower the number of salmon returning to the lakes. The recent decrease in δ15N values is greater than at any time in the past 300 years, and is despite warming marine waters that should lead to increased abundance. This decline tracks the history of commercial fishing and is only found in lakes having salmon runs, suggesting a disruption of the salmon-based nutrient cycle, which may hinder the recovery of salmon populations in such lakes.

Fluctuations in δ15N and lake productivity are also evident during the ~2,000 years before commercial fishing (Finney et al., 2000, 2002; Gregory-Eaves et al., 2003). Clearly, natural factors can cause significant variations in salmon abundance. Temporal variations in multiple lakes are similar to those of proxy paleoclimate information for the North Pacific region, and thus consistent with climatic change being an important driver of salmon abundances. Temporal changes in Alaskan salmon abundance also match those in other marine proxy records, such as sardine and anchovy populations off southern California, highlighting the widespread effect of climatic change on marine and coastal ecosystems in the North Pacific region.

Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
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study. Some new insights relevant to the multicellular fossil record are highlighted below.

1. What proportion of local and regional diversity is captured by the fossil record? In marine benthic systems, actualistic studies indicate that 75-100 percent of living marine macrobenthic invertebrate species that have biomineralized hard parts (e.g., shelled invertebrates such as most mollusks, sea urchins, barnacles, and corals as well as mineral-secreting algae and various unicellular benthos) are present as dead skeletal material in immediately surrounding sediments, and a similar proportion of living genera have known fossil records (Schopf, 1978; Valentine, 1989; Kidwell and Flessa, 1995; Kidwell, 2002a). These numbers are quite high, given that the likely preservation potential of any given shelled individual is probably quite low. However, a better estimate of the genus-level completeness of the entire macrobenthic fauna is ~40 percent, which is a much-cited determination of the proportion of shelled taxa, as opposed to taxa that are only sparsely mineralized (e.g., starfish and some polychaetes, arthropods, and sponges; ~30 percent of these genera have a fossil record), and taxa composed exclusively of soft tissues (very few modern genera are known in the fossil record [Schopf, 1978]). Few comparable studies have been attempted for terrestrial animal communities, although one regional study of land mammals found a high overall fidelity but a strong correlation with body size (Behrensmeyer and Dechant Boaz, 1980; and see review in Kidwell and Flessa, 1995; Behrensmeyer et al., 2000). Thus, although more research is needed on how these percentages vary among environments and groups, modern death assemblages as well as the fossil record can capture a significant proportion of biological diversity among mineralized metazoans.

Animal and plant groups having only refractory organic coats and skeletons (e.g., pollen, cystate microbes, woody plants, arthropods) are less durable and require particular conditions for preservation, but individual instances of good fossil preservation can be highly resolved (in taxonomy, morphology, and temporal precision). Some intervals of good fossil preservation can be both temporally extensive and highly detailed. For example, sedimentary cores of lakes provide centennial resolution of vegetation changes for the last 12,000 years using a combination of pollen and plant macrofossils; lake records are sufficiently abundant to provide 100-km geographic resolution of vegetational and biogeographic patterns for most of eastern North America; and high-elevation lake records together with low-elevation packrat midden records provide similar resolution in the western United States (Betancourt et al., 1990; Webb, 1993; S.T. Jackson et al., 1997; Williams et al., 2004). Truly extraordinary conditions are required to fossilize soft tissues for geologically significant

Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
×

periods of time (pre-Quaternary), but dozens of such “Fossil Lagerstätten” are known (e.g., Allison, 1988; Selden and Nudds, 2004; Allison and Briggs, 1993). Although generally only sparsely fossiliferous in terms of numbers or density of specimens, they can be invaluable sources of novel morphologic, ecologic (e.g., preserved gut contents, pollen on fur), biogeographic (e.g., hummingbirds in the Eocene of Germany, whereas today they are strictly a New World group), and evolutionary information (Gould, 1989).

2. How common is the transport of dead remains to settings outside the species’ original habitat or geographic range? Homogenization of the original spatial distributions of taxa due to postmortem transport and mixing is a persistent concern among paleontologists. Such transport does occur, but its effects are predictable and quantifiable. For example, pollen of wind-pollinated plants is more readily and widely transported than that of animal-pollinated plants, but the area of source forest whose pollen is sampled by the sedimentary record of a given lake is proportional to the surface area of the lake—more detailed understanding of vegetation type and distribution can only be acquired from small basins (S.T. Jackson, 1994; Davis, 2000) or by using plant macrofossils. Out-of-habitat transport and mixing of benthic macro- and microfauna in shallow marine (estuarine to continental shelf) settings is less significant than might be predicted—facies-scale differences in fauna typically persist even in relatively high-energy, level-bottom settings. Environments having high proportions of allochthonous taxa are steep-gradient seafloors and narrow shelves, high-energy tidal channels, and deepwater turbidite fans, which are readily recognized from independent geologic evidence (e.g., Parsons and Brett, 1991; Kidwell and Bosence, 1991; Kidwell and Flessa, 1996). Transport among level-bottom habitats does occur, but “exotic” taxa are more likely (1) to be small-bodied than large-bodied, (2) to have lived attached to the seafloor rather than burrowed within it, and (3) to be represented by relatively few individual specimens. Actualistic research further indicates that many taxa that are “suspicious” because they are poor ecological fits to modern environmental conditions are instead non-transported ecological relicts from former community states (e.g., persistence of epiphytic benthic foraminifers in surficial sediments of seafloors that are no longer vegetated; presence of intertidal mollusks in subtidal sediments in the wake of rapid marine transgression [Anderson et al., 1997; Flessa, 1998; Roy et al., 2001]). Among terrestrial vertebrates, predators and scavengers can be as important—or more important—than physical processes in transporting bones, but again most taxa are found within or close to their original habitats (e.g., Dunwiddie, 1987; Greenwood, 1991; Hadly, 1999; Behrensmeyer et al., 2000; Birks, 2003).

Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
×

The record of the broader geographic distribution of fossil species is affected by the uneven distribution, preservation, and sampling of sedimentary environments in the geologic record. Nevertheless, shifts in the geographic range of species in response to climate change are readily detected in many fossil records, with most study focused on Quaternary pollen, mammals, and planktonic foraminifera (e.g., COHMAP, 1988; Faunmap Working Group, 1994; CLIMAP, 1976; Roy et al., 2001). The geographic distribution of pre-Quaternary fossils has frequently been used to recognize major immigration events in both the marine and terrestrial realms (e.g., Stehli and Webb, 1985; Vermeij, 1991; Jablonski and Sepkoski, 1996; Clark et al., 1998); to examine the responses of terrestrial faunas to tectonic and climate change (e.g., Carrasco et al., 2005); and to track the geographic patterns in recovery from mass extinction (e.g., Jablonski, 1998). Biases caused by incomplete preservation and or incomplete sampling of geographic regions can be recognized by careful consideration of the expected statistical distribution of fossil samples under varying degrees of completeness (e.g., Holland, 2000; Smith et al., 2001).

3. How faithful are the density and relative abundances of species, and the relative frequencies of feeding types, life histories, and body sizes in fossil assemblages relative to those in the original biota? Organisms composed exclusively of soft (non-mineralized, volatile) tissues require unusual and rapidly acting processes to be preserved, and thus assemblages rich in such specimens should be virtual snapshot records of standing communities. Assemblages composed of taxa having refractory (chitinous, woody) or mineralized skeletal parts have greater potential to be biased by differential preservation processes, including postmortem transport among taxa and age classes, as well as mixing of generations from time-averaging (see below). In what detail can community structure and change be recognized by tracking only one comparably preserved portion (e.g., only pollen and wood, or only well-mineralized taxa); and among the preserved fraction, how faithful are quantitative data to that portion of the original source community? Studies in modern depositional environments comparing local death assemblages with surrounding live communities indicate that among the intrinsically preservable taxa, the original proportions of taxa—and thus many aspects of community structure—are surprisingly well recorded, even by the time-averaged death assemblages that dominate the fossil record of many groups (Kidwell and Flessa, 1995; Kidwell, 2001a; 2002a; see Box 2.6). Mollusks, pollen, and mineralizing microbial groups (e.g., foraminifera, diatoms, and radiolarians) have been the subjects of extensive research on this issue (e.g., Donovan, 1991; S.T. Jackson, 1994; Sugita, 1994; Donovan and Paul,

Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
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BOX 2.6
Ecological Information from Time-Averaged Death Assemblages

The fidelity of the taxonomic composition, spatial distribution, and abundance of taxa in fossil assemblages relative to the source living populations is particularly problematic in assemblages of the well-mineralized taxa that dominate the fossil record. Such taxa have generally high preservation potential, but their hard parts are consequently also likely to survive postmortem transport and—whether transported or not—accumulate in time-averaged death assemblages where the number and relative abundance of taxa might be distorted by their differing rates of mortality and relative postmortem fragility (Vermeij and Herbert, 2004).

Paleoecological fidelity has been tested for many decades primarily by estimating the live-dead agreement in modern environments; dead data are generated by sieving dead remains from sediments accumulating in the target habitat or region, and live data may be derived from the same samples (e.g., benthic organisms) or from various kinds of line-transect, quadrant, trap, or aerial surveys of the surrounding area. Shelled mollusks (bivalves and gastropods) have been the subject of an especially large number of such studies (e.g., see figure below).

Percentage of live species of shelled mollusks also found as dead remains in marsh, intertidal, coastal, and open marine habitats. Vertical line at top of bar indicates standard error; number above habitat name indicates the number of studies analyzed. SOURCE: Kidwell (2001b); used with permission.

Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
×

To acquire a quantitative estimate of these effects, as well as a robust estimate of true death assemblage fidelity, Kidwell (2001b, 2002a,b) reanalyzed raw data from 19 molluscan live-dead studies using a standard set of measures of live-dead agreement, and then subjected these to both conventional statistics and meta-analysis.

The analysis yields fundamentally good news about marine sedimentary environments as sinks of locally produced molluscan hardparts (Kidwell, 2002b):

  1. Much previously reported variation among datasets is due to differences in sample size—where data are based on at least 100 live and 100 dead individuals, live-dead agreement for all metrics becomes far more consistent among datasets.

  2. Death assemblages capture local live diversity efficiently—88 percent of species documented living in a habitat are also present dead in the same set of samples.

  3. Seventy-six percent of all dead individuals are from these same species, that is, species that are only found as dead remains (and thus that might be transported exotics) are each represented by very few individuals.

  4. Despite the high variability among studies, species’ dominance in death assemblages is in fact highly correlated with those species’ dominance or rarity alive.

  5. Sediment grainsize and other measures of environment fail to bias preservation in any strong or consistent way, contrary to expectations.

  6. All ecological metrics tested so far show a strong, mesh-size effect; the fidelity of molluscan death assemblages to their local live communities is significantly higher if the analysis focuses only on the late juvenile to adult individuals, rather than if it includes ecologically and taphonomically volatile larvae and early juveniles.

The pervasiveness and magnitude of the mesh-size effect suggests a simple protocol for improving the recovery of accurate ecological data from molluscan death assemblages, namely, target the late juvenile to adult segment of the age- and body-size frequency distribution, which is ecologically and taphonomically relatively stable. This basic approach may well prove useful in other animal groups, inasmuch as mortality and postmortem destruction both tend to be focused on the ontogenetically youngest members of populations. These results are very encouraging, both for using dead remains for rapid biodiversity assessment in modern environments, and for building prehistoric time series using sedimentary records.

Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
×

1998; Martin, 1999; Behrensmeyer et al., 2000; Davis, 2000). Detailed investigations of the taphonomy of woodrat middens (e.g., Betancourt et al., 1990; Frase and Sera, 1993; Lyford et al., 2004) have validated their utility in reconstructing Quaternary vegetation change in arid regions, and leaf litter captures the richness of surrounding woody plants to moderate to high degrees (Burnham, 1993; Meldahl et al., 1995). Although other major mineralizing groups (corals, echinoderms, land mammals, reef and freshwater mollusks; see Kidwell and Flessa, 1995; Behrensmeyer et al., 2000) have received less attention, results so far are very encouraging. The ability to reconstruct the characteristics of ecological relationships, relative abundances, and biomass is important, because such information is the key to answering many important questions about the structure of communities and the persistence of species under the stress of environmental change.

4. To what extent are the remains of multiple generations or successive community states mixed into single fossil assemblages, thereby blurring the details of events and transitions? Studies of modern communities and environments have been critical to quantifying these values, and despite the complexities the results are encouraging. For example, radiocarbon dating and other chronologic methods indicate that individual cm-scale samples in Quaternary lake records represent only approximately one decade of time-averaging of pollen input from the local community; in many instances, annual resolution is possible, but as in modern monitoring time series, it is often easier to analyze trends when annual data are blurred into decadal time-windows (running averages). Decadal resolution in sedimentary cores can therefore be highly valuable. Annual to decadal resolution is not uncommon in estuarine settings (e.g., Brewster-Wingard and Ishman, 1999), even in rocks that are hundreds of millions of years old (e.g., Lanier et al., 1993). In fully marine settings characterized by slower sediment accumulation or more vigorous postmortem mixing, time-averaging is more prolonged; for example, specimens from shelled-mollusk assemblages can range up to a few thousand (in nearshore settings such as bays and lagoons) to ~10,000 years old (on sediment-poor continental shelves), due to the strong mixing of shell generations by burrowing organisms and storm reworking of the seafloor (Flessa and Kowalewski, 1994; and see Martin et al., 1996, for mixed mollusk-foraminifer assemblages) (see Box 2.7). However, even in such time-averaged samples, most specimens are quite young (at most a few hundred years), and thus the effective time-averaging of information on species’ isotopic signatures and relative abundance is probably much less than predicted from the maximum shell age in the collection (Olszewski, 1999; Kidwell, 2002b). Again, mollusks, pollen, and foraminifera have

Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
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BOX 2.7
How Much Time Is Represented in Accumulations of Shells in Modern Marine Environments?

Flessa and Kowalewski (1994) surveyed published radiocarbon dates for shells from nearshore and offshore habitats on today’s continental shelves. They found that nearshore accumulations generally represented a total of ~1,000 years of accumulation while those from shelf habitats had shells averaging ~10,000 years in age (see figure below). These results provide estimates of the degree of time-averaging in comparable fossil deposits.

Histograms showing differences in the time-averaged age distribution of shells from modern nearshore and shelf environments. SOURCE: Reprinted from Flessa and Kowalewski (1994)1; by permission of Taylor & Francis AS.

1  

“Shell survival and time-averaging in nearshore and shelf environments: estimates from the radiocarbon literature” by K.W. Flessa and M. Kowalewski, from Lethaia www.tandf.no/leth, 1994, vol 27, pp 153-165.

Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
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received the greatest research attention, but preliminary results for other major groups are promising.

5. How faithful are observed appearances in the fossil record to the actual evolutionary duration and temporal sequence of species? What is the likelihood that the observed first or last appearance of a species in the fossil record reflects its true time of origination or extinction? Thus, what are the confidence limits on our knowledge of species’ evolutionary durations and temporal sequence? Such information is critical for relating particular paleoenvironmental changes to their evolutionary effects. Knowledge of raw extinction and speciation rates allows the ranking of clades, functional groups, and habitats for relative extinction risk; permits the discrimination of pulsed and non-pulsed nature of biotic originations and extinction; and addresses many other questions of biosphere dynamics on evolutionary timescales. These issues have been examined aggressively over the last 15 years by a combination of statistical analysis of large empirical datasets and model simulations that use increasingly realistic assumptions for biases in geohistorical records (Marshall, 1990, 1997; Holland, 1995, 2000, 2003; Foote, 2003). Indeed, even when confidence limits cannot be assigned, cohort analysis (e.g., Foote, 2001a,b) provides valuable estimates of evolutionary rates that are relatively insensitive to incomplete stratigraphic ranges. Despite concerns about continent-to-continent differences in environmental history, and an increase in the volume of fossiliferous rocks toward the present, many of the first-order features of the fossil record (repeated mass extinctions of global biota, secular increase in global diversity, differences in evolutionary rates, bursts of diversification) are robust to sampling and taphonomic biases (e.g., Foote, 2003).

6. To what extent are isotopic, biomolecular, and other chemical signatures of particular taxa or functional groups (e.g., plankton, eukaryotes, C3 photosynthesizers) modified by postmortem and burial processes? And what factors determine the fidelity of the chemical proxy record of past species and communities? The comparatively easy preservation of refractory and mineralized skeletons favors the preservation of biological and environmental information, including extremely high-resolution isotopic data from single skeletons as a result of the accretionary growth of corals, mollusks, and trees, and including isotopic variation in tree rings (e.g., Poussart et al., 2003). Concern with the post-depositional stability of these signatures dates to their first application in the 1950s, and this remains an important aspect of research into the development of new methods and appropriate selection of samples (Grossman et al., 1996; Kohn et al., 1999; Pearson et al., 2001). Improvements in instruments

Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
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increasingly permit analysis of single skeletons or accretionary skeletal layers rather than bulk samples, and analysis of single organic compounds, most notably the comparatively stable lipids. These methodological improvements are dramatically increasing the resolving power and confidence in paleoecological and paleoenvironmental reconstruction from chemical proxies in general (e.g., Norris and Corfield, 1998; Hinrichs et al., 2001), as are efforts to develop criteria for selecting materials for analysis (Koch, 1998; Dawson et al., 2002).

In addition, the original mineralogy of hard parts is sometimes completely altered, although such alteration is more common in older sediments. For example, taxa whose skeletons were originally composed of the biomineral aragonite are rarely preserved in that form in Paleozoic age rocks, but instead are usually demineralized (occur as open molds in the rock), recrystallized to more stable calcite, or replaced by some other mineral; Schubert et al. (1997) note that as much as 20 percent of Paleozoic fossils are preserved as silica rather than their original mineralogies. While such alteration precludes isotopic analyses of the hard parts, and is thought to shorten observed stratigraphic ranges (e.g., Briggs and Crowther, 2001; Donovan and Paul, 1998) and bias proportional abundances within individual assemblages (e.g., Cherns and Wright, 2000; Wright et al., 2003), the quantitative impacts are not yet fully explored and other paleoecological information (e.g., geographic distribution, relative abundance, ontogenetic age, repair scars) may still be obtainable.

Information on Microbial Life

The past decade has seen an increased appreciation for the importance of microorganisms in Earth’s biotic systems. Microorganisms—microscopic, often unicellular organisms, both prokaryotic (exclusively clonal) and eukaryotic (capable of sexual reproduction)—were virtually the only form of life during the first four-fifths of Earth history, and their biogeochemical conditioning of Earth’s surface environments was critical to the evolution of higher life as it is known on this planet, including the evolution of multicellular animals (in the late Precambrian) and the invasion of land (in the early Phanerozoic). Microbial organisms still dominate primary productivity on Earth today (that is, photosynthetic and other autotrophic means of synthesizing organic matter from inorganic compounds) despite the evolutionary diversification and prominence of higher plants. Microbial life also dominates the recycling of organic detritus (breakdown of dead or discarded biological tissues into simpler organic and inorganic compounds) (Fenchel, 1988; Perry et al., 2002).

Although multicellular plants and animals participate significantly at lower trophic levels and dominate all higher trophic levels (herbivores,

Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
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carnivores, omnivores) that we exploit directly for food and other sustenance, microbes are critical components in the functioning of Earth’s biosphere, and in biogeochemical cycling in particular. Understanding the functioning of microbial communities, especially in systems that include multicellular life, is an important aspect of understanding ecological dynamics. A major direction of research lies in the development of techniques to identify, from geohistorical records, the presence and role of microorganisms in ecosystems. As an essential aid to interpreting that record we also need to know how the organic and mineral records of microbial activity are modified by subsequent burial, diagenesis, and metamorphism.

Most of life on Earth over the history of the planet has been microbial. Certainly, all early recognizable life was unicellular, but the evidence for this from rock textures (>3 Ga) as well as microfossils and isotopic signatures has remained controversial (Grotzinger and Rothman, 1996; Reid et al., 2000; Brasier et al., 2002; Garcia-Ruiz et al., 2003; Schopf et al., 2002). The record of Proterozoic microbial life is much better, providing an increasingly rich record of the diversification of both prokaryotic and early eukaryotic lineages.

In contrast, microbial groups having biomineralized tests (including photosynthetic diatoms and calcareous nannoplankton, and heterotrophic [animal-like] radiolarians, silicoflagellates, and foraminifers) have an excellent fossil record. With the exception of diatoms and benthic foraminifera, these Phanerozoic groups largely have been exploited for their usefulness in biostratigraphic correlation and as carriers of isotopic-proxy information on environments, rather than to gain insights into their roles in ecological dynamics. Analysis of calcareous and siliceous microfossils continues to be of incalculable value for paleoceanographic investigations (particularly by the ocean drilling community), providing much of the chronostratigraphic framework and environmental information for the recognition of climate change, productivity, changes in circulation, and major evolutionary events. As many studies illustrate, the attributes that make these microfossils ideal for biostratigraphic and environmental analysis of small-volume, high-resolution core samples—their microscopic individual size, relatively large population sizes, widespread occurrence, mineralized skeletons identifiable to species level, and skeletal capture of isotopic signatures—also make them valuable sources of ecological insights. These insights range from the local and regional to the global-scale shifts of planetary respiration associated with mass extinctions and other extraordinary episodes (e.g., Falkowski et al., 2004). Furthermore, most of these biomineralizing groups already have been subject to taphonomic evaluation in modern environments (discussed above in the section on multicellular life).

Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
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Although the record of non-mineralizing microbial life is better in the more recent record, organic molecules of definite biological origin can be well preserved even in the pre-Phanerozoic record. Much of what is known about the timing of major microbial innovations in the deep past and allied changes in biogeochemical cycling comes from the geologic record of lipid biomarkers and isotope signatures. For example, data for lipids recovered from the rock record have made it possible to establish the first appearance of the Archaea and of eukaryotes (Brocks et al., 1999; Summons et al., 1999), and allowed recognition of some significant evolutionary expansions (e.g., of marine Archaea in the mid-Cretaceous). Lipids make excellent molecular fossils and proxies for biogeochemical processes because their structures are resistant to degradation. Some have molecular structures that are unique to the organisms that make them (that is, serve as biomarkers; e.g., Brocks et al., 1999; Huang et al., 1999; Pancost et al., 2002), and most can preserve C- and H-isotope signatures.

Biomarkers are proving to be increasingly valuable in reconstructing the input of organic matter and microbial dynamics of ecosystem change in Quaternary environments, especially in lakes and coastal estuaries of high societal and economic value (e.g., Boon et al., 1979; Laureillard and Saliot, 1993; Canuel et al., 1995; Mudge and Norris, 1997). There, high levels of primary production, caused by elevated nutrient loading (eutrophication) and/or diminished ability of herbivores to crop production (because of overfishing and top-down stresses), can lead to increased delivery of organic matter to the lake bottoms or seafloors and a cascading series of responses (the Baltic Sea, Adriatic Sea, and Chesapeake Bay are good illustrative examples where considerable controversy exists over both natural versus human, and bottom-up versus top-down drivers; see Malone et al., 1999; J.B.C. Jackson et al., 2001; Boesch et al., 2001). Biomarkers now permit the terrestrial, algal, and bacterial components of the total organic rain to be differentiated and tracked over time (e.g., in the Chesapeake; see Zimmerman and Canuel, 2000, 2002). These components change in absolute and relative abundance as the microbial loop grows to dominate the food web, eventually leading to low-oxygen conditions at the lake bottom or seafloor and in some portion of the water column. Such low-oxygen conditions are recognizable by laminated sediments, dark sediment color, biomarkers of anaerobic bacteria, carbon and nitrogen isotopic changes, iron and other metal speciation chemistry, and—in estuarine environments—by distinctive sulfur chemistry (Jonsson et al., 1990; Cornwell et al., 1996; Struck et al., 2000; Voss et al., 2000; Shen et al., 2002). Establishing when these transitions from complex food webs to much simpler Precambrian-like microbial loops occur relative to human activities (e.g., the onset of intense agricultural fertilization, commercial shell or fin fisheries, urbanization) is critical for evaluating the roles of

Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
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anthropogenic and other stresses in estuarine and lacustrine deterioration, as well as for achieving a more general understanding of the dynamics of microbial communities in ecosystems containing higher organisms.

There is potential to use these biomolecular methods in deeper time both in these and other environments and to develop proxies for a larger array of organisms, both microbial and multicellular. Conclusions based on lipid biosignatures are still limited by incomplete knowledge of how lipids are transformed upon burial (lipid diagenesis), which organisms make various lipid types (taxonomic or metabolic specificity), and the extent to which these organisms might fractionate associated isotopes (vital effects). Nevertheless, these questions are tractable, and improved understanding of the diagenesis of biomarkers and the development of new biomarkers, especially to diagnose the arrival and expansion of invasive and harmful taxa, deserve priority in future research.

Isotopic signatures can be preserved in organic molecules from geohistorical records that range from decades to billions of years in age. For those signatures that are biological in origin, the magnitude of the isotopic fractionation potentially can discriminate among a variety of candidate organisms. For example, bacteria that aerobically oxidize methane can use one of two enzymes, and the isotopic fractionation depends upon the enzyme used. At present we do not yet completely understand the full array and combined effects of the many complex factors that determine the isotopic signatures preserved in the rock record, and thus this remains an important area for future research. For example, considerable progress is being made in the analysis of isotopic fractionation by higher organisms (e.g., distinguishing C3 versus C4 photosynthesis in plants), but further work is required to understand the trophic significance of fractionation and its post-burial preservation, for both microbial and multicellular organisms.

Genomic data should facilitate a more complete analysis of the diversity and distribution of enzymes involved in important biogeochemical processes today, and permit their calibration to isotopic signatures. Because gene sequences also contain within them a record of phylogenetic relationships among microbial groups (and thus implicitly their temporal order of appearance), genomic data have the potential to greatly improve our understanding of the evolution of biogeochemical pathways and their isotopic signatures. This is a valuable tool to combine with the preserved geologic record of molecular fossils, from which the absolute timing of microbiological and biogeochemical innovations can be deduced.

PRECISION AND ACCURACY OF GEOHISTORICAL RECORDS

Measuring and documenting rates of change are some of the primary goals of both biology and paleontology, and thus the fineness (precision)

Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
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and truth (accuracy) with which the relative timing and absolute ages of events can be determined are critical. Techniques for measuring geologic age vary greatly in the materials dated, the methodology, and the temporal range. Figure 2.2 shows the time depth and resolution of a variety of useful techniques, and each has power in a particular set of sample materials, environments, and depth of time. It is useful to distinguish among methods that directly date the biological materials themselves, those that date associated inorganic materials, and those that establish the contemporaneity of events.

Direct Dating of Biological Materials

Tree rings and their analogs in coral, mollusk, and other accretionary hard parts provide a chronology based on the periodicity of accretionary growth in these organisms. In addition, variation in growth rates or biogeochemical composition can document environmental variation at annual and subannual temporal scales. In effect, the preserved remains themselves provide a clock and a calendar that can be used to date the remains directly. The use of dendrochronology (tree ring dating) is currently restricted to habitats with strong seasonal variation in growth, and can be extended through cross-dating to thousands of years before present. Corals are largely limited to tropical, shallow marine habitats, with individual sclerochronologies able to be taken as far back as a few centuries, and older time series able to be developed using radiometric dating. Decadal-, centennial-, and millennial-scale modalities generally can be recognized from these annual records. The development of chronologies based on accretionary growth in mollusks are in their infancy, but long-lived species show promise for ~1,000 year chronologies (Marchitto et al., 2000), and even short-lived species provide useful decadal-scale chronologies (Jones et al., 1989; Schöne, 2003). What chronologies based on rapidly growing species lack in total duration, they often make up for in sub-annual resolution.

Although the use of internal growth periodicities for absolute chronologies is restricted to the late Quaternary (e.g., dendrochronology), most of these methods can be applied in the deeper past where appropriate materials are preserved. These high-resolution time series are not always tied to a comparably resolved absolute timescale, but they do provide valuable estimates of past seasonal and environmental variability in temperature, salinity, and nutrient cycling (e.g., Purton and Brasier, 1999; Steuber, 1996).

Radiocarbon analysis, using the unstable 14C isotope, is the primary method for direct dating of organic materials less than 40,000-50,000 years old. Radiocarbon ages traditionally have been based upon measurement

Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
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FIGURE 2.2 Effective temporal range (vertical axis shows millions of years into the past) and precision (width of horizontal bars represents 2-sigma confidence intervals) of some dating techniques commonly used to date geologic materials. The numbers below each technique indicate 2σ variation expressed as a percent of calculated age; horizontal scale bars below each expanding triangle express the 2σ variation in millions of years. SOURCE: Doug Erwin, National Museum of Natural History; used with permission.

Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
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of β-particle emission during decay of 14C isotopes. Such measurement requires large amounts (usually ≥1 g) of carbon, which often necessitates mixing of sediment constituents or organic materials of different ages. Some aquatic plants and algae use HCO3 and CO3= in photosynthesis, which may derive from dissolution of ancient carbonate-rich bedrock or surficial materials. Many organic-rich materials (bone, peat, shell) can be contaminated by intrusion and replacement by younger or older carbon. Thus, conventional β-decay dates are subject to imprecision and error beyond the standard counting error. Development of accelerator mass spectrometry (AMS) as an alternative means of assessing isotopic composition of carbon samples helps circumvent some of these problems. Much smaller quantities of carbon are required (as little as 10 µg), which minimizes time-averaging , and has other practical advantages:

  1. Dates can be obtained on compound-specific organic fractions from samples, using the most stable organic molecules and avoiding those most likely to exchange carbon with the surrounding environment. This has been especially useful in the dating of bone, shells, and concentrations of pollen and other microfossils.

  2. Dates can be obtained on one or a few specimens (plant fossils, bones, shells), separate portions of long-lived individuals, and even individual carbon compounds (e.g., Ohkouchi et al., 2003).

  3. Dating can be restricted to materials that are most likely to yield reliable dates (e.g., terrestrial plant macrofossils, charcoal).

  4. Dates can be obtained on sediment samples spanning as little as a few mm of deposition, rather than several cm.

The advent of AMS dating has yielded more precise and accurate sediment chronologies and age estimates for fossils, and has made it possible to assess whether fossils occurring in the same stratum were truly contemporaneous (or, alternatively, represent contamination or time-averaging ).

Because the production rate of 14C atoms in the atmosphere varies owing to variation in cosmic radiation, the 14C content of the atmosphere varies through time. Radiocarbon dates are thus not equivalent to calendar-year dates. However, variations in atmospheric 14C content for the past 20,000 years are well characterized, and so it is possible to estimate the calendar-year age of a sample given its radiocarbon age2 (Stuiver and Reimer, 1993).

Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
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Uranium/thorium techniques also can be used to date fossil material as old as ~300,000 years. Although molluscan aragonite does not appear to provide a closed system and often yields unreliable dates, coral skeletons have proven to be especially suitable for U-series dating. The technique also has been applied to tooth enamel and bone, but additional work is needed to refine the analyses of these materials.

Amino acid racemization techniques (Wehmiller, 1993; Rutter and Blackwell, 1995; Goodfriend et al., 2000) also can be applied directly to skeletal materials (bone, shell, eggshell), and when calibrated (typically with radiocarbon or uranium/thorium techniques) can yield estimates of calendar age (e.g., Kaufman, 2003; Kowalewski et al., 1998). Within the range of radiocarbon dating, calibrated amino acid dates can be reliable and economical alternatives to radiocarbon dating for particular sites. Amino acid racemization dating is also valuable beyond the range of the radiocarbon technique, although precision is lower. Electron spin resonance can also be applied directly to fossil materials, such as shell and enamel, but the method requires further calibration and dates are rarely as precise as those produced by radiocarbon dating (Blackwell, 1996).

Dating Associated Geologic Materials

Other techniques of dating (luminescence, lichenometry, cosmogenic nuclides, paleomagnetism, astronomical cyclicity, radiometric other than radiocarbon and uranium/thorium) cannot be applied directly to fossil specimens but can date material associated with the fossil in some way, and this is the primary means of age determination for fossils that cannot be dated directly. For example, cave deposits (speleothems) associated with fossils can be dated using uranium/thorium.

Age estimation of sediments spanning the past two to five centuries using radiocarbon-based models is rendered difficult by several factors. First, landscape disturbance frequently causes changes in sediment accumulation rates in lake and wetland basins, and most parts of the world have been cleared or cultivated in the past few centuries. Second, the precision of radiocarbon age estimates has a finite limit, typically ± 50-100 years except in a few unusual cases. However, other dating methods can be applied. For example, varve counts can provide precise chronologies for lakes with annually laminated sediments. Certain short-lived isotopes that accumulate in sediments can be used for dating. Primary among these is 210Pb, which can provide reliable age models for sediments of the last 100-150 years (Noller, 2000). Cesium-137, which is produced during nuclear explosions, is useful for very recent sediments. Microscopic and macroscopic carbonaceous/metallic particles produced by industrial combustion can sometimes be used as stratigraphic markers, as can

Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
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biostratigraphic markers (e.g., Ambrosia, Plantago, Salsola, Cannabis, and other indicators of land clearance or cultivation).

Uranium-lead and argon-argon are two of the more widely used systems for absolute age determination in older, pre-Quaternary rocks. Both generally depend on samples of detrital volcanic materials, generally from ash falls preserved in the sedimentary rock record. Minerals studied include monzanite, zircon, apatite, and uraninite. Both systems are highly accurate, and the two techniques complement each other by providing important cross-checks on dates. U/Pb encompasses two decay systems (uranium-235 and uranium-238), thus providing an internal check on the reliability of results; argon lacks this ability. Rocks amenable to argon dating are somewhat more common, increasing the scope of the technique. There is also the prospect that continuing uncertainties about the precise potassium-40 decay constant will be resolved in the next few years, thereby opening up a further technique for developing accurate chronologies.

Two distinct techniques are applied to the U/Pb system—Super High-Resolution Ion Microprobe (SHRIMP) and Isotope Dilution Mass Spectrometry (ITDMS). SHRIMP studies have the advantage of analyzing extremely small areas on single zircon grains, allowing identification of later overgrowths on the rim of grains, a complication that can reduce the precision of results. SHRIMP studies are particularly powerful for Archean and Proterozoic rocks, although they are also useful through much of the Phanerozoic.

Where dates are not available for every level in a local sedimentary sequence, it is generally possible to develop an age model to estimate the ages of intervening levels, especially when the record is from depositional systems that have generally uniform sedimentation rates (most notably lake and deep marine sediments). Rates of sedimentation are calculated by interpolation between the few dates that are available. This rate then can be used to model the chronology for intervening intervals and, by extrapolation, through deeper parts of the local record if this extends beyond the limits of a single dating technique (e.g., radiocarbon).

Milankovitch periodicities (periodic oscillations induced by changes in orbital parameters occurring at 23 Ka, 41 Ka, 100 Ka, and 400 Ka timescales; see Hays et al., 1976) have been recognized in sedimentation patterns, isotopic ratios, and other features back in time to at least the Miocene (and sporadically to the Triassic). Such variations have been very useful for calibrating many long-term records, and are now routinely employed in paleoclimatology and have been successfully employed in paleoceanography and stratigraphy (e.g., Zachos et al., 2001; House and Gale, 1995; Hinnov, 2000).

Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
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Correlation Techniques

Even when absolute ages cannot be determined for fossil deposits, it is often possible to correlate intervals between localities and to establish contemporaneity. When absolute ages are known in one location, correlation allows projection of those dates to a different location. Techniques used in correlation between localities are varied.

Biostratigraphy relies on broadly distributed, short-ranging species that allow correlation between rock units deposited during the same interval of time. Pelagic marine microfossils and to a lesser extent pollen are the most useful such fossils because of their broad geographic range and rapid evolutionary turnover, but a wide variety of fossils have been used. Biozonation schemes are well established for some fossils (e.g. pollen, foraminifera, radiolarians, nannofossils), in large part because of the immense efforts during the Deep Sea Drilling Project (DSDP, from 1968 to 1983) and Ocean Drilling Program (ODP, from 1985 to 2003). In continental deposits of the Cenozoic and latest Cretaceous, land-mammal ages—intervals of time characterized by distinctive assemblages of mammals (Lindsay, 2003; Woodburne, 2004)—have proven especially useful. Many biozonation schemes are increasingly well integrated with radiometric age determinations of associated rocks and with zonations of relative time based on past reversals in Earth’s magnetic field.

More recently, other correlation techniques have been applied: (1) using distinctive isochronous events (isochrons) that occurred at a single, discrete point in time; (2) wiggle matching; and (3) quantitative biostratigraphy. These approaches are especially valuable tools because they permit correlation between marine and terrestrial sequences, resolve the ordering of events, and greatly improve the fidelity of correlation over standard zonations.

Isochrons are a powerful tool because if correctly identified, they represent a single time horizon. Examples include volcanic ash beds and rapid shifts in carbon isotopes that permit correlation over large distances. The boundaries of magnetic reversal intervals can also serve as isochrons; although individual reversals are probably drawn out over hundreds to thousands of years, this fuzziness will in many cases be insignificant given the level of resolution that is otherwise possible. Epiboles—thin, widespread beds of a single species—can similarly serve as time horizons (Brett and Baird, 1997), but are typically restricted to intra-basinal rather than global correlation. In those rare instances where the horizon of interest contains material that can be dated to a high level of resolution (e.g., the volcanic ashes that coincide with the latest Permian mass extinction in south China [Bowring et al., 1998]), the isochrons also provide constraints on the rates of ecological or evolutionary response to environmental

Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
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change. Long-term records of changing isotopic values (typically of oxygen, carbon, sulfur, or strontium) in stratigraphic sequences have proven to be especially valuable for correlation because many such shifts are global. Variations in isotopic values in a new sequence are matched to corresponding isotopic variations in a well-dated (calibrated) sequence using a procedure informally known as wiggle matching (e.g., Hoek and Bohncke, 2001). In the Quaternary, variation in atmospheric 14C concentrations can be used to increase dating precision in some circumstances. Such wiggle matching, in which a tightly spaced series of AMS 14C ages is plotted against sediment depth, and then matched to the 14C-calendar-year curve, can yield subdecadal precision for Sphagnum peats (Kilian et al., 2000; Blaauw et al., 2004). However, this application requires large numbers of AMS dates (10 to 100 or more per core), at high expense. Strontium isotopes in the oceans have varied through time as a result of changes in the composition of continental rocks, the extent of hydrothermal activity, and rates of continental weathering. However, because the change in strontium isotope composition was not monotonic during the Phanerozoic, and isotopic values show some scatter at any particular time, this approach does not yet yield high-resolution dates, and it cannot be used for high-resolution correlation.

Newly developed techniques and applications in geochronology and quantitative biostratigraphy (see Agterberg and Gradstein, 1999; Buck and Millard, 2003; Sadler, 2004) seek to both order and correlate by integrating, sequencing, and calibrating a large number of events. These approaches are based on algorithms that emphasize the temporal ordering of events occurring in different locations, rather than the establishment of simultaneity of events, as with the search for isochrons. Thus, the principal result is an integrated stratigraphy that can then be assigned a temporal framework using high-precision dating. Such ordination techniques have been used with a variety of taxonomic groups in a variety of stratigraphic settings (e.g., Guex, 1991; Alroy, 1994; Sadler and Cooper, 2003). These approaches have the potential to improve temporal resolution to 10,000 to 50,000 years over time spans from 50 million to 100 million years (Sadler, 2004). As biostratigraphic databases increase in number and size, a growing challenge will be the provision of computing facilities to handle these computationally intensive procedures.

The most recent geologic timescale (see Figure 2.3; Gradstein and Ogg, 2004; Gradstein et al., 2004) integrates currently available geochronologic and stratigraphic information and represents perhaps a two-fold increase in refinement over the timescale available 20 years ago. For example, 40-Ka resolution is now available in the Neogene, more than 200 radiometric dates provide calibration points, and error bars are now provided to estimate uncertainties associated with boundaries. Continued refinement of

Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
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FIGURE 2.3 The geologic timescale of Gradstein and Ogg (2004), with numerical ages from Gradstein et al. (2004). The “golden spikes” represent stratigraphic unit boundaries that are located by a particular point in an actual stratigraphic section. SOURCE: International Commission on Stratigraphy; used with permission.

Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
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the geologic timescale is now being systematically pursued3 and is likely to provide even greater precision and accuracy for future studies of the geologic record of ecological dynamics.

IMPACT OF DATABASE TECHNOLOGIES AND SYNTHETIC STUDIES

The development of databases and database technologies during the past 20 years has facilitated analyses and syntheses at broad geographic and temporal scales, stimulated broad, community-based research projects, and enabled cross-disciplinary work. The addition of web-based technologies for rapid data transfer and communication has greatly improved scientific communication and dramatically improved the dissemination of research results.

Databases and associated research projects abound in both the biological and geological sciences. Examples in the biological sciences most relevant to this report are databases on the distribution of plants (e.g., SALVIAS—Spatial Analysis of Local Vegetation Inventories Across Scales4), fish (FishBase5), phylogenetic analyses (e.g., Tree of Life6), and many others available through links from the Natural Science Collections Alliance.7 The importance of the analysis and synthesis of wide-ranging datasets and databases to the ecological and evolutionary sciences has been recognized by the establishment of the NSF-supported National Center for Ecological Analysis and Synthesis (NCEAS—see Chapter 4) and the recent NSF funding of a Center for Synthesis in Biological Evolution (CSBE). In the geological sciences most relevant to this report, examples include databases for paleoclimatic reconstructions (National Oceanic and Atmospheric Administration’s [NOAA] World Data Center for Paleoclimatology8), the JANUS9 database of results from the Ocean Drilling Program (see Chapter 4), resources for paleogeographic reconstructions (e.g., Paleogeographic Atlas Project10 and PALEOMAP project11), data on the diversity of fossils through geologic time (Paleobiology Database12

Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
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FIGURE 2.4 The “Sepkoski curve” showing numbers of genera of marine animal fossils through Phanerozoic time. The blue portions of the curve indicate time intervals chosen for more detailed study of diversity change by Alroy et al. (2001). SOURCE: Newman (2001); used with permission.

see Chapter 4), information on the stratigraphic distribution of fossils (e.g., NEPTUNE13 and Paleostrat14), and many more specialized databases (e.g., Panama Paleontology Project15 and Neogene fossil mammals of the Old World16).

There is probably no better example of how data compilations and syntheses can shape entire disciplines than the graphs illustrating the diversity of families and genera of marine fossils through geologic time (e.g., see Figure 2.4). Laboriously assembled from the print literature by Sepkoski (1982, 1993, 2002), these simple figures summarized the state of

Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
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knowledge of diversity change though geologic time, and in many regards they have proven robust to taxonomic revision and phylogenetic approaches (e.g., Sepkoski and Kendrick, 1993; Robeck et al., 2000; Adrain and Westrop, 2000). They have attracted broad attention in the biological sciences because of the near-plateau in diversity over several hundred million years in the Paleozoic, the abrupt extinctions and rapid rebounds that punctuate the time series, and the dramatic post-Paleozoic diversity increase to the present day. Interpretation of all these features remains controversial, and countless research projects have been engendered by new approaches to the biological and geological factors underlying the entire curve and its many taxonomic and ecologic components (e.g., Sepkoski, 1984, 1996; Raup and Boyajian, 1988; Valentine et al., 1991; Eble, 1999; Bambach et al., 2002; Alroy et al., 2001; Foote, 2003; Jablonski et al., 2003). Sepkoski’s efforts helped launch a new, integrative, and synthetic style of research in paleontology, and were instrumental in the formation of the community-based Paleobiology Database, which has initiated a next-generation compilation using spatially explicit occurrence data.

SUMMARY AND OUTLOOK—STRATEGIES FOR GEOHISTORICAL ANALYSIS

Geohistorical data are essential for answering many kinds of questions, especially when the aims are either to discriminate between anthropogenic and non-anthropogenic effects or to understand phenomena that cycle or emerge over periods greater than a few years. Such discrimination is extremely difficult without recourse to historic records in the broadest sense—pre-scientific documents, archaeological materials, natural sedimentary records. Therefore, a rigorous strategy for evaluating ecological dynamics using geohistorical records—and for integrating geological and biological methods and insights—is essential.

Intensified research on these issues over the last 15-20 years now enables the exploitation of uniquely valuable data lodged in geohistorical records (summarized from above):

  • Important environments (notably lakes, peatlands, estuaries, and deep ocean basins) typically yield high-resolution time series data ideal for tracking ecosystem change. Valuable and unique data also can be acquired even from isolated before-and-after sample sets, and the time-averaging that characterizes many geological samples of past ecosystems is advantageous for statistical analyses of broad-scale changes and patterns.

  • Newly developed geological and, especially, geochemical methods provide an increasing number of proxy indicators of environmental change. Thus, changes in the composition, relative abundance, distribu-

Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
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tion, and diversity of fossil biotas can now be used as measures of the biotic response to environmental change, rather than indicators of both the change and the response.

  • An increasing number of ecologically valuable metrics can now be evaluated confidently, either using the conventional fossil record of preserved individuals, or using various chemical proxies of former biological populations (biomarkers, isotopic signatures).

  • Relative and absolute age dating of geologic materials and time series has improved greatly in the last 20 years, and has crossed a key threshold with respect to analyzing ecological dynamics. Resolution now matches or exceeds what would be possible using conventional biostratigraphic methods, liberating geohistorical analysis of ecological dynamics from pitfalls of circularity in age and rate determinations.

  • New database and web technologies have stimulated community-based efforts to assemble and analyze large amounts of ecological, paleoecological, and evolutionary data relevant to understanding the geologic record of ecological dynamics. Such efforts at synthesis are essential partners in efforts to acquire new data.

The geologic column provides a wealth of geohistorical records that are ready to yield important data on ecological dynamics. It is not the record that is inadequate but rather the availability of resources to extract and analyze the record. Using geohistorical records to their fullest potential will require an effort focused on the biological analysis of groups known to have reasonable fossil records, and on the development of proxy methods for groups of great biological interest but poor fossilization potential. Biologists and earth scientists, working together, will need to frame research questions so that the answers take maximum advantage of the great potential of geohistorical analysis to provide major insights.

Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
×
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Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
×
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Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
×
Page 32
Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
×
Page 33
Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
×
Page 34
Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
×
Page 35
Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
×
Page 36
Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
×
Page 37
Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
×
Page 38
Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
×
Page 39
Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
×
Page 40
Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
×
Page 41
Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
×
Page 42
Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
×
Page 43
Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
×
Page 44
Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
×
Page 45
Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
×
Page 46
Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
×
Page 47
Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
×
Page 48
Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
×
Page 49
Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
×
Page 50
Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
×
Page 51
Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
×
Page 52
Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
×
Page 53
Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
×
Page 54
Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
×
Page 55
Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
×
Page 56
Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
×
Page 57
Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
×
Page 58
Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
×
Page 59
Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
×
Page 60
Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
×
Page 61
Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
×
Page 62
Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
×
Page 63
Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
×
Page 64
Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
×
Page 65
Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
×
Page 66
Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
×
Page 67
Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
×
Page 68
Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
×
Page 69
Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
×
Page 70
Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
×
Page 71
Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
×
Page 72
Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
×
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Suggested Citation:"2 The Nature and Value of Geohistorical Information." National Research Council. 2005. The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change. Washington, DC: The National Academies Press. doi: 10.17226/11209.
×
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In order to answer important questions about ecosystems and biodiversity, scientists can look to the past geological record—which includes fossils, sediment and ice cores, and tree rings. Because of recent advances in earth scientists’ ability to analyze biological and environmental information from geological data, the National Science Foundation and the U.S. Geological Survey asked a National Research Council (NRC) committee to assess the scientific opportunities provided by the geologic record and recommend how scientists can take advantage of these opportunities for the nation’s benefit. The committee identified three initiatives for future research to be developed over the next decade: (1) use the geological record as a “natural laboratory” to explore changes in living things under a range of past conditions, (2) use the record to better predict the response of biological systems to climate change, and (3) use geologic information to evaluate the effects of human and non-human factors on ecosystems. The committee also offered suggestions for improving the field through better training, improved databases, and additional funding.

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