National Academies Press: OpenBook

Effects of Past Global Change on Life (1995)


Suggested Citation:"INTERPRETATIONS AND CONCLUSIONS." National Research Council. 1995. Effects of Past Global Change on Life. Washington, DC: The National Academies Press. doi: 10.17226/4762.
Page 61
Suggested Citation:"INTERPRETATIONS AND CONCLUSIONS." National Research Council. 1995. Effects of Past Global Change on Life. Washington, DC: The National Academies Press. doi: 10.17226/4762.
Page 62
Suggested Citation:"INTERPRETATIONS AND CONCLUSIONS." National Research Council. 1995. Effects of Past Global Change on Life. Washington, DC: The National Academies Press. doi: 10.17226/4762.
Page 63

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GLOBAL CHANGE LEADING TO BIODIVERSITY CRISIS IN A GREENHOUSE WORLD: THE CENOMANIAN-TURONIAN 61 (CRETACEOUS) MASS EXTINCTION lutionary rate. However, the same scale deviates by 440,000 to 510,000 yr from ages calculated by the same method for early Turonian ammonite biozones. This suggests that postextinction ammonite evolution takes place at much more variable rates than found among pre-extinction ammonite lineages. In both Milankovitch cycle-based time scales, therefore, the assumption that ammonite biozones have approximately equal PBC 4, and relatively stable evolutionary rates through the C-T mass extinction-recovery interval is not supported. The calibrated time scale based on integrating new 40Ar-39Ar ages with the 100,000-yr Milankovitch cycle history, with calculations starting from a radiometric age in the middle of the cyclic depositional sequence, is thus the most accurate. This scale is used below to calibrate the physical, chemical, and biological components of the Cenomanian-Turonian mass extinction-recovery interval. Even finer resolution for dating and correlation can be obtained by utilizing physical and chemical event deposits/ horizons within the same section. Most of the 112 event beds/horizons in the Bridge Creek Limestone are bentonites, trace element, stable isotope, and organic carbon enrichment levels (spikes), and sea-level or climate cycle boundaries with regional to interregional dispersion. Within the time scales developed for this interval from radiometric and climate cycle data, the event-bounded intervals of the Bridge Creek Limestone Member, spanning the C-T boundary, have an average duration of 17,000 yr. This highly refined C-T chronology for the Western Interior Cretaceous Basin, integrating radiometric, event/ cycle chronostratigraphic, and biostratigraphic data, closely parallels common time scales utilized in Quaternary studies of global change (Figure 3.1), ecosystem destruction, and biodiversity loss (e.g., see papers in Bradley, 1991). A TIMETABLE FOR CENOMANIAN-TURONIAN MASS EXTINCTION The integration of high-resolution stratigraphic, geochemical, and paleobiological data depicting late Cenomanian-early Turonian global change, with a real time scale combining 40Ar-39Ar volcanic ash ages with Milankovitch climate cycle deposits of known time duration, makes it possible to evaluate the rates, patterns, and timing of the C-T mass extinction and associated environmental perturbations within a greenhouse world (see Appendix). Figures 3.3 and 3.4 show the integrated high-resolution data for the study of the C-T mass extinction at Pueblo, Colorado. Figure 3.5 shows the derivation of the real time scale from this data base. Together, these allow derivation of the timetable for environmental perturbations and mass extinction in the C-T boundary interval at Pueblo (shown in the Appendix). From oldest (1) to youngest (22), these divisions are based collectively on physical, chemical, and biological characteristics that are unique to each interval. Figure 3.6 summarizes these relationships through time. These data can be used to model the patterns of extinction, survival, and recovery in a greenhouse world, and to enhance the development of predictive models for the modern global crisis. INTERPRETATIONS AND CONCLUSIONS The Cenomanian-Turonian mass extinction is among the best-studied biotic crises in Earth history, and is unusual in that it occurred during one of the greatest eustatic highstands of the Phanerozoic, in a relatively equable, warm, greenhouse world usually considered favorable for the diversification of life. High sea-level and widespread development of epicontinental seas in this interval have ensured the preservation of complete C-T boundary sections worldwide. In most sections, there is little or no sedimentologic expression of the extinction interval, which is preserved primarily within relatively monotonous, fine- grained neritic, hemipelagic, and pelagic facies characterized by well-defined Milankovitch bedding cycles. However, integrated geochemical and paleobiological data, obtained through very high-resolution (1 to 10 cm) stratigraphic sampling, reveal dynamic environmental changes in ocean-climate systems during this global mass extinction interval. The C-T boundary interval initiates with a series of tropical extinctions among rudistid bivalves and other reef- associated taxa, beginning at the end of the early Cenomanian (95.9 Ma) and culminating 0.5-1.0 m.y. below the C-T boundary (about 93.9 to 94.4 Ma); dating of this final tropical extinction event and its correlation with well-studied temperate records, however, is imprecise. Johnson and Kauffman (1990) recorded a loss of three rudistid genera (0.1 per taxon extinction rate), and 32 species (0.2 per taxon extinction rate), which comprise 43% of the lower Cenomanian rudistid species, 26% of the middle Cenomanian species, and 47% of the upper Cenomanian rudistid species in the Caribbean Province. Tropical extinction patterns are similar in various well-studied Italian and French sections, where no reefs are known within 3 to 10 m of the C-T boundary. In the Western Interior of North America, where the best integrated physical, geochemical, and temperate to subtropical biological record of the C-T extinction interval has been compiled, an integrated dating matrix of 40Ar-39Ar radiometric ages, and bedding rhythms, representing the 41,000-yr and 100,000-yr Milankovitch climate cycles, has allowed development of a relatively precise timetable for Cenomanian-Turonian boundary extinctions and their probable causal mechanisms. The Pueblo, Colorado, ref

GLOBAL CHANGE LEADING TO BIODIVERSITY CRISIS IN A GREENHOUSE WORLD: THE CENOMANIAN-TURONIAN 62 (CRETACEOUS) MASS EXTINCTION erence section is utilized to illustrate this chronology and its application to interpreting rates, patterns, and probable causes of C-T mass extinction. The first geochemical perturbations associated with the C-T extinction interval in this setting occur 880,000 yr below the C-T boundary; the first molluscan extinction event (step) occurs 520,000 yr below the C-T boundary, coeval with initiation of a series of 10 successive trace element enrichment horizons, probably representing an oceanic advection sequence. The first five of these enrichment intervals have Ir peaks that are two to five times greater than background levels, and are precisely coeval with the first five C-T extinction steps; two of these levels have recently yielded possible microtektites and rare shocked quartz grains in Colombia and Colorado, respectively. Similar trace element enrichment patterns occur at many sections in North America, western Europe, Colombia, and North Africa. The first C-T extinction events at Pueblo also mark the first extraordinary peaks of stable isotope and organic carbon fluctuations, relative to background values. An overall enrichment of Corg across the C-T boundary records development of an oceanic anoxic event (OAE II). Ocean-climate systems are therefore highly perturbed, with the rates and magnitude of change in temperature and ocean chemistry well above Cretaceous background levels. In the North American Interior, extinction among subtropical to temperate lineages spans 1.46 m.y. from the latest Cenomanian through the middle-early Turonian, and appears punctuated or stepwise in nature based on regionally composited, robust range-zone plots of both macro- and microfossil species. The major part of the extinction interval spans 520,00 yr of the latest Cenomanian and comprises five steps of macrofaunal (mainly molluscan) extinction and two microfossil extinction levels. Extinction events occur successively, therefore, about every 100,000 yr. Three smaller macrofossil and two microfossil extinction events occur 230,000 to 940,000 yr above the C-T boundary within the survival and early recovery intervals. Given that Cenomanian marine organisms over much of the world had evolved over millions of years into a warm, more equable greenhouse world characterizing Cretaceous eustatic sea-level rise, it is probable that most lineages had relatively narrow adaptive ranges with sharp ecological thresholds, especially in terms of thermal range and water chemistry, and that they were thus extinction prone. These biotas would have been easily stressed by even small-scale environmental perturbations of the water column. The very close correlation of mass extinction steps with short-term trace element enrichment events (advection events?), and with rapid, large-scale fluctuations in the temperature, salinity, and oxygen of the water column, suggests that the extraordinary rate and magnitude of these environmental shifts directly caused the series of ecologically graded C-T extinction events, or steps, as they progressively breached the narrow adaptive ranges of Cenomanian-Turonian species, and ecosystems, adapted to an equable greenhouse world. Predictably, extinction initiated among tropical reef ecosystems, and terminated with more temperate-adapted and cosmopolitan groups. Yet the cause of these marine environmental perturbations remains an enigma; several viable hypotheses exist. Hallam (1984) and others have suggested that many mass extinctions, including the C-T boundary event, resulted from oxygen depletion in oceans and relatively deep epicontinental seas associated with expanding oxygen minima zones and the establishment of oceanic anoxic events. This mechanism would be especially effective during global greenhouse conditions, when ocean circulation was sluggish and may have been driven largely by low-oxygen warm saline bottom waters derived from marginal tropical evaporite belts, and when productivity was high. Widespread dysoxic to anoxic conditions would ecologically stress global normal marine biotas and could have contributed to C-T mass extinction. However, this hypothesis is rejected as the primary cause for C-T extinction because (1) widespread oxygen depletion in the oceans characterized most of the time interval between the Aptian Mass Extinction and the C-T boundary interval, including the long Albian OAE Ic, without causing mass extinction. If anything, these were longer and more severe intervals of oxygen depletion than the one that coincided with C-T extinction. Instead, C-T extinction comes at the end of the dysoxic to anoxic interval (Hay, 1988, Figure 3.3), during a set of dynamic geochemical changes leading to reoxygenation of deeper portions of the oceans. C-T extinction events were more likely related to the rate and magnitude of these changes than to oxygen depletion itself. (2) Because of the long middle Cretaceous interval of oceanic oxygen depletion associated with global greenhouse warming and elevated sea-level, a large proportion of deeper water marine biotas became adapted to low-oxygen conditions and are commonly associated with organic-rich black shales (e.g., many Inoceramidae and other bivalves; Kauffman and Sageman, 1990; Sageman et al., 1991, 1994). Yet many of these same lineages became extinct in the C-T boundary interval as conditions shifted rapidly toward more oxygenated oceans in the basal Turonian. If anything, a case for oxygen poisoning might be made for the extinction of some low-oxygen adapted taxa. A second explanation for C-T marine mass extinction might lie with trace element poisoning associated with advection events from the deep ocean and from epicontinental basins, as suggested for Paleozoic mass extinctions by Berry et al. (1990) and Wilde et al. (1990). Potentially toxic trace elements were greatly enriched in the oceans

GLOBAL CHANGE LEADING TO BIODIVERSITY CRISIS IN A GREENHOUSE WORLD: THE CENOMANIAN-TURONIAN 63 (CRETACEOUS) MASS EXTINCTION and deeper portions of epicontinental seas during accelerated outgassing associated with large-scale middle Cretaceous plate reorganization and development of the Pacific superplume (Larson, 1991a,b). These trace elements could have been concentrated as oxides and carbonates in basinal Cretaceous sediments in the presence of at least moderate amounts of benthic oxygen, especially during high sea-level and offshore sediment starvation; trace elements could also have been sequestered in solution in low-oxygen to anoxic water masses (OMZs). Expansion and benthic touchdown of the oceanic oxygen minimum zone during the Cenomanian-Turonian OAE would have remobilized sequestered trace elements from the seafloor and caused progressive advection of potentially toxic chemicals through the water column, with profound effects on the global marine biota. If the base of the food chain was affected by these advection events, it would have caused a far- reaching set of negative ecological feedback loops within the trophic web. The precise correlation of the first five C-T extinction steps, and two of the succeeding steps, with trace element enrichment horizons in epicontinental and continental shelf deposits of four continents, strongly supports the hypothesis of advection and trace element poisoning as a partial cause of C-T mass extinction. The relatively shallow water trace element enrichment layers found to be associated with C-T extinction events in high-resolution stratigraphic analysis, probably represent depositional sites of advecting trace elements from deeper ocean sources, situated at and above the oceanic redoxcline between the OMZ and the mixing zone. A third hypothesis for C-T mass extinction, which draws on events described in the two previous hypotheses, focuses on the extraordinary rates and magnitudes of ocean-climate changes associated with the 1.46-m.y.-long C-T boundary interval, and their effects on a global biota narrowly adapted for the most part to the equable greenhouse environments that had been developing throughout the early and middle Cretaceous. Data from Pueblo, Colorado (Figures 3.3 and 3.4) are characteristic of many global boundary sections and show a series of exceptionally rapid, large-scale shifts in organic carbon values (representing rapid shifts in benthic oxygen levels); in δ13C values (representing changes in carbon cycling) within the global positive δ13C excursion; in δ18O values (possibly representing rapid salinity and/or temperature changes); and in trace element values (probably representing one or more oceanic advection sequences). Virtually all late Cenomanian extinction events, and some lesser ones in the early Turonian, are correlative with one or more of these rapid, large-scale geochemical fluctuations. Whereas these geochemical signals can be strongly modified by diagenetic processes, the fact that similar changes occur in virtually all well-studied global C-T boundary sections suggests that they represent a primary ocean-climate signal. Major changes in ocean chemistry and temperature around the C-T boundary could well have been the primary cause of extinction events as they progressively exceeded the narrow adaptive ranges of many stenotopic Cenomanian-Cretaceous lineages. Of special interest in this theory is the general correlation of many apparently rapid environmental fluctuations with bedding rhythms representing 41,000- and 100,000-yr Milankovitch climate cycles (Barron et al., 1985; Fischer et al., 1985; Kauffman, 1988a; Glancy et al., 1993; Figures 3.3 and 3.5 herein). On the one hand this may suggest diagenetic modification of a primary signal in carbonate-rich versus carbonate-poorer facies of the C-T boundary interval, but to the degree that it represents a primary signal, it suggests that the Milankovitch climate cyclicity may have acted as an independent catalyst that drove an environmentally perched, greenhouse ocean-climate system to even greater levels of change, at rates dictated by the climate cycles themselves. Finally, the possibility of extraterrestrial influences on the C-T mass extinction cannot be ruled out. The precise correlation of the first five late Cenomanian extinction events, or steps, with Ir enrichment of two to four or five times background levels leaves open the possibility of extraterrestrial sources for the iridium. Orth et al. (1989, 1990, 1993) have been cautious in suggesting extraterrestrial origins, pointing out instead an apparent similarity of the overall C-T trace element suite to those originating from deep mantle outgassing, and the fact that the C-T boundary interval was also a time of major plate rearrangement and superplume development (Larson, 1991a,b). On the other hand, four temporally clustered late Albian to late Cenomanian terrestrial impact craters are known with age error bars that overlap the C-T boundary (Grieve, 1982), suggesting an impact storm, or shower (sensu Hut et al., 1987); this is a conservative estimate of terrestrial impacts, when considering the amount of Cenomanian surface that has been subducted or is now covered by younger sediments/strata, vegetation, ice, and especially water. Because impacting is predictably spatially random (Grieve, 1982), and the late Cenomanian world was 80 to 82% covered by water near eustatic highstand, it is likely that at least four of five potential impactors would have fallen in the world seas and oceans during this impact shower. This statistically projects at least 20 impacts during the Cenomanian and early Turonian interval, the majority of which would be aquatic. Oceanic impacting would cause repeated, short-term stirring events. This hypothesis further predicts an extended duration for the Cenomanian impact shower, with high probability that it would overlap the C-T mass extinction interval. Recent discoveries of possible microtektites (Colombia) and multilamellate shocked quartz grains (Colorado) at two

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What can we expect as global change progresses? Will there be thresholds that trigger sudden shifts in environmental conditions—or that cause catastrophic destruction of life?

Effects of Past Global Change on Life explores what earth scientists are learning about the impact of large-scale environmental changes on ancient life—and how these findings may help us resolve today's environmental controversies.

Leading authorities discuss historical climate trends and what can be learned from the mass extinctions and other critical periods about the rise and fall of plant and animal species in response to global change. The volume develops a picture of how environmental change has closed some evolutionary doors while opening others—including profound effects on the early members of the human family.

An expert panel offers specific recommendations on expanding research and improving investigative tools—and targets historical periods and geological and biological patterns with the most promise of shedding light on future developments.

This readable and informative book will be of special interest to professionals in the earth sciences and the environmental community as well as concerned policymakers.

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