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Solid-Earth Sciences and Society 3 The Global Environment and Its Evolution ESSAY A familiar image shows the Earth hanging against the black marbled coldness of deep space. The blues sparkle, barely dulled by patches of brown. Swirling white whorls veil the bright sphere only slightly. Twenty-five years ago that image had not yet been seen. Furthermore, the idea of the Earth as an integral unit was not a prevalent one. Study of the planet proceeded at local or regional scales. Then plate tectonics began to weave regional studies into one planetwide dynamic model. The past two decades have also seen the emergence of a new perspective in the earth sciences—or to use a more recent term, earth system science—emphasizing changes in the global environment that occur over spans of geological time. The changing environments leave geological evidence that permits investigation of a wide range of geographic, oceanographic, climatic, and biotic transitions. Such evidence includes information about environmental changes that cannot be directly observed today. The record of the rocks reveals that certain factors force changes in the global environment and that some ecosystems are more sensitive than others to those changes. The surface has been changing for over 4.5-billion-years. Many of these changes are fluctuations within definable extremes. A familiar example is that of water. Sediments record changes in the hydrologic cycle during which rising seas engulfed extensive continental tracts and then drained away. Some of these changes resulted from ice ages that left wide swaths of continental shelf exposed to the air during glacial accumulation and sent torrents to the oceans during intervals of melting. Other cyclic fluctuations recorded in the geological record include geochemical exchanges through reservoirs: atmosphere, ocean, biomass, sediment, crust, and mantle. Ocean basins rise and sink and expand and contract in cycles, and mountain ranges thrust upward and then waste away. While cyclic changes leave recognizable patterns in the geological record, they continually alter the components that are recycled. Inevita-
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Solid-Earth Sciences and Society bly, new conditions are created by these natural movements. The original state can never be exactly regained. The new conditions are the result of what is referred to as secular change—change with time. The most obvious secular change that has taken place during earth history is the early transformation of its surface from a landscape of naked rock, barren seas, and toxic atmosphere to a landscape seething with life, with organisms that exist on a variety of scales and in a medley of forms. As the cycles have churned away and new secular changes have occurred, sporadic catastrophic events have thrown the whole dynamic system into chaos. Geoscientists use an assortment of techniques and instruments to investigate the complex interactive systems that have created the surface environment. A 200-year-old tradition of field mapping and detailed description offers a solid foundation—called ground truth—for new technologies like remote sensing and for new conceptual models such as ones that explain how biological evolution has altered the chemistry of the atmosphere. Remote-sensing technology is not limited to satellite imagery and geodesic laser measurements; remotely sensed magnetic and gravitational anomalies help trace the vertical and horizontal movements of the crust. Fine-tuned seismic reflection research has also provided a new way to "see" inside the Earth. Seismic reflection can now produce subsurface images that rival, in an areal extent, drilled cores for locating geological boundaries; the cores, however, are often needed to provide ground truth. Tomography combines sets of seismic reflectance data to create cross sections through various planes. Mapping also reveals patterns of past environments, ranging from tropical seas to jungles and deserts. These environments are identified by a great variety of evidence, including fossil occurrences, key sedimentary rocks, and the isotopic composition of shells. Maps of fossil occurrences show how organisms were distributed in space and where they lived bears directly on how they evolved. Once geologists have determined what the patterns are, they can study how those patterns changed; on a planetary scale, this dual effort is at the heart of studies of global change. Geochronology provides the framework for arranging temporal sequences in the geological record. It also provides estimates for rates of chemical, physical, and biological change. Paleontology and stratigraphy led to the first arrangement of geological time scales. Intervals of the Earth's past gained names like Precambrian, Devonian, Cretaceous, and Pleistocene. Today, new quantitative methods for analyzing fossil occurrences are improving our ability to compare ages of strata throughout the world. Techniques that use naturally occurring radioactive isotopes can now provide dates within a 2-million-year range of accuracy for events that affected particular materials 2.5-billion-years ago. At the other extreme of recency and resolution, close documentation of tree-ring patterns yields chronologies accurate to less than 1 year. Reconstruction of ancient seas, climates, and continental geography for key intervals of the past provides data for constructing and testing
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Solid-Earth Sciences and Society numerical and conceptual models that portray global circulation patterns for ancient oceans and atmospheres. Such models are shedding light on the agents that triggered ice ages and other shifts to new environmental states on a global scale. Ancient life has tracked changes in habitats—sometimes migrating, sometimes evolving, and sometimes disappearing from the Earth. In fact, fossils provide the only direct record of large-scale evolution and extinction, and this record can be understood only in the context of past global change. Evolutionary theorists depend on the paleontological record to test their hypotheses, and the application of innovative quantitative techniques is providing new insights into rates, patterns, and modes of evolution. The unifying theory of a dynamic Earth and the vast scale of coverage provided by satellite imagery complement recent advances in international cooperation at all levels. Geoscientists have organized the International Geological Correlation Program (IGCP) and the Global Sedimentary Geology Program (GSGP) and have helped set up the International Geosphere-Biosphere Program (IGBP). Over the past 25 years, internationally based ocean drilling programs have concentrated on obtaining cores from the ocean floor—nearly three-quarters of the surface—that have improved patchy data and supported stratigraphic correlations with an unexpected degree of success. The drill ship has been to geologists what the telescope is to astronomers, allowing geologists to study the most remote parts of their domain. Such systematic examinations of the Earth are filling in gaps in knowledge as though they were pieces of a giant spherical jigsaw puzzle. Investigators who correlate the geological information from remote regions like the deep ocean find signs of environmental change that affected the whole planet. Some of these sweeping changes periodically caused widespread extinction of species. The study of such extensive occurrences is called global event stratigraphy. The most familiar example of global event stratigraphy is the evaluation of the iridium anomaly found in widely distributed strata that date to 66-million-years ago. The iridium anomaly may signify a meteorite impact that resulted in the extinction of as much as 66 percent of the species, including all the dinosaurs. The most pervasive extinction occurred 225-million-years ago when perhaps as much as 95 percent of all species died off. More recently, 75 percent of the giant mammal species that roamed the spacious North American plains south of the Laurentide ice sheet disappeared 11,000 years ago as the ice sheet melted away. This extinction may have had other causes besides the climatic fluctuations that occurred in the wake of the retreating ice front. Additional threats were posed by the hunting skills of human beings. Anthropogenic factors—those caused by the activities of humans—affect the surface environment on a vast scale. As modern technology offers striking images of a bright blue globe against a black abyss, it also provides evidence of environmental crises, some caused by humankind. While efforts continue to monitor global change and to attribute the causes to humans or to nature, no consequences can be predicted without
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Solid-Earth Sciences and Society an understanding of the records of past global change. Through the record in the rocks, the past has become the key to the future. THE GLOBAL ENVIRONMENTS: A GEOLOGICAL PERSPECTIVE The Changing Land Surface The processes of weathering, erosion, and soil formation that together degrade upland areas have operated throughout earth history. Variations in the way the processes operate have generally been dominated by climate. Glaciers form in polar or subpolar regions or at high altitudes; deserts develop around 20° from the equator; and rain forests, with their great rivers, grow in equatorial and temperate latitudes. Against this image it has been surprising to learn from new methods of dating and from quantitative studies of material fluxes that the times in which we live are unusual—not for one but for two reasons. The first is that since Northern Hemisphere glaciation developed about 2.5-million-years ago, fluctuations in ice volume have been large enough to repeatedly change world sea level by as much as 100 m over time scales of tens to hundreds of thousands of years. Despite recent fluctuations, Antarctic ice caps have remained stable for as long as 40-million-years. For this reason the dominant major cause of sea level change during the past 2.5-million-years has been the accumulation and ablation of Northern Hemisphere ice sheets. As an example of fast change, consider that 18,000 years ago, when sea level was about 100 m lower, rivers reached the sea at the edge of the continents. Since then they have retreated so much in response to rising sea level that some river mouths (e.g., the Susquehanna) now lie far back from the continental shelves in estuaries like Chesapeake Bay. Such short-term changes prevent the establishment of an equilibrium in weathering, erosion, and deposition, and during the past 2.5-million-years they have rendered the surface an unusually dynamic place. We ourselves provide the other reason for unusual conditions. The condition of the soil and processes of erosion have been substantially changed since agriculture began. These changes have increased in recent decades because of such processes as dam building, forest destruction, widespread irrigation, and flood control. For instance, sediments that formerly were carried to the Mississippi delta are now being impounded behind dams; this is contributing to the encroachment of the Gulf of Mexico upon the delta. The problems of marine transgression along the Gulf Coast are discussed throughout this report; here, attention is drawn to progress in understanding the major processes that dominate the change in land surface above sea level, with emphasis on the peculiar problems and opportunities that result from the apparently exceptional time in which we live. Landforms Landforms are continually changing, but most changes are subtle and generally escape notice. Although great attention is rightly given to catastrophic events such as earthquakes, volcanic eruptions, and landslides, the time scales of importance in geomorphology—the study of landforms and the processes that shape them—range from seconds to millions of years, and the space scales range from single hillsides to global dimensions. The challenge is to characterize the ways in which landforms respond to both common and uncommon events. The geomorphic record contains information about the ways in which present and past environmental changes have modified the processes operating at the surface, both in intensity and duration. A long-term view is essential because the time spans of contemporary monitoring are too short to represent the range of possible conditions. Long-range perspectives from the geomorphic record permit testing of models of environmental change, whether they apply at global, regional, or local scales. A landscape can provide information about the magnitudes and return frequencies of natural processes. This information can lead to identification of geomorphological thresholds that precede disastrous events. Some responses are immediate—floods, landslides, and debris flows—but others can be spread over years or decades—upland soil erosion, glacial to interglacial cycles, river-channel change, and sea level change. The rates of some of these processes have been greatly accelerated by human activities. Geomorphic events of the past, which are recorded in landforms and stratigraphy, can provide usable analogs of anticipated environmental change.
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Solid-Earth Sciences and Society For example, the rate of sea level rise in the next century could become 1 cm per year, a rate not unlike that at which sea level rose at the end of the last ice age. Study of the coasts drowned at that time will provide information relevant both to biotic response to a future sea level rise and to the anticipated acceleration of sedimentation in the lower reaches of river systems. Geomorphologists and geochemists have been using particular isotopes, produced in the atmosphere and in rocks by cosmic radiation, to determine ages of landforms and to date events such as floods, landslides, fault movements, lava and debris flows, and the onset of glaciation. The application of accelerator mass spectrometry to carbon-14 (14C) dating provides a means of dating samples both older and smaller—by a thousand times—than the type of sample conventionally used. Various new methods are being developed to determine the age of a landform, measuring the time elapsed either since the rocks forming it were deposited or since they were exposed by erosion. These methods represent a breakthrough in quantifying landform dynamics, permitting more precise dating of key events in the evolutionary development of landforms and yielding erosional rates for the various features of a landscape. The Himalaya, for example, have long been considered not only the highest but also the fastest rising mountain range in the world (Figure 3.1). Just how fast they are rising is something we are only beginning to understand. Uplift rates may be as great as 5 mm per year—5 km per million years—in places along the front of the mountain belt during the past 20-million-years. Crystals of microcline were eroded from the surface and deposited in the sediments of the Bengal deep-sea fan south of Sri Lanka only a few million years after they became cool enough to stop losing radiogenic argon by diffusion, which occurred at a depth of 5 km inside the mountain belt. Weathering and Soil Formation The interaction of the atmosphere and hydrosphere—in the form of groundwater—with the rocks at the surface is very complex, partly because organisms ranging in size from bacteria to trees are involved but largely because the relevant time scales vary so widely. Water has a residence time as short as a single storm and typically cycles on an annual scale; trees have lives of decades to centuries; and the minerals formed in the weathering processes can have residence times in the soil of as high as thousands to millions of years. Varieties of soil are strongly controlled by local climate, and with the growing interest in global change soils are being looked at anew to learn what they record about past climatic changes on time scales from decades to millions of years. Soils lie at the interface between the geosphere, biosphere, and atmosphere. They have unique properties that derive from the intimate mixing of partly weathered geological substrata, dead organic matter, live roots, microorganisms, and an atmosphere high in carbon dioxide, nitrogenous gases, and moisture. Ions of potassium, sodium, calcium, magnesium, sulfur, and phosphorus are released from minerals by hydrolytic weathering. Many of these ions, as well as carbon and nitrogen, are also released from dead organic matter by microbial digestion. The geological composition of the soil passively influences soil biota through ionic deficiencies or release of toxic elements. The soil biota actively influence weathering rates through respiration and production of organic acids. Soils are open systems that gain and lose energy and matter as they evolve through time (Figure 3.2). Gains, losses, translocations, and transformations occur continuously in the soil column; the relative magnitudes of these processes determine the types of horizons formed along the column. If the vectors of these processes remain relatively constant, the intensity of their expression in horizons is time dependent. Time-dependent soils are useful for correlating geographically separated geomorphic surfaces. This property of soils has been widely used in studies concerned with the rates and timing of tectonically and climatically driven geomorphological processes. In the past, most research on rates and pathways of soil development centered on careful chemical, physical, and microscopic dissection of soil horizons developed during known time intervals. Future research will emphasize collection of in situ measurements to document fluxes of gases and solutes, rates of mineral weathering, and biological interactions. Already, these studies reveal the physiology of soil to be modulated by complex feedback mechanisms that are nonetheless subject to catastrophic breakdown under external influences such as fire, erosion, or dramatic climatic change. If we are to understand the contribution of soils to ecological stability, we must also study the effect of anthropogenic disruptions, such as toxic chemical discharges, on soil processes. Soils are great purifiers, but we do not know what level of disruption constitutes a lethal dose.
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Solid-Earth Sciences and Society FIGURE 3.1 View of the Himalaya. Rivers and Material in Transit from Mountains to Sedimentary Basins The surface is shaped locally by weather. The temperature and moisture regimes, with the weather intensities and rates of change, represent climate. The effects of climate change are becoming more clearly understood through the use of general circulation models. Climatologists, who develop general circulation models to predict the consequences of global change, test their models by comparing results obtained with variables that define past conditions against the evidence found in the geological record. That evidence includes the landforms themselves, sedimentary sequences, and fossil biotas. Closer study of the response generated on the surface by climatic changes that last for short intervals, up to hundreds of years, or long intervals,
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Solid-Earth Sciences and Society FIGURE 3.2 Flow of the major processes in soil development. hundreds of thousands of years, allows prediction of how future climatic fluctuations can alter the landscape. For example, the sizes of river channels formed in the past 10,000 years and the character of their contained sediments have been used to reconstruct a history of long-term change in the magnitudes of high-frequency floods in the Upper Mississippi valley (Figure 3.3). That record helps to test general circulation models of climatic fluctuations over the past ten centuries. Stratigraphic evidence also extends the existing record of river behavior beyond the limits of data collected through observation over the past 100 years. Advances have recently been made in estimating the size and frequency of ancient floods, effectively extending hydrologic records for up to thousands of years, by combining interpretations of river and sediment behavior with results from geological dating methods. The intensity and sequence of climatic events are crucial factors in molding the landscape into a distinctive form and producing a recognizable geological pattern. The lone occurrence of a moderate flood might accomplish little erosion and deposition, but the clustered occurrence of two or three moderate floods can destabilize entire channel systems and cause them to become sensitive to the erosional effects of even small floods. Studies of the erosion, transportation, and deposition of sediments in contemporary drainage sys- FIGURE 3.3 Variations in the size of annual and biennial floods in the upper Mississippi Valley over the past 9,000 years; the size is expressed as a percentage of the present flood size.
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Solid-Earth Sciences and Society tems are showing departures from the steady state. Modern watersheds are accumulating more sediment than they pass on. In many rivers a large proportion of the sediment is transported by flood events that prevail only a few days of the year. How the climatic conditions that favor these events relate to the magnitudes and frequencies of erosional and transportation episodes is emerging as an area of great scientific interest. Erosion by other agents, such as glaciation and wind, plays some part in the degradation of topography, but most eroded material is carried to the sea by rivers either in solution or as detrital sediment. River transport varies enormously with both climate and source area; the Huang He (Yellow River) alone, for example, accounts for 6 percent of the world's total suspended matter river load, because this river drains the readily eroded windblown material of the loess plateau in the Chinese interior. In addition, the beginnings of agriculture more than 4,000 years ago appears to have greatly increased the rate at which sediment is moving into the river. Winds and Glaciers: More Material in Transit Although most of the material carried into sedimentary basins is transported by rivers, appreciable amounts are carried by wind and glaciers. Deserts, where wind erosion and deposition in the form of sand dunes are most important, have become the subject of increased research activity. This activity has been sparked by such diverse influences as the rapid extension of the Sahara into the Sahel within the past 30 years, the discovery of wind-made landforms on Mars, and the availability of satellite and radar images of the Earth's remote deserts (Figure 3.4). Again, new techniques of age determination have yielded exciting results. Recent measurements of the thermoluminesence of rocks from surfaces buried beneath dunes in the high plains of the American West have shown that the dunes were moving in response to desert winds much more recently than had previously been realized. Windblown dust mixed into the deep-sea sediments of the North Pacific helps to show how continental climatic fluctuations in China relate to the orbitally induced climatic changes that are well known from the deep-sea record. On a longer time scale, windblown dust in sediments from the deep Atlantic indicates that the Sahara first became a huge desert about 10-million-years ago, possibly as a result of changes in atmospheric circulation related to the uplift of the Tibetan plateau. Glaciers and glacial deposits reflect the tremendous fluctuations in the climate of the current ice age. Only 20,000 years ago glaciers extended as far south as New York City, and there may have been as many as a dozen comparable advances and retreats of Northern Hemisphere ice during the past 2.5-million-years. A new research effort to integrate continental and oceanic data from the past 2.5-million-years should produce a picture of how the surface environment adapts to rapid climate change. Glacier ice provides a unique record of short-term change, which is discussed in the part of this chapter dealing with cyclical change. Lakes: Interruptions in Transit Lakes represent a peculiar part of the earth system. If the solar-driven heat engine entails erosion of mountains, transit of eroded material from the mountains to the sea, and deposition of sediments at the edges of the continents and on the ocean floor, then lakes represent an interruption to the smooth flow of the system. As such, lakes are usually quite short lived because they fill with sediments. The familiar outlines of North America's Great Lakes are less than 20,000 years old and are unlikely to last more than another 20,000 years. Only a few of the world's existing lakes are more than a million years old, and the oldest, Lake Baikal in Siberia and the Great Rift Valley lakes of East Africa, are found in places where the continents are being ripped apart by tectonic forces. Lakes and lake sediments provide crucial information about climatic change during the past 2.5-million-years. In recently glaciated areas, some lakes have produced annual cycles in sedimentary layers called varves. Some of these varve-layer exposures indicate thousands of years of continuous deposition. The most productive method of retrieving environmental information from lake deposits is the analysis of fossil pollen, which provides a record of changes in nearby vegetation. In tropical areas, lakes expand during intense monsoonal episodes, while in temperate latitudes high lake levels may indicate more rain or less evaporation. Fluctuations in shoreline elevations are used with the pollen record to reconstruct recent environmental changes. The long-term rock record contains scattered evidence of ancient lakes; the oldest of these is more than 2-billion-years old, half as old as Earth itself. The study of old lake deposits has become a major research activity. Interest has been stimulated not only by studies of the geologically recent lakes but also by the discovery that lake beds are commonly
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Solid-Earth Sciences and Society FIGURE 3.4 SIR-A radar scan (diagonal band) reveals aggraded valley segments that were barely perceptible on Landsat images of eastern Sahara in northwest Sudan (19.7°N, 25.2°E). a source of petroleum—an idea promoted by Chinese geologists and now broadly accepted. Some of the finest remains of early humans have been found around the shores of old lake beds, and often the richest sites for dinosaur fossils are associated with the rocks along ancient shores.
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Solid-Earth Sciences and Society FIGURE 3.5 Atlantic-type (passive) continental margin along the eastern United States. Continental deposits accumulated in fault-bounded basins as the early stages of rifting separated North America from Africa. When the continents separated enough so that seawater could enter to form the Atlantic Ocean, marine deposition commenced. Deltas and Estuaries Rivers reach the sea in deltas and estuaries. Deltas form at river mouths when the prevailing current becomes too slow to carry detritus, so that enormous amounts of sediment are dumped close to the continental margin. Where deltaic deposition has continued for tens of millions of years, deltas have extended onto the deep ocean floor. Huge petroleum resources in the Mississippi, Niger, Orinoco, and other deltas are currently being developed, and related understanding has contributed to the way in which deltas are perceived from the complementary resource, environment, and hazard viewpoints. Estuaries, in contrast to deltas, are broad embayed river mouths that have been flooded since the end of the most recent glaciation when sea level rose and the river's sediment load diminished. Water circulation in estuaries is much more restricted than in the open sea, and, as a result, sensitivity to environmental modification is very high. For this reason estuaries are important targets for interdisciplinary research in biogeochemical dynamics. Beneath the Sea The large-scale structure of the ocean basins has been established by the operation of the Earth's internal heat engine, which causes rupture and drift of continents and island arcs, formation of new ocean floor at spreading centers, and establishment of new arc systems where plates converge. The operation of these processes leads inexorably to the opening and closing of oceans, to island-arc and continental collisions, to the assembly of continents, to the addition of new arc material to existing continents, and to recycling of both ocean-floor rocks and continental material into the mantle. Solar heat modifies the ocean floor mainly by deposition of detrital sediment eroded from the land and by precipitation of calcium carbonate and silica from oceanic waters—partly by marine organisms—to form limestone and chert. Together, these processes degrade the thermally generated submarine relief, not so much by erosion, the process dominant above sea level, as by deposition that smooths the topography. Deposition at Atlantic-Type Margins Sedimentary deposition below sea level is controlled primarily by the tectonic framework of the ocean basins. The largest volumes of sediments accumulate at the rifted continental margins, called Atlantic-type margins because they are best developed around the Atlantic Ocean (Figure 3.5). The most rapid additions to these types of margins at present come from rivers draining the world's largest areas of high elevation—from the Mississippi and Mackenzie rivers that drain western North
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Solid-Earth Sciences and Society America and from the Ganges and Indus rivers that drain the Himalaya and Tibet. Accumulations of rock as much as 15 km thick can develop along Atlantic-type margins. These, the greatest thicknesses of sediments in the world, are accommodated by subsidence as the lithosphere—thin and hot at continental rupture—cools and thickens. The load of accumulating sediments depresses the lithosphere farther and amplifies this subsidence. Broad continental shelves extending for many tens of kilometers from the edge of the ocean at depths of only a few tens of meters below sea level are characteristic of Atlantic-type oceanic margins, because of deposition by powerful river systems and perhaps because of extensive lithospheric thinning at the time of continental rupture. Those off the coast of New England and the Alaskan coast of the Bering Sea are typical. The recent sea level changes that mark responses to glaciation and deglaciation have led to repeated erosive episodes of the unconsolidated sediments of continental shelves. Large masses of such sediments have been off-loaded through submarine canyons onto deep-sea fans and abyssal plains. Limestone shelves develop where there is little sediment eroded from the land and in areas of abundant biological activity. Sediments originate mainly from the calcareous skeletons of shallow-water, bottom-dwelling marine organisms. These limestone shelves respond to sea level change in a way very different from sand and mud shelves. When sea level falls, exposure to fresh water as rain or runoff produces cementation that binds the loose skeletal sediment. As a result, carbonate-dominated banks and shelves reflect conditions of deposition during high stands of the sea and of subsequent cementation during low stands. Much of Florida and all of the Bahama banks were produced by this process. Submarine canyons carry sediments from the continental shelves to the deep oceans and the submarine fans, where that sediment settles. When they were first recognized about 50 years ago, the prime question asked was how such enormous features could form. Computation of the huge volume of sediments in the fans showed that turbid sediment-laden flows pouring from the continental shelves in times of glacially controlled low stands of the sea could have readily carved even the greatest of submarine canyons, many of which are much larger than the Grand Canyon. Modern research on both submarine fans and canyons is accelerating because of the availability of new instrumental capabilities. Deep-sea drilling, multibeam echo sounders, side-look scanning sonars, and manned and remotely controlled submersibles are providing a much more detailed picture than was formerly obtainable. Research on the sedimentary development of the Atlantic-type margins has expanded enormously during the past decade, largely in response to two stimuli: an appreciation, following the plate tectonic revolution, of how continental rupture happens and an understanding of how the sediment wedge at the continental margin evolves through time. The latter owes much to oil exploration, which led to the development of the technique of sequence stratigraphy, where coherent packages of distinctive strata in reflection seismic data—calibrated against the record of local oil wells—can be used to establish a detailed history of the transgression and regression of the sea. Lively controversy persists as to exactly how and whether the seismic stratigraphic records can be linked to global sea level fluctuations. Deposition at Convergent Plate Boundaries The greatest variations in topographic relief are produced at convergent plate boundaries. The giant peaks of the Himalaya, 8 km high, result from the collision of India with Asia, and the 11-km extreme of oceanic depth is found in the Marianas Trench, where subduction is carrying the Pacific plate into the mantle. Two dominant sedimentary depositional environments are associated with these immense topographic contrasts: trenches and foreland basins. Subduction-zone trenches contain substantial sediment accumulations only where the supply of sediments is large enough to exceed the rate of removal by subduction (Figure 3.6). This type of accumulation is occurring, for example, at the eastern end of the Aleutian Trench close to sediment sources in the North American continent and at the southern end of the Lesser Antilles Trench close to the South American continent at the mouth of the Orinoco River. Sediments accumulate at the front of the related arc system to form a thick wedge, or accretionary prism, that extends along the trench. An exciting challenge—currently being addressed by deep-sea drilling, multibeam echo sounding, and other new techniques—is to establish exactly how unconsolidated sediment entering a deep-sea trench becomes solid rock in the accretionary prism, a process that involves both intense deformation and the expulsion of vast quantities of water. The other sites of substantial sediment accumulation in convergent plate boundary zones are the
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Solid-Earth Sciences and Society mark the geological record beginning 34-million-years ago exemplify these points. This crisis is especially amenable to study because it happened relatively recently. The stratigraphic record is of high-quality, and the well-established pattern of magnetic reversals provides an excellent temporal framework. Fossil records of planktonic foraminifera, calcareous nannoplankton, and terrestrial mammals show that the crisis was protracted, suggesting multiple pulses of extinction. As we have seen, changes in terrestrial paleofloras indicate that climates cooled in many areas; shifts in oxygen isotopes of foraminiferan shells reveal that water masses underwent major changes; and plate reconstructions suggest that the Antarctic cooling system for the deep-sea originated at this time. Nonetheless, the ultimate cause of this crisis remains controversial. Pulses of extinction, when they have removed large numbers of species by imposing unusual and lethal conditions, can reset biological systems in ways that may have had nothing to do with the victim's ability to adapt successfully to ordinary conditions. This circumstance is epitomized by the extinction of the dinosaurs, which emptied ecospace to the benefit of the mammals. In recent years, dinosaurs have been recast as active, ecologically adept creatures—not backward lumbering forms that were inherently inferior to mammals. A continuing question about global biotic crises in general is whether, through geological time, they form the tip of one tail in a unimodal distribution of extinction rates or whether they represent a statistical outlier; a sparse data base currently frustrates valid statistical testing. The second condition would imply unique causation rather than simply an accentuation of normal agents of extinction. On the other hand, periodic spacing of mass extinctions may suggest a highly abnormal cause. The possible regularity of these crises stimulates current debates. At the other end of the distribution of extinction rates lies another question: When the record shows extinctions occurring at minimal rates, do species still disappear in groups or do they die off independently in piecemeal fashion? The former condition would require an expansion of catastrophic ideology to embrace even relatively calm intervals of biotic history. Finer resolution of the stratigraphic record may solve this basic problem of normal, or background, extinction levels. Macroevolutionary Trends Macroevolution transcends species boundaries, involving changes in the more generalized taxonomic levels, such as genus or family. Macroevolutionary changes include those trends in the marine realm that have been driven by the expansion of sophisticated predators on the seafloor over the past 100-million-years. Phyletic evolution can produce macroevolutionary trends, as can differences in rates of extinction and speciation among groups of species. An important question here relates to the relative importance of phyletic evolution in producing trends in the history of life. If it has been of secondary importance, as asserted in the punctuational model, differential rates of extinction and speciation gain importance. A further question concerning phyletic evolution addresses the degree to which it has been gradual and the degree to which it has followed a stepwise course, in which established species have undergone most of their changes during very brief intervals of geological time. Trends can be documented only on the basis of careful taxonomic and biostratigraphic studies, and they can be interpreted only by considering the functional morphology and ecology of component species. Origins of Major Biological Categories Fossil data provide estimates of the times when higher biological groups evolved. For groups with excellent fossil records, early estimates remain unchanged even though large volumes of new data have accumulated. For those with poor fossil records, the estimates are vulnerable to new discoveries. Thus, although Archaeopteryx is remarkable for its intermediate morphological position between dinosaurs and birds, the early fossil record of birds is so poor that a recent claim for a much earlier bird cannot be rejected out of hand. The fossil record offers a unique opportunity to assess the origins of taxonomic groups at general levels—the origins of kingdoms in Precambrian time and then of phyla. Times of origin for the more general groups, which are estimated from molecular data, do not always agree with those from fossil data. Molecular data clearly have great potential for the assessment of relative times of origin, but only well-dated fossils will estimate actual rates of divergence between forms. The environmental sites of major evolutionary breakthroughs are difficult to predict. These events may be concentrated in relatively unstable habitats, such as those of high latitudes or nearshore marine habitats, or in more stable habitats, such as those of the tropics or offshore marine habitats at middle or low latitudes. Hostile habitats generally offer more vacant ecospace, but more
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Solid-Earth Sciences and Society hospitable habitats sustain more varieties of life and harbor more biological groups, which means that they can support greater total rates of speciation. Origins of higher groups frequently reflect the evolution of adaptive innovations. Studies of ancestors and descendants in the fossil record can reveal the morphogenetic mechanisms that gave rise to innovations. Among these mechanisms are changes in the relative timing for development of different morphological features. This area of research offers great potential for fruitful interaction between paleobiologists, developmental biologists, and geneticists. Phylogenetic Reconstruction During the past two decades, cladistic analysis—the study of similarities resulting from common origins—has emerged as a powerful quantitative method for reconstructing the genealogical relationships called phylogenies. It is based on assessment of how traits that have evolved only once are distributed among taxonomic groups. Some proponents of cladistic analysis disregard the stratigraphic distribution of groups, grounding reconstruction of phylogenies on judgment as to which characteristics are primitive and which are derived. An alternative approach reconstructs phylogenies by evaluating stratigraphic and morphological distances between groups. Comparisons have shown that the two approaches sometimes yield identical results. Such comparisons are of great value, and methods that combine the two approaches warrant further examination. Molecular data also can reconstruct phylogenies; however, some of the techniques, including DNA hybridization, remain controversial, and their results must be compared to those achieved with the other approaches. Catastrophes in Earth History Geologists call sudden violent changes catastrophes, and they contrast catastrophes with the changes in the rock record attributed to constant gradual processes. There is obviously a continuum between frequent events, moderate events, and the occasional violent happening, and this simple relationship is readily expressed in empirical laws. Such relationships can, for example, link earthquake frequency with earthquake magnitude. Nearly two centuries ago polarized positions were assigned to geologists—they were either catastrophists or uniformitarians. Since then geological interpretation has accommodated the occurrence of occasional violent events, of the kinds experienced within our own lifetimes—hurricanes, earthquakes, and volcanic eruptions. A uniformitarian approach has dominated because it is very effective in analyzing the rock record and in making correlations. The possibility that the four or five great biological extinctions of the past few hundred-million-years marked catastrophic events never received serious attention because no evidence of a promising catastrophic mechanism had been recognized. That changed a decade ago when researchers, studying a few sites scattered over the globe, reported that strata that marked the mass extinction of about 66-million-years ago contained anomalously high concentrations of iridium and related platinum-group metals. Similarily, high concentrations have now been reported from dozens of localities worldwide of the same age. They are considered strong evidence of a catastrophic event close to the time of extinction of many groups of animals, of which dinosaurs are the most notable. This latter extinction event is one of the five largest extinction events to have occurred since fossils became abundant—the largest, 250-million-years ago, is represented by a less detailed record. But many things were happening 66-million-years ago; evidence indicates that the atmosphere and ocean were cooling and that the sea level was rising again after having fallen. Because the last vestiges of some life forms ceased to appear in strata that date to before the time of the iridium anomaly, it is argued that a catastrophic event may have been the final blow at a time of general environmental deterioration. Two kinds of nonbiological catastrophic perturbations that have been suggested are a global volcanic episode and the impact of a meteorite. A high platinum-metal concentration could indicate origins from within the mantle, though such a volcanic episode would have to be extremely intense, and one of the largest lava eruption events of the past few hundred-million-years formed the Deccan basalts in India at just the right time. Catastrophic perturbation by extraterrestrial impact seems more probable, not only because meteorites exhibit high concentrations of platinum-group metals in the right proportion but also because of an association of the iridium-rich horizon with quartz grains showing the effects of intense shock. Shock features are commonly found with meteorite impacts but are unknown in volcanos. And perhaps even more significant is the existence of small beads of glassy material of the type produced by the heat released by meteorite impacts—microtectites. Several mechanisms have been proposed that show how meteorite impact could
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Solid-Earth Sciences and Society result in sufficient environmental modification to cause catastrophic mortality. One computer simulation portrays the ejection of a dust layer into the high atmosphere that would cut out sunlight for months at a time, ending photosynthesis and sinking all latitudes into a deep cold. After the dust settled, high levels of atmospheric carbon dioxide would warm the Earth by producing a greenhouse effect. Another model suggests that the fast-traveling ejecta from the impact site could have been at temperatures high enough to cause atmospheric oxygen and nitrogen to combine, forming clouds that would precipitate into nitric acid rain. A third hypothesis, which is gaining increased acceptance, is that an impact of the proper age, formed the 180-km-diameter Chicxulub crater in the Yucatan of Mexico. Chicxulub strata is composed of thick sulfate-rich evaporite and carbonate deposits; an impact into such deposits could eject huge amounts of sulfate aerosols into the stratosphere, resulting in large-scale global cooling and several years of acid-rich rains as the aerosols settled out of the atmosphere—both the cooling and acid rains could have devastated the food chain. Several issues complicate the impact scenario—one strong argument cites evidence of animal groups that suffered heavy extinction before the primary iridium anomaly settled into place. Regardless of what the eventual consensus on its cause may be, the iridium anomaly has stimulated intense geological research. Not only have the contemporary rocks been studied closely—the anomaly has been documented at about 100 sites throughout the world—but the deep ocean record has been scrutinized, revealing a perturbation of the earth system that lasted half a million years. The most significant result of this intense geological research may be that serious consideration of sudden global catastrophes is now acceptable. All meteorite impacts are now receiving great attention. The roughly 100 known impact craters on Earth are being looked at anew, their ages are being reassessed, and attempts are being made to see whether their incidence could be periodic. An impact site in Sweden, 50 km in diameter, has been drilled to a depth of more than 6 km, allowing observations about the effects of large-body impact at depth. The discovery of iridium anomalies in some of the oldest rocks strongly suggests a flurry of meteorite impacts as late as 3.6-million-years ago. And the most drastic of catastrophes, collision between the Earth and a Mars-sized body before 4-billion-years ago, is now considered a possible explanation for the Moon's formation. Observations of asteroids show that the current meteorite flux is likely to be higher than had been considered, and in 1989 one carefully observed asteroid passed closer to us than any other in the past 50 years. There is a calculable, if remote, chance that the Earth will be struck by an object more than a few kilometers in diameter in the foreseeable future. Such an event would cause massive destruction, and a case can be made for assessing the possibility of diverting an incoming object to avert such a potential collision. No other global iridium anomalies have been recognized, although a locally defined anomaly dated at 34-million-years ago corresponds to a substantial extinction. No iridium or shocked quartz horizons have been found for the greatest extinction ever, which was 240-million-years ago; however, the absence of evidence, especially in older rocks, should not lead to a positive conclusion. There is certainly evidence that other ancient mass extinctions were complex events, extending over intervals of several million years. The mainstream of the earth sciences has shifted away from the extreme uniformitarian position, which attempts to explain all phenomena in terms of directly observed processes. Now researchers, confident in the soundness of their inductive methodology, can consider the possibility of exceptional events. There is, of course, an appropriate reluctance to invoke exceptional events as causal mechanisms, and intense skepticism deservedly awaits all such suggestions. The study of catastrophic effects on earth systems requires further work. Promising directions for general research include their continuing consequences, such as how perturbations propagate through the earth system and how secular change is altered permanently by catastrophe. MODELING THE EARTH SYSTEM An Incomplete Record Solid-earth scientists, like all other scientists, have long used conceptual models as an aid in understanding. For example, it was recognized in ancient times that the occurrence of seashells in rocks on the continents required a model in which the disposition of sea and land changed with time. By the nineteenth century this idea had developed further with the recognition that there had been widespread episodes of continental flooding that were correlatable over great distances. In the mid-twentieth century the repeated flooding and emergence of North America over the past 550-million-years were found to provide a strong framework for under-
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Solid-Earth Sciences and Society standing historical geology, and, during the past 20 years, flooding and emergence have been found to be approximately simultaneous for at least three continents. This coincidence has led to the suggestion that the main driver of sea level change is within the world ocean. The amount of seafloor spreading going on at a particular time seems a strong candidate for the dominant control. Young seafloor is hot and shallow, and aged seafloor is cold and lies deep. The average rate of spreading and the total length of spreading center at any one time can be quantitatively associated with the extent of flooding of the continents. Conceptual models that are progressively refined may be considered successful for parts of the earth system but are not really comprehensive. The question of continental flooding helps to show why this is so. The amount of water that is locked up in ice has decreased since about 20,000 years ago with the melting of North American and Eurasian ice sheets. This decrease has accounted for about 100 m of sea level rise over the same interval. This is a very large change in sea level compared with the range of continental elevation. About 80 percent of the continental area lies within 1 km of the present sea level. Although we can estimate the change in the average age of the ocean floor for the past 100-million-years we cannot do so well for the amount of water locked up in ice sheets. Evidence that there were no major ice sheets 100-million-years ago is strong, but it is not clear how long ago the great Antarctic ice sheet formed, nor can we tell whether it has fluctuated much or even disappeared and been renewed. This example illustrates a general situation. Comprehensive modeling of earth systems is very difficult, because although some parts of the system are understood, fluctuations in critical variables for other parts of the system are too poorly known to justify the construction of elaborate models. The challenge represented by the fact that some parts of the earth system can be readily modeled whereas others cannot has been met by concentrating on modeling those subsystems that can be handled well and by seeking innovative ways of quantifying information about subsystems that cannot. A further problem is linking the models of system parts that operate at different rates, such as the atmosphere, which changes on a very short time scale, and the ocean, which changes more slowly. The most successful and productive models reconstruct cyclic changes involving limited components of the earth system. Only a few components, or variables, can be tested at one time, but every successful simulation provides further understanding. Some aspects of paleoceanography and paleoclimatology lend themselves to quantitative modeling in isolation from the whole. However, most are not, and, because of the interdependencies, the models should be global in scale, link adjacent water masses, and couple the atmosphere with the oceans, so great is the interdependence of these two great fluid envelopes. Atmosphere and ocean are linked especially by exchanges of heat, momentum, carbon dioxide, and, of course, water. The surface temperatures of the ocean influence these fluxes. The salinity of waters that reach the ocean bottom, a variable related to climate, also affects oceanic circulation. And changes in global sea level affect the levels of atmospheric carbon dioxide, which in turn influence global climate through the greenhouse effect. Outstanding successes include modeling of the glacial cycles of the past 0.8-million-years, as controlled by variations in orbital parameters, and modeling of sedimentary basin fill, as controlled by loads applied to the lithosphere. Modeling of atmospheric systems, which mainly use energy balance models and general circulation models (GCMs), has grown to be extremely sophisticated in recent years, and solid-earth scientists have found some community of interest with atmospheric modelers. For example, at the time of the most recent glacial maximum, the solar energy input at the surface was less than it is today. A GCM of that time also indicates wind and temperature distributions that are very different from those of today. These indications have been compared with the geological record of the most recent glacial maximum with some success. Simplified energy balance models for the atmosphere for 100 million and 375-million-years ago, when continental distributions were very different and much of the surficial climate was warmer than it is now, have also met with some qualified success. There are some problems, too—for example, existing GCMs for the glacial maximum will not predict conditions that would grow the large ice sheets that existed then. Intellectual Frontiers Understanding the processes that are active today in establishing the surface environment and understanding how those processes have operated throughout geological time are the basic challenges addressed in this chapter. Intellectual frontiers relate to questions about the surface environment that have not been asked in the past, either because it was not possible to ask them or because it was not
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Solid-Earth Sciences and Society recognized that they had meaningful answers. A distinctive new development is the perspective that views the environment as a complex of interactive systems. Now specific problems are posed, and solved, as part of a broader framework of global understanding. Remotely sensed imagery from space and organized international cooperation have done much to stimulate the global approach. In the next decade the operation of higher-resolution instruments on advanced space platforms, such as those envisaged for the Earth Observing System, will enhance the global perspective. But perhaps the most important efforts toward global understanding are made through programs that depend on international cooperation among scientists. The Ocean Drilling Program exemplifies this trend, as does the innovative International Geological Correlation Program and the International Geosphere-Biosphere Program. Dating Past Events Because geochronology scales physical, chemical, and biological events against time, it plays a fundamental role in the earth sciences. To appreciate what has happened, we need to know the sequence of events and the rates of change. Quantitative biostratigraphic techniques yield correlations with accuracies approaching a few hundred thousand years for bodies of rock that are hundreds of millions of years old and lie thousands of kilometers apart. These methods of correlation are integrated with others, including paleomagnetic methods and radiometric dating of marker beds such as volcanic ashes. Together they encourage the search for high-frequency events and for regular patterns in such events. Correlation techniques and isotopic dating serve as checks on each other. Isotopic dating methods can focus on scales from billions to mere thousands of years, but when possible they should be integrated with other dating methods. Global event stratigraphy correlates the worldwide expression of certain events. It provides a framework of additional instantaneous markers against which intervening events can be calibrated. The stratigraphic evidence of rapid global sea level change falls within this category, as does the identification of global chemical signals. The chemical signals include both narrow stratigraphic markers that formed during brief moments of geological time and long-term secular trends that trace continuing developments. The iridium anomaly, which marks the mass extinction of 66-million-years ago and may signal the impact of a huge meteorite, is the most famous geochemical marker in the stratigraphic record. But others have been, and will continue to be, discovered. All of the challenging areas of research described in the following paragraphs—historical studies of oceans and atmospheres, terrestrial environments, and life on Earth—depend on advances in geochronology. Atmospheric and Oceanic Chemistry On the longest time scale, the geological record indicates a gross change from a reducing to an oxidizing state of the linked atmosphere-ocean system. The details of timing and the reason for this secular change still provide topics for lively debate. On shorter time scales, the record of the rocks preserves evidence of cyclical changes. Geologists can trace variable concentrations of carbon dioxide in the atmosphere and ocean on time scales ranging up to hundreds of millions of years. They have also distinguished episodes during which much of the deeper ocean was anoxic. Intervals when widespread anoxia in deep waters expanded to flood broad areas of the continents are especially interesting, because they resulted in the massive accumulation of valuable hydrocarbons from sources in black anoxic sediments. Some researchers think that the storage of so much organic carbon implies the possibility of an increased oxygen content in the atmosphere at such times in the past. Others consider that an inflammatory concept. Past oceanic composition, recorded within ancient sediments, reflects many aspects of the global environment. These include the mantle contributions through volcanism and continental input through erosion, the global climate, the presence or absence of ice caps, and the level and kinds of biological activity. The history of ocean chemistry can be established from the rock record-an endeavor that is rewarding because it has been so successful. For the past 150 million years, the interval when the sediment now carpeting the deep-seafloor has been accumulating, ocean chemists can study changes among the individual water masses that together make up the world ocean. The most fundamental variable controlling atmospheric and oceanic chemistry has been the temperature of the deep sea. But patterns of upwelling and shallower currents and high biological productivity have undergone dramatic shifts at frequent intervals. Changes in oceanic conditions, especially sea level, have exerted a strong control over evolution
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Solid-Earth Sciences and Society and extinction and also over the formation of valuable resources. Dynamics of the Global Environment The evolution of the environment is an important area of research in modern earth sciences. The study of ancient conditions, when climates were warmer than today, offers insight into potentials of modern greenhouse warming. In addition, studies of different global ecosystems of the past may reveal peculiarities of the present world that render it especially vulnerable to certain forcing factors. The most recent segment of the geological record provides the temporal resolution and geographic control needed to identify very sudden environmental changes. During the first 4-billion-years of earth history, life arose and evolved through many intermediate stages to a point at which a variety of multicellular plants and animals existed. Evolution and extinction during this interval were tightly interwoven with profound changes in the physical nature of our planet, especially its atmospheric chemistry. This intimate relationship between life and environment serves to underscore an important point about future directions of research. Different intervals of earth history require distinct scientific strategies, because the intervals were characterized by different kinds and degrees of environmental change and are represented by different kinds of geological records. The prospects are exciting. But they require a prodigious amount of research to chronicle global environmental change for a variety of past intervals with the resolution required. Once models can accurately simulate environmental conditions of key intervals during earth history, they can be used for predicting future conditions. Collaborative modeling projects that unite workers in the geological sciences with meteorologists and oceanographers are beginning to yield results. Life Through Time Numerous advances have breathed new life into paleontology. The contributions of the fossil record to the study of evolution and extinction uniquely document myriad forms unknown in the modern world. A cumulative chronology indicates the rates of evolution and extinction, and the timing of major events in the context of environmental change. Such a chronology establishes fluctuations and patterns that can point to new questions about the history of life. The development and application of quantitative techniques will continue to play a prominent role in future studies of life through time. Key methods will assess morphological change; patterns of evolution and extinction; and theoretical modeling of taxonomic, stratigraphic, and environmental data obtained from the fossil record. The fossil record also provides evidence of the timing of evolutionary branching. The molecular clocks used to estimate the times when certain extant groups branched from others must be calibrated against fossil data, and conclusions must be tested against macroscopic evidence. Of special importance are fossil data that reveal the occurrence of adaptive breakthrough—evolutionary innovations that ushered in new modes of life that transformed the ecosystem. These can be recognized in the fossil record only by inferring function from morphology, an activity that merits increased support. Many adaptive breakthroughs have triggered adaptive radiations—diversification from a single life form that results in the invasion of a variety of ecological niches. Such radiations have special significance because they account for most of the major evolutionary changes in the history of life. Understanding the general diversification of life on Earth requires an understanding of adaptive radiations. Sudden extinctions—the negative equivalent of adaptive radiations—have repeatedly reordered the global ecosystem and opened the way for new evolutionary directions. Mass extinctions can be understood only in the context of global environmental change. The taxonomic, temporal, and geographic patterns that have characterized these events are especially significant; compilation of new data demands the continued generation of high-quality biostratigraphic and taxonomic information. Whether global biotic catastrophes have occurred in combination with background extinction or instead have been quantitatively or qualitatively distinct is a question that can be answered only by understanding the patterns and causes of noncrisis extinctions. The Most Recent Past Geologists know that the record of the most recent past is exceptional because its complex history can be better established than that of any earlier period. Powerful new techniques are determining the ages, isotopic compositions, and temperatures of materials deposited during the past 2.5-million-years. But the most significant recent development is the worldwide awareness of changes in the global environment caused by humanity. This new awareness has stimulated a need to better understand the
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Solid-Earth Sciences and Society TABLE 3.1 Research Opportunity Framework Research Opportunities Research Areas A. Understand Processes B C D I. Paleoenvironment and Biological Evolution ■: Soil development, history, and contamination ■: Glacier ice and its inclusions ■: Quaternary record ■: Recent global changes ■: Paleogeography and paleoclimatology ■: Paleoceanography ■: Forcing factors in environmental change ■: History of life ■: Discovery and curation of fossils ■: Abrupt and catastrophic changes ■: Organic geochemistry II. Global Geochemical and Biogeochemical Cycles ■: Geochemical cycles: atmospheres and oceans III. Fluids in and on the Earth ■: Analysis of drainage basins ■: Mineral-water interface geochemistry IV. Crustal Dynamics: Ocean and Continent ■: Landform response to change ■: Quantification of thresholds, response rates and feedback mechanisms for landforms ■: Mathematical and computer modeling of landform changes ■: Sedimentary basins ■: Sequence stratigraphy V. Facilities, Equipment, Data Bases ■: Exploit new tools and techniques (e.g., isotopes, trace compounds, DNA sequencing and hybridization, digitizing techniques) ■: Exploit new dating techniques (e.g., radiometric methods, trends in isotope ratios, biostratigraphic correlation, chemical markers in stratigraphy) ■: Acquire high-quality data bases and establish information systems immediate past that is forging closer links between geologists and other earth scientists. The history of human evolution is beginning to unfold in sufficient detail to reveal the kind of environmental influences that affected human ancestors. All the challenges that have been identified here as intellectual frontiers have special possibilities for resolution when addressed in light of our understanding of the ongoing global changes during this most recent geological period. RESEARCH OPPORTUNITIES The Research Framework (Table 3.1) summarizes the research opportunities identified in this chapter, with reference also to other disciplinary reports and recommendations. These topics, representing significant selection and thus prioritization from a large array of research projects, are described briefly in the following section. The relevant processes operate near the surface for the most part, although there is no sharp boundary between surficial geology and the deep-seated processes covered in Chapter 2. There are excellent prospects for generating better models of the earth system, both present and past, including global tectonic models, coupled ocean-atmosphere biogeochemical models of the fluid envelope, and paleoecological models of the biosphere. The fossil record of the biosphere provides information about evolution as well as evidence of environmental changes and migration of continents. The relevant Research Areas for this chapter are interrelated, and many of the research topics relate to more than one area. The carbon cycle (Area II),
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Solid-Earth Sciences and Society for example, links to life and biological evolution (Area I) and has influenced paleoatmospheres and paleoceans theme (Area I), as well as playing a major role in continental weathering theme (Area III). It also has connections to the mantle through the igneous and metamorphic processes in the lithosphere and crust (Area IV), as discussed in Chapter 2. Some of the many applications of these Research Areas are outlined in Chapters 4 and 5. Research Area I. Paleoenvironment and Biological Evolution Soil Development, History, and Contamination If extended to soils, the new age-determination capabilities that are being applied to landforms and drainage basins could quantify temporal aspects of soil development that have been the subject of much speculation. More complete understanding of soil processes as aspects of environmental, especially climatic, evolution will improve both paleosoil and soil-contamination studies. Glacier Ice and Its Inclusions Given the important role of glaciers in controlling sea level and climate, and considering the evident instability of glacial volume, it is essential that we gain a clearer picture of the history of glaciation since the onset of the recent ice age about 2.5-million-years ago. Especially important too is further study of the annual layers in ice cores, including analysis of oxygen isotopes, carbon dioxide, and dust to obtain a record of the past few thousand years. We need a much better understanding of glacial expansion and contraction and the role of environmental thresholds in governing these processes. Quaternary Record The record of change in the Quaternary (the past 1.6-million-years) is important because it is the most complete available record for any part of the past and thus provides the best picture of environmental change. Synthetic and snyoptic studies of Quaternary history (e.g., CLIMAP and COHMAP) have led the way. Extending studies of this kind with better spatially distributed data and higher temporal resolution will help to show how phenomena occurring simultaneously in different areas were related to each other, and will indicate sequences of events. This kind of information is needed to test atmospheric and oceanic models that are being used to assess what might happen in future global environments. Recent Global Changes The evolution of the ocean, atmosphere, and life during the first 4-billion-years of the planet's history is of great intellectual appeal and deserves continued emphasis. Nonetheless, of greatest practical value are studies of global change during the most recent interval of geological time. The geological record of the past 10,000 years has great potential for shedding light on the ways in which climatic changes affect life. In light of the global warming anticipated for the coming decade, warm intervals of the past should be scrutinized for possible lessons for the future. In addition, intervals exhibiting large-scale environmental change or mass extinction of life warrant detailed scrutiny for patterns and causes. Paleogeography and Paleoclimatology The past few years have seen major advances in paleoclimatology and terrestrial paleogeography. Sedimentary indicators and fossil biotas, especially floras, continue to shed light on climates of the past, but there is also a need to refine the existing kaleidoscopic picture of changing continental configurations, especially for the long pre-Mesozoic (older than 66-million-years) interval of earth history. Global paleogeographic reconstructions must continue to be developed and refined in all possible ways, including paleogeographic interpretations, global paleobathymetric reconstructions, identification of accreted terranes and microcontinents both through anomalous fossil distributions and paleomagnetic determinations. Determination of the rates of geochemical cycling requires knowledge of the sizes, elevations, and positions of continents in the past. Paleoceanography Changing climatic regimes, plate positions, and levels of land and sea have had powerful effects on the thermal structure and current patterns of the world ocean. At the forefront of paleoceanography today are studies of the three-dimensional oceanic structure, including thermohaline circulation, conditions in the deep-sea, upwelling, and vertical zonation of plankton. The Ocean Drilling Program should continue to play a key role in this kind of research for the most recent 150-million-years of
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Solid-Earth Sciences and Society earth history, as should the use of isotopic data. The global influence of events in polar regions is in special need of further study. Forcing Factors in Environmental Change We are only beginning to understand the interrelationships between continental configurations, the dynamics of the ocean and atmosphere, and the distributions of life on Earth. In modeling global environmental change, sensitivity experiments that suggest what forcing factors have pushed environmental conditions across thresholds to new states are often more successful than detailed global simulations. At present, models are outstripping the data needed to constrain and test them, and research that will provide additional data is badly needed. History of Life Inasmuch as the fossil record represents a unique store of information on rates, trends, and patterns of evolution and extinction, we must continue to exploit it to understand these aspects of the history of life. There is still no consensus on such issues as the incidence or cause of evolutionary stasis, the degree to which extinction occurs in pulses, or what environmental changes trigger rapid evolution, including bursts of speciation. New quantitative techniques, including morphometric methodologies, must play an important role in research here. There is also a need to develop methods of phylogenetic analysis that integrate morphological and molecular approaches with stratigraphic data. Interactions between life and the environment—for example, how much the changes in atmospheric composition have been responses to evolutionary change and how atmospheric change has influenced evolution—must be established. Discovery and Curation of Fossils Our understanding of the processes of biological evolution continues to be refined by the discovery and description of fossils that fill gaps in the record. Examples of great steps forward made within the past decade are the discovery of new localities and material indicative of the diversity of marine life 550-million-years ago, identification of the conodont animal and appreciation that it was a vertebrate, critical new specimens of the first terrestrial vertebrates, and (stepping back into the ocean?) a whale with vestigial limbs. The kind of relatively unglamorous fieldwork that leads to these successes requires ongoing commitment. Abrupt and Catastrophic Changes Sudden events have a lot to teach earth scientists, particularly where their record extends over the whole world or at least very large areas. What are the causes of these events? Are they of impact, volcanic, or other origins? Does the rock record indicate precursory phenomena? Was there subsequent environmental change? If there is evidence of change, how long did it last? The behavior of the environment under stressed or extreme conditions is likely to be informative and rapid climatic changes are of special interest in the field of global change research. It is worth noting that had this report been written a dozen years ago little emphasis would have been given to catastrophes. Organic Geochemistry There are diverse ways in which organic geochemistry is yielding new information. New techniques for isotopic analysis of specific organic compounds, ''chemical fossils," provide opportunities for reconstructing the temperatures, compositions, and oxidation states of the ancient ocean. Working out the role of ancient microbial communities in sediments and illuminating the thermal histories of sedimentary basins are some of the challenges. Research Area II. Global Geochemical and Biogeochemical Cycles Geochemical Cycles: Atmospheres and Oceans It is crucial that we improve our understanding of geochemical cycles to learn how the atmosphere and oceans have changed in the past and how they may change in the future. Controls of atmospheric carbon dioxide, and the resulting greenhouse effect, are of especially great significance. Geochemical cycles are complex dynamic systems that entail geological, biological, and extraterrestrial processes. Changes in fluxes and in sizes of chemical reservoirs must be more accurately quantified for the geological past. Among the controls needing further study are the compositions and abundances of sedimentary rocks, magmatic and metamorphic degassing, biological uptakes and emissions, sea level change, rates of weathering, and isotopic shifts for key elements. Even fluxes to and from the modern ocean are poorly known, as are the contributions of relevant
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Solid-Earth Sciences and Society organic biogeochemical processes. Mathematical techniques for modeling biogeochemical cycles can be greatly improved. Research Area III Fluids in and on the Earth Analysis of Drainage Basins To understand past changes in our terrestrial habitat and to anticipate future changes and their consequences, it is imperative that geomorphologists undertake quantitative analysis of drainage basins. This analysis should quantify linkages and pathways of weathering sediment erosion, storage, transportation, and contamination. Links to sequence stratigraphy (see Chapter 4) will come from identifying the impact of base-level changes on fluvial systems. One important question is how the history of river flooding for the past few thousand years has related to climatic change. On long time scales there is a need to relate the rates of surficial processes to tectonic activity. Mineral-Water Interface Geochemistry The rapid growth in this field will lead to a more fundamental understanding of weathering, the chemistry and physics of mineral-water interfacial phenomena, how chemical species partition between minerals and aqueous fluids in crust, and how the hydrosphere interacts with crustal rocks. Research Area IV Crustal Dynamics: Ocean and Continent Landform Response to Change Landforms represent the products of tectonic, climatic, and hydrologic processes. The ability to date surfaces and shallow sediments using cosmogenically generated nuclides and other direct and indirect methods provides the opportunity for improving understanding of how these processes generate landforms. The current strong interest in how tectonic, climatic, and hydrologic processes change with time makes this opportunity particularly timely. Quantification of Thresholds, Response Rates, and Feedback Mechanisms for Landforms Further definition is needed for stability/instability thresholds, response and recovery times for landforms following destabilizing events, and the significance of return frequencies of key climatic events for landform stability. Mathematical and Computer Modeling of Landform Changes Modeling will elucidate landform changes over longer periods of time than it is possible to observe them. Process-based models are essential for understanding mechanisms and for predicting future changes. Exciting progress has been made with, for example, hillslopes and river channel patterns. Sedimentary Basins Depositional basins are likely to reward detailed study because they record so many diverse kinds of information. The integrated approach to basin studies—which embraces structural, erosional, and depositional evolution as well as thermal, chemical, and fluid-flow history and uses field, seismic, well-log, geochemical, and isotopic data—is proving extremely powerful. Foreland basins and rifted continental margin basins, including great deltas, harbor the world's largest volumes of hydrocarbons. We are learning about how the sediment accumulation in these environments relates to tectonic and sea level changes and apparently to orbital parameters. Sequence Stratigraphy The identification of sediment packages that are separated by what appear to be abrupt temporal boundaries is an exciting new tool because it permits characterization of environmental changes that can be linked to episodes outside the area of deposition. Some researchers correlate boundaries in sequence stratigraphy globally, but this practice is questioned by others. Improved access to well-calibrated sequence data may help to resolve this issue. Facilities, Equipment, and Data Bases Data Bases in Well-Managed Computer-Based Information Systems The major problem facing paleogeographers, paleoclimatologists, and paleoceanographers is that the ability to predict and understand the earth system at high-resolution is outstripping the availability of the data required to test the methods and models. Vigorous collection of and rigorous quality control on data from the ancient record is urgently
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Solid-Earth Sciences and Society needed. Data and samples taken on continental margins and from wells drilled by industry represent an invaluable resource, as do many kinds of fossil data. The data base available to scientists could be expanded by several orders of magnitude. New Tools and Techniques New equipment offers much potential that has yet to be exploited in paleontology in the areas of isotopic analysis; identification of trace organic and inorganic compounds, DNA sequencing and hybridization, new multivariate techniques, digitizing techniques for three-dimensional quantification of morphology, application of geographic information systems (GIS) to quantitative historical biogeography, and computer-interfaced digitizers (some three-dimensional and some linked to light microscopes or scanning electron microscope systems) for morphometric studies. New Dating Techniques During the past few years, new dating techniques have greatly improved resolution in the temporal correlation of geological and biological events and in the measurement of rates for a wide variety of processes. These techniques include identification of key chemical markers in stratigraphic sequences, quantitative biostratigraphic correlation, application of new radiometric methods, and utilization of secular trends in stable isotope ratios. Not only the development of new dating techniques, but also the refinement and ingenious application of existing methodologies, will benefit numerous areas of research in the decades to come. BIBLIOGRAPHY National Research Council Reports NRC (1982). Studies in Geophysics: Climate in Earth History, Geophysics Study Committee, Geophysics Research Board, National Research Council, National Academy Press, Washington, D.C., 198 pp. NRC (1983). Opportunities for Research in the Geological Sciences, Committee on Opportunities for Research in the Geological Sciences, Board on Earth Sciences, National Research Council, National Academy Press, Washington, D.C., 95 PP. NRC (1986). Global Change in the Geosphere-Biosphere: Initial Priorities for an IGBP, U.S. Committee for an International Geosphere-Biosphere Program, Commission on Physical Sciences, Mathematics, and Resources, National Research Council, National Academy Press, Washington, D.C., 91 pp. NRC (1989). Technology and Environment, Advisory Committee on Technology and Society, National Academy of Engineering, National Academy Press, Washington, D.C., 221 pp. NRC (1989). Margins: A Research Initiative for Interdisciplinary Studies of Processes Attending Lithospheric Extension and Convergence , Ocean Studies Board, National Research Council, National Academy Press, Washington, D.C., 285 pp. NRC (1990). Research Strategies for the U.S. Global Change Research Program, Committee on Global Change, U.S. National Committee for the IGBP, National Research Council, National Academy Press, Washington, D.C., 291 PP. NRC (1990). Studies in Geophysics: Sea Level Change, Geophysics Study Committee, Board on Earth Sciences and Resources, National Research Council, National Academy Press, Washington, D.C., 234 pp. NRC (1991). Toward Sustainability: Soil and Water Research Priorities for Developing Countries, Committee on International Soil and Water Research and Development, Water Science and Technology Board, National Research Council, National Academy Press, Washington, D.C., 65 pp. Other Reports The International Geosphere-Biosphere Programme: A Study of Global Change (IGBP) of the International Council of Scientific Unions (1992). The Pages Project: Proposed Implementation Plans for Research Activities, Pages Scientific Steering Committee, 110 pp. NASA (1988). Earth System Science: A Closer View, Earth System Sciences Committee, NASA Advisory Council, Washington, D.C., 208 pp. Office of Science and Technology Policy (1992). Our Changing Planet: The FY 1992 U.S. Global Change Research Program, Committee on Earth and Environmental Sciences, 90 pp.
Representative terms from entire chapter: