Solid-Earth Sciences and Society (1993) / Chapter Skim
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3 The Global Environment and Its Evolution
3 The Global Environment and Its Evolution
Pages 91-136
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From page 91... ...
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.
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From page 92... ...
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 lanciscape 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.
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From page 93... ...
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.
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From page 94... ...
THE GLOBAL ENVIRONMENT: 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.
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From page 95... ...
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.
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From page 96... ...
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.
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From page 97... ...
. LEACH I NG ions i= _ ~FIGURE 3.2 Flow of the major processes in soil development effectively extending hydrologic records for up to thousands of years, by combining interpretations of river and sediment behavior with results from geological dating methods.
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From page 98... ...
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.
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From page 99... ...
THE GLOBAL ENVIRONMENT AND ITS EVOLUTION 99 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 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)
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From page 100... ...
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 SOLID-EARTH SCIENCES AND SOCIETY Continental shelf \ Continental slope \ Continental rise 'it" / / Continental deposits / Continental shelf Continental and slope rise prism deposits ocean floor at spreading centers, and establishment of new arc systems where plates converge.
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From page 101... ...
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.
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From page 102... ...
During most of the geological past, when sea level was higher, foreland basins accumulated marine sediments. Recent research in foreland basins emphasizes an integrated approach that models how episodes of uplift in the mountain belts modify sediment supply and interact with sea-level changes and with thrusting of the mountain load over the basin.
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From page 103... ...
The rise and fall of sea level take place on time scales ranging from days to tens of millions of years; superimposed on these cyclical changes are noncyclic or secular changes, such as biological evolution, that have been occurring throughout earth history. The way in which the cyclical processes operate has been modified to some extent by secular changes, but it is one of the more exciting
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From page 104... ...
The rock record contains evidence of cycles operating on daily scales, such as tidally controlled sedimentation; annual scales, such as varved lake deposits; and tens to hundreds of thousands of years, such as cycles controlled by variations in the Earth's orbit around the Sun. Sea levels change over millions of years due to variations in the average age of the ocean floor; ocean basins open and close over tens to hundreds of millions of years; and the isolation of major compositional reservoirs in the mantle may be the result of cycles that take billions of years.
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From page 105... ...
These cycles cause subtle changes, particularly in high latitudes, in the seasonal variation of the incoming solar radiation, called insolation, and may be reliably calculated from celestial mechanics. The ice ages themselves are recorded in the ratio of oxygen isotopes in deep-sea sediments.
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From page 106... ...
Remote sensing, the set of processes by which we observe large areas of the Earth from outer space, also provides valuable information for assessing environmental changes of the recent past. Combinations of different perspectives and frequent coverage inform us about rates of tectonism, volcanism, and other processes that have altered landforms.
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From page 107... ...
Closing oceans, such as the Pacific and the Mediterranean, are bounded largely by convergent plate boundaries. Researchers concluded that perhaps earth history operates within a framework of complex cycles of opening and closing ocean basins.
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From page 108... ...
SOLID-EARTH SCIENCES AND SOCIETY North Pole _ ~- ~ _ - ~,~ South Pole Arabia - -I Australia voirs appears to be considerably perturbed by the rapid rise in the atmospheric component; model simulations indicate that it could take hundreds of years to restore equilibrium to this subsystem. The geological record offers information about a past scenario that involved disequilibrium among these same three reservoirs.
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From page 109... ...
Paleoceanography: Cycles in the History of Oceanic Waters Recognition of changes in variables such as the chemistry of the oceans, the global sea level, the configuration of ocean basins, the three-dimensional thermal structure of the ocean, and the history of marine organisms permits the description of ancient conditions, which even during the past 18,000 years have undergone remarkable transformations. On a broader scale of time, changes have been even more profound.
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From page 110... ...
However, conditions within ancient oceans can be reconstructed by using patterns of modern oceanic circulation to reassemble the thermal structure and dominant currents in ancient oceans and by selecting especially important physical, chemical, and biological indicators in the geological record to plot distributions. Nowhere has this approach been un SOLID-EA1RTH SCIENCES AND SOCIETY dertaken more effectively than in the Climate: Long-Range Investigation, Mapping, and Prediction (CLIMAP)
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From page 111... ...
The Arctic Ocean became connected to the Atlantic over the Iceland sill, allowing cold Arctic surface waters to descend into the deep sea. Throughout earth history a cold basal layer of ocean water must have formed each time at least one of the poles became frigid.
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From page 112... ...
In the near future, fossil alkenones may yield a general temperature map for the 50-million-year-old global ocean. At any time in earth history, deep ocean water is generated from the densest water masses that develop at shallower levels and have access to the major ocean basins.
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From page 113... ...
· Land Plants: Terrestrial floras are excellent indices of paleoclimates. Flowering plants are especially useful because of their conspicuous fossil record, which extends for about 100 million years.
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From page 114... ...
Testing the models for earlier periods of earth history becomes even more dii31~cult because climatic indicators are less precisely documented. But general climatic models developed for periods approaching half a billion years offer provocative results that must be taken seriously.
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From page 115... ...
Beginning around 550 million years ago, preskeletal multicellular animals left an abundant fossil record consisting of tracks, trails, and body imprints. Study of early environments, and the organisms that evolved in them, is intertwined with the study of secular trends.
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From page 116... ...
Rapid convection in the mantle would have led to rapid operation of cycles that open and close ocean basins, although there is no way of knowing when rigid extensive plates of lithosphere first characterized the surface. An intermediate arrangement could have involved a less organized convective system dominated by numerous Hawaii-like hot spots.
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From page 117... ...
Among the plants, ferns and conifers evolved, and animal life developed from marine invertebrates to fish, insects, amphibians, reptiles, and mammals. Interpretation of change between 600 million and 150 million years ago profits from the richness of the fossil record and the relatively accurate reconstructions of former continental positions based in part on paleomagnetic data (see Figure 3.8~.
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From page 118... ...
Recognizing Environmentsfrom the Geological Record Our picture of global environmental conditions is a composite of local studies. Many of these studies are rooted in detailed analyses of depositional environments environments in which sediments, and often fossils, have accumulated.
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From page 119... ...
Paleogeograpl~y Researchers have proposed that not only the positions of the continents but also the uplift of mountains exert control over global climate patterns. Mechanisms that physically alter environments on a regional scale have traditionally been accepted, but theories that suggest that tectonic forces may cause climatic changes on a global scale still inspire controversy.
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From page 120... ...
Throughout earth history, lateral plate movements have caused profound environmental changes. Paleolatitudinal data indicate that between about 450 million and 400 million years ago, the great supercontinent of Gondwana encroached on the south pole and underwent major climatic changes that appear to have influenced all latitudes.
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From page 121... ...
This changed ocean circulation and may have led to the accumulation of continental ice sheets. Specifically, we need to find out the climatic and vegetational histories of the Amazon Basin and of similar rain forests in western Africa and Asia.
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From page 122... ...
Whatever the causes of the various extinctions during the current ice age, the fossil record gives some indication that the modern world is biotically deprived. This paleontological perspective makes the faunas and floras that survive today especially precious The magnificent mammals of the Serengeti Plain provide an unusual glimpse of a rich Savannah fauna, but even this environment is depleted in diversity compared to the fauna that inhabited the same area 2 million or 3 million years ago.
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From page 123... ...
Intrinsic Biological Traits The fossil record reveals that rates of speciation and extinction correlate in predictable ways with geographic dispersal, characteristic size and stability of populations, and behavioral complexity. For example, weak dispersal can favor speciation by fostering the frequent isolation of small populations and can favor extinction by restricting geographic distribution.
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From page 124... ...
This occurred when the fragmentation of tropical rain forests in Africa and South America during dry intervals of the current ice age led to the origin of many new species in the small remnants of forest that survived until the return of better times. Adaptive Radiation Rapid proliferation of species within a group constitutes adaptive radiation.
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From page 125... ...
Normally, extinctions occur on local scales at a regular rate, but mass extinctions affect many groups throughout broad regions of the globe. The record of mass extinctions shows more sudden and drastic change 125 · i _ 25 FIGURE 3.14 A proliferation of Time new species of seed-eating rodents (Millions of and song birds fostered a great Increase In the diversity of predatory snakes.
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From page 126... ...
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.
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From page 127... ...
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.
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From page 128... ...
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.
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From page 129... ...
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.
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From page 130... ...
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.
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From page 131... ...
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.
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From page 132... ...
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.
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From page 133... ...
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 pateogeography.
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From page 134... ...
Forcing Factors in Environmental Cltange 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.
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From page 135... ...
Sequence Stratigrapl~y 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.
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From page 136... ...
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.
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