ters, which in turn prevented the efficient consumption of organic carbon and eventually resulted in the accumulation and preservation of material now used as a nonrenewable resource.

Such episodes of enhanced burial of organic carbon represented globally significant perturbations of the carbon cycle. This change is reflected in the carbon isotope record, which indicates that the fraction of carbon buried in organic form was 50 percent higher during anoxic events than during the intervals immediately preceding and following them. As a result, atmospheric levels of oxygen must have risen even as the deep seas became oxygen depleted. Atmospheric levels of CO 2 declined, contributing to global cooling and the consequent termination of the conditions that had prevailed. Though many factors related to anoxic events can be identified and discussed, the details are not yet fully understood.

Paleoceanographers are investigating the changes over the past 100-million-years in the vertical thermal structure of the oceans at increasingly finer scales of resolution. The heart of this research, which addresses biogeographic patterns as well, concentrates on diagnostic elemental isotopes. The isotopes distinguish plankton living at particular water depths characterized by unique thermal conditions. Areas of upwelling leave their own kinds of evidence, including phosphate deposits along ancient continental margins. Many of these deposits have considerable economic value, and paleoceanographic models contribute to their discovery.

Paleoclimatology: Cycles in Past Climates

A number of important climatic indicators help to establish how climate has changed. Most of them are applicable to the record of the past few hundred-million-years, and some portray conditions that dominated billions of years ago. These indicators are coals, soils, evaporites and sand dunes, glacial deposits, marine reefs and bedded carbonate deposits, and land plants.

  • Coals: Present in the modern world as peat, coals form from organic accumulations in areas combining high rain and poor drainage. The optimal conditions are located in equatorial rain forests or in moist areas at higher latitudes best represented today in zones about 55° north and south of the equator.

  • Soils: Soils rich in the clay kaolinite develop in warm humid climates. Associated with these soils are laterites, in which iron and aluminum are concentrated. Bauxite, the main ore of aluminum, also characterizes very hot moist conditions. All of these climatic indicators are end products of protracted weathering processes and resist subsequent alteration. Despite later cooling or aridity, such evidence of tropical climate can be preserved in the rock record for vast stretches of geological time.

  • Evaporites and Sand Dunes: These sedimentary features reflect arid conditions. Thick evaporite deposits require continuing replenishment from seas or lakes and extremely dry air. Dunes imply strong winds, a source of sand, and a lack of stable vegetation. They are best developed in the subtropical dry climatic zones but can form at mid-latitudes in landlocked regions such as the Gobi Desert or in rainshadows of mountains such as the Sierra Nevada.

  • Glacial Deposits: Glaciers leave boulders in erratic deposits and produce icebergs that release dropstones to lake bottoms and seafloors. Their most characteristic traces, however, are striations—the scratch marks found on pebbles or boulders that glaciers transported or on bedrock that glaciers scoured. Only continental glaciers have broad climatic significance because mountain glaciers commonly form at high altitudes, even near the equator.

  • Marine Reefs and Bedded Carbonate Deposits: Today, limestones accumulate mainly within about 40° of the equator. Those formed primarily by calcareous algae are probably restricted to this zone in part by sunlight requirements. Others, including massive organic reefs built by organisms specific to different time periods, are limited by thermal requirements. Modern coral reefs are confined to within about 30° of the equator.

  • 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. Climatic conditions are reflected in the basic leaf morphology (Figure 3.10) of flowering plants. Perhaps most valuable is leaf outline—a strong, positive, linear relationship exists between the percentage of species in fossil floras with smooth leaf margins and the mean annual temperature of the habitat. While the slope of the curve may have varied with time, the kind of gradient that we observe today has almost certainly characterized flowering plants since early in their history. The visible characteristics and the composition of fossil vegetation, as determined from pollen, spores, and seeds as well as leaves, give a general picture of climatic changes in North

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