FIGURE 1.3 Distribution of laterite (shaded) on land masses and estimated major ocean surface currents about 80 million years ago. Laterite are soils that form under relatively uniform maritime conditions on windward sides of continents within the humid intertropical, or tropical forest, zone. From Climate in Earth History, National Academy Press (1982).

ciety. For example, convection in the molten iron-alloy core produces a magnetic field that protects most organisms from lethal solar radiation, provides a basis for navigation, and occasionally disrupts electromagnetic communication systems.

Priority Theme A-I: Global Paleoenvironments and Biological Evolution

Ongoing programs have reconstructed climatic and oceanographic conditions during ancient intervals that were much different from those of today. These varying conditions range from intervals when the deep-sea was much warmer than at present and its oxygen content was depleted to times when vast ice sheets spread to regions far from the poles. Efforts to piece together the kaleidoscopic movements of the continental lithosphere, from climate change evidence found in the rock record and from mountain-building and ocean-forming processes, are progressing rapidly. Researchers have established the relative positions of most of the larger parts of the continents with respect to each other and with respect to the North and South poles for the past 500-million-years.

This record of moving plates provides a meaningful framework for the study of paleogeography, paleobiology, paleoclimatology, and paleoceanography. This framework has so revolutionized these fields that they are, for all intents and purposes, new and budding disciplines. Paleogeography produces maps that show not only the relative positions of continental masses but also the locations of high mountain ranges and shallow seas. Finer details, gleaned from paleoclimatic evidence, indicate specific environments such as deserts, coal swamps, and glacial ice (Figure 1.3). Paleoenvironmental maps represent beautiful reconstructions from scientific detective work, and they have become important guides in exploring for minerals and energy resources. These reconstructed maps lead explorers to potential sites of valuable concentrations, formed during continental development, of material such as petroleum and phosphates. The discovery of the submarine hydrothermal vents found at oceanic spreading centers and the associated mineral deposits has revolutionized interpretations of the origins of many ancient ore concentrations.

One important endeavor is unraveling the record of past continental movements. Another is constructing circulation models for ancient oceans and atmospheres. Reconstruction of such circulation models may illuminate distinctive transitions from one stable environment to another, as well as the forcing factors responsible for such transitions. New approaches to the study of global geochemical cycles contribute to these reconstructions of large-scale environmental change. Researchers are only beginning to appreciate how much life-supporting oxygen and heat-trapping carbon dioxide have fluctuated.

The Earth's dynamic environments form the backdrop against which the history of life unfolds. The rock record is a vast repository of information documenting natural processes of trial and error. These natural proving grounds can be considered experiments that offer lessons about the relationship between life and environments—lessons that will serve humans well in their attempt to confront global change in the decades ahead. The most important of these lessons will come from understanding the events of the past 2.5-million-years. The record of the most recent climatic lurch—when deglaciation accelerated around 15,000 years ago—lies right at the surface. The evidence is in the landscape, on the ocean floor, and within the top layers of the remnant ice sheets. Pertinent information is garnered from ice and ocean cores, tree rings,



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