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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