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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