Geochemical Archives Encoded in Deep-Sea Sediments Offer Clues for Reconstructing the Ocean's Role in Past Climatic Changes

Paleoceanographers are trying to understand the causes and consequences of global climate changes that have occurred in the geological past. One impetus for gaining a better understanding of the factors that have affected global climate in the past is the need to improve our predictive capabilities for future climate changes, possibly induced by the rise of anthropogenic carbon dioxide (C[O.sub.2]) in the atmosphere.

The relatively recent geological past (the last 1.6 million years), known as the Quaternary period, is characterized by large climatic swings from ice age to warmer interglacial periods similar to the present one. During the ice ages, large ice sheets accumulated on the northern continents, and sea level dropped by as much as 120 meters. Analysis of air bubbles trapped in the Antarctic and Greenland ice caps documents significantly lower atmospheric C[O.sub.2] levels during the cold glacial periods compared to the warm interglacials [ILLUSTRATION FOR FIGURE 1 OMITTED]. Since carbon dioxide is a well known "greenhouse" gas, whose presence in the atmosphere traps heat near the earth's surface, its lower concentration in the glacial atmosphere could have contributed to the cold climate of the ice ages.

Establishing the exact role that atmospheric C[O.sub.2] played in past natural climatic oscillations, however, is not a simple matter. Changes in atmospheric C[O.sub.2] may have been more a response to climate change than a forcing mechanism. On the other hand, while we know that the pace of Quaternary glaciations was primarily driven by variations in Earth's distance from the sun and in the angle of Earth's axis of rotation, the resulting changes in incoming solar radiation to the planet's surface are too small to account for the large climate variability observed. This implies that the effect of these orbital parameters must have been amplified by some internal feedback mechanisms within the earth's environment, and we suspect that atmospheric …