Geochemical Archives Encoded in Deep-Sea Sediments Offer Clues for Reconstructing the Ocean's Role in Past Climatic Changes
Francois, Roger, Bacon, Michael P., Oceanus
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 C[O.sub.2] may be a major factor. In view of its obvious connection to present societal concerns, this particular problem has elicited a lot of attention in the paleoceanographic community.
In the modern ocean, factors affecting atmospheric C[O.sub.2], such as export flux of organic carbon and carbonate to the deep sea, dissolution of calcium carbonate shells in the deep sea, and deep water circulation, can be measured directly. A variety of incubation techniques are used to measure production of organic matter in surface waters, and broad views of surface water production at a given time can now be obtained from satellite imagery [ILLUSTRATION FOR FIGURE 2 OMITTED]. As several articles in this issue attest, sediment traps are deployed to estimate the export and recycling of organic matter and calcium carbonate from surface waters to the deep sea, and thermohaline circulation is becoming increasingly well constrained, both in terms of flow rates and pathways (see Oceanus Vol. 37, No. 1 and Vol. 39, No. 2). For past oceans, however, these variables cannot be measured directly but must be inferred from proxy analysis (a marker in the sediments from which the variables can be inferred indirectly). A fraction of the biogenic particles produced in surface water survives degradation or dissolution in the deep sea and gradually accumulates on the seafloor. As many oceanic processes leave a chemical imprint in this material, a very complex but rather comprehensive chemical archive, which can be dated and deciphered, is continuously buried in deep-sea sediments.
It is probably safe to say that every element of the periodic table, every isotope, and an assortment of specific organic molecules that survive sediment burial have some potential for providing information on how past oceans operated. It is for us to discover the processes that regulate their distribution in the sediment, how well the chemical signals are preserved during burial, and whether they can then be used to infer past changes in the processes that generated them. …