Occupying more than 70% of the Earth's surface, the oceans are a very important source of paleoclimatic information. Between 6 and 11 billion metric tons of sediment accumulate in the ocean basins annually, and this provides an archive of climatic conditions near the ocean surface or on the adjacent continents. Sediments are composed of both biogenic and terrigenous materials (Fig. 6.1). The biogenic component includes the remains of planktic (near surface-dwelling) and benthic (bottom-dwelling) organisms, which provide a record of past climate and oceanic circulation (in terms of surface water temperature and salinity, dissolved oxygen in deep water, nutrient or trace element concentrations, etc.). By contrast, the nature and abundance of terrigenous material mainly provides a record of humidity-aridity variations on the continents, or the intensity and direction of winds blowing from land areas to the oceans, and other modes of sediment transport to, and within, the oceans (fluvial erosion, ice-rafting, turbidity currents, etc.).

The CLIMAP research group (Climate: Long-range Investigation, Mapping, and Prediction) contributed greatly to our understanding of past changes in ocean surface temperatures, generating many important new insights and hypotheses about orbital forcing and mechanisms of glaciation, such as the rates and timing of icesheet growth and decay (Mclntyre et al., 1975, 1976; CLIMAP Project Members,

FIGURE 6.1 Pelagic sedimentation in the ocean (Hay, 1974).

1976, 1981, 1984; Hays et al, 1976; Ruddiman and Mclntyre, 1981a). Subsequently, the SPECMAP (Spectral Mapping Project) focused on determining the spectral characteristics of the ocean sediment-based paleoclimatic record, and establishing a basic timeframe for past climatic events (Imbrie et al, 1984; Martinson et al., 1987). The contribution of these projects to paleoclimatic research has been immense, but inevitably new research has raised questions about the validity of earlier results, especially the reconstructions of tropical and equatorial SSTs at the last glacial maximum (LGM). In the CLIMAP SST reconstructions, it appeared that SSTs at low latitudes changed very little at the time of the LGM, but current research indicates that, in some areas, temperatures may have been considerably lower (by 3-5 °C) at that time (Guilderson et al, 1994; Stute et al, 1995). This is a very controversial, but extremely important issue (discussed further in Sections 6.4 and 6.5) as it has significant implications for understanding not only past climatic changes, but also the potential magnitude of any future anthropogenic global warming. This is because low latitude SSTs play a key role in water vapor/cloud feedback within the climate system and are thus critical to assessing the overall climate sensitivity to particular forcing mechanisms. Current research also focuses on the three-

dimensional structure of the world's oceans through time, in an effort to reconstruct not only surface water conditions in the past, but also the deepwater circulation that plays a critical role in energy transfer and the sequestration of carbon dioxide in the abyss.

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