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PALEO-SEA SURFACE TEMPERATURE ESTIMATIONS: ORGANIC GEOCHEMISTRY AND PALEOCLIMATES The need for an organic paleotemperature marker Sea surface temperature (SST) is an extremely good and easily determined measure of modern climatic conditions, and an understanding of paleo-SST is essential for understanding natural climate change. It is known that the Earth's climate has gone through a series of large fluctuations over approximately the last 3 million years, cycling from climatic modes similar to today (interglaciations) to glacial regimes that were generally colder and more severe than at present. Accurate reconstructions of global climate change provide important tests of the robustness of global coupled atmosphere-ocean general circulation models that are employed in studies of contemporary and future climatic conditions. Until the late 1980s only two quantitative SST estimation techniques were in use: paleontological faunal assemblages and foraminiferal oxygen isotopes. Faunal-based SST estimates are presently the most accurate and robustly tested paleotemperature estimators available. Paleotemperature estimates based on these techniques still dominate estimates of paleotemperatures in the literature. However, these established techniques have significant limitations, especially in the tropical and polar oceans where temperature changes are thought to be small on glacial to interglacial time scales. Estimates of past SST using faunal assemblages (CLIMAP, 1981) for the last glacial maximum indicate differences in global average SST which contrast with land temperature estimates (based on, for example, pollen and tropical snow line depression). It has been suggested that assemblage techniques may either over- or underestimate temperature changes by reflecting large thermocline effects or because individual species may modify their depth or season of growth to follow a preferred growth temperature. The 6 18 0 signal preserved in a foraminifer test is perhaps the most widely
recognized quantitative paleotemperature estimates and, thus, for quantitative paleoclimatic assessment. Alkenones as a quantitative temperature estimator Volkman et a!. (1979) identified an unusual class of lipids, long chain unsaturated (C 37 -C 39 ) alkenones and alkenes, as constituents of the cosmopolitan alga Emiliania huxleyi. Marlowe eta!. (1984) and Brassell eta!. (1986) suggested that the 'unsaturation levels' in the alkenones demonstrated a potential for paleotemperature estimation. The unsaturation level, U~ 7 , is defined as the ratio of the concentration of di-unsaturated alkenones (those with 2 double bonds) less the 37:4 alkenone abundance, to the total concentration of C 37 alkenones (di, tri, or tetra-unsaturated: c37:2, c37:3, c37:4) found in the sample (Brassell eta!., 1986):
u k37 =
[C37:2]- [C37:4l [C37:2l + [C37:3] + [C37:4]
-----=---=cc=::___::_~'-'----
(PI)
Prahl eta!. (1988) defined the parameter U~~ which eliminates c37:4 alkenones from the u~7 equation, because this improves the linearity of the correlation in cu
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