EVOLUTION OF EARLY CENOZOIC MARINE TEMPERATURES

The equator to high southern latitude sea surface and vertical tempera ture gradients are reconstructed from oxygen isotope values of plankto nic and benthic foraminifers for the following five time intervals: la te Paleocene, early Eocene, early middle Eocene, late Eocene, and earl y Oligocene. Paleotemperatures are calculated using standard oxygen is otope/temperature equations with adjustments to account for (1) variat ions in sea water deltaO-18 related to changes in global ice volume ov er time and (2) latitudinal gradients in surface water deltaO-18. Thes e reconstructions indicate that sea-surface temperatures (SST) of the Southern Oceans in the early Eocene were as high as 15-degrees-C, wher eas temperatures during the late Paleocene and early middle Eocene rea ched maximum levels of 10-degrees-12-degrees-C. By the late Eocene and early Oligocene high latitude SST had declined to 6 and 4-degrees-C, respectively. For most of the early Paleogene, low latitude sub-tropic al temperatures remained constant and well within the range of Holocen e temperatures (24-degrees-25-degrees-C) but by the late Eocene and ea rly Oligocene declined to values in the range of 18-degrees to 22-degr ees-C. The late Paleogene apparent decline in tropical temperatures, h owever, might be artificial because of dissolution of near-surface for aminifera tests which biased sediment assemblages toward deeper-dwelli ng foraminifera. Moreover, according to recent plate reconstructions, it appears that the majority of sites upon which the late Eocene and e arly Oligocene tropical temperatures were previously established were located either in or near regions likely to have been influenced by up welling. Global deepwater temperature on average paralleled southern o cean SST for most of the Paleogene. We speculate based on the overall timing and character of marine sea surface temperature variation durin g the Paleogene that some combination of both higher levels of greenho use gases and increased heat transport was responsible for the excepti onal high-latitude warmth of the early Eocene.