The equator to high southern latitude sea surface and vertical temperature gradients are reconstructed from oxygen isotope values of planktonic and benthic foraminifers for the following five time intervals: late Paleocene, early Eocene, early middle Eocene, late Eocene, and early Oligocene. Paleotemperatures are calculated using standard oxygen isotope/temperature equations with adjustments to account for (1) variations in sea water δ18O related to changes in global ice volume over time and (2) latitudinal gradients in surface water δ18O. These reconstructions indicate that sea-surface temperatures (SST) of the Southern Oceans in the early Eocene were as high as 15°C, whereas temperatures during the late Paleocene and early middle Eocene reached maximum levels of 10°–12°C. By the late Eocene and early Oligocene high latitude SST had declined to 6 and 4°C, respectively. For most of the early Paleogene, low latitude sub-tropical temperatures remained constant and well within the range of Holocene temperatures (24°ndash;25°C) but by the late Eocene and early Oligocene declined to values in the range of 18° to 22°C. The late Paleogene apparent decline in tropical temperatures, however, might be artificial because of dissolution of near-surface foraminifera tests which biased sediment assemblages toward deeper-dwelling foraminifera. Moreover, according to recent plate reconstructions, it appears that the majority of sites upon which the late Eocene and early Oligocene tropical temperatures were previously established were located either in or near regions likely to have been influenced by upwelling. Global deepwater temperature on average paralleled southern ocean SST for most of the Paleogene. We speculate based on the overall timing and character of marine sea surface temperature variation during the Paleogene that some combination of both higher levels of greenhouse gases and increased heat transport was responsible for the exceptional high-latitude warmth of the early Eocene.

Evolution of Early Cenozoic marine temperatures

Glassy Turonian foraminifera preserved in clay-rich sediments from the western tropical Atlantic yield the warmest equivalent ?18O sea-surface temperatures (SSTs) yet reported for the entire Cretaceous-Cenozoic. We estimate Turonian SSTs that were at least as warm as (conservative mean ~30 °C) to significantly warmer (warm mean ~33 °C) than those in the region today. However, if independent evidence for high middle Cretaceous pCO2 is reliable and resulted in greater isotopic fractionation between seawater and calcite because of lower sea-surface pH, our conservative and warm SST estimates would be even higher (32 and 36 °C, respectively). Our new tropical SSTs help reconcile geologic data with the predictions of general circulation models that incorporate high Cretaceous pCO2 and lend support to the hypothesis of a Cretaceous greenhouse. Our data also strengthen the case for a Turonian age for the Cretaceous thermal maximum and highlight a 20–40 m.y. mismatch between peak Cretaceous-Cenozoic global warmth and peak inferred tectonic CO2 production. We infer that this mismatch is either an artifact of a hidden Turonian pulse in global ocean-crust cycling or real evidence of the influence of some other factor on atmospheric CO2 and/or SSTs. A hidden pulse in crust cycling would explain the timing of peak Cretaceous-Cenozoic sea level (also Turonian), but other factors are needed to explain high-frequency (~10–100 k.y.) instability in middle Cretaceous SSTs reported elsewhere.

Testing the Cretaceous greenhouse hypothesis using glassy foraminiferal calcite from the core of the Turonian tropics on Demerara Rise

Evolving ideas about the Cretaceous climate and ocean circulation

Although our knowledge of living planktonic foraminifera is limited, useful paleoecologic information can be obtained from ancient assemblages. During the mid-Cretaceous, as today, the simple, inflated morphotypes inhabited the nearsurface waters while the flatter, keeled forms probably occupied deeper habitats. Three faunal groups are proposed, an Epicontinental Sea Fauna (ESF) characterized by species of Gubkinella, Guembelitria and Heterohelix, an open marine Shallow Water Fauna (SWF) composed primarily of species of Hedbergella and Globigerinelloides, among others, and finally an open marine Deep Water Fauna (DWF) represented by species of Planomalina, Rotalipora and Praeglobotruncana (keeled taxa). Open ocean assemblages of DSDP Sites 545 and 547 are numerically dominated by species of the SWF in the >63-,um size fraction while the ESF comprises about 5% of the assemblages, and the biostratigraphically important DWF generally comprises less than 2%. These assemblages demonstrate remarkable stratigraphic consistency. In contrast to open ocean assemblages, epicontinental sea assemblages of the Vocontian Basin are characterized by loss of the DWF with decreasing water depth and an increase in the proportion of the ESF. The ESF:SWF ratio also becomes increasingly variable with decreasing water depth. Upwelling also has pronounced effects on the composition of planktonic assemblages as suggested by an increase in the proportion of the ESF, interpreted to include opportunistic species.

Paleoecology of Mid-Cretaceous planktonic foraminifera; a comparison of open ocean and epicontinental sea assemblages

•Present address: Department of Geological and Geophysical Science, Princeton University, Princeton, New Jersey 08544.

Samuel M Savin

Papers

Oxygen isotopic evidence for the depth stratification of tertiary and cretaceous planktic foraminifera