AN ISOTOPE-BASED PALEOTEMPERATURE RECORD FOR THE PALEOGENE SUBTROPICS DERIVED FROM SHELF MACROFAUNA, US GULF COASTAL PLAIN

Improvements over the last decade in our ability to approximate past levels of atmospheric carbon dioxide have made it even more important to obtain quantitative estimates of temperature during past greenhouse intervals, so as to understand the sensitivity of the climate system to changes in pCO₂. The Eocene is one such time, when pCO₂ was high and global temperatures were warm. Recent work has suggested very warm temperatures (>30°) even at high latitudes. While polar amplification does seem to characterize times of greenhouse warming, these temperatures would suggest even warmer conditions at lower latitudes. Here, we present a paleotemperature record for the latest Cretaceous through the early Oligocene for the subtropics (~30°N paleolatitude) that derives from stable isotope analysis of biogenic carbonates on the US Gulf Coastal Plain shelf.

Consistency in mean and seasonal range of isotope values among shells from correlative units in multiple localities spanning 100s of kilometers along the shelf suggests that shelf data from marine assemblages are useful for regional climate reconstructions and may suffer less from local heterogeneity in water composition than previously suspected. Oxygen isotope values through time generally follow the benthic δ¹⁸O stack, and are depleted by approximately 3 ‰. Temperatures are calculated using standard biogenic carbonate equations. Water composition is assumed to be the global average corrected for latitude, and several estimates of changing global ocean composition are considered for the late Eocene and early Oligocene, when ice growth in Antarctica increased seawater values from the ice-free average of -1‰. Published comparison of δ¹⁸O values with clumped isotope analyses on the same shell carbonate and tetraether lipid indices of co-occurring sedimentary organic matter in the early Eocene Hatchetigbee Formation support use of the latitudinal correction to the global seawater value and anchor δ¹⁸O-based temperatures.

Calculated temperatures in the early Eocene are in the upper 20s and fall to about 20°C in the late Eocene and early Oligocene. At no point do they exceed 30°C, making warmer values at much higher latitudes difficult to explain. Interproxy differences may account for some of the discrepancy.