HOW WARM WAS THE EARLY EOCENE? PALEOTEMPERATURE RECONSTRUCTIONS FROM US GULF COAST VENERICARDIA

Much effort has been focused on the paleoclimate record of the early Eocene, which encapsulates the warmest interval of the past 65 million years (the early Eocene climatic optimum, EECO), and estimating its absolute warmth. Low-latitude temperatures have been a particular source of debate due to conflicting interpretations of planktonic foraminiferal oxygen isotope records. Marine mollusks provide an alternative source of paleoclimate information and do not suffer from the same diagenetic concerns as microfossils. The US Gulf Coastal Plain preserves a record of marine shelf deposition through the early Eocene characterized by excellent preservation of primary shell aragonite and minimal diagenesis; sediments are generally unlithified even after 55 million years. Oxygen isotope values from the bivalve Venericardia of the Hatchetigbee Formation, spanning the EECO, yield warm paleotemperatures with seemly high seasonal temperature ranges that have been attributed to freshwater mixing. Here, we increase the stratigraphic coverage of sampling and couple δ¹⁸O analyses with strontium isotope ratios in order to better understand the degree to which freshwater mixing contributes to δ¹⁸O-derived paleotemperatures from these bivalves. Transects across growth bands of two shells from each of seven stratigraphic horizons within the formation were microsampled for stable oxygen isotope analyses. Data from all shells consistently yield winter temperatures of ~24°C, suggesting minimal if any influence of fresh water. Summer temperatures are more variable, but most range between ~33°C and 36 °C assuming normal marine salinity. Strontium isotope ratios of growth increments recording maximum and minimum δ¹⁸O values yield slightly more radiogenic values in dark (negative δ¹⁸O) bands, suggesting the potential for some freshwater mixing, but the difference is not significant. Given that low-latitude paleotemperatures may have been substantially warmer than previously thought, it may not be unreasonable for warm shallow waters of the early Eocene Gulf to have reached temperatures of ~33°C.