SEASONAL VARIATIONS OF TEMPERATURE AND PRECIPITATION IN ANTARCTICA DURING THE EOCENE GREENHOUSE

As we head toward a warmer world, it is increasingly important to understand anticipated changes in temperature and precipitation and consequent shifts in storm trajectory and intensity. One of the best tools we have for predicting future conditions is the study of warm intervals in Earth’s geologic past, such as the Eocene Epoch (56-34 Ma), characterized by the warmest temperatures of the Cenozoic. While the record of average temperature is fairly well constrained for most of the globe throughout the Eocene, relatively little is known about the accompanying seasonal changes, particularly in the high latitudes. An assessment of Eocene high latitude seasonality is especially pertinent in light of several recent studies that indicate increased seasonal precipitation in high latitudes during the globally warm early–middle Eocene, suggesting the possibility of monsoon-like conditions nearer the poles during greenhouse climate regimes. As modern monsoons are restricted to low latitudes, this points to a significant departure from the present-day climate system, adding another layer of complexity for predictions in a future warmer world. Here, we present high-resolution oxygen isotope data from fossil bivalves belonging to the genera Cucullaea and Eurhomalea collected from the nearshore Eocene La Meseta Formation, Seymour Island, Antarctica. The use of multiple taxa with variable timing of minimum and maximum growth allows us to reconstruct the full extent of seasonal isotopic variations. Mean values of late Eocene shells are ~1.6‰ more positive than middle Eocene shells, and the seasonal amplitude collapses by ~1.2‰ between the middle and late Eocene. The muted seasonal cycle and enriched isotopic mean of late Eocene shells might be achieved via summertime cooling, as polar waters become less seasonal as climate cools. If due to temperature alone, these data indicate a shift in the annual range from 12.7 – 20.1 ^oC in the middle Eocene to 8.2 – 10.0 ^oC in the late Eocene. Alternatively, high amounts of summer precipitation and runoff into nearshore settings could inflate the seasonal amplitude of middle Eocene shells. With the addition of seasonally resolved clumped isotope data, we will be able to distinguish between the influences of temperature and seawater composition, allowing us to better evaluate model results.