HIGH-LATITUDE SEASONALITY IN A GREENHOUSE EARTH: VARIATIONS IN TEMPERATURE AND PRECIPITATION THROUGHOUT THE EOCENE INFERRED FROM ANTARCTIC BIVALVES

Climate change is one of the most pressing issues facing societies around the globe, yet changes in seasonality and precipitation that accompany greenhouse conditions are still poorly understood. Because global warming is amplified at the poles, studying high-latitude settings during warm intervals in Earth’s geologic past can provide insights about future conditions. Several recent studies have reported increased seasonal precipitation in high latitudes during the globally warm Eocene Epoch (55-34 Ma), suggesting the possibility of monsoon-like conditions near the poles during greenhouse climate regimes and hence a significant departure from the present-day climate system. Monsoon-like conditions would result in increased runoff during the summer months. This may create a layer of less dense brackish water on the sea surface that would limit vertical mixing within the water column, keeping shallow seawater warm. We test this hypothesis using seasonally resolved δ¹⁸O_aragonite data from fossil bivalves from the La Meseta Formation, Seymour Island, Antarctica, a nearshore shallow-marine unit. Seasonal runoff should yield lower δ¹⁸O_water values in summer relative to winter, thereby amplifying the range of seasonal variation in shell oxygen isotopic composition. Late Eocene cooling should reduce summer precipitation and thus the contribution of fresh water to summer shell δ¹⁸O_aragonite, so that seasonal variation in the shell should decline more than one might anticipate from temperature alone. Seasonal oxygen isotope data from 38 years in 6 shells in the warmer middle Eocene range from -1.57 to 1.35‰ and average 0.39‰, while those from 16 years in 3 shells in the cooler late Eocene range from 0.62 to 2.06‰, and average 1.19‰. Samples from the late Eocene thus have a more attenuated seasonal amplitude and higher average δ¹⁸O value, consistent with the hypothesis that seasonality in precipitation decreases with decreasing mean annual temperature, with cooling mostly in summer. By combining these oxygen isotope data with clumped isotope thermometry, we can determine water temperature during seasonal extremes and calculate the composition of the water in both summers and winters, to examine the assumption concerning the δ¹⁸O_water and better quantifying the precipitation regime.