MULTI-PROXY INVESTIGATION OF THE CENOMANIAN WESTERN INTERIOR SEAWAY USING OYSTER MACROFOSSILS (~94.5 MA)

Quantifying ocean temperatures during geologic intervals of extreme warmth, like the Cenomanian and Turonian stages (~100-90 Ma), has implications for paleocirculation patterns, paleoecology, and thermal thresholds in the development of ocean anoxia. Strata deposited in the shallow, expansive Cretaceous Western Interior Seaway (WIS) of North America preserve a rich archive of climate and environmental information from a past greenhouse interval and can teach us about natural limits within the Earth system. The use of traditional oxygen isotope paleothermometry in the WIS is challenging due to known but unquantified variations in the oxygen isotopic composition and salinity of seaway waters. Here, we analyze oyster macrofossils distributed across the WIS and reconstruct paleoceanographic and paleoclimatic conditions during the mid-Cretaceous thermal maximum using multiple geochemical proxies. High-resolution sub-annual 𝛿¹⁸O and 𝛿¹³C isotope analyses of oyster macrofossils from the genera Exogyra and Pycnodonte measure paleoseasonality. These are among the first isotope-derived estimates of Cenomanian-Turonian seasonality and allow us to determine the degree of seasonality during the hothouse climate of the mid-Cretaceous in the mid-latitudes of the WIS. Also, carbonate clumped isotope analyses of the oysters provide rare Δ₄₇-derived estimates of Cenomanian-Turonian temperature. These findings put quantitative constraints on the maximum seaway temperature during Ocean Anoxic Event 2, a hyperthermal episode during the mid-Cretaceous greenhouse which is hypothesized to have been the hottest time interval on Earth in the past 100 Ma.