MULTI-PROXY SEASONALITY INVESTIGATION OF THE MID-CRETACEOUS WESTERN INTERIOR SEAWAY USING OYSTER MACROFOSSILS

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 (δ¹⁸O_w) and salinity of seaway waters. Here, we analyze oyster macrofossils distributed across the WIS to reconstruct paleoceanographic and paleoclimatic conditions during the mid-Cretaceous thermal maximum using multiple geochemical proxies. High-resolution sclerochronologic (sub-annual) 𝛿¹⁸O and 𝛿¹³C isotope analyses of oyster macrofossils from the genera Exogyra and Pycnodonte measure paleoseasonality in the mid-latitudes of the WIS. When combined with carbonate clumped isotope paleotemperatures of the fossil specimens, the degree of temperature and δ¹⁸O_w seasonality is quantified during the hothouse climate of the mid-Cretaceous. These are among the first isotope-derived estimates of Cenomanian-Turonian seasonality, including the Ocean Anoxic Event 2 hyperthermal interval.