LATE CRETACEOUS LATITUDINAL GRADIENTS IN TEMPERATURE AND SEASONALITY USING THE MARINE OYSTER EXOGYRA CANCELLATA

There is great significance in studying geohistorical analogues to our present interval of greenhouse warming and associated climatic effects. Parameters such as seasonal temperature range and the pole-to-equator gradient in mean temperature and seasonality have a major impact on the ecology of organisms and larger patterns of biodiversity and species turnover, but the response of these parameters to greenhouse conditions has yet to be fully constrained. The shell chemistry of marine mollusks records the water temperature of growth for the living environment. If shell accretion is continues throughout the year, it is possible to recover not only mean temperature but seasonal extremes in temperature as well. Thus, fossil mollusks offer much potential for resolving important questions relating to temperature and seasonality. We incrementally sampled shells of the calcitic Late Cretaceous marine oyster Exogyra cancellata from localities spanning ~10 degrees of paleolatitude along the US Atlantic Coastal Plain so as to explore how Late Cretaceous greenhouse conditions differed from today. At present, mean annual sea surface temperature cools by ~1.5⁰C per degree of latitude over this same region, and seasonal range at the surface increases with latitude. With respect to depth on the shelf, mean temperature and seasonal range decrease primarily due to a drop in summer temperatures with depth, and this effect is more pronounced at higher latitudes. Paleodepth is therefore important to consider in interpreting climate information derived from benthic mollusks. Because the water column is nearly isothermal in winter, winter temperatures recovered from fossil mollusks should be directly comparable across latitudes irrespective of depth, and can therefore be used to reconstruct meridional gradients in deep time. The 15⁰C difference in winter temperatures from the northern to southern extent of the sampled area today should be easy to resolve with δ¹⁸O, enabling comparison to Cretaceous values. Differences in seasonality with latitude are more sensitive to depth but can be approximated if the depositional setting is held as constant as possible. Multiple years sampled from multiple shells at each locality allow for a statistically robust comparison of winter temperatures and seasonality across latitudes.