Seasonally Resolved Multiproxy Paleotemperatures from the Campanian North Atlantic Coastal Plain

The late Cretaceous represents a time of significant warmth in Earth history. Proxy and modeling studies are bringing this interval into increasingly better focus, but considerable uncertainty still persists regarding the distribution of temperature in space and time, due both to sparse and unevenly distributed data and to uncertainty about how to compare results from different proxies. Seasonally resolved δ¹⁸O data from benthic skeletal macrofossils allow for correction of bias due to differences in habitat and timing of growth, and provide internally consistent and climatologically robust data for both mean and seasonal extremes that can then be compared to other proxies and climate model simulations. Clumped isotope data from the same fossils can constrain average temperature and seawater composition and provide the framework within which to interpret seasonal data. We present such data from large, marine, fossil bivalves of the genus Exogyra collected along the Atlantic and Gulf coastal plain in the US and Mexico ( 27 to 38 °N paleolatitude). Samples all come from a 72 Ma time plane, as constrained by biostratigraphy and strontium isotope ratios; Fe and Mn concentrations are within the ranges observed in modern calcitic bivalves. Winter temperatures on the shelf today regardless of water depth approximate those at the surface in most settings. Winter paleotemperatures from 103 annual cycles derived from shell data reveal a late Campanian winter SST gradient comparable to today's globally-averaged winter marine gradient over the same range of latitude. Steepening of the regional gradient, as observed today along the Atlantic shelf, must have happened after the Campanian, likely in association with opening of the basin to southward flow from the Arctic in the Eocene. Calculated winter temperatures, assuming conventional seawater estimates, are on par with today, yet clumped isotopes temperatures from the same shells yield a comparable gradient but temperatures 10°C warmer, suggesting underestimation of regional seawater δ¹⁸O values. Reconciling these paleotemperature estimates requires evaporative enrichment on the coastal plain during the late Cretaceous, and may indicate formation of warm saline intermediate waters on the low-to-mid-latitude shelf.