CONTINENTAL CLIMATE AND HYDROLOGY IN THE CAMPANIAN WESTERN INTERIOR BASIN INFERRED FROM CLUMPED ISOTOPE PALEOTHERMOMETRY

Vast epicontinental seaways are one of the defining features of Cretaceous paleogeography. The North American Western Interior Seaway (WIS) received inputs of water from the proto-Gulf of Mexico, Arctic Ocean, and continental freshwater in the Campanian (~83-72 Ma). The temperature and isotopic composition of freshwater responded to overall climate, overland precipitation patterns, and diachronous uplift during the Sevier and Laramide orogenies. We measure Δ₄₇ in unionid bivalve shells to constrain temperature and δ¹⁸O_water of river and pond environments along the western shore of the WIS (~36.7°N-48.7°N) in the late Campanian (~75 Ma). Supplemental ⁸⁷Sr/⁸⁶Sr measurements provide additional context about depositional environment. We report surface water temperatures ranging from 23°C to 37°C and a reduced latitudinal temperature gradient in this hothouse world compared to modern. We use these new Δ₄₇ temperatures, which indicate shell growth outside the optimal temperature range observed for modern unionids, to recalculate previously published δ¹⁸O_water values for Campanian freshwater environments (n = 244). The Δ₄₇ temperatures shift δ¹⁸O_water values higher than previous estimates, resulting in a bimodal distribution of values centered around -14‰ and -7‰. More radiogenic ⁸⁷Sr/⁸⁶Sr values are associated with lower δ¹⁸O_water values and vice versa, broadly corroborating the hypothesis that major trunk rivers include the extreme highlands in their catchments and the catchments of smaller streams are limited to the foreland basin. The lowest δ¹⁸O_water values we observe in trunk rivers bolster previous estimates of >3500m elevation in the Campanian Proto-Cordillera. This high elevation in the Campanian is consistent with previous paleoelevation estimates for the Nevadaplano in the Maastrichtian and early Cenozoic. Taken together it appears the Nevadaplano was a long-standing topographic feature, consistent with other structural and sedimentological data.