We have developed a coupled ocean-atmosphere general circulation model, the GENESIS-MOM model, with the ability to transport and fractionate water isotopes in the ocean and atmosphere. The model is used to predict modern and Cretaceous precipitation and seawater δ18O. The model reproduces the large-scale modernday isotopic distribution. In the zonal mean, the difference between simulated and observed seawater δ18O is within 0.2‰ in the low and middle latitudes and within 1‰ at high latitudes. In comparison to modern, simulated Cretaceous surface seawater δ18O is systematically depleted by 0.3‰ at low and middle latitudes. These differences are attributed to equilibrium fractionation during surface evaporation at low latitudes and an increased partitioning of 18O from the surface into the deep ocean due to intermediate and deep water formation in subtropical basins in the Cretaceous. We also find that regional seawater δ18O is significantly influenced by the paleobathymetry and the resolution of oceanic gateways, boundary conditions that are not well known for the past. Our simulation of Cretaceous seawater δ18O has major implications for oxygen isotope paleothermometry. We conclude that conventional assumptions of past seawater δ18O may lead to an overestimate of Cretaceous sea-surface temperatures, especially at middle and high latitudes.
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