Simulation of modern and middle Cretaceous marine δ18O with an ocean-atmosphere general circulation model

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 delta O-18. The model reproduces the large-scale modern-day isotopic distribution. In the zonal mean, the difference between simulated and observed seawater delta O-18 is within 0.2% in the low and middle latitudes and within 1% at high latitudes. In comparison to modern, simulated Cretaceous surface seawater delta O-18 is systematically depleted by 0.3 parts per thousand at low and middle latitudes. These differences are attributed to equilibrium fractionation during surface evaporation at low latitudes and an increased partitioning of O-18 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 delta 18O 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 delta O-18 has major implications for oxygen isotope paleothermometry. We conclude that conventional assumptions of past seawater delta O-18 may lead to an overestimate of Cretaceous sea-surface temperatures, especially at middle and high latitudes.