GCM SIMULATION OF THE δ18O CONTENT OF MIDDLE CRETACEOUS CONTINENTAL PRECIPITATION AND COMPARISON WITH PALEOSOL SIDERITE δ18O

The δ¹⁸O of paleosol siderite spherules archive past changes in the cycling of meteoric water. Comparisons with simulated δ¹⁸O of precipitation in climate models have the potential to constrain uncertain boundary conditions and physical processes in past climates that were much different than modern. We use the GENESIS atmospheric general circulation model (AGCM) with water isotopic transport and fractionation capabilities to quantify the influence of atmospheric CO₂, sea level, and topography of the Western Cordillera (WC) on the δ¹⁸O of middle Cretaceous precipitation. The model predicts a systematic increase of up to 3‰ in the δ¹⁸O of North American precipitation due to an increase in CO₂ from 2 to 12x pre-industrial levels. In contrast, the specification of lowstand conditions and a high ancestral WC in GENESIS reduces the δ¹⁸O of North American precipitation locally by up to 6 and 8‰.

We compare the simulated δ¹⁸O of precipitation with the δ¹⁸O of paleosol siderite spherules and find very good agreement between the proxy data when the model includes lowstand conditions and a high ancestral Brooks Range. Unlike previous studies, our model results indicate that large-scale fractionation of water isotopes did not change substantially during this greenhouse climate. Our results indicate that the GCM physics are adequate for simulating first-order aspects of greenhouse climates and demonstrate the utility of model-proxy δ¹⁸O comparisons for constraining paleogeographic features.