General circulation model simulation of the [[delta].sup.18]O content of continental precipitation in the middle Cretaceous: a model-proxy comparison

We use the GENESIS atmospheric general circulation model (GCM) with water isotopic transport and fractionation capabilities to quantify the influence of atmospheric C[O.sub.2], sea level, and elevation of the Western Cordillera on the [[delta].sup.18]O of middle Cretaceous precipitation. The model predicts a systematic increase of nearly 3 [per thousand] in the [[delta].sup.18]O of North American precipitation due to warming associated with an increase in C[O.sub.2] from 2 to 12 times pre-industrial levels. In contrast, the specification of lowstand conditions and a high ancestral Western Cordillera reduces the [[delta].sup.18]O of North American precipitation locally by as much as 6%0 and 8 [per thousand]. We compare the simulated [[delta].sup.18]O of precipitation with the [[delta].sup.18]O of paleosol siderite spherules and find good agreement only when the model includes lowstand conditions and a high ancestral Western Cordillera. Our results imply either that Cretaceous high-latitude paleosol [[delta].sup.18]O was influenced by orographic precipitation and the Western Interior Seaway, or that the GCM's hydrological cycle is deficient at high pC[O.sub.2]. Additional paleosol data are needed to resolve this issue. Keywords: paleoclimate, Cretaceous, siderite, [[delta].sup.18]O, general circulation model.