We present a paleolatitudinal precipitation reconstruction for the greenhouse setting of mid-latitude North America based on the oxygen isotopic composition of sphaerosiderites found in middle Cretaceous wetland paleosols. Our reconstructed middle Cretaceous δ18O values of precipitation are ∼4‰ less than values from comparable modern low-elevation coastal settings free of monsoons. The data fit a conceptual model in which the precipitation source for the eastern margin of the Cretaceous Western Interior Seaway of North America is an 18O-enriched oceanic coastal jet. In this subtropical-tropical setting, mid- Cretaceous precipitation rates are interpreted to range from ∼2500 to ∼4100 mm/yr.

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