EARLY EOCENE AND MIDDLE CRETACEOUS PALEOSOL AND MODEL-BASED RECONSTRUCTIONS OF GREENHOUSE PALEOPRECIPITATION

This study focuses on oxygen isotopic records of paleoprecipitation derived from middle Cretaceous and early Eocene paleosol siderite spherules obtained along a paleolatitudinal transect in North America, although data obtained globally from coeval deposits are considered. The late Paleocene-early Eocene Earth is considered to have been the warmest of the Cenozoic and is perhaps second only to the middle Cretaceous greenhouse as a period of extreme global warmth during the past 100 million years.

The d18O data are used to benchmark a stable isotope tracer version of the GENESIS v. 2 general circulation model. Both the early Eocene and middle Cretaceous oxygen isotope profiles display a pronounced south to north depletion, and are depleted relative to similar modern latitudinal distributions. These results indicate that early Eocene paleotemperatures may have been as warm as those reconstructed for the middle Cretaceous and provide evidence for an amplified atmospheric hydrologic cycle during these episodes of extreme warmth. Our modeling results support these conclusions and indicate that early Eocene and middle Cretaceous Precipitation minus Evaporation (P-E) profiles were substantially altered compared to modern profiles: in tropical and middle latitude regions, P-E values are 2-3X greater than today, whereas in the subtropics, P-E values are up to 3X less than today. Excess tropical precipitation (relative to modern) “rains out” over the oceans, whereas at subtropical latitudes, higher net evaporation leads to a transfer of moisture to land.

A discrepancy between paleosol and model-derived d18O values exists at high latitudes; the model values are substantially enriched relative to the paleosol data. We explore several explanations for this discrepancy including geological considerations, the use of a modeled seasonal cycle rather mean annual values, and “super-rotation”, i.e. localized tropical heating in the model atmosphere, a mechanism that has been observed to cause down welling and surface warming at high latitudes in planetary dynamical studies.