| Co-Authors: | Pollard, D., EMS Environment Institute, Penn State, pollard@essc.psu.edu; Barron, E., College of Earth and Mineral Sciences, Penn State, eric@essc.psu.edu |
| Keywords: | Cretaceous, greenhouse, hydrologic, North America, WIB paleosols, siderite, oxygen isotopes, precipitation |
| Abstract: | The oxygen isotopic composition of siderite spherules from middle Cretaceous wetland paleosols have been used to reconstruct paleolatitudinal precipitation gradients for the greenhouse setting of mid-latitude North America. These published results (White et al., 2001, Geology, v. 29, no. 4, p. 363-366) indicate that the oxygen isotopic composition of precipitation was ~4 per mil depleted relative to comparable modern low-elevation coastal settings free of monsoons. More specifically, along the mid-latitude eastern margin of the North American Western Interior Seaway, middle Cretaceous precipitation rates are interpreted to have ranged from ~2500 to ~4100 mm/yr.
Here we present our new data from coeval east coastal plain deposits of North America, and published values from the North Slope, Alaska, to complement the Western Interior data set. These new data indicate that our earlier assessment of the Western Interior, extended north using the published North Slope data, are accurate. That is an enhanced greenhouse hydrologic cycle, and the presence of the Western Interior Seaway, led to much higher precipitation rates than those observed in comparable settings today.
Some of the east coast data (Nova Scotia, New Jersey, Delaware) are compatible with a latitudinal gradient in precipitation model. However, in Alabama samples, and some of the New Jersey and Delaware samples, calculated oxygen isotopic values for middle Cretaceous precipitation can be substantially more depleted (by up to 3 per mil) than the values observed at comparable latitudes in the Western Interior. An argument can be made for a "reversed" latitudinal gradient along the middle Cretaceous east coast, from that observed in Western Interior strata.
We explore these relationships using a stable isotope version of the GENESIS atmospheric general circulation model. The modeling effort uses relative sea-level highstand and lowstand settings, and 4X present-day atmospheric carbon dioxide contents. We first specify sea surface oxygen isotopic compositions as uniformly -1.2 per mil across the middle Cretaceous oceans. Second, we impose a sea-surface oxygen isotopic profile based on literature compilations and GENESIS-derived sea-surface temperatures. Our initial results indicate that the Western Interior data sets mostly match the model simulations. Model runs are ongoing. |