Stable Isotope Tracers of Cretaceous Arctic Paleoprecipitation

We report estimated stable isotope compositions of depositional waters and paleoprecipitation from the Cretaceous Arctic to further elucidate the role of the global hydrologic cycle in sustaining polar warmth during that period. Estimates are based on new hydrogen isotopic analyses of n-alkane biomarkers extracted from Late Cretaceous and mid-Cretaceous terrestrial deposits in northern Alaska and the Canadian High Arctic. We integrate these new results with earlier published work on oxygen isotopic analyses of pedogenic siderites, dinosaurian tooth enamel phosphates, and pedogenic clay minerals from the same field areas. Average Late Cretaceous delta D values of -143 parts per thousand VSMOW corresponded with average delta O-18 values of -24.1 parts per thousand VSMOW, and average mid-Cretaceous 6D values of -106 parts per thousand VSMOW corresponded with average delta O-18 values of -22.1 parts per thousand VSMOW. The distributions of water isotope delta D and delta O-18 values from Cretaceous Arctic deposits do not intersect with the Global Meteoric Water Line, suggesting an apparent deuterium excess ranging from about 40 to 60 per mil. We considered several possible explanations for these Cretaceous results including (1) mass-balance changes in zonal patterns of evaporation and precipitation at lower latitudes, (2) concentration of H-2 in leaf tissue waters from continuous transpiration by coniferous paleofloras during the Arctic growing season, and (3) concentration of H-2 in the groundwaters of methane-emitting Arctic soils.