Stable Isotope Tracers of Cretaceous Arctic Paleoprecipitation

Abstract

We report estimated stable isotope compositions of depositional waters and paleoprecip-itation 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 pub-lished work on oxygen isotopic analyses of pedogenic siderites, dinosaurian tooth enamel phos-phates, and pedogenic clay minerals from the same field areas. Average Late Cretaceous δD values of −143‰ VSMOW corresponded with average δ18O values of −24.1‰ VSMOW, and average mid-Cretaceous δD values of −106‰ VSMOW corresponded with average δ18 O values of −22.1‰ VSMOW. The distributions of water isotope δD and δ18 O values from Cretaceous Arctic deposits do not intersect with the Global Meteoric Water Line, suggesting an apparent deuterium excess rang-ing 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 of2 H in leaf tissue waters from continuous transpiration by conif-erous paleofloras during the Arctic growing season, and (3) concentration of2H in the groundwaters of methane-emitting Arctic soils.