THE PARADOX OF HYDROGEN AND OXYGEN ISOTOPIC COMPOSITIONS OF CRETACEOUS ARCTIC PALEOPRECIPITATION (Invited Presentation)

The development of extreme polar warmth during warm periods of Earth History like those of the Cretaceous is a firmly established fact in published scientific literature. Stable isotope proxy data for Cretaceous Arctic paleoprecipitation (δ¹⁸O and δD) are now available from pedogenic carbonate and clay minerals, vertebrate tooth enamel phosphates, and long-chain n-alkane biomarkers from fossil leaf waxes, all of which can be used to examine the relationship between extreme Cretaceous polar warmth and the coeval global hydrologic cycle. These data address a major Earth System Science question concerning the applicability of the classical modern empirical relationship between local mean annual surface air temperatures and the local weighted mean annual δ¹⁸O and δD compositions of precipitation first described by Dansgaard (1964). This relationship associates higher water δ¹⁸O and δD isotope values with higher temperatures, and lower water δ¹⁸O and δD values with lower temperatures. Can the extrapolation of this modern relationship to the warmed Cretaceous Arctic successfully predict higher Cretaceous Arctic paleoprecipitation δD and δ¹⁸O values? We recently reported coordinated paleoprecipitation isotopic data from Late-Cretaceous terrestrial deposits from the Colville Basin in northern Alaska and mid-Cretaceous terrestrial deposits from the Sverdrup Basin and Eclipse Trough of the Canadian High Arctic Archipelago (Ludvigson, et al., 2022, Geosciences. 12(4):143. https://doi.org/10.3390/geosciences12040143). These units yield higher than modern Arctic water δD values, but these δD data are also associated with lower water δ¹⁸O values than would be expected from intersection with the Global Meteoric Water Line. The apparent deuterium excess ranges from about 40 to 60 per mil. Some possible explanations for these results include: (1) mass-balance changes in zonal patterns of evaporation/precipitation at lower latitudes, (2) concentration of ²H in leaf-tissue waters from continuous transpiration during the Arctic growing season, and (3) concentration of ²H in the groundwaters of methane-emitting Arctic soils.