ADDRESSING THE PROXY DATA-MODEL MISFIT REGARDING LOW OXYGEN ISOTOPIC VALUES OF CRETACEOUS ARCTIC PALEOPRECIPITATION: NEW DATA FROM THE BASTION RIDGE FORMATION, SVERDRUP BASIN OF NORTHERN CANADA

Pedogenic carbonates (calcites) and siderites formed in Cretaceous paleosols from paleoequatorial to paleopolar latitudes in North America have provided useful stable isotopic paleohydrologic data to help constrain the oxygen isotope mass balance of the mid-Cretaceous hydrologic cycle. Data are extremely sparse in polar locales, and there is an urgent need for more of these data to advance this line of inquiry. Glacier Fiord at Axel Heiberg Island, Nunavut, Canada contains a Cretaceous (Valanginian to Campanian) record of deposition in the Sverdrup basin. A Cenomanian siderite-cemented siltstone from the Bastion Ridge Formation produces δ¹³C values of +4.0‰ ± 0.30‰ VPDB and δ¹⁸O values of -18.9‰ ± 0.33‰ VPDB. The positive δ¹³C values from freshwater siderites in the Bastion Ridge Formation are shared in common with all other sampled Cretaceous units from north of 50°N paleolatitude, and are indicative of methane-emitting high latitude terrestrial paleoenvironments. At an estimated range of 68 to 72°N paleolatitude for the Bastion Ridge Fm, we calculate a Cretaceous mean annual paleotemperature range of 12.6 to 13.7°C, with a range of paleoprecipitation δ¹⁸O values from -23.3 to -23.0‰ VSMOW. These results plot in the lower range of other published proxy polar δ¹⁸O precipitation values, and are consistent with other estimates for Cretaceous precipitation produced from the Colville Basin in northern Alaska. Results from Axel Heiberg Island further build the case for low δ¹⁸O values for polar paleoprecipitation during warm periods in Earth History. Proxy polar Cretaceous δ¹⁸O paleoprecipitation datasets that are indicative of low δ¹⁸O values (≤ -20‰ VSMOW) are not consistent with results from GCM simulations. Namely, the GCMs with water isotope modules along with the classic modern empirical δ¹⁸O-MAT relationship of Dansgaard (1964, Tellus 16:436-468) suggest that much higher Cretaceous δ¹⁸O values should be expected. Shallow marine to nonmarine deposits in the Sverdrup Basin accumulated in a passive continental margin, and orographic influences on the δ¹⁸O values of shallow groundwaters are not a plausible mechanism to explain δ¹⁸O values. We interpret the δ¹⁸O data from the Bastion Ridge Formation to be truly indicative of zonal mean Cretaceous Arctic paleoprecipitation.