PROBING A CLIMATE MODEL-DATA MISFIT REGARDING THE OXYGEN ISOTOPIC COMPOSITION OF ARCTIC PRECIPITATION IN THE CRETACEOUS GREENHOUSE WORLD

There has been a recent emphasis in climate change research to validate computer climate models (GCMs) by simulating paleoclimatic datasets. The Arctic region has long been an area of special interest because of model-data misfits, principally involving the strong empirical evidence for polar warmth in greenhouse worlds like those of the Cretaceous Period. In recent years, published empirical datasets on the δ¹⁸O values of Cretaceous Artic paleoprecipitation have been the subject of GCM simulation exercises, and once again, there appear to be misfits between model results and empirical data. The GCM simulations of Poulsen et al. (2007; Geology 35:199-202) suggested that the pedogenic siderite δ¹⁸O data for Cretaceous paleoprecipitation in North Slope Alaska (Ufnar et al., 2004; GSA Bull. 116:463-473) are too light, and might not actually be representative of Cretaceous Arctic climate—thus identifying an important research problem. The data of Ufnar et al. (2004) were produced from the Albian Nanushuk Formation in the USGS Grandstand core, located about 80 km from the Brooks Range thrust front. In order to test the idea of local orographic influences on the Ufnar et al. (2004) dataset, we collected new data from the Nanushuk Formation in the USGS Tunalik and Wainwright drillcores from the National Petroleum Reserve in Alaska (NPRA; drillsites 210-240 km from Brooks Range thrust front). Moreover, we also collected new data from exposures of the Maastrichtian Prince Creek Formation along the Colville River, North Slope Alaska. From these formations, eight paleosol horizons were used, and their δ¹⁸O values range between -10.00‰ and -15.61‰ VPDB with standard deviations ranging from 0.13 up to 0.65 per mil within individual horizons. The δ¹³C values are much more variable, ranging from +1.62‰ to +13.88‰ VPDB. Carbon-oxygen isotope plots of the data show evidence for fluid mixing between modified marine and meteoric phreatic fluids during siderite precipitation in some horizons. The information gathered in this study support the findings of Ufnar et al. (2004), and shows that the Arctic precipitation δ¹⁸O values simulated by Poulsen et al. (2007) are heavier than any results produced from a now more robust Cretaceous sampling network in North Slope Alaska.