Probing a Paleoclimate Model-Data Misfit in Arctic Alaska from the Cretaceous Greenhouse World
Lollar, Joseph Chad
University of Kansas
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Abstract Joseph Chad Lollar, M.S. Department of Geology, June 2011 University of Kansas There has been a recent emphasis in climate change research on developing computer models that can simulate past and future climatic settings. These models have never been able to accurately recreate many known details of the Earth System. In particular, the late 20th-century Arctic warming and the even more pronounced polar warming during past greenhouse periods in Earth History, such as the Cretaceous, are major climate model-data misfits. The difficulty in simulating the polar warmth phenomenon might be partly attributable to uncertainties concerning the role of the hydrologic cycle in the global climate system. This failing is also exacerbated by a dearth of empirical paleoclimatic data from the Polar Regions that would better constrain our understanding of the hydrologic cycle during these time periods. To address this problem and to better constrain the boundary conditions for climate models, this study focused on broadening the distribution of empirical oxygen isotopic data from high latitude locations of the mid-Cretaceous (Albian-Cenomanian) greenhouse world by measuring the carbon-oxygen isotopic compositions of seven pedogenic siderite paleosol horizons from the Nanushuk Formation in North Slope Alaska. Pedogenic siderite is used because it is common in the geologic record, and is widely used as a proxy for paleoprecipitation. Sedimentologic logging and petrographic analysis of samples from the Tunalik #1 (Haywood, 1983) and Wainwright #1 cores (Lepain and Decker, in press) suggests that these siderites developed in poorly drained, reducing soils that formed in coal-bearing delta plain facies. The isotopic data from horizons sampled display slightly varying ä18O values with more highly variable ä13C values, with a positive covariance. This positive covariance probably results from mixing between modified marine and meteoric pore fluids during the precipitation of the siderite. The ä18O values range from -9.57 / to -13.58 / VPDB, while the ä13C values range from +1.32 / to +14.34 / VPDB. The siderites are elementally impure, having varying substitutions of Ca, Mg, Mn, and Sr for Fe, thus further suggesting the influence of modified marine fluids upon siderite precipitation. In order to deconvolve the fluid mixing processes and better estimate the end-member freshwater ä18O compositions, fluid mixing modeling was carried out. Fluid-mixing modeling indicates fresh-water ä18O ranges between -21.00 to -15.35 / VSMOW; with an average of -18.91 / VSMOW. Moreover, the corresponding siderite fresh-water end-member MSL ä18O values are between -16.22 to -10.54 / VPDB; with an average of -14.12 / VPDB. This proxy record of paleoprecipitation supports earlier published results that proposed an amplified hydrologic cycle that transported more latent heat from the equator to the poles than in the present climate system. In addition to expanding the distribution of zonal empirical data from the Cretaceous Arctic, this study shows that the range of ä18O values for Cretaceous siderite-bearing soil horizons of the North Slope, Alaska, from the Tunalik #1 and Wainwright #1 drillcores do display some overlap with the earlier published results of Ufnar et al. (2004b), but do not match those predicted by the Earth System modeling of Poulsen et al. (2007). These new data clearly emphasize the need for much more additional sampling in order to adequately characterize the hydrologic cycle of the Cretaceous Arctic, and thus better constrain Earth System Models of Cretaceous climate.
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