Evidence for a regionally persistent carbon isotopic chemostratigraphy in the Neogene High Plains succession of western Kansas
by Greg A. Ludvigson, Jon J. Smith, Peter A. Macfarlane, Rolfe Mandel, Adel E. Haj, Laura Murphy, and Arlo McKee
Geological Society of America, Abstracts with Programs, v. 40, no. 5, p. 24.
Presented at the 42nd Annual Meeting of the North-Central Section of the Geological Society of America, 24-25 April, 2008
Calcic paleosols occur with high stratigraphic frequency in Miocene, Pliocene, and Pleistocene fluvial strata of the High Plains Succession (HPS), and work by Fox and Koch (2003) demonstrated potential for dating HPS deposits in Kansas using the δ13C values of pedogenic carbonates and organic matter. Their work showed a systematic long-term increase in δ13C values over the Late Miocene-Pleistocene interval, a trend related to the evolutionary innovation of the C4 grassland prairie biome. Their dataset over the last 10 Ma are used to formulate a stratigraphic age model correlating age of deposit with δ13C of pedogenic carbonates with the second order polynomial regression: y = 0.11052x2 - 1.7546x + 0.037283; where x = Age (Ma); y = δ13C of pedogenic carbonate; with R2 = 0.75. The pedogenic carbonate correlation curve rises from a Late Miocene baseline value of -7‰ VPDB up to Pleistocene values greater than 0‰ VPDB. A test of this model is being carried out on a research borehole drilled by USGS in the Cimarron National Grassland (CNG site; McMahon et al., 2003, USGS WRIR 03-4171). Our δ13C analyses of bulk organic carbon throughout the section are at 30 cm sample spacings, and pedogenic carbonates at spacings of 0.3-3 m. Results from lower parts of the CNG borehole show baseline bulk organic δ13C values ranging between -27 to -26‰ VPDB, and pedogenic carbonate δ13C values of about -7‰ VPDB. These results are compatible with Neogene paleofloras dominated by C3 biomass, and by analogy to Fox and Koch (2003) suggest correlation with Late Miocene portions of the Ogallala Formation. The upper part of the CNG borehole shows bulk organic δ13C values up to -17‰ VPDB, while carbonates from this interval are still being analyzed. A fine-scale intricate chemostratigraphic structure from δ13C and δ18O data in the CNG profile offers prospects for developing high-resolution paleoclimatic correlations. Chronostratigraphic calibrations of the chemostratigraphy by ash-bed chronology are pending.
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