A RECORD OF CARBON AND OXYGEN ISOTOPE EXCURSIONS FROM PEDOGENIC CARBONATES OF THE NEOGENE HIGH PLAINS SUCCESSION IN WESTERN KANSAS

Fox and Koch (2003) sampled Neogene paleosols from throughout the High Plains region with well-established ages and showed a long-term increase in δ¹³C values from pedogenic carbonates and soil organic matter (SOM) starting at ~10 Ma. They interpreted this to reflect the Neogene expansion of C₄ grasses. More recently, Martin et al. (2008) showed that Plio-Pleistocene carbon isotope excursions (CIEs) from south Kansas are associated with δ¹⁸O shifts of up to 10‰, suggesting that these record changes in paleotemperatures and/or evaporation effects. Borehole cuttings from three USGS research wells in southwestern Kansas (CNG, CAL-121, and CAL-122) provide an opportunity to test the regional expression of these chemostratigraphic trends in High Plains deposits. Carbonate nodules from the CNG borehole show a long-term upward increase in δ¹³C values with a starting baseline value of -6‰ VPDB. The CAL-121 and CAL-122 profiles also show increases in δ¹³C values, but with baseline values of -4‰ VPDB. The SOM profiles for the CNG and CAL boreholes show long-term increases in δ¹³C values with starting baseline values of -27‰ VPDB. The δ¹³C values of SOM extracted from pedogenic CaCO₃ differ from those in bulk sediments by as much as 2‰. These relationships suggest that there were long-term differences in paleohydrologic or soil gas processes between the CNG and CAL sites. Pedogenic carbonates show three well-developed positive CIEs in the CNG profile, with possible correlatives in the CaCO₃ chemostratigraphy of the CAL boreholes. Carbonate CIEs are variably expressed in corresponding oxygen isotope profiles as well. Positive CIEs correspond with positive δ¹⁸O shifts of <1‰ in the CNG borehole, but with larger positive shifts of up to 2‰ in the CAL boreholes. Conversely, some negative CIEs in the CAL boreholes are associated with positive δ¹⁸O shifts, likely recording a different set of processes than those in positive CIEs. These features did not originate simply from changes in paleovegetation or temperature; other paleoclimatic, paleohydrologic, or diagenetic processes were likely involved.