CONTRASTING PALEOSOL OXYGEN ISOTOPE RECORDS FROM THE GREAT PLAINS OF SOUTHERN AND NORTHERN HEMISPHERIC GLACIATIONS DURING THE CENOZOIC

The two most significant changes in Earth's climate during the Cenozoic were the onset of Southern Hemisphere glaciation (Oi-1, earliest Oligocene, 33.0-33.5 Ma) and the onset of sustained Northern Hemisphere glaciation (late Pliocene, 2.7 Ma). The oxygen isotope composition (δ¹⁸O) of authigenic paleosol carbonates in stratigraphically well-constrained Eocene-Oligocene and Plio-Pleistocene sections in the Great Plains provides a means to examine climate in the interior of a continent during these two intervals of major global change. Paleosol carbonate δ¹⁸O values are controlled by water δ¹⁸O and soil temperature, but the effect of water composition dominates. Calcareous paleosols in the White River Group and superposed Sharps Formation in Toadstool Park, NE and the Big Badlands, SD provide a mostly complete paleoclimate record from the late Eocene to the early Oligocene. Carbonate δ¹⁸O values (n=172) are variable in the latest Eocene, do not change abruptly during Oi-1, and increase by 2-3‰ after the onset of Oi-1. Correlative Oligocene values in NE and SD differ by several permil, possibly due to differences in local soil hydrology. Other paleoclimate proxies in the region imply cooling and drying, so higher carbonate δ¹⁸O values probably reflect evaporative enrichment of soil water in ¹⁸O, possibly in response to onset/intensification of the North American monsoon rather than directly due to climate changes attendant on Southern Hemisphere glaciation. In contrast, Plio-Pleistocene paleosol carbonates in the Meade Basin, KS (n=173) decrease steadily by >3‰ from ca. 24‰ below an ash dated at 2.1 Ma to ca. 21‰ above an ash dated to 1.7 Ma, with the exception of three stratigraphically successive values with unusually high δ¹⁸O values. The decrease reflects the end of the Pliocene warm period and onset of major Northern Hemisphere glaciation, driven by summer cooling and an increase in the proportion of winter precipitation, which would lead to lower meteoric water δ¹⁸O values and lower carbonate δ¹⁸O values. The contrast in the two records indicates the potential complexity of paleosol δ¹⁸O as a paleoclimate proxy and the possibility that regional rather than global climatic signals may dominate a given record.