BULK GEOCHEMISTRY AND MAGNETIC SUSCEPTIBLY OF PALEOSOLS ACROSS THE EOCENE-OLIGOCENE TRANSITION OF THE NORTHERN GREAT PLAINS (WHITE RIVER GROUP), USA

The White River Group (WRG) preserves one of the most complete terrestrial records of the Eocene-Oligocene Transition (EOT). Climate signals from this period of global cooling are preserved in paleosols of the Chadron and Brule Formations of the WRG. A shift from well-developed soils with root traces up to 3cm in diameter, to weakly formed soils dominated by thin, hair-like root is seen from the Eocene into the Oligocene. Previous research paired these physical observations with geochemical weathering proxies to suggest a climate-forced ecological shift from forested canopy cover to open landscapes. To further test this hypothesis, we examined paleosols from Toadstool Geologic Park, NE, ranging in age from 36.1-30.9 Mya, with a goal of assessing the extent of aridity across the EOT in this region.

Molecular weathering ratios derived from the bulk geochemistry of paleosols detail maturity, calcification, salinization, leaching, and translocation, all of which can be used to infer the intensity of soil drainage. Mean annual precipitation (MAP) is approximated from both B-horizon geochemistry (chemical index of alteration) and depth to Bk-horizon. Results suggest stable sub-humid conditions throughout the late Eocene (maximum MAP of 912.8 ± 181mm/yr) with an abrupt decrease to 404.3 ± 181mm/yr in the early Oligocene. Low frequency (465Hz) magnetic susceptibility (MS) replicates the sharp decrease seen in MAP in the early Oligocene.

Previous paleoclimate interpretations range from no resolvable change in aridity to a distinct reduction in wetter habitats. In order to test the fidelity of individual paleoclimate proxies, our results from bulk geochemistry and MS will be compared with biogeochemical indicators of water stress and paleoecology (leaf wax-n alkanes), soil productivity (soil organic matter), and MAP (𝛿¹³C of soil organic matter). Our study indicates increased aridity in the early Oligocene and correlates with the marine 𝛿¹⁸O timeline of the EOT.