A MULTIPROXY APPROACH TO INTERPRETING ARIDITY ACROSS THE EOCENE-OLIGOCENE TRANSITION OF THE NORTHERN GREAT PLAINS (WHITE RIVER GROUP), NORTH AMERICA

Abstract

The White River Group (WRG) represents consistent alluvial to eolian deposition across the North American Great Plains region during the late Eocene and early Oligocene, creating one of the most complete terrestrial records of the Eocene-Oligocene Transition (EOT). The WRG is an archive of paleosols and biogeochemical signals, including vertebrate fossils and plant-derived organic compounds, that provide useful paleoclimate proxies. Contextualizing these proxies with newly dated tuffaceous ash beds allows for a high-resolution climate reconstruction that can be correlated to other terrestrial sections and the marine record. This study combines paleosol derived climate proxies with leaf wax n-alkanes and magnetic susceptibility to create a multi-proxy record related to aridity during the EOT. Paleosol and leaf wax analyses suggest that a stable subhumid environment, dominated by forested landscapes, was consistently present until the onset the EOT-1. During this time, n-alkane average chain length ranges from 29.29-29.92 and suggest minimal input from grasses. Between 35.224 - 33.939 Ma, a decrease in ACLs is concurrent with increased mean annual temperature and precipitation, negating previous assertions of prolonged late Eocene climate degradation. An abrupt 550.29 ± 147 mm drop in MAP is coincident with the EOT-1. This is the first terrestrial EOT study in the Great Plains region to produce a drop in MAP associated with the EOT that falls outside the statistical margin of error. In the earliest Oligocene, increased ACLs suggest a transition to grass-like landscapes. A change in landscape vegetation is supported by paleosol characteristics that suggest a shift to mollisols, including dense drab halo root traces and phosphorus trends that indicate mollic epipedons. CIA-K of B-horizons also indicate an abrupt shift from alfisols to mollisol across the Eocene-Oligocene boundary. MS is strongly correlated with MAP in the late Eocene and shows the potential for a regionally specific transform function for estimating precipitation when MAP is > 700 mm/yr. Additionally, this study demonstrates that accurate interpretation of leaf wax n-alkanes relies on understanding the original n-alkane position within the paleosol profile and the impact of depositional events on plant succession. The negligible drop in MAT from the paleosol proxies in this study do not support the findings of Zanazzi et al. (2007). However, this study closely agrees with Boardman and Secord (2013), which does not show any meaningful decrease in δO18 from fossil tooth enamel. Boardman and Secord (2013) also interpret a shift towards open biomes and reduced "wetter habitats," which is consistent with the shift away from Alfisols in the Eocene, to inceptisols and mollisols in the early Oligocene as described in this study.