MAGNETIC SUSCEPTIBILITY OF PALEOSOLS IN THE WHITE RIVER GROUP (NORTHWEST NEBRASKA) REFLECTS REGIONAL EOCENE-OLIGOCENE PALEOCLIMATE

The White River Group (WRG) of northwest Nebraska represents consistent alluvial to eolian deposition across the Eocene-Oligocene Transition (EOT). Recent studies of regional paleosols underscore the impact of diagenesis on magnetic susceptibility (MS) as a paleoclimate proxy. Our goal was to interrogate whole profile trends in MS and interpret the effects of soil depth and horizon position on the relationship between MS and mean annual precipitation (MAP). We present an analysis of twelve paleosols that formed across the EOT, spanning the Chamberlain Pass Fm (Chadronian) to Brule Fm (Whitneyan). Statistics and whole profile trends of paired MS and geochemical indices are combined with climofunctions, petrography, and clay mineralogy to create a multi-proxy record during the EOT. Findings are supported by preliminary depth-resolved carbon and oxygen stable isotope data of carbonate nodules and matrix.

MS is strongly correlated with MAP in the late Eocene (r² = 0.71, p = 0.007). At this time, MAP ranges from ca. 774 -1217 mmyr^-1, reflecting a stable subhumid climate. δ¹⁸O_carb and δ¹³C_carb support late Eocene stability, with all profiles showing average values of -9.5‰ and -6.8‰, respectively. Paired B-horizon measurements of MS and MAP consistently covary with depth. In the early Oligocene, coincident with a ~550 mmyr^-1 drop in MAP, MS values are anomalously high and are no longer described by the trend established in the late Eocene. The shift in MAP and MS is contemporaneous with the first step of the EOT (~33.8), and other trends in soil features (micromorphology, clay mineralogy, soil type), that suggest a coherent regional hydroclimate response to climate change. We posit climate-driven wild fires as a possible reason for the spike in MS during periods of increased aridity, due to the potential conversion of hematite to magnetite during burning. We suggest charcoal and PAH analysis as a way to test this hypothesis. Paired MS and geochemical indices for all soil horizons (n=44) reveal no relationships between MS and CIA-K, Ba/Sr, Bases/Ti, or total iron. The lack of correlation suggests the relationship between MS and MAP is not an artifact of any underlying soil process, and that late Eocene MS is a function of MAP. However, anomalously high values in the Oligocene indicate MS is susceptible to other environmental factors.