DIFFERENTIATING IN SITU VERSUS CATCHMENT-DERIVED BIOMARKER SIGNALS OF PALEOCLIMATIC CHANGE ACROSS THE TERRESTRIAL EOCENE-OLIGOCENE TRANSITION OF NORTHWEST NEBRASKA

The White River Group of northwest Nebraska preserves alluvial to eolian deposition across the Eocene-Oligocene Transition (EOT). We investigated paleosols and leaf wax n-alkanes to (a) reconstruct climate and vegetation changes associated with the EOT, and (b) characterize sampling biases associated with soils formed on alluvial parent materials. Analyses were conducted on nine paleosols that formed between ~36.9 – 33.83 Ma. Climate proxies derived from paleosol geochemistry, petrography, and clay mineralogy were combined with n-alkane data to create a multi-proxy record during the EOT.

The shortest n-alkane average chain length (ACL) occurs at 36.9 Ma, coincident with soil features that indicate a humid period. The dominant pedotypes at this time are ultisols, with vosepic clay fabrics and a kaolinite-dominated clay mineralogy. Estimated mean annual precipitation (MAP) ranges between 1114.98 – 1217.43 ± 182 mm/yr. Between 35.22 - 33.94 Ma, a decrease in A-horizon ACL 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 EOT-1 (~33.8 Ma), at which A-horizon ACL values increase to 30.8 which is consistent with a transition to grass-like landscapes, supported by dense drab halo root traces and phosphorus trends indicating mollic epipedons. Additionally, CIA-K of B-horizons indicate an abrupt shift from alfisols to mollisols.

Covariation and agreement between A-horizon n-Alkane distributions and soil proxies demonstrate consistency between independent climate signals and support interpreting A-horizon n-Alkanes as reflective of in-situ vegetation. Conversely, n-alkanes sampled from B and C horizons in this study often show ACL values distinct from (generally higher) A-horizon values, suggesting n-alkanes in lower soil horizons represent a reworked signal from elsewhere in the basin. δD_wax data offers a similar insight, detailing more negative values at depth, the inverse of observed diagenetic trends. This study highlights the importance of viewing terrestrial sections through a pedological lens and accurately identifying soil horizons to untangle catchment versus in-situ signals within terrestrial systems.