ORGANIC MATTER DISTRIBUTION IN OXIDIZED PALEOSOLS OF THE EARLY OLIGOCENE BRULE FORMATION, WHITE RIVER GROUP, NORTHWEST NEBRASKA

The terrestrial White River Group (WRG) of the North American midcontinent contains a vast paleopedological and paleobiological archive preserved across the Eocene-Oligocene Transition (EOT). Current understanding of vegetative reaction to the EOT rests primarily on indirect evidence through paleosol analysis and δ¹³C of mammalian bioapatite because ancient soils were oxidizing and plant remains are rare. Here we report the first observation of charcoalified organic matter in WRG paleosols. Microcharcoal grains are common in paleosols in the Early Oligocene and are differentiable from opaque heavy minerals based on their gritty grey to black appearance in reflected light and frequently frayed grain boundaries. Microcharcoal grains and microscopic leaf fragments were handpicked from the clay-size fraction of disaggregated bulk rock samples using standard techniques and were used as a visual comparison to in situ grains. Paleosols developed on fluvial deposits contain charcoal grains up to ~50 µm, while those formed on eolian deposits contain generally more angular grains that reach 150 µm in diameter. This taphonomic bias suggests that larger, less abraded charcoal grains in eolian environments preserve a more local vegetative signal.

Dendritic to amorphous opaque coatings are also visible in thin section on bone fragments, along the axis of root voids and within the matrix of well-developed paleosols formed on eolian silt. These coatings are morphologically similar to mycorrhizal fungi. Raman spectra of in situ microcharcoal grains and possible fungal remains contain peaks at ~1300 and ~1600 cm^-1, consistent with disordered organic carbon. Electron microscopy will be used to validate these initial interpretations. These organic materials may offer researchers an additional geochemical archive for future paleoclimatic reconstructions.