Impact of Surficial Weathering on the Magnetic Properties of Paleosols: a Core to Outcrop Comparison from the Bighorn Basin, WY

Abstract:

Environmental magnetic studies of soils have generally focused on providing insights into the climate and environmental conditions that produce modern soils and geologically young paleosols (<5 Ma), but environmental magnetic analysis of more ancient paleosols has been limited. The Bighorn Basin Coring Project (BBCP) recently recovered ~300 m of sediment core spanning the Paleocene-Eocene boundary from the Polecat Bench locality in the Bighorn Basin, WY. Core scan images show that oxidative weathering extends some 20-30 m below the surface, suggesting that recent surficial weathering could have significantly altered the magnetic signals preserved in paleosols that crop out in this region. This study presents the first detailed core to outcrop comparison of paleosol magnetism of which we are aware. Nine paleosols from Polecat Bench, spanning the Paleocene-Eocene Thermal Maximum (PETM), have been sampled at high-resolution from outcrop trenches and core sediments. Importantly, these paleosols are stratigraphically below the weathering front in the core sediments, suggesting that they preserve an unweathered magnetic signature. We present results of preliminary comparison between the magnetic signals preserved in core sediments and the same sediments exposed as outcrops using detailed measurements of low-field magnetic susceptibility, frequency dependence of susceptibility, isothermal remanent magnetization (IRM), and anhysteretic remanent magnetization (ARM). We explore the implications of surficial weathering for environmental magnetism, including the application of magnetic paleoprecipitation proxies calibrated using modern soils to more ancient paleosols.