ACTIVE SILICATE WEATHERING ON FLOODPLAINS DURING THE PALEOCENE-EOCENE THERMAL MAXIMUM: INSIGHTS FROM THE BIGHORN BASIN, WYOMING, USA

Silicate weathering leads to the sequestration of atmospheric CO₂ on 10⁵–10⁶-year timescales. During Cenozoic hyperthermal events, such as the Paleocene-Eocene Thermal Maximum (PETM), silicate weathering is thought to increase and offset the rapid and massive input of CO­₂ into the atmosphere and oceans. However, few nonmarine records have been used to quantify changes in silicate weathering on continents during the PETM. In this study, we measure Li isotope ratios of clay-sized sediments in floodplain deposits (n = 31) and bedrock (n = 12) in the Bighorn Basin, Wyoming (USA) to quantify silicate weathering intensity across the PETM. The difference between modern river suspended load δ⁷Li values and δ⁷Li_bedrock values inversely correlates with silicate weathering intensity, which we apply to an ancient alluvial system. For each floodplain deposit, we compute a Li-weighted average δ⁷Li_bedrock value by using immobile element ratios (Ti/Al, Zr/Al, Cs/Al) to partition the source of clay sediments between crystalline basement rock and Cretaceous shale endmembers. We observe a sharp and sustained decrease in δ⁷Li_clay values (from ~0.5 to -2.2‰) after the onset of the PETM while δ⁷Li_bedrock values remain steady at ~1.0‰. The decreasing Δ⁷Li_clay-bedrock values demonstrate that silicate weathering intensity increased during the PETM and stayed high into the initial recovery phase of the event, even as atmospheric pCO₂ decreased. When sedimentological information is considered, we determine that silicate weathering in soils that formed farthest from ancient river channels were more sensitive to hydroclimate changes than near-channel soils, demonstrating that these changes were controlled at least in part by in situ floodplain weathering. The simplest explanation for these changes relates to increased mean annual temperature and pCO₂ alongside seasonal fluctuations in water table height, all of which promote mineral dissolution and precipitation reactions in floodplain soils. These findings show that silicate weathering in floodplains responds to climate change and that the limits of weathering in floodplains are linked to hydroclimate and soil hydrology.