Swift Weathering Response on Floodplains During the Paleocene‐Eocene Thermal Maximum.

Silicate weathering is thought to increase and offset the rapid, massive input of CO2 into the atmosphere and ocean during the Paleocene‐Eocene Thermal Maximum (PETM), but few nonmarine records have been used to quantify this. We probe changes in silicate weathering intensity by measuring Li isotope ratios of bedrock and ancient floodplain deposits spanning the PETM in the Bighorn Basin, Wyoming (USA). Our results reveal a rapid increase in silicate weathering intensity during the PETM that remained high during at least the initial stage of climate recovery. Additionally, we determine that soils that formed farthest from ancient river channels underwent larger weathering changes than near‐channel soils. Alongside increased temperatures and pCO2, the simplest explanation for this response relates to increased seasonal fluctuations in water table height in the floodplain that promote dissolution and precipitation reactions. These findings newly demonstrate that weathering on floodplains, like mountain hillslopes, responds to climate change. Plain Language Summary: The chemical breakdown of silicate minerals on continents promotes the withdrawal of CO2 from the ocean and atmosphere. This process is thought to be enhanced when CO2 rapidly enters the ocean and atmosphere, such as during past climate change events like the Paleocene‐Eocene Thermal Maximum (PETM). Although this enhancement of silicate weathering is found in global carbon cycle models and the chemistry of marine rocks, less terrestrial evidence exists for how, where, and to what extent silicate minerals weathered during the PETM. In this study, we measured the chemistry of bedrock and ancient floodplain sediment that span the PETM in the well‐studied Bighorn Basin, Wyoming (USA) to quantify changes in silicate weathering intensity. We find that silicate weathering intensity increases rapidly (within 7,000 years of the onset of the PETM) and remains elevated even as climate has begun to return to its pre‐perturbed state. We also determine that soils that formed farthest from ancient river channels underwent larger weathering changes than near‐channel soils, suggesting active weathering responses on floodplains. Alongside high CO2 contents and warmer temperatures, the simplest explanation for this weathering response relates to soil hydrology, where increased water flow through soils caused by fluctuating water tables enhanced weathering. Key Points: A terrestrial silicate weathering response quantified during the Paleocene‐Eocene Thermal Maximum (PETM)Weathering intensity increases rapidly after PETM, with stronger responses found in floodplain soils farthest from ancient channelWeathering response in floodplain linked to hydroclimate and soil hydrology [ABSTRACT FROM AUTHOR]