T180 Weathering and Soils: Advances in Understanding Rates, Mechanisms, Controlling Factors and Feedbacks

Tuesday, 12 October 2021
1:30 PM - 5:30 PM
D139/D140

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

Paper number: T180-2
Program: Technical Sessions
Time: 1:35 PM – 1:50 PM

Authors

Abstract

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 δ7Libedrock 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 δ7Libedrock 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 δ7Liclay values (from ~0.5 to -2.2‰) after the onset of the PETM while δ7Libedrock values remain steady at ~1.0‰. The decreasing Δ7Liclay-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.

Geological Society of America Abstracts with Programs. Vol. 53, No. 6

doi: 10.1130/abs/2021AM-368278

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