T62 Recent Advances in Soil and Paleosol Science

Sunday, 15 October 2023
1:30 PM - 5:30 PM
320

REVISITING THE SIWALIKS SOIL CARBONATES: USING TRIPLE OXYGEN ISOTOPES TO DISTINGUISH PALEOCLIMATIC PROCESSES ACROSS THE C3-C4 EVOLUTIONARY TRANSITION

Paper number: T62-10
Program: Technical Sessions
Time: 3:40 PM – 3:55 PM

Authors

Abstract

Preliminary triple oxygen isotope measurements of soil carbonates of the Siwaliks sequence (Potwar Plateau) show no distinguishable differences in Δ′17O with age across the C3-C4 transition which marked the appearance of grasses on the ancient Gangetic floodplain. A subset of samples with coeval changes in δ18O and δ13C from ~9 to 1 Ma were analyzed to test the hypothesis that this emergence paired either with changes in aridity or a shift in the hydrological processes affecting the region. The δ18O of soil carbonates are sensitive recorders of the formation temperature and water composition, however, subsidiary processes such as evaporation can modify the original soil water δ18O values. Triple oxygen isotopes include both 17 and 18O which for waters follow a covaried slope of 0.529. Kinetic effects due to diffusion of the lighter isotope during evaporation follows a slope of 0.5185. As a result, the δ18O value increases while Δ′17O decreases below the range for meteoric waters, making this tool useful for discriminating an evaporative effect upon pedogenic carbonate formation.

Our results consist of δ18O values from 20 to 26 (‰ vs. VSMOW) distinguished by two groups, both having an average Δ′17O value of -0.130 (+/- 0.02 ‰ vs. VSMOW, λ=0.528). Lower δ18O values are typically older than 7Ma except for one sample, while higher δ18O are <7Ma, where the onset of δ13C values recording the C4 pathway appeared in the soil carbonate record. Stationary Δ′17O values at variable δ18O suggest changes in the hydrological processes at the Potwar Plateau rather than increasing aridity resulting from more strongly evaporated waters. However, the extremely negative Δ′17O values for all samples cannot be explained by calcite-water equilibrium from an unevaporated source water alone and requires constraint on pedogenic parameters during carbonate formation such as relative humidity, evaporative flux, and soil temperature.

Geological Society of America Abstracts with Programs. Vol. 55, No. 6

doi: 10.1130/abs/2023AM-396088

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