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

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.