Triple oxygen and clumped isotopes in modern soil carbonate along an aridity gradient in the Serengeti, Tanzania

Abstract

The isotopic composition of paleosol carbonates is used extensively to reconstruct past vegetation, climate, and altimetry, but poor constraints on soil evaporation and temperature have limited the utility of oxygen isotopes in the studies. Recent advances in carbonate clumped isotope thermometry (TΔ47) allow for independent controls on temperature, but the influence of evaporation remains unresolved. However, the sensitivity of 18O-17O-16O distributions to kinetic fractionation makes it possible to use triple oxygen isotopes (Δ′17O) to track evaporation in water. Recent work shows the sensitivity of Δ′17O to evaporation in lakes and lacustrine carbonates, but little is known about variation of Δ′17O in soil carbonates and their potential to track evaporation. For this study, we sampled soils across an aridity gradient in the Serengeti, Tanzania to evaluate how soil carbonate Δ′17O tracks soil water evaporation. Modern soil carbonates were collected from 11 sites across a transect of the Serengeti Ecosystem where mean annual precipitation and aridity index range from 499 to 846 mm yr−1 and 0.33 to 0.55, respectively. δ13C values range from −2.7 to 1.8‰ and reflect C4 dominated grasslands, whereas δ18O values of soil carbonates vary by ∼8‰ along a gradient in aridity. TΔ47 from these soil carbonates average 23°C (1σ± 4°C), which does not vary significantly across sites or with depth, likely due to minimal annual variation in temperature at the equator. Using these temperatures for each carbonate, reconstructed δ18O values of soil water are up to 6‰ higher than δ18O values of local precipitation and springs, indicating considerable soil water evaporation. The Δ′17O values of these soil carbonates range from −162 to −106 per meg and decrease as both aridity and δ18O values increase. Our results support the hypothesis that soil water evaporation drives the variance in δ18O and Δ′17O of soil carbonate in arid climates, demonstrating the potential for soil carbonate Δ′17O to track paleoaridity and constrain interpretations of paleosol carbonate δ18O records.