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

The isotopic composition of paleosol carbonates is usedextensively 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 (T-Delta 47) allow for independent controls on temperature, but the influence of evaporation remains unresolved. However, the sensitivity of O-18-O-17-O-16 distributions to kinetic fractionation makes it possible to use triple oxygen isotopes (Delta'O-17) to track evaporation in water. Recent work shows the sensitivity of Delta'O-17 to evaporation in lakes and lacustrine carbonates, but little is known about variation of Delta'O-17 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 Delta'O-17 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. delta C-13 values range from -2.7 to 1.8 parts per thousand and reflect C-4 dominated grasslands, whereas delta O-18 values of soil carbonates vary by similar to 8 parts per thousand along a gradient in aridity. T-Delta 47 from these soil carbonates average 23.C (1 sigma +/- 4 degrees 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 d18O values of soil water are up to 6 parts per thousand higher than d18O values of local precipitation and springs, indicating considerable soil water evaporation. The Delta'O-17 values of these soil carbonates range from -162 to -106 per meg and decrease as both aridity and delta O-18 values increase. Our results support the hypothesis that soil water evaporation drives the variance in delta O-18 and Delta'O-17 of soil carbonate in arid climates, demonstrating the potential for soil carbonate Delta'O-17 to track paleoaridity and constrain interpretations of paleosol carbonate delta O-18 records. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

The study investigates the relationship between Delta'O-17 in soil carbonates and soil water evaporation, finding that in arid climates, the variance in soil carbonate Delta'O-17 is driven by soil water evaporation, showing the potential to track paleoaridity.