The isotopic composition of paleosol carbonates are 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 (T∆47) 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 (∆ʹ17O) to track evaporation in water. Recent work shows the sensitivity of ∆ʹ17O to evaporation in lakes and lacustrine carbonates, but little is known about variation of ∆ʹ17O 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 ∆ʹ17O 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. T∆47 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 ∆ʹ17O 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 ∆ʹ17O of soil carbonate in arid climates, demonstrating the potential for soil carbonate ∆ʹ17O to track paleoaridity and constrain interpretations of paleosol carbonate δ18O records.

Reconstructing water availability in terrestrial ecosystems is key to understanding past climate and landscapes, but there are few proxies for aridity that are available for use at terrestrial sites across the Cenozoic. The isotopic composition of tooth enamel is widely used as paleoenvironmental indicator and recent work suggests the potential for using the triple oxygen isotopic composition of mammalian tooth enamel (∆’17Oenamel) as an indicator of aridity. However, the extent to which ∆’17Oenamel values vary across environments is unknown and there is no framework for evaluating past aridity using ∆’17Oenamel data. Here we present ∆’17Oenamel and δ18Oenamel values from 50 extant mammalian herbivores that vary in physiology, behavior, diet, and water-use strategy. Teeth are from sites in Africa, Europe, and North America and represent a range of environments (humid to arid) and latitudes (34S to 69N), where mean annual δ18O values of meteoric water range from -26.0‰ to 2.2‰ (VSMOW). ∆’17Oenamel values from these sites span 146 per meg (-283 to -137 per meg, where 1 per meg = 0.001‰). The observed variation in ∆’17Oenamel values increases with aridity, forming a wedged-shape pattern in a plot of aridity index vs. ∆’17Oenamel that persists regardless of region. In contrast, the plot of aridity index vs. δ18Oenamel for these same samples does not yield a distinct pattern. We use these new ∆’17Oenamel data from extant teeth to provide guidelines for using ∆’17Oenamel data from fossil teeth to assess and classify the aridity of past environments. ∆’17Oenamel values from the fossil record have the potential to be a widely used proxy for aridity without the limitations inherent to approaches that use δ18Oenamel values alone. In addition, the data presented here have implications for how ∆’17Oenamel values of large mammalian herbivores can be used in evaluations of diagenesis and past pCO2.