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