T11 “Curse Your Sudden but Inevitable Betrayal": Advances and Challenges in Terrestrial Paleoclimate

Monday, 26 October 2020
1:30 PM - 5:30 PM
Meeting Rooms

A NEW PROXY FOR REGIONAL PALEO‑ARIDITY USING CLUMPED AND TRIPLE OXYGEN ISOTOPES OF MODERN SOIL CARBONATES FROM THE SERENGETI ECOSYSTEM, TANZANIA

Paper number: T11-10
Program: Technical Sessions
Time: 3:30 PM – 3:45 PM

Authors

Abstract

Paleosol carbonates are used extensively in the rock record to reconstruct vegetation using carbon isotopes, but poor constraints on temperature and soil evaporation has limited the use of oxygen isotopes in these reconstructions. Advances in carbonate clumped isotope thermometry allow for independent controls on temperature (T∆47), but proxies for evaporation remain elusive. The sensitivity of 18O-17O-16O distributions to kinetic fractionation makes it possible to track evaporation using triple oxygen isotope distributions (∆ʹ17O). Recent work on lacustrine carbonates shows that ∆ʹ17O is sensitive to evaporation, but little is known about variation of ∆ʹ17O in soil carbonates or their potential to track evaporation. Here we report isotopic data from modern soil carbonates collected from 11 sites along a transect in the Serengeti Ecosystem where Mean Annual Precipitation (MAP) and Aridity Index (AI = MAP/PET) 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 grasslands. δ18O values vary by ~8‰ along the transect and increase with increased aridity (lower MAP, higher AI). Mean T∆47 from these soil carbonates is 23 ± 4˚C and is within error of mean annual air temperature (MAAT) or 1-2˚C warmer at all sites. T∆47 does not vary significantly across sites or with depth, and soil temperature measurements and modeling suggest that this invariability is likely due to small changes in temperature at the equator. Using T∆47 temperatures, reconstructed soil water δ18O values are up to 6‰ greater than δ18O values of local precipitation and springs, indicating considerable evaporation in the soils. The ∆ʹ17O values of these soil carbonates range from -161 to -111 per meg and decrease with increasing aridity and increasing δ18O. This clear trend in ∆ʹ17O values across an aridity gradient supports the hypothesis that soil water evaporation drives the variance in δ18O of soil carbonate and indicates the promise of using ∆ʹ17O values of soil carbonates to track paleoaridity in eastern Africa.

Geological Society of America Abstracts with Programs. Vol. 52, No. 6

doi: 10.1130/abs/2020AM-353566

© Copyright 2020 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.