RECONSTRUCTING ARIDITY AND EVAPORATION IN EAST AFRICA USING TRIPLE OXYGEN ISOTOPES IN SOIL CARBONATES

Paleosols are part of the standard toolkit for studying paleoenvironments at and near paleoanthropological sites throughout Africa. The majority of the studies focus on reconstructing paleo-vegetation (C₃ vs. C₄ plants) using δ¹³C values from paleosol organic matter and paleosol carbonates. In the past 25 years, paleopedology has expanded to include proxies for precipitation and temperature. However, calibration of many of the paleosol-based proxies are based on North American soils, which limits their utility for reconstructing environments in tropical Africa. We have started to address this issue by sampling modern soils from Serengeti National Park, which is close to many paleoanthropological sites and often used as an analog for the environments in which early humans lived. Eleven Serengeti soil trenches were described and sampled across a SE-NW transect that follows a precipitation gradient of 500 to 1000 mm yr^-1. Here, we present a portion of this work and focus on the isotopic composition of carbonates from these trenches. We aim to improve our understanding of δ¹⁸O values of soil carbonates from modern and ancient systems and to explore the potential to reconstruct aridity via a combination of clumped (Δ₄₇) and triple oxygen isotope (Δ¹⁷O) measurements in soil carbonates. Preliminary results from modern Serengeti soils indicate that δ¹³C_sc ranges from -2.6 to +1.8‰_VPDB and δ¹⁸O_sc ranges from -5.0 to +4.3‰_VPDB. Δ₄₇ temperatures from soil carbonates for all sites average 23°C (±5°C), similar to modern mean annual temperatures in the Serengeti, which ranges from 19°C in the SE to 23°C in the NE. Using the Δ₄₇ temperatures and δ¹⁸O_sc values, reconstructed soil water δ¹⁸O values (δ¹⁸O_sw) are up to 8‰ higher than δ¹⁸O values of local meteoric water. Such elevated δ¹⁸O_sw values may be a response to soil water evaporation and aridity but we cannot know this with δ¹⁸O_sc and Δ₄₇ values alone. Our preliminary measurements of Δ¹⁷O values of these carbonates suggest that the soil water is evaporated relative to local meteoric waters. These data suggest that Δ¹⁷O may be a valuable addition to the paleopedology toolkit for reconstructing paleo-aridity at paleoanthropological sites in East Africa and other long-term records of δ¹⁸O_sc values.