Environmental controls on the hydrogen isotopic composition of volcanic glass from the Southern Afar rift, eastern Ethiopia

Abstract

Hydrogen stable isotope values of hydrated volcanic glass as a proxy for the isotopic composition of past meteoric waters provide an opportunity for reconstructing past climates. Here we present new hydrogen stable isotope values from 63 individual tuffs from the Afar region in the lower Awash Valley of eastern Ethiopia. The hydrogen isotopic results from volcanic glass spanning the last 6.4 Ma show a wide distribution with values ranging between −89 and − 32 ‰ (VSMOW). The variability is consistent with the observed paleo-depositional setting (i.e., lower values for fluvial settings while higher values are recorded in lacustrine settings). The reconstructed hydrogen isotopic values of parent waters are considerably lower than those of modern meteoric waters, suggesting a bias toward lower values during the hydration of volcanic glass.Reconstructed hydrogen stable isotope values of water derived from volcanic glass differ from other proxies of regional climate in northeast Africa, pointing to the controls of local meteoric waters on the hydrogen isotopic composition of volcanic glass. There is agreement between the reconstructed isotopic composition of parent waters and the lowest hydrogen isotopic values of modern precipitation in the Awash catchment that correspond to periods of large rainfall. This correspondence probably indicates that volcanic glass is preferentially hydrated during the wettest seasons. To test this idea, we compared the isotopic results from volcanic glass with the reconstructed isotopic composition of surface waters from soil carbonates deposited during the Pliocene and Pleistocene in the lower Awash Valley. The comparison reveals a considerable difference between proxies, with volcanic glass recording lower isotopic values than soil carbonates. Evaporative isotopic enrichment of water in shallow (<1 m) soil profiles probably accounts for the elevated values in soil carbonates. In contrast, the presence of smectite-rich vertisols above volcanic deposits appears to retard infiltration of meteoric waters deep into the subsurface. During rainfall events, vertisols swell and prevent all but the largest rainfall events from penetrating deep (>1 m) into the profiles and promoting the hydration of volcanic glass that display systematically lower isotopic values of meteoric waters than the mean annual rainfall.