Estimating the hydrogen isotopic composition of past precipitation using leaf-waxes from western Africa

Abstract

The hydrogen isotopic composition of plant leaf-wax n-alkanes (δDwax) is a novel proxy for estimating δD of past precipitation (δDp). However, vegetation life-form and relative humidity exert secondary effects on δDwax, preventing quantitative estimates of past δDp. Here, we present an approach for removing the effect of vegetation-type and relative humidity from δDwax and thus for directly estimating past δDp. We test this approach on modern day (late Holocene; 0–3 ka) sediments from a transect of 9 marine cores spanning 21°N–23°S off the western coast of Africa. We estimate vegetation type (C3 tree versus C4 grass) using δ13C of leaf-wax n-alkanes and correct δDwax for vegetation-type with previously-derived apparent fractionation factors for each vegetation type. Late Holocene vegetation-corrected δDwax (δDvc) displays a good fit with modern-day δDp, suggesting that the effects of vegetation type and relative humidity have both been removed and thus that δDvc is a good estimate of δDp. We find that the magnitude of the effect of C3 tree – C4 grass changes on δDwax is small compared to δDp changes. We go on to estimate δDvc for the mid-Holocene (6–8 ka), the Last Glacial Maximum (LGM; 19–23 ka) and Heinrich Stadial 1 (HS1; 16–18.5 ka). In terms of past hydrological changes, our leaf-wax based estimates of δDp mostly reflect changes in wet season intensity, which is complementary to estimates of wet season length based on leaf-wax δ13C.