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

The hydrogen isotopic composition of plant leaf-wax n-alkanes (delta D-wax) is a novel proxy for estimating delta D of past precipitation (delta D-p). However, vegetation life-form and relative humidity exert secondary effects on delta D-wax, preventing quantitative estimates of past delta D-p. Here, we present an approach for removing the effect of vegetation-type and relative humidity from delta D-wax and thus for directly estimating past delta D-p. We test this approach on modern day (late Holocene; 0-3 ka) sediments from a transect of 9 marine cores spanning 21 degrees N-23 degrees S off the western coast of Africa. We estimate vegetation type (C-3 tree versus C-4 grass) using delta C-13 of leaf-wax n-alkanes and correct delta D-wax for vegetation-type with previously-derived apparent fractionation factors for each vegetation type. Late Holocene vegetation-corrected delta D-wax (delta D-vc) displays a good fit with modern-day delta D-p, suggesting that the effects of vegetation type and relative humidity have both been removed and thus that delta D-vc is a good estimate of delta D-p. We find that the magnitude of the effect of C-3 tree - C-4 grass changes on delta D-wax is small compared to delta D-p changes. We go on to estimate delta D-vc 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 delta D-p mostly reflect changes in wet season intensity, which is complementary to estimates of wet season length based on leaf-wax delta C-13. (c) 2013 Elsevier Ltd. All rights reserved.