The stable hydrogen isotopic composition of sedimentary plant waxes as quantitative proxy for rainfall in the West African Sahel

Various studies have demonstrated that the stable hydrogen isotopic composition (delta D) of terrestrial leaf waxes tracks that of precipitation (delta D-precip) both spatially across climate gradients and over a range of different timescales. Yet, reconstructed estimates of delta D-precip and corresponding rainfall typically remain largely qualitative, due mainly to uncertainties in plant ecosystem net fractionation, relative humidity, and the stability of the amount effect through time. Here we present delta D values of the C-31 n-alkane (delta D-wax) from a marine sediment core offshore the Northwest (NW) African Sahel covering the past 100 years and overlapping with the instrumental record of rainfall. We use this record to investigate whether accurate, quantitative estimates of past rainfall can be derived from our delta D-wax time series. We infer the composition of vegetation (C-3/C-4) within the continental catchment area by analysis of the stable carbon isotopic composition of the same compounds (delta C-13(wax)), calculated a net ecosystem fractionation factor, and corrected the delta D-wax time series accordingly to derive delta D-precip. Using the present-day relationship between delta D-precip and the amount of precipitation in the tropics, we derive quantitative estimates of past precipitation amounts. Our data show that (a) vegetation composition can be inferred from delta C-13(wax), (b) the calculated net ecosystem fractionation represents a reasonable estimate, and (c) estimated total amounts of rainfall based on delta D-wax correspond to instrumental records of rainfall. Our study has important implications for future studies aiming to reconstruct rainfall based on delta D-wax; the combined data presented here demonstrate that it is feasible to infer absolute rainfall amounts from sedimentary delta D-wax in tandem with delta C-13(wax) in specific depositional settings. (C) 2016 Elsevier Ltd. All rights reserved.