Near-constant apparent hydrogen isotope fractionation between leaf wax n-alkanes and precipitation in tropical regions: Evidence from a marine sediment transect off SW Africa

Abstract

A transect of marine surface sediment samples from 1°N to 28°S off southwest Africa was analysed to verify the application of hydrogen isotope compositions of terrestrial plant-wax n-alkanes preserved in ocean sediments as a proxy for continental hydrological conditions. Conditions on the adjacent continent range from humid evergreen forests to deciduous forests, wood- and shrub land and further to arid grasslands and deserts. The hydrogen isotope values for the dominant n-alkane homologues (C29, C31 and C33) vary from −123‰ to −141‰ VSMOW and correlate with the modelled hydrogen isotope composition of mean annual and growing season precipitation of postulated continental source areas (r up to 0.8, p<0.01). The apparent hydrogen isotope fractionation between alkanes and mean annual precipitation is remarkably uniform (−109‰ on average, σ⩽5‰, n=27). Potentially, effects of aridity on the apparent hydrogen isotope fractionation are concealed by the contribution of different plants (C3 dicotyledons vs C4 grasses). Thus, isotope ratios of leaf wax n-alkanes preserved in ocean margin sediments in these and similar tropical regions may be directly converted to δD ratios of ancient precipitation by employing a constant hydrogen isotope fractionation.