Hydrogen isotope fractionation of leaf wax n-alkanes in southern African soils

摘要

The hydrogen isotope composition of plant leaf wax (delta D-wax) has been found to record the isotope composition of precipitation (delta D-p). Hence, delta D-wax is increasingly used for palaeohydrological reconstruction. It is, however, also affected by secondary factors, such as vegetation type, evapotranspiration and environmental conditions, complicating its direct application as a quantitative palaeohydrological proxy. Here, we present delta D-wax data from soils along vegetation gradients and climatic transects in southern Africa to investigate the impact of different environmental factors on delta D-wax. We found that delta D-wax correlated significantly with annual delta D-p (obtained from the interpolated Online Isotopes in Precipitation Calculator data set) throughout eastern and central South Africa, where the majority of the mean annual precipitation falls during the summer. We found evidence for the effect of evapotranspiration on delta D-wax, while vegetation change was of minor importance. In contrast, we found that delta D-wax did not correlate with annual delta D-p in western and southwestern South Africa, where most of the annual precipitation falls during winter. Wide microclimatic variability in this topographically variable region, including distinct vegetation communities and high vegetation diversity between biomes as well as a potential influence of summer rain in some locals, likely compromised identification of a clear relationship between delta D-wax and delta D-p in this region. Our findings have implications for palaeoenvironmental investigations using delta D-wax in southern Africa. In the summer rain dominated eastern and central region, delta D-wax should serve well as a qualitative palaeohydrological recorder. In contrast, the processes influencing dDwax in the winter rain-dominated western and southwestern South Africa remain unclear and, pending further analyses, potentially constrain its use as palaeohydrological proxy in this region. (C) 2017 Elsevier Ltd. All rights reserved.