n-Alkane parameters from a deep sea sediment transect off southwest Africa reflect continental vegetation and climate conditions

An isobathic transect of marine surface sediments from 1 degrees N to 28 degrees S off southwest Africa was used to further evaluate the potential of the chain length distribution and carbon stable isotope composition of higher plant n-alkanes as proxies for continental vegetation and climate conditions. We found a strong increase in the n-C29-33 weighted mean average delta C-13 values from -33 parts per thousand near the equator to around -26 parts per thousand further south. Additionally, C25-35 n-alkanes reveal a southward trend of increasing average chain length from 30.0 to 30.5. The data reflect the changing contribution of plants employing different photosynthetic pathways (C-3 and C-4) and/or being differently influenced by the environmental conditions of their habitat. The C-4 plant proportions calculated from the data (ca. 20% for rivers draining the rainforest, to ca. 70% at higher latitude) correspond to the C-4 plant abundance in continental catchment areas postulated by considering prevailing wind systems and river outflows. Furthermore, the C-4 plant contribution to the sediments correlates with the mean annual precipitation and aridity at selected continental locations in the postulated catchment areas, suggesting that the C-4 plant fraction in marine sediments can be used to assess these environmental parameters. (C) 2012 Elsevier Ltd. All rights reserved.