Biocoenosis response to hydrological variability in Southern Africa during the last 84 ka BP: A study of lipid biomarkers and compound-specific stable carbon and hydrogen isotopes from the hypersaline Lake Tswaing

摘要

Sea surface temperature (SST) changes in the SW Indian and SE South Atlantic Ocean are proposed to have controlled environmental changes at the southern tip of the African continent during the past. For a better understanding of the hydrological variations and their environmental impact during the past 84 ka before present (BP), we analyzed lipid biomarkers and their stable carbon (δ~(13)C) and hydrogen (δD) isotopic compositions in sediments from Lake Tswaing (25°24'30" S, 28°04'59" E). 8D values of long-chain n-alkanes from higher plants varied between -155 and - 82‰ and matched well with SST gradient changes between the tropical and subtropical SW Indian Ocean. Increased SSTs correlate with deuterium-depleted 8D values of n-alkanes and indicate a higher convection transporting more moisture across South Africa in the time from 84 to 80 ka BP, during the marine isotope stage (MIS) 4 and from 50 to 39 ka BP. During these humid periods high amounts of organic matter (up to 9.8% total organic carbon (TOC)) were deposited in Lake Tswaing. Periods with deuterium-enriched 8D values of n-alkanes (76 ka BP, 73 to 69 ka BP, 55 to 51 ka BP, and 31.4 to 12.6 ka BP), indicating dry climate and increased lake water salinity, were associated with lower TOC contents and a higher degradation state of the terrestrial organic matter. We occasionally detected algae-derived C_(37)- and C_(38)-alkenones in the core, particularly in times when lake salinity and alkalinity were elevated. The disappearance of alkenones around 25 ka BP was accompanied by highly depleted δD values of n-alkanes from aquatic organisms (- 180‰ for n-C_(23)) indicating a substantial freshening of the lake water. The vegetation surrounding Lake Tswaing during the last 84 ka BP consisted mainly of savanna-type plants, such as trees and shrubs with varying contributions from C_4 grasses. The latter predominated particularly at the end of MIS 5 and during arid climatic conditions in MIS 3. We show that the changing vegetation controls the n-alcohol to n-alkane ratio of the leaf waxes; this is reflected by the higher plant alkane (HPA) index which has potential as a new climate-controlled vegetation proxy; in semi-arid periods HPA index values were elevated reflecting savanna vegetation which is adapted to drier climate, whereas HPA index values were lower during more humid conditions.