Holocene sea level and environmental change at the southern Cape – an 8.5?kyr multi-proxy paleoclimate record from Lake Vo?lvlei, South Africa

摘要

South Africa is a key region to reconstruct and understand past changes in atmospheric circulation, i.e. temperate westerlies and tropical easterlies. However, due to the scarcity of natural archives, South Africa's environmental evolution during the late Quaternary remains highly debated. Many available sediment archives are peri-coastal lakes and wetlands; however, the paleoenvironmental signals in these archives are often overprinted by sea-level changes during the Holocene. This study presents a new record from the coastal wetland Vo?lvlei, which is situated in the year-round rainfall zone of South Africa on the southern Cape coast. It presents an ideal sedimentary archive to investigate both sea level and environmental changes. A 13?m long sediment core was retrieved and analysed using a multi-proxy approach. The chronology reveals a basal age of 8440? + 200 / - 250 ?cal?BP. Paleoecological and elemental analyses indicate marine incursions from ca. 8440 to ca.?7000?cal?BP with a salinity optimum occurring at 7090? + 170 / - 200 ?cal?BP. At ca.?6000?cal?BP, the basin of Vo?lvlei was in-filled with sediment resulting in an intermittent (sporadically desiccated) freshwater lake similar to present. In contrast to previous investigations which used indirect proxies for hydrological reconstructions, here we apply a combined biomarker–sedimentological approach that allows the potential identification of precipitation sources, in combination with relative estimates of moisture availability. Increasing moisture is observed throughout the record starting from 8440 + 200 / - 250 ?cal?BP with contributions from both westerlies and easterlies from ca. 8440 to ca. 7070?cal?BP. Westerly-derived rainfall dominates from ca.?7070 to ca. 6420?cal?BP followed by a distinct shift to an easterly dominance at ca. 6420?cal?BP. An overall trend to westerly dominance lasting until ca. 2060?cal?BP is followed by a trend towards an easterly dominance to the present, but both phases show several intense, short-term variations. These variations are also evident in other regional studies, highlighting that the source and seasonality of precipitation has varied distinctly on the southern Cape during the Holocene. Comparison of the Vo?lvlei record with other regional studies suggests a coherent trend in the overall moisture evolution along the southern Cape coast during the past 8500?years.