Abstract
Abstract. 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 Voëlvlei, 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 Voëlvlei 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 Voëlvlei record with other regional studies suggests a coherent trend in the overall moisture evolution along the southern Cape coast during the past 8500 years.
Highlights
A record-breaking drought occurred in South Africa from 2015 to 2017, and future climate projections predict increasing drought for large parts of the country (Engelbrecht and Engelbrecht, 2016; Engelbrecht et al, 2011)
Three charcoal and seven n-alkane samples were analysed with the Mini Carbon Dating System (MICADAS) accelerator mass spectrometry (AMS) coupled to an element analyser (Ruff et al, 2010; Salazar et al, 2015; Szidat et al, 2014) at the LARA AMS Laboratory, University of Bern, Switzerland. 14C results from the LARA AMS were reported as F14C and corrected for cross-contamination and constant contamination after Salazar et al (2015)
As we cannot calculate the precise contribution of terrestrial organic carbon, these 14C ages were excluded from the age model in a second modelling iteration (Heaton et al, 2020; Hogg et al, 2020)
Summary
A record-breaking drought occurred in South Africa from 2015 to 2017, and future climate projections predict increasing drought for large parts of the country (Engelbrecht and Engelbrecht, 2016; Engelbrecht et al, 2011). Southern Africa’s past and present climate has been driven by complex interactions between two major oceanic and atmospheric circulation systems, i.e. the Benguela and Agulhas currents, and the westerlies and easterlies (Tyson and Preston-Whyte, 2000) (Fig. 1a). While the eastern and central parts of the country receive most rainfall (> 66 %) from tropical moisture-bearing atmospheric circulation systems during austral summer (summer rainfall zone, SRZ), a narrow belt along the west coast receives most rainfall (> 66 %) from temperate westerlies during the austral winter (winter rainfall zone, WRZ) (Fig. 1a). An intermediary area between the SRZ and WRZ receives rainfall from both systems throughout the year (year-round rainfall zone, YRZ) (Fig. 1a) (Engelbrecht et al, 2015; Scott and Lee-Thorp, 2004), and this includes the southern Cape coast, which is the focus area for this study
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