Holocene climate variability in the winter rainfall zone of South Africa
1Department of Earth Science, University of California, Santa Barbara, CA 93106-9630, USA
2NIOZ, Royal Netherlands Institute for Sea Research, Texel, The Netherlands
3MARUM, Center for Marine Environmental Sciences, University of Bremen, Bremen, Germany
4Institute of Geosciences, University of Kiel, Kiel, Germany
5Department of Geosciences, University of Tübingen, Tübingen, Germany
Abstract. We established a multi-proxy time series comprising analyses of major elements in bulk sediments, Sr and Nd isotopes, grain size of terrigenous fraction, and δ18O and δ13C in tests of Neogloboquadrina pachyderma (sinistral) from a marine sediment sequence recovered off the Orange River. The records reveal coherent patterns of variability that reflect changes in wind strength, precipitation over the river catchments, and upwelling of cold and nutrient-rich coastal waters off Western South Africa. The wettest episode of the Holocene in the Winter Rainfall Zone (WRZ) of South Africa occurred during the "Little Ice Age" (700–100 yr BP). Wet phases were accompanied by strengthened coastal water upwellings, a decrease of Agulhas water leakage into the Southern Atlantic, and a reduced dust incursion over Antarctica. A continuous aridification trend in the WRZ and a weakening of the Southern Benguela Upwelling System (BUS) between 9000 and 5500 yr BP parallel with evidence of a poleward shift of the austral mid-latitude westerlies and an enhanced leakage of warm Agulhas water into the Southeastern Atlantic. The temporal relationship between precipitation changes in the WRZ, the thermal state of the coastal surface water, and variation of dust incursion over Antarctica suggests a causal link that most likely was related to latitudinal shifts of the Southern Hemisphere westerlies and changes in the amount of Agulhas water leakage into the Southern BUS. Our results of the mid-Holocene time interval may serve as an analogue to a possible long-term consequence of the current and future southward shift of the westerlies that may result in a decline of rainfall over Southwest Africa and a weakened upwelling with implication for phytoplankton productivity and fish stocks. Furthermore, warming of the coastal surface water as a result of warm Agulhas water incursion into the Southern BUS may affect coastal fog formation that is critical as moisture source for the endemic flora of the Namaqualand.