Holocene palaeoclimatic variations as recorded offshore south-western South Africa

The BMBF funded project RAIN (Regional Archives for Integrated iNvestigations) attempts to reconstruct the complex climate history of southern Africa at the continental – marine interface. The unique positions of cores GeoB 8331-4 and GeoB18308-1 offshore of the Oranje and Gouritz Rivers facilitate the simultaneous analysis of climate signals from two different climate zones. On the one hand, the sediment influx from these river mouths is driven by precipitation changes in the source region; the east African summer rainfall zone (Compton and Maake 2007) driven by shifts in the ICTZ. On the other hand, variations in local riverine input and marine productivity reflect changes in the coastal oceanic and atmospheric circulation systems such as the Southern Hemispheric Westerlies, the Agulhas and the Benguela currents. We aim to capture the various signals in the sediment and attribute them to source regions and driving factors.

Here we present first insights into variations in the carbonate, organic and clastic sediment components provided by initial XRF and infrared spectrometry analysis. According to lipid biomarker analysis the organic material at the core sites are often of terrestrial origin. Furthermore, diatom analyses reveal that a substantial part of the biogenic silica may be associated to terrestrial diatom species and phytoliths. The organic sediment content reflected by the biogenic silica and total organic carbon concentrations and the Br/Ti ratio, is thus assumed to be a terrestrial signal. In core GeoB8331-4. It shows a decreasing trend throughout the Holocene which may reflect a decrease in vegetation in either the east African Oranje River catchment area or the catchment area of local west coast rivers such as the Holgat River. This trend is mirrored in the Ti/Rb ratio as the main source of clastic material at the core site is fluvial. A lower fluvial transport capacity during a drier late Holocene would have reduced the terrigenous input to core site. The carbonate sediment content recorded in the Ca/Ti ratio and the total inorganic carbon concentration reveals an opposing, increasing trend since the early Holocene. This may be driven by dilution/dissolution effects from an increased fluvial/organic input during the mid-early Holocene.

In concordance with this study, wetter than present conditions have previously been suggested for the early-mid Holocene (11-6ka) in the eastern South African summer rainfall zone (e.g. Dupont et al. 2011). This may have produced the strong early-mid Holocene terrestrial/fluvial signal at the core site, as the rainfall and vegetation would have been increased in the eastern Oranje River catchment. Furthermore, it has been suggested that the Southern Hemispheric Westerlies were shifted to the north in the aftermaths of the last glacial, producing heavier winter rainfalls along the western South African coast (Chase and Meadows, 2007). This could have led to an increased terrestrial input from local rivers to the sediment site in the early-mid Holocene. Core GeoB 18308-1 captures the regional climate signal of the transitional climate zone in the Southern Cape area. Here, increases in terrigenous material as opposed to marine input (Ca/Ti ratio) and predominantly terrestrial organic matter characterize warm/wet periods such as the Medieval Warm Period around AD 1100 to 1200 (Tyson et al. 2000).