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BioArchive Tswaing Crater

This project is part of the joint research initiative of the German and South African Earth science communities Inkaba yeAfrica.

Today the African climate system is significantly controlled by atmospheric key elements like monsoon (in NE Africa), El-Nino-Southern Oscillation (SE and S Africa) as they both exert control on the seasonal precipitation regime. To what extend the Antarctic Vortex plays its role is not clear yet. What are the links between atmospheric circulation and the oceans? A warmer tropical Indian Ocean is often associated with dry conditions over S Africa and wet conditions over E Africa. Modelling studies testing the sensitivity to sea surface temperature anomalies confirm that moisture convergence and rainfall over S Africa is indeed reduced during Indian Ocean warm events. As one of the few long and continuous terrestrial records in southern Africa, the sediments from the Tswaing Crater can contribute reliable evidence for global climate change over the past ~200,000 years.

With a multi-proxy approach including TIC, TOC, TN, XRF-scanning, organic petrology, Rock-Eval pyrolysis, biomarker and isotope analyses, reflecting carbonate production, detrital input, salinity and in-situ (algal and bacterial) bioproductivity, we traced changes in water column stratification and rainfall rates during the past 70 kyrs.

The biomarker characteristics and their specific δ13C signal from the present-day lake-water body and the adjacent vegetation are used for comparison to highlight environmental changes in the past as recorded in biomarkers of the sediments. Characteristics of typical organisms living today in the Tswaing crater ecosystem (e.g. higher-plants versus cyano-bacteria) are displayed in Figure 2.

The modern carbon cycle of the terrestrial and aquatic Tswaing biota is mainly controlled by C3-, C4-plants and aquatic micro-organisms. Diploptene, a marker for bacterial activity, shows a distinctly different isotopic composition than other marker molecules. The low δ13CDiplo- ratios indicate that activities of methanotrophic micro-organisms play a role at the sediment/water interface.

The relative abundance and δ13C-ratios of selected biomarkers indicate several changes in the Tswaing crater ecosystem between ~14,000 to 2,000 yr BP (see figure 3): From ~14.000 to 13,000 years BP (end of Termination I) and at the beginning of the Holocene (~11,000 years BP), the lake water body was well ventilated, as indicated by reduced organic matter preservation. These periods reflect moist and possibly cooler conditions while in between and from ~10,000 and 8,000 yr BP the climate was dry and organic matter was preserved in a stagnant water column. In the earlier Holocene, arid conditions reduced grass and probably also macrophyte densities along the shore. As a follow-up the nutrient supply to the lake decreased and aquatic productivity was diminished. At the end of this arid period the aquatic productivity increased again and reached a maximum ~5,500 yr BP. Especially bacterial activity is growing after ~8,000 yr BP and methanotrophic micro-organisms become an important constituent of the bacterial lake community. We propose that during the more arid conditions of the earlier Holocene the ITCZ has shifted northward, an interpretation, which is corroborated by other African climate studies.

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