U-series dating of terrestrial carbonates for the chronology of human evolution and Quaternary climate variability in southern Africa.

Abstract:

South Africa preserves a rich terrestrial carbonate record of palaeoclimates over long timescales (up to 3 Ma) to investigate questions changes in hydroclimate and their relation to the global climate system during the Quaternary. All of the carbonates under consideration here are from palaeoanthropological or archaeological sites, meaning that there is a direct link between the palaeoclimate archive and local record of human evolution. The U-Pb chronometer opens up the prospect of dating carbonates older than 500 ka, making investigations over longer timescales achievable. This technique has reached a level of maturity where it can be applied both routinely and precisely and there is a considerable collection of cave carbonates already U-Pb dated from South Africa. These ages represent the first direct chronology for the early hominin bearing caves in South Africa, and provide a record of repeated wet and dry phases from 3 Ma to 1 Ma. Such old speleothems have under gone varying degrees of diagenesis, meaning that thin section petrography and fabric analysis is imperative to prior to interpreting palaeoclimate proxies, such as stable light isotopes. Trace elements appear to be more faithful recorders of what is likely variation in drip water composition, in turn driven by changing external conditions. Laser ablation data reveals incredibly detailed trace element records, hinting at very high-resolution records. While not yet routine, U-Th dating of pedogenic, lacustrine and other terrestrial carbonates is showing great promise, with early indications of closed system behaviour on a small (mm) scale. Careful thin section petrography is a necessary precursor to any dating analyses, aided by laser ablation trace element mapping to select domains of low Th and high U for dating. There is a rich archaeological record of the emergence of early modern humans in South Africa; the focus of almost all this research has been along the southern coast. New excavations of arid, inland sites in the southern Kalahari basin have revealed well preserved Middle Stone Age deposits, which are coeval with massive relict tufa deposits. Tufa deposition is punctuated over the last 120 ka and provides up to 10 ka long high-resolution records of local hydroclimate variability.