Reconstruction of ancient environments using stable isotope analysis of archaeological charcoal from Sibudu Cave, KwaZulu-Natal

• Main Author: Hall, Grant
• Format: Others
• Language: en
• Published: 2010
• Online Access: http://hdl.handle.net/10539/8330

The stable carbon isotopic analysis of archaeological charcoal has the potential to provide an archive of environmental change during the Middle Stone Age occupation of Sibudu Cave, KwaZulu-Natal, South Africa. A wide array of evidence from the site suggests that profound environmental and cultural changes took place in the region through the past 75ka. The aim of this project is to develop a methodology to obtain such isotopic evidence and to test the validity of this evidence through comparison with additional proxy environmental data from the site. Analogue data from modern tree species, Mimusops caffra, Podocarpus latifolius and P. falcatus provide an absolute annual chronological isotopic record of environmental response to prevailing climatic conditions. High precision radiocarbon dating of growth rings demonstrates that these species produce annual growth rings. Wholewood samples provide an accurate record of climatic conditions. Direct comparison of δ13C value of M. caffra from northern KwaZulu-Natal with the historic climatic record, shows a strong inter-annual response to rainfall variability. Signal processing approaches allow the rainfall response to be separated from long-term anthropogenic influences. The δ13C values of P. latifolius trunk cores and corresponding branch samples, from KwaZulu-Natal, preserve the same environmental record, correlating with humidity and temperature data from the region. It is thus possible to obtain a record of past climatic conditions from the growth rings of branches. A P. falcatus disc from the Baviaans Kloof (Eastern Cape) provides a δ13C time series for a moisture-restricted environment, responding to annual rainfall variation and provides a contrasting analogue to the KwaZulu-Natal trees. Branch samples from P. latifolius, representative of the size class of wood fuel likely utilised by MSA inhabitants of Sibudu Cave, were combusted under oxidising and reducing conditions. Their respective δ13C values are more negative with respect to the source material, but reflect the same response to prevailing climatic conditions. An experiment determined the range of isotopic variability in products released during various stages of combustion. The results indicate that combustion temperature has a significant affect on the carbon isotope signature of the various products released during combustion, but the isotopic composition of the remaining wood tissue remains relatively constant, preserving seasonal and inter-annual isotopic trends. These modern analyses indicate that it is possible to obtain an isotopic record of past climatic conditions from archaeological charcoal. The isotopic variability of archaeological Podocarpus and Celtis charcoal, from the MSA layers of Sibudu Cave reflect past environments. During the Howiesons Poort (65ka-62ka) the δ13C charcoal data indicate a cool, humid forested environment predominated. δ13C data from the post-Howiesons Poort (~58ka) and late MSA (~48ka) occupations show that conditions shifted to a dry, open grassland/woodland mosaic community with remnants of riverine forest. These interpretations were validated through comparison with additional faunal, botanical and sedimentological proxy data from Sibudu. The environmental record from Sibudu Cave and palaeoenvironmental proxy data from seven South African MSA sites provide evidence for the local manifestation of large scale climate events between 70ka and 50ka. During Marine Isotope Stage (MIS) 4 it appears that southern Africa experienced conditions similar to those during the Last Glacial Maximum. At Sibudu the environment changed from a predominantly forested community to more open grass/wood land mosaic. Such environmental change is thought to be due to a weakening of the Agulhas Current and eastward shift of the Agulhas Retroflection resulting in lower sea surface temperatures and a corresponding decrease in humidity and rainfall. Adverse environmental conditions ultimately led to Sibudu being abandoned from about 37ka until approximately 1000 BP.