Reconstruction of ancient environments using stable isotope analysis of archaeological charcoal from Sibudu Cave, KwaZulu-Natal
Date
2010-07-30T09:10:58Z
Authors
Hall, Grant
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Abstract
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.