Thursday, 7 March 2013

Can clam shells explain the demise of a civilisation? by Prof Melanie Leng

“Clam shells used for food, jewellery and in the wall covering of shelters found at the world famous Çatalhöyük UNESCO archaeological site in Central Turkey between 8-9,000 years ago give a unique insight into the demise of a short lived civilisation”

One of the key archaeological questions at Çatalhöyük is why the site was occupied for such a short amount of time (c. 8,000-9,000 years ago) in human occupation terms. I was invited to join an International team of scientists (from Turkey, Israel, the US and UK) to investigate climate signals from the site, inparticular I was tasked with using the geochemistry of the freshwater clam shells that were found around the site to investigate wettness. It is thought that the clams were collected for food and that the shells were subsequently used as jewellery and in the wall covering of shelters at the site.

We collected dozens of the best preserved clam shells and used a dental drill to take small samples from the shell in a timeline from the juvenile “umbo” part of the shell to the last growth before death (the shell edge). The sample holes represent a few weeks/months of growth within the 1-3 years life span of each shell. We then measured the oxygen and carbon isotope composition of the samples. These data are “wetness indicators” in this environment and the fine scale sampling shows extreme changes in the wetness of the region between winter and summer over the life span of each clam.
Mass spectrometers within the British Geological Survey used to measure the isotope composition of the clam shells

The results show that the winters were extremely wet, the area was probably like a wetland with rivers, streams and ponds, all interconnecting. It would have been a very damp environment especially in early spring when food crops would have been planted. On top of this the data show very dry and arid summers, making the growing season for crops very short. This combination of wet winters and arid summers may have made central Turkey a very difficult place to occupy, especially for ancient civilisations.

Some of the clam shells used to investigate seasonal climate variation at Çatalhöyük. The very fine sampling pits along the growth of the shells provide geochemical evidence for changes in wetness between winters and summers about 9,000 years ago

Overall while we cannot conclusively say why Çatalhöyük was occupied for such a short amount of time, we have clear evidence from the chemistry of the clam shells that growing food would have been difficult due to the extreme variations in wetness over the seasons. Maybe after a few particular bad years the occupants moved on to higher ground where drainage and summer aridity might have been less of an issue.

By Mel Leng
Preliminary findings have been published in Journal of Archaeological Science
A full report of the Çatalhöyük site investigation will be available  later this year:
Bar-Yosef Mayer, D. E., Leng, M. J., Aldridge, D.C., Arrowsmith, C., Gümüş, B.A., Sloane, H.J. 2013. Unio shells from Çatalhöyük: Preliminary palaeoclimatic data from incremental isotopic analyses along the growth axis of the shells. in: I. Hodder (ed.) Humans and landscapes of Çatalhöyük: reports from the 2000-2008 seasons. Çatalhöyük research project Volume 8. British Institute At Ankara. BIAA Monograph Series. 

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