Paper No. 1
Presentation Time: 8:05 AM
STABLE ISOTOPE COMPOSITION OF LAND SNAILS FROM AN ~8KA B.P. ARCHAEOLOGICAL SITE IN EASTERN ALGERIA
Early Holocene aragonitic shells of the land snail Helix melanostoma ranging in age from 7280 ± 115 to 9590 ± 155 B.P. (14C years) were collected in the 1970s from Aïn Misteheyia, a Capsian escargotière in northeastern Algeria (Lubell et al., 1976). Measured d18O values of these snail shells ranged from -2.9 to 0.0 (PDB), whereas d13C values ranged from -9.9 to -5.8 (PDB). Live specimens of H. melanostoma (collected in 1973) had a d18O value of 0.8 and a d13C value of -10.2. The d18O values of the ancient shells exhibit an apparent local minimum which may correspond to a decrease in d18O observed around 8.2ka B.P. in Greenland ice cores. Average d18O values (-4.9) of winter precipitation from an Algerian IAEA site were combined with the oxygen isotope flux balance model of Balakrishnan and Yapp (2004) to calculate expected values of snail shell d18O as a function of relative humidity at 14°C (modern winter temperature) and 9°C (a hypothetical, cooler winter). Similar calculations were performed using a more negative hypothetical precipitation d18O value of -6.9. Comparison of predicted shell d18O values with measured values suggests that a decrease of shell d18O from 0.0 to -2.9 could be associated with an increase in relative humidity of ~0.05 depending on changes in d18O of winter precipitation and/or temperature. The d13C values of the ancient shells are consistent with a diet of predominantly C3 organic matter. Although there is no obvious relationship between d13C and d18O in the present data set, the d18O indications of a North African climatic perturbation at ~8ka B.P. are consistent with other proxy indicators (palynological, limnological, geomorphological, archaeological). Such findings have implications for the development of human cultures in the Maghreb.
M. Balakrishnan and C.J. Yapp (2004) Geochim. Cosmochim. Acta 68, 2007-2024.
D. Lubell, F.A. Hassan, A. Gautier & J.-L. Ballais (1976) Science 191: 910-920.