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.