Rainfall in the Negev Desert during the Middle Holocene, Based on13C of Organic Matter in Land Snail Shells

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Abstract
Analysis of stable carbon isotope ratios (13C/12C) of organic matter in land snail shells is used to infer middle Holocene rainfall amounts in the Negev Desert by reconstructing the distribution of C4plants in the family Chenopodiaceae. The organics are derived from the diet of the snails, which consists of plant material, and are enriched in13C where C4plants are present. A survey of modern plant communities indicates that in areas receiving ≥300 mm mean annual rainfall, nearly all plant communities consist of C3species only (no C4chenopodes), whereas in areas under ≤230 mm rainfall, most plant communities contain one or more C4chenopode species. In between is a transition zone consisting of a mosaic of both pure C3and mixed C3+ C4plant communities. Isotopic results for fossil land snails indicate a consistent geographic pattern throughout the middle Holocene, from ca. 6500 to 3000 yr B.P., with the transition zone located ca. 20 km south of its present position. This implies a near doubling of rainfall within this region as compared to present.