Mid-Holocene rainfall in the Negev Desert from ¹³C of land snail shell organic matter

Abstract

Analysis of the ¹³C/¹²C ratio of organic matter in fossil bones has been used in a number of studies for reconstruction of palaeodiets, especially the presence or absence of plants having a C₄ photosynthetic pathway (enriched in ¹³C relative to C₃ plants)^1,2. This isotope method has also been used to detect organic matter derived from C₄ plants in materials such as sediments^3–5 and desert rock varnishes⁶. In the present study, the ¹³C/¹²C ratio of the organic matrix of fossil land snail shells from the Northern Negev Desert in southern Israel was used to map out the distribution of C₄ shrubs in the past. Because in this region C₄ shrubs are generally restricted to the arid zone (≤ 280 mm mean annual rainfall), shifts in rainfall patterns could be inferred from the changes in plant distributions. Analysis of land snail shells dated to the period ∼2,800–4,000 yr BP showed that during this time the northern limit of C₄ shrubs was shifted 20–30 km south of its present position, implying substantially wetter conditions in the Northern Negev than occur there at present.