No abstract

This study reassesses and refines the use of crop carbon stable isotope values (Δ 13 C) to reconstruct past water availability. Triticum turgidum ssp. durum (durum wheat), Hordeum vulgare (six-row barley) and Sorghum bicolor (sorghum) were experimentally grown at three crop research stations in Jordan for up to three years under five different irrigation regimes: 0% (rainfall only), 40%, 80%, 100% and 120% of the crops' optimum water requirements. The results show a large variation in carbon stable isotope values of crops that received similar amounts of water, either as absolute water input or as percentage of crop requirements. We conclude that C 3 crop carbon stable isotope composition should be assessed using a climate zone specific framework. In addition, we argue that interpretation should be done in terms of extremely high values showing an abundance of water versus low values indicating water stress, with values in between these extremes best interpreted in conjunction with other proxy evidence. Carbon stable isotope values of the C 4 crop Sorghum were not found to be useful for the reconstruction of water availability.

Crop irrigation has long been recognized as having been important for the evolution of social complexity in several parts of the world. Structural evidence for water management, as in the form of wells, ditches and dams, is often difficult to interpret and may be a poor indicator of past irrigation that may have had no need for such constructions. It would be of considerable value, therefore, to be able to infer past irrigation directly from archaeo-botanical remains, and especially the type of archaeo-botanical remains that are relatively abundant in the archaeological record, such as phytoliths. Building on the pioneering work of Rosen and Wiener (1994), this paper describes a cropgrowing experiment designed to explore the impact of irrigation on the formation of phytoliths within cereals. If it can be shown that a systemic and consistent relationship exists between phytolith size, structure and the intensity of irrigation, and if various taphonomic and palaeoenvironmental processes can be controlled for, then the presence of past irrigation can feasibly be inferred from the phytoliths recovered from the archaeological record.

Water availability and water management systems were critical for the success of past agricultural societies. One way to determine past water availability is through phytolith ratios 2 as demonstrated by research conducted on modern C3 plants. In order to determine if phytolith ratios in C4 plants are similarly affected by plant water availability, the C4 plant Sorghum bicolor was experimentally grown at three different crop growing stations over a two year period in Jordan. The husks, leaves and stems of the plants grown under the 0% and 100% irrigation regimes were processed and analysed for their sensitive to fixed form phytolith ratio. These results were then compared to results of those conducted using C3 plants. Our results showed that while there were differences in ratios between growing years and the crop growing site, the greatest difference in the ratios was in irrigation regime. Our results also showed, however, that the ratio of sensitive to fixed forms for the samples taken from the husks was far higher than the ratios found in the leaves and stems and far higher than those found in previous studies on C3 grasses. We suggest that if this method is to be used to interpret archaeological phytolith assemblages, an assessment of the likely taxa and plant part composition of the assemblage should first be undertaken through phytolith and macro-botanical analysis.