Crop water status from plant stable carbon isotope values: A test case for monsoonal climates

Stable carbon isotope analysis is increasingly used in archaeology as an indicator of crop water status and/or water management regime. While the technique shows promise, robust modern baseline studies are required to inform and validate archaeological interpretations. Here, we test stable carbon isotope values as a crop water status proxy in a monsoonal climatic context for the first time. Specifically, we test the relationship between grain stable carbon isotope values (δ^13C_grain), water availability, irrigation and soil type in barley (Hordeum vulgare L. (Zohary and Hopf.)) in north-west India, with the aim of deriving a locally-appropriate model for isotopic interpretation. We test this relationship across a substantial rainfall gradient (200–1000mm/year) and find a negative logarithmic relationship between climatic water availability and δ^13C. However, there is significant noise in the relationship, and we report δ^13C_grain variation of over 3‰ amongst samples drawn from similar climatic contexts. Soil type, irrigation type and irrigation frequency have no clear modifying effects. We conclude that: (1) barley stable carbon isotope values can act as an archaeological water status proxy in monsoonal areas, but will be most sensitive in areas receiving <450mm rainfall per year; and (2) it is not possible to precisely infer water management regimes. On the basis of our results, we propose guidelines for archaeological barley stable carbon isotope interpretation in north-west India and analogous monsoonal climates.