Stable isotope evidence for salt-marsh grazing in the Bronze Age Severn Estuary, UK: implications for palaeodietary analysis at coastal sites

Stable isotope analyses of modern coastal and salt-marsh plant species (‘salt-loving’ plants or halophytes) have demonstrated that these are significantly enriched in 15N compared to other terrestrial plants. Coastal salt-marshes were far more extensive in the past than they are today. They represented a vast and much-exploited resource in many areas of the UK and north-western Europe and were considered to be prime land for the grazing of animal stock.This paper aims to test whether the unusual isotope chemistry of halophytes and other coastal plants allows for the identification of salt-marsh grazing in the past through the stable isotope analysis of herbivore skeletal remains. We present the results of carbon and nitrogen stable isotope analysis of bone collagen of herbivores from the middle and late Bronze Age sites of Brean Down, Redwick and Peterstone in the Severn Estuary, UK. Here, direct archaeological evidence indicates salt-marsh grazing as a deliberate herding strategy.The animal bone isotope data from these three Severn Estuary sites are significantly enriched in 15N in comparison to other Holocene faunal data-sets from the UK. We interpret this as evidence that the isotopic signatures of coastal and salt-marsh plants are passed on through the food chain and conclude that stable isotope analysis of animal bone collagen has great potential for investigating salt-marsh or coastal exploitation in the past. Our results also demonstrate that herbivore stable isotope values may vary significantly between different sites and regions within the UK. They underline the necessity for complementary faunal data-sets when undertaking palaeodietary analyses of human remains.