Archaeological herbivore δ13C and δ34S provide a marker for saltmarsh use and new insights into the process of 15N-enrichment in coastal plants

Abstract

Saltmarshes are tremendously important culturally and ecologically because they can provide abundant nutrient-rich fodder for grazing livestock, with relatively little resource investment. They also perform critical ecological services, including detoxifying water and stabilizing coastlines. For these reasons, methods for investigating the nature and extent of past saltmarsh use have significant potential to provide new insights into patterns in coastal adaptations among past agricultural societies as well as paleoenvironmental change. Stable nitrogen isotope (δ15N) analyses of animal bone collagen have been used to investigate ancient saltmarsh use based on the idea that plants growing in saline and/or coastal regions can have higher δ15N values. However, interpretation of δ15N evidence for saltmarsh use can be challenging because variation in terrestrial nitrogen sources and cycling can result in similarly elevated in δ15N values in animals that did not feed in saltmarshes. Ecological studies show that stable carbon (δ13C) and sulfur (δ34S) isotope analyses are well suited for investigating saltmarsh use because key species (particularly those in the genus Spartina) that dominate these ecosystems in many areas of the world are C4 plants capable of incorporating sulfuide-derived, 34S-depleted sulfur. We investigate the utility of δ13C, δ34S, and δ15N for tracking saltmarsh use in archaeological herbivores (n = 80) from seventeenth-to-eighteenth-century Acadian settlements in Canada, which were renowned for their agricultural use of saltmarsh ecosystems. Results show extreme variation in δ13C and δ34S consistent with feeding along C3-to-C4 and sulfate-to-sulfide-derived sulfur continuums. Significant correlations between δ13C and δ34S suggests that variation in these isotopic compositions reflects the relative importance of Spartina in past Acadian animal husbandry. These findings indicate that, even in areas where other 13C-eniched food sources are available (e.g., seaweeds, maize), combined δ13C and δ34S analyses should be able to identify Spartina use in coastal regions. A strong negative correlation was also found between δ34S and δ15N, suggesting that processes driving coastal sulfur and nitrogen cycles in the region are coupled (independently of Spartina consumption) and provide new insights into the mechanism behind 15N enrichment observed in saltmarsh plants