CARBON ISOTOPES (δ 13 C AND Δ 14 C) IN SHELL CARBONATE, CONCHIOLIN, AND SOFT TISSUES IN EASTERN OYSTER (CRASSOSTREA VIRGINICA) Carla S Hadden 1* • Kathy M Loftis 1 • Alexander Cherkinsky 1 Center for Applied Isotope Studies, University of Georgia, Athens, GA 30602, USA. ABSTRACT. Biogeochemical analyses of eastern oysters (Crassostrea virginica) are frequently included in environ- mental monitoring and paleoecological studies because their shells and soft tissues record environmental and dietary signals. Carbon isotopes in the mineral phase of the shell are derived from ambient bicarbonate and dissolved inorganic carbon (DIC), while organic carbon present in soft tissue is of dietary origin. Mineral-bound organic matter within the carbonate shell matrix (“conchiolin”) is studied less frequently. The purpose of this study was to compare carbon isotope composition (δ 13 C and Δ 14 C) of conchiolin to those of shell carbonates and soft tissues in eastern oysters and assess the extent to which conchiolin can provide insight into paleoecological records. Eleven oyster speci- mens were live-collected from Apalachicola Bay, USA, as well as a set of environmental samples (water, sediment, and terrestrial plants). Overall, the δ 13 C values in all studied oyster tissue types record environmental signals related to carbon sources, with conchiolin being enriched in 13 C by an average of 2.3‰ relative to bulk soft tissues. Δ 14 C values in oyster shell carbonates generally reflect the marine versus riverine source of DIC, while conchiolin Δ 14 C values are impacted by variable relative contributions of young and old organic matter. Environmental samples indicate a significantly large difference in Δ 14 C among sources, from –127‰ in particulate organic matter to approxi- mately +15‰ in DIC. Conchiolin is significantly depleted in 14 C relative to other tissue types, by as much as 56.6‰, posing a major obstacle to the use of conchiolin as an alternative material for radiocarbon dating. KEYWORDS: carbon cycling, environmental reconstruction, marine shell, shell. INTRODUCTION Biogeochemical analyses of mollusks are used frequently in environmental, paleontological, archaeological, and paleoenvironmental studies. Typically, such studies take advantage of the ecological and dietary signals that are recorded in the soft tissues of the animal or the climatic and environmental signals that are recorded in the carbonate mollusk shell. Mineral-bound organic matter within the carbonate shell matrix (“conchiolin”) has, historically, been studied less frequently. The purpose of this study was 1. to compare carbon isotope signatures (δ 13 C and Δ 14 C) in shell carbonate, conchiolin, and soft tissues in eastern oyster (Crassostrea virginica) from Apalachicola Bay, USA; 2. to assess the usefulness of oyster conchiolin as an ecological and dietary proxy analogous to soft tissue; and 3. to evaluate the potential of oyster conchiolin as an alternative material for radiocarbon ( 14 C) dating. Eastern oyster is considered a “keystone species” for estuarine environments along the eastern coast of North America. It has been an important food resource for native peoples in the region for at least the past 8000 years, and has continued to be an important fishery and food source in historical and modern times. As such, oyster shell middens are a potentially powerful environmental and ecological archive that extend over the entire history of human occupation in eastern North America. Shell carbonates are in isotopic equilibrium with seawater. δ 13 C and 14 C in aquatic mollusk shells are typically assumed to reflect the carbon isotopic composition of the dissolved inorganic carbon (DIC) in the surrounding water, although a small proportion of shell carbon is obtained *Corresponding author. Email: hadden@uga.edu. Radiocarbon, 2018, p. 1–13 DOI:10.1017/RDC.2018.27 Selected Papers from the 2nd Radiocarbon in the Environment Conference, Debrecen, Hungary, 3–7 July 2017 © 2018 by the Arizona Board of Regents on behalf of the University of Arizona at https://www.cambridge.org/core/terms. https://doi.org/10.1017/RDC.2018.27 Downloaded from https://www.cambridge.org/core. University of Georgia Libraries, on 27 Apr 2018 at 13:53:26, subject to the Cambridge Core terms of use, available