SPATIOTEMPORALVARIABILITY IN ΔR IN THE NORTHERN GULF OF MEXICO, USA Carla S Hadden* • Alexander Cherkinsky Center for Applied Isotope Studies, University of Georgia, Athens, GA 30602, USA. ABSTRACT. Strombus alatus and Busycon sinistrum are large marine gastropods that are frequently recovered from archaeological contexts in southeastern North America. We previously proposed a reservoir age offset (ΔR) for B. sinistrum from the northern Gulf of Mexico region based on known-age pre-bomb 20th-century specimens. We also reported significant variability in radiocarbon both among and within S. alatus specimens, which precluded a reliable estimation of ΔR for this taxon. In this paper, we present a complementary data set from archaeological contexts to re-evaluate marine reservoir effects in the northern Gulf Coast region at multiple spatial and temporal scales. The new data set consists of a total of 13 14 C age determinations from well-associated marine (B. sinistrum and S. alatus) and terrestrial (Odocoileus virginianus) samples from a closed context at the Bayou St. John (1BA21) archaeological site. We suggest a slightly updated ΔR value of –2 ± 53 14 C yr for late Holocene-age B. sinistrum from the northern Gulf Coast region. S. alatus, and possibly other species of strombid conchs, are poor candidates for 14 C dating due to the highly variable 14 C content observed within and among specimens. Though subregional variability in inputs of 14 C-depleted waters is likely, life-history factors related to ontogenetic niche and/or habitat shifts appear to be a major influence in shell 14 C for S. alatus. KEYWORDS: reservoir age, marine shell, Gulf of Mexico. INTRODUCTION Marine shell is one of the most abundant organic materials available for radiocarbon dating in coastal regions. Obtaining accurate ages from marine shell is complicated by a variety of factors. Ocean waters are depleted in 14 C relative to the atmosphere, and, as a result, samples of marine origin yield 14 C ages older than do contemporaneous samples of terrestrial origin (Stuiver and Polach 1977; Stuiver et al. 1986). Differences in coastal geomorphology, ocean circulation, upwel- ling, and freshwater inputs produce localized deviations from the global-averaged marine reservoir age. The correction for local offsets, termed ΔR, is estimated by measuring the 14 C content of known-age marine materials, or through comparison of 14 C ages of contemporaneous terrestrial and marine sample pairs (Ascough et al. 2005). Reimer and Reimer (2001) maintain an online database of published ΔR values, accessed via mapping software, which 14 C researchers can access to facilitate calibration of marine-influenced 14 C ages. As a first approximation, ΔR is assumed to be a constant for a given region (Reimer and Reimer 2001), although researchers working in diverse coastal settings have demonstrated considerable variability over both long-term and short-term timescales (Austin et al. 1995; Druffel 1997; Kennett et al. 1997; Deo et al. 2004; Culleton et al. 2006). 14 C researchers have begun to examine marine reservoir effects in southeastern North America only recently (Thomas 2008; Shanks and Byrd 2012; Thomas et al. 2013; Cherkinsky et al. 2014; Hadden and Cherkinsky 2015). These studies suggested that estimates of ΔR in this region must take into account the marine species being studied, as well as variability in local conditions such as source and volume of freshwater inputs. Hadden and Cherkinsky (2015) previously reported 30 14 C dates obtained from modern, pre-bomb marine shells (Busycon sinistrum and Strombus alatus) of known ages collected from northwestern Florida, USA, in order to estimate species-specific ΔR corrections for those taxa. We also presented data on intrashell variability in 14 C content of these short-lived (~10–20 yr) mollusks. In this paper, we present a com- plementary data set of 14 C ages from archaeological specimens from this same region to examine variability in marine reservoir effects along the northern Gulf of Mexico, USA, over both short- and long-term timescales, i.e. years and centuries. *Corresponding author. Email: hadden@uga.edu. Radiocarbon, 2016, p. 1–11 DOI:10.1017/RDC.2016.65 Selected Papers from the 2015 Radiocarbon Conference, Dakar, Senegal, 16–20 November 2015 © 2016 by the Arizona Board of Regents on behalf of the University of Arizona