SPATIOTEMPORALVARIABILITY IN ΔR IN THE NORTHERN GULF OF MEXICO, USA

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 (Delta R) 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 Delta R 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 C-14 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.R value of -2 +/- 53 C-14 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 C-14 dating due to the highly variable C-14 content observed within and among specimens. Though subregional variability in inputs of C-14-depleted waters is likely, life-history factors related to ontogenetic niche and/or habitat shifts appear to be a major influence in shell C-14 for S. alatus.