Radiocarbon in Marsh Periwinkle (Littorina Irrorata) Conchiolin: Applications for Archaeology

Abstract

In coastal and island archaeology, carbonate mollusk shells are often among the most abundant materials available for radiocarbon (14C) dating. The marsh periwinkle (Littorina irrorata) is one of these such species, ubiquitously found along the Atlantic and Gulf coasts of the United States in both modern and archaeological contexts. This paper presents a novel approach to dating estuarine mollusks where rather than attempting to characterize the size and variability of reservoir effects to “correct” shell carbonate dates, we describe a compound-specific approach that isolates conchiolin, the organic matter bound with the shell matrix of the L. irrorata. Conchiolin typically constitutes <5% of shell weight. In L. irrorata, it is derived from the snail’s terrestrial diet and is thus not strongly influenced by marine, hardwater, or other carbon reservoir effects. We compare the carbon isotopes (δ13C and Δ14C) of L. irrorata shell carbonate, conchiolin, and bulk soft tissue from six modern, live-collected specimens from Apalachicola Bay, Florida, with samples that represent possible sources of carbon within their environment including surface sediments, marsh plant tissues, and dissolved inorganic carbon (DIC) in water. Ultimately, this paper demonstrates that samples obtained from wet chemical oxidation of L. irrorata conchiolin produces accurate 14C dates.

COMPARING DIRECT CARBONATE AND STANDARD GRAPHITE 14C DETERMINATIONS OF BIOGENIC CARBONATES

“…The presence/absence of the acid dissolution step is a key difference between two methods and might have interesting implications regarding the sources of carbon measured by the two methods. Various studies suggest that mollusk shells (and other biominerals) contain a few tenths of a percent up to 5% by mass organic material, or "conchiolin" (Fremy 1855), which is an integral structural component within the biomineral (Galstoff 1964;Keith et al 1993;Cuif et al 2004;Zhang and Zhang 2006;Hadden et al 2019). It is unlikely that the phosphoric acid dissolution of a biomineral during standard graphite 14 C processing oxidizes organic carbon to gaseous CO 2 .…”

“…It is unlikely that the phosphoric acid dissolution of a biomineral during standard graphite 14 C processing oxidizes organic carbon to gaseous CO 2 . Converting the residual acid insoluble organics to CO 2 requires the use of a strong oxidizer like sodium persulfate (e.g., Mills and Quinn 1979;Hadden et al 2019) or additional purification and combustion (e.g., Hadden et al 2018). However, we are aware that the Cs sputtering of carbonate powder could liberate negative carbon ions from a shell's organic fraction.…”

“…Thus, we suspect that conchiolin-bound carbon could be an additional source of carbon present in direct carbonate 14 C measurements. Previous studies have Comparing Graphite 14 C Determinations 391 shown that paired shell and conchiolin 14 C ages (or 14 C activities) are similar (e.g., Berger et al 1965;Burliegh 1983;Haynes and Mead 1987;Hadden et al 2019). In some environments, however, organisms may preferentially incorporate significant amounts of 14 C-dead carbon in their conchiolin that is not present in their soft tissues or shell carbonate (Masters and Bada 1977;Hadden et al 2018).…”