A New Radiocarbon Pretreatment Method for Molluscan Shell Using Density Fractionation of Carbonates in Bromoform

Abstract

ABSTRACT. Coastal archaeological sites that lack organic remains for radiocarbon dating are often abundant sources of molluscan shells. As a substitute for materials such as bone and charcoal, shells can be analyzed with 14 C dating to determine a site's age. Despite their being convenient, non-mobile archaeological artifacts, molluscan shells are plagued by several issues, including carbonate remodeling, in which aragonite in shells is converted to calcite as predicted by thermodynamics. We present here a carbonate density separation technique that addresses the issue of carbonate remodeling. Using a density fractionation with bromoform, aragonite concentrations are enriched in shells that have undergone significant remodeling. The technique has been applied to archaeological shells and has returned dates that are younger than those previously determined for the same shells.

DATING THACH LAC: CRYPTIC CaCO3 DIAGENESIS IN ARCHAEOLOGICAL FOOD SHELLS AND IMPLICATIONS FOR 14C

“…Measurement of sequential CO 2 fractions removed by stepped hydrolysis, following the methods of Burr et al (1992) and Yokoyama et al (2000), can be used to investigate the nature and age of secondary recrystallization and help assess the reliability of the final result (see also Lindroos et al [2014] who used similar techniques to date burnt shell in mortar). Treatments that take advantage of differences in density between aragonite and calcite are promising but untested on a wide range of shellfish (Douka et al 2010;Russo et al 2010) and are unlikely to be useful where cryptic recrystallization has occurred.…”

Recrystallization of pedogenic and biogenic carbonates in soil: Environmental controls, modelling and relevance for paleoenvironmental reconstructions and dating

“…Their carbonate fraction represents a useful paleoenvironmental and chronological proxy Pigati, 2013;. The CaCO 3 fraction of shells can be especially useful for such investigations if the preservation of organic compounds is poor, such as in arid environments or coastal regions (Russo et al, 2010;Zazzo and Saliège, 2011). Under such circumstances, shell carbonate can be the only alternative to paleoenvironmental and chronological studies (Chappell and Pollach, 1972;Újvári et al, 2014).…”

“…Mollusk shells are usually well preserved in sediment after burial , but their elemental and/or isotopic composition can be influenced by recrystallization processes Collins, 2012). Recrystallization occurs following soil dryness, increased Ca 2+ concentration and/or a drop in soil CO 2 partial pressure (Chappell and Pollach, 1972;Russo et al, 2010). Since the amount of soil CO 2 and its isotopic composition is in equilibrium with CO 2 respired by roots and rhizosphere organisms, the isotopic signature (δ 13 C, Δ 14 C) of recrystallized carbonate will equilibrate with soil CO 2 .…”

“…Considering the significant effect of modern C on radiocarbon dating, various techniques have been proposed to assure the fidelity of geochemical signals. Evidence of recrystallization can be detected using optical and electron microscopes (Cochran et al, 2010), X-ray analysis (Chappell and Pollach, 1972;Piepenbrink, 1989;Cochran et al, 2010), trace element measurements (Shemesh, 1990;Oliver et al, 1996;Cochran et al, 2010) and density analysis (Russo et al, 2010).…”