Improved AMS¹⁴C Dating of Shell Carbonates Using High-Precision X-Ray Diffraction and a Novel Density Separation Protocol (Cards)

Abstract

ABSTRACT. One critical variable in the successful application of radiocarbon dating is the effective removal of carbonaceous contaminants. In the case of marine carbonates, contamination appears usually in the form of secondary low-magnesium calcite, the stable polymorph of calcium carbonate and byproduct of the post-mortem recrystallization or replacement of the autochthonous phase, originally in the form of high-magnesium calcite or aragonite. Depending on the nature of the depositional environment, the secondary phase may be contemporary in age with the original shell carbonate and may have even been derived from it by dissolution-recrystallization processes, or can be an exogenous contaminant of younger or older age. The limited ability of current pretreatment protocols to detect and remove the secondary mineralogical phases prior to dating carbonates has been one of the reasons marine shell and coral 14 C determinations are often difficult to validate in terms of their reliability. We have developed a new pretreatment protocol designed to achieve greater reliability and accuracy in the dating of this material. The method entails 2 steps. The first one involves the improved detection and quantification of secondary calcite in aragonite using X-ray diffraction, at a precision of ~0.1% and ~0.8%, respectively. Next, where this is required, a novel density separation step using non-toxic heavy liquids (CarDS) is applied to the diagenetic sample. This enables the clear separation of calcite and aragonite, with only the latter kept for dating. We have applied the new steps, screening and separation, on standard and archaeological examples and our initial results suggest that it is successful and reproducible. In this paper, we describe the method and initial results.

Radiocarbon Dating of Late Pleistocene Marine Shells from the Southern North Sea

“…A second possibility is that biogenic reprecipitation occurred rapidly after mollusk death when conditions were still (nearly) identical to original mollusk living environment (cf. Douka et al 2010). Evidence for rapid recrystallization (within several millennia) is shown by the SEM analysis of our Holocene mollusk species.…”

“…Post-mortem development of inorganic intracrystalline carbonate cement within a shell structure is regarded a natural cause of introducing younger carbon (Mangerud 1972;Aitken 1990;Webb et al 2007;Douka et al 2010). In these studies, recrystallization is essentially described as an inorganic process (only) controlled by the concentrations of both free carbonate and Ca 2+ ions.…”

“…Contamination effects and the resulting apparent young ages have been described for marine shells (Mangerud 1972;Nadeau et al 2001). Carbonate diagenesis is generally regarded as the cause for incorporation of younger 14 C into the crystal structure (Mangerud 1972;Goslar and Pazdur 1985;Aitken 1990;Nadeau et al 2001;Magnani et al 2007;Webb et al 2007;Douka et al 2010;Price et al 2011). …”

Testing of a single grain OSL chronology across the Middle to Upper Palaeolithic transition at Les Cottés (France)

“…Although dating itself cannot answer these questions, it is undeniable that a reliable chronological framework for archaeological industries such as the Aurignacian and the Châtelperronian is paramount in our quest to answer these questions. Over the past two decades significant advances have been made in both 14 C dating of charcoal (e.g., Bird et al, 1999), shell (e.g., Douka et al, 2010) and bone (e.g., Higham et al, 2006) and in optically stimulated luminescence (OSL) dating of sedimentary quartz (e.g., Murray and Wintle, 2000;Jacobs and Roberts, 2007) and feldspar mineral grains (Li and Li, 2011;Buylaert et al, 2012). When applying these two methods to Palaeolithic or Stone Age sites, 14 C dating is restricted to sites younger than its effective upper limit of~50 ka, and OSL dating is more commonly applied to sites older than 50 ka (i.e., where 14 C dating cannot be used).…”

The chronostratigraphy of the Haua Fteah cave (Cyrenaica, northeast Libya) — Optical dating of early human occupation during Marine Isotope Stages 4, 5 and 6

“…The shells were scanned for post-depositional alteration using high-precision X-Ray diffraction (Douka et al, 2010a). No secondary calcite was identified and the samples were considered suitable for dating using the routine protocol of the ORAU, which involves phosphoric acid dissolution, and CO 2 purification prior to graphitization and AMS measurement (Brock et al, 2010).…”