Diachronic quantification of the local marine reservoir effect (MRE) using Loripes orbiculatus shells from Late Holocene lagoonal deposits at Puntone di Scarlino (Central Tuscany, Italy): Proposed roles of microbial diagenesis and sedimentation rates

With the aim of dating the early salt production at Puntone di Scarlino (Central Tuscany, Italy) and establishing the environmental history of this coastal site, a sediment core was studied, taken from the lagoon next to the archaeological site. Diachronic radiocarbon dating of terrestrial plant macro remains and Loripes orbiculatus (Poli, 1795) shells, a burrowing lucinid bivalve occurring throughout the sediment cored, revealed a Marine Reservoir Effect (MRE) that varied markedly over time. Between ca. 4000 and 2500 cal BP, the oR values ranged between -50 and + 500 14Cyears, thus rendering the Loripes shells truly unsuited for independent radiocarbon dating. Extensive geochemical and palaeoecological study of the core and its environment showed that none of the ubiquitous explanations for this highly variable MRE, such as 'hard water' or 'upwelling old seawater', can be valid. We attribute the phenomenon to the uptake by this lucinid mollusc of 'old carbon' from the sediment column into which it had burrowed, released by diagenetic microbial decomposition processes such as methanogenesis. The age of this inorganic carbon varied, being linked to the sedimentation rate: with decreasing sedimentation rate its impact will increase, whereas at high sedimentation rates its impact will likely be minimal. Our results raise serious doubts about the suitability for radiocarbon dating of benthic fauna from shallow coastal environments and point at these diagenetic processes as potentially important sources of 'old carbon'.