Dating Marine Shell: A Guide for the Wary North American Archaeologist

Radiocarbon dates on marine shell and other materials of marine origin appear significantly older than contemporaneous samples of terrestrial/atmospheric origin. Misunderstandings regarding the mechanisms that give rise to this "marine reservoir effect" (MRE), the terminology used to define it, and the mathematics used to describe it cause many coastal archaeologists to distrust or misinterpret marine shell dates. The recent release of a reformulated C-14 calibration curve for marine samples (Marine20), which necessitates recalculation of all local reservoir age corrections, may add to the confusion. Here, we review the benefits of dating shell; provide a plain-language explanation of the mechanical, chemical, biological, and cultural processes that give rise to age disparities associated with the MRE; and offer advice to archaeologists intending to date marine shell. Our hope is that these comments will not only aid archaeologists in the planning and interpretive stages of research but also assist in assessing the reliability of legacy chronologies based on marine materials. More broadly, we encourage careful evaluation of all sources of uncertainty in all C-14 chronologies, whether based on terrestrial or marine materials.