An investigation into ¹⁴C offsets in modern mollusc shell and flesh from Irish coasts

Our ability to reliably use radiocarbon dates of mollusc shells to estimate calendar ages depends on the feeding preference and habitat of the particular species and the geology of the region (Mangerud, 1972). Gastropods which feed by scraping are particularly prone to incorporation of carbon from the substrate into their shells as evidenced by studies comparing the radiocarbon dates of shells and flesh from different species from the same location and the same species on different substrates (Dye, 1994; Hogg et al., 1998). Limpet shells (Patella sp.) are commonly found in prehistoric midden deposits in the British Isles and were presumably part of the palaeodiet, however these shells have been avoided for use in radiocarbon dating in regions of limestone outcrops. Preliminary results from limpets (Patella vulgata) collected alive on limestone and granite substrates on the west coast of Ireland indicate that the shells were formed in equilibrium with the seawater, with no significant ¹⁴C offsets. Limpets collected from the east coast of Northern Ireland have elevated ¹⁴C due to the output of Sellafield nuclear fuel reprocessing plant. In all locations, the flesh was depleted in ¹⁴C compared to the shells. Mussels (Mytilus edulis), which are suspension feeders, were also collected alive from the same locations and these yielded statistically indistinguishable results to the limpets suggesting that there are no offsets specific to the feeding ecology of these two species. The results will have an important consequence for radiocarbon dating of midden deposits as well as the bones of humans and animals who fed on the limpets.