The potential of shells from Mytilus edulis for retrospective analysis of marine ¹⁴C discharges from nuclear power plants

Carbon-14 (14C) is an important contributor to the collective effective dose to the public due to releases from nuclear power plants (NPPs). In Sweden, only airborne emissions of 14C from NPPs are currently routinely monitored, and the existing data on waterborne 14C discharges are limited. A recent study of 14C in brown algae (Fucus spp.) in Swedish coastal waters showed higher F14C values collected at Ringhals NPP, on the Swedish west coast, than expected. Therefore, this study aimed at assessing if blue mussels (Mytilus edulis) could be used to retrospectively estimate the 14C concentration of dissolved inorganic carbon (DIC) in seawater at three sites. A method was developed to extract the fibrous layer that forms visible annual structures in the shells. All samples were analyzed with accelerator mass spectrometry and the results compared with 14C data from Fucus spp. For one of the analyzed shells (structures from 1974-1978), from the site Särdal, F14C in Fucus spp. and M. edulis agreed very well. For another shell (1972-1978), shell structures from some of the earlier years displayed up to 6% lower F14C than Fucus spp. F14C in one shell from a remote site, Båteviken, only had small annual variations (2017-2022: F14C = 1.070 ± 0.015 (1 σ)). Two shells from Ringhals NPP had higher average F14C, and a significant temporal variability (2014-2022: F14C = 1.427 ± 0.268 (1 σ)). Difficulties in unambiguous identification of the annual structures in the shells, as well as the future potential of this method, are discussed.