Sea shells record large biases from the marine bomb-14C curve in NW European seawater between the late 1960s and 2019

The Northeast Atlantic alone has received 1.2 PBq of ¹⁴C as liquid and gaseous releases from European nuclear fuel reprocessing plants (NRPs) between the 1950s and present. The input of reprocessing-¹⁴C has the potential to elevate the regional ¹⁴C content of seawater, sediments and marine biota above the ambient levels expected from the bomb-¹⁴C. Yet, a comprehensive assessment of the time evolution of F¹⁴C in seawater is still missing for the Northwestern European Seas. Moreover, the least-well studied period of time (1990’s onward) corresponds to the largest liquid ¹⁴C releases reported by the Sellafield and La Hague NRPs. In this study, we aim at better constraining the temporal changes of F¹⁴C between the late 1960s and 2019, and to delimit the area of influence of reprocessing discharges with regard to ¹⁴C. To this end, we combine Accelerator Mass Spectrometry techniques and a novel archive of bivalve shells that inhabited the Irish Sea, the North Sea, Norway and the Bay of Biscay throughout the main period of reprocessing-¹⁴C discharge. The shells are made of aragonite, and thus, they can be used as an analogue of the past seawater F¹⁴C. The shell-based F¹⁴C data can be accurately placed in the temporal context because the animals have a known capture date and short lifespan of two years. The reconstructed F¹⁴C values vary between ~1 and ~3 after the 1970s. This range of F¹⁴C values is even larger than the one displayed by the atmospheric bomb peak (1 - 1.9). To investigate if the excess ¹⁴C is related to the reprocessing releases, we use a simple box model that simulates the seawater F¹⁴C by mixing bomb and reprocessing-¹⁴C, as well as the naturally occurring ^12,14C. In shells from the southern North Sea, the F¹⁴C increases 0.1-0.4 above ambient levels after the mid-1990s in response to increased discharge rates of liquid ¹⁴C from the La Hague plant. Similarly, the shells collected in the Irish Sea show two consecutive peaks in the mid-1990s (F¹⁴C ~ 2.0) and 2000s (F¹⁴C ~ 2.2) that can be attributed to peak discharge rates of liquid ¹⁴C reported by Sellafield. The F¹⁴C in shells from the eastern coast of the UK and Norway are within the range of the ambient values, which indicates the expected rapid dilution of the reprocessing signal with open ocean waters. In previous studies, the bomb-¹⁴C marine curve has been used as a benchmark, among others, to estimate the age and growth rate of calcifying animals, to date marine sediments, and to investigate water mass mixing and circulation timescales. Given the biases from the marine bomb-¹⁴C curve unraveled by the shell data, we suggest that liquid releases from the NRPs should not be disregarded when applying ¹⁴C as a chronological or circulation tool to marine samples collected in the Irish Sea and parts of the North Sea over the last 5 decades.