Abstract
Marine biogenic materials such as corals, shells, or seaweed have long been recognized as recorders of environmental conditions. Here, the bivalve Cerastoderma edule is used for the first time as a recorder of past seawater contamination with anthropogenic uranium, specifically 236U. Several studies have employed the authorized radioactive releases, including 236U, from nuclear reprocessing plants in La Hague, France, into the English Channel, and Sellafield, England, into the Irish Sea, to trace Atlantic waters and to understand recent climate induced circulation changes in the Arctic Ocean. Anthropogenic 236U has emerged over recent years as a new transient tracer to track these changes, but its application has been challenged owing to paucity of fundamental data on the input (timing and amount) of 236U from Sellafield. Here, we present 236U/238U data from bivalve shells collected close to La Hague and Sellafield from two unique shell collections that allow the reconstruction of the historical 236U contamination of seawater since the 1960s, mostly with bi-annual resolution. The novel archive is first validated by comparison with well-documented 236U discharges from La Hague. Then, shells from the Irish Sea are used to reconstruct the regional 236U contamination. Apart from defining new, observationally based 236U input functions that will allow more precise tracer studies in the Arctic Ocean, we find an unexpected peak of 236U releases to the Irish Sea in the 1970s. Using this peak, we provide evidence for a small, but significant recirculation of Irish Sea water into the English Channel. Tracing the 1970s peak should allow extending 236U tracer studies into the South Atlantic Ocean.
Highlights
Recycling of spent nuclear fuel is carried out to separate and recover the fissionable nuclides and involves the authorized discharge of lowlevel radioactive effluents into the environment
We report 236U/238U ratios measured in C. edule shells, sediment, and seawater collected from regions exposed to radionuclide discharges from Sellafield and La Hague
C. edule living in Traeth Melynog between 2016 and 2019 display an average 236U/238U ratio of (61 ± 2) ×10‐9 (n=4), which is in agreement with a 236U/238U ratio of (59 ± 2) ×10‐9 (n=1) measured in the overlying seawater collected in January 2019
Summary
Recycling of spent nuclear fuel is carried out to separate and recover the fissionable nuclides and involves the authorized discharge of lowlevel radioactive effluents into the environment. The point-source and temporally variable release of anthropogenic radionuclides from reprocessing plants has labelled European coastal waters with a different isotopic composition since the 1950s or late 1960s (Fig. 1a and b). Based on this principle, several studies have used the released nuclides (e.g. 60Co, 106Ru, 125Sb, 129I, 137Cs and 3H) to investigate water circulation transport pathways and timescales by tracing the labelled waters downstream of the discharge locations (e.g., Bailly du Bois and Guéguéniat, 1999; Smith et al, 2011; Yiou et al, 1994). The combination of 236U from La Hague and Sellafield with Atlantic waters carrying a background value of 236U that results from the atmospheric nuclear weapon tests (Casacuberta et al, 2014; Sakaguchi et al, 2009; Winkler et al, 2012) forms the so-called tracer input function, representing a valuable tool to calculate tracer ages and to estimate circulation timescales and mixing regimes of Atlantic waters in the Arctic and sub-Arctic regions (Casacuberta et al, 2018; Castrillejo et al, 2018; Wefing et al, 2019)
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