High-resolution nitrogen stable isotope sclerochronology of bivalve shell carbonate-bound organics

Nitrogen stable isotope ratios (?15N) of organic material have successfully been used to track food web dynamics, nitrogen baselines, pollution, and nitrogen cycling. Extending the ?15N record back in time has not been straightforward due to a lack of suitable substrates in which ?15N records are faithfully preserved, thus sparking interest in utilizing skeletal carbonate-bound organic matter (CBOM) in mollusks, corals, and foraminifera. Here we test if calcite Pecten maximus shells from the Bay of Brest and the French continental shelf can be used as an archive of ?15N values over a large environmental gradient and at a high temporal resolution (approximately weekly). Bulk CBOM ?15N values from the growing tip of shells collected over a large nitrogen isotope gradient were strongly correlated with adductor muscle tissue ?15N values (R2=0.99, n= 6, p<0.0001). We were able to achieve weekly resolution (on average) over the growing season from sclerochronological profiles of three shells, which showed large seasonal variations up to 3.4‰. However, there were also large inter-specimen differences (up to 2.5‰) between shells growing at the same time and location. Generally, high-resolution shell ?15N data follow soft-tissue ?15N values, but soft-tissues integrate more time, hence soft-tissue data are more time-averaged and smoothed. Museum-archived shells from the 1950s, 1965, and 1970s do not show a large difference in ?15N values through time despite expected increasing N loading to the Bay over this time, which could be due to anthropogenic N sources with contrasting values. Compiling shell CBOM ?15N data from several studies suggests that the offset between soft-tissue and shell ?15N values (?tissue-shell) differs between calcite and aragonite shells. We hypothesize that this difference is caused by differences in amino acids used in constructing the different minerals, which should be specific to the CaCO3 polymorph being constructed. Future work should use compound specific isotope analyses (CSIA) to test this hypothesis, and to determine whether certain amino acids could specifically track N sources or possibly identify amino acids that are more resistant to diagenesis in fossil shells. In conclusion, bivalve shell CBOM ?15N values can be used in a similar manner to soft-tissue ?15N values, and can track various biogeochemical events at a very high-resolution.