Organic Phases in Bivalve (Arctica Islandica) Shells: Their Bulk and Amino Acid Nitrogen Stable Isotope Compositions

The stable nitrogen isotope composition of bivalve shell organics serves as a proxy for nitrogen fluxes in modern and past ecosystems. An essential prerequisite to reconstruct environmental variables from delta N-15 values of bivalve shells is to understand if pristine isotope signals can be retrieved from shell organics after sample pretreatment. delta N-15 analyses of fossil shells should be limited to the intra-crystalline organic matrix (intra-OM), which is trapped within biomineral units and less likely contaminated or diagenetically overprinted than inter-crystalline organics (inter-OM). However, it remains unclear whether the different shell organic phases (insoluble/soluble inter-OM, intra-OM) are isotopically distinct and whether delta N-15 values of intra-OM agree with those of bulk organic matter. These questions were tackled by applying different solvents (H2O, HCl, H2O2, NaOCl) to homogenized shell powder of a modern Arctica islandica. Milli-Q water did not alter bulk delta N-15 values indicating the dissolution of the inter-OM was negligible. Acid-extracted intra-OM exhibited a larger isotope variation within replicates and showed a minor but significant fractionation in bulk delta N-15 values related to the loss of acid-soluble components. Compared to H2O2, NaOCl oxidative treatment was more effective in cleaning inter-OM and produced reliable bulk and amino acid (AA)-specific delta N-15 data of intra-OM. Furthermore, differences in the relative abundance and delta N-15 values of individual AAs suggested that the N isotope composition is not uniform within shells, and the N-bearing content and AA composition differ between organic phases. Future studies should test the capability of bulk and CSIA-AA delta N-15 proxies in fossil shells as paleoenvironmental archives.