Organic phases in bivalve (Arctica Islandica) shells: their bulk and amino acid nitrogen stable isotope compositions

Abstract

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 δ15N values of bivalve shells is to understand if pristine isotope signals can be retrieved from shell organics after sample pretreatment. δ15N 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 δ15N 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 δ15N 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 δ15N 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 δ15N data of intra-OM. Furthermore, differences in the relative abundance and δ15N 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 δ15N proxies in fossil shells as paleoenvironmental archives.