Nitrogen isotope (δ15N) records preserved in mollusc shells have strong potential to reconstruct changes in nitrogen sources, transformations and transport in the ocean beyond instrumental observations. This is because mollusc shells can be precisely dated and often preserve highly temporally resolved records. In addition to bulk nitrogen isotope analysis, compound-specific nitrogen isotope analysis of amino acids (CSIA-AA) further enables the separation of biogeochemical processes from the trophic effects. However, the extent to which bulk and amino acid-specific δ15N are preserved in shell-bound organic matter in (sub-)fossil materials over the multi-millennial to multi-million-year timescales remains poorly understood. Furthermore, intra-crystalline organic matter (intra-OM), which is encapsulated inside calcium carbonate biomineral units, has not been systematically evaluated for its ability to preserve original isotopic signals relative to total shell organic matter (total-OM). Here, we investigate the effects of diagenesis on bulk and amino acid-specific nitrogen isotope compositions in the shell-bound organic matter (both intra-OM and total-OM) of the long-lived bivalve Arctica islandica, a well-established geochemical archive in the North Atlantic. We analysed live-collected and sub-fossil shells spanning the Holocene from the northern North Sea and fossil specimens from several Crag formations in East Anglia, UK, dating to Plio-Pleistocene. Using spectroscopic, chemical, and isotopic approaches, we assessed the preservation of the biomineral structure, shell organic chemistry and nitrogen isotope compositions in bulk shell organic matter (δ15NBulk) and their amino acids (δ15NAA). Only minimal modifications of the original aragonitic shell mineralogy were detected as indicated by low (<5 %) calcite content in all specimens analysed from the different studied time periods (Plio-Pleistocene, Early or Mid-Holocene, Late Holocene and Modern). Then we identified a diagenetic trajectory in which inter-crystalline organic matter was progressively lost through open-system interactions during taphonomy, reflected by declining nitrogen content and convergence of amino acid compositions between total-OM and intra-OM. In contrast, intra-OM remained physically protected within the biomineral matrix and retained nitrogen over ca. 2–3 million years, despite internal molecular reorganisation including peptide hydrolysis and selective loss of labile amino acids (e.g., serine and threonine). Given lack of net nitrogen loss, intra-OM showed robust preservation of δ15NBulk. However, while δ15NAA of intra-OM in Holocene shells retained patterns consistent with limited diagenetic alteration, CSIA-AA δ15N patterns in Plio-Pleistocene shells deviated from the Holocene pattern, even within intra-OM, with changes in the relative isotope spacing among amino acids associated with degradation. These findings provide mechanistic insight linking organic matter preservation and nitrogen isotope preservation in molluscs shell biomineral archives and highlight intra-OM as a promising substrate for reconstructing past marine nitrogen cycling within certain diagenetic limits.
CORE (COnnecting REpositories)
CORE (COnnecting REpositories)