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

• Published in: Geochemistry, Geophysics, Geosystems
• Main Authors: Huang, Qian, Agbaje, Oluwatoosin B. A., Conti, Martina, Schöne, Bernd R.
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2023
• Subjects: amino acids; bivalve shells; nitrogen isotope composition; pretreatment protocols; Arctica islandica
• Online Access: https://curis.ku.dk/portal/da/publications/organic-phases-in-bivalve-arctica-islandica-shells(5d5a2ffc-08eb-4daf-8b56-c1f7a7026c3f).html; https://doi.org/10.1029/2023GC011147; https://curis.ku.dk/ws/files/372324701/Geochem_Geophys_Geosyst_2023_Huang_Organic_Phases_in_Bivalve_Arctica_Islandica_Shells_Their_Bulk_and_Amino_Acid.pdf

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 δ 15 N values of bivalve shells is to understand if pristine isotope signals can be retrieved from shell organics after sample pretreatment. δ 15 N 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 δ 15 N values of intra-OM agree with those of bulk organic matter. These questions were tackled by applying different solvents (H 2 O, HCl, H 2 O 2 , NaOCl) to homogenized shell powder of a modern Arctica islandica. Milli-Q water did not alter bulk δ 15 N 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 δ 15 N values related to the loss of acid-soluble components. Compared to H 2 O 2 , NaOCl oxidative treatment was more effective in cleaning inter-OM and produced reliable bulk and amino acid (AA)-specific δ 15 N data of intra-OM. Furthermore, differences in the relative abundance and δ 15 N 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 δ 15 N proxies in fossil shells as paleoenvironmental archives.