Journal Article

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

Schöne, Bernd R., Huang, Qian, Agbaje, Oluwatoosin B. A., Conti, Martina

2023

Overview

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. Plain Language Summary The nitrogen isotope ratio (15N/14N) of organics embedded in bivalve shells can be used to understand biogeochemical processes in the environment. When applying chemical solvents to clean the shells, some organic phases are inevitably lost. To evaluate the potential effects of chemical cleanings on nitrogen isotope composition (δ15N) in shell organics, we applied various solvents to modern shell samples. Most of the shell organic phases carry specific distinct amino acid (AA) compositions and therefore have different nitrogen isotope compositions. These results imply that the nitrogen‐bearing contents (such as AA) and their isotope compositions are not uniform within shells. Specifically, we compared the organics entrapped inside crystals (intra‐OM) with the total shell organics (raw‐OM). The offsets in bulk δ15N data between the two phases were mainly driven by the difference in AA proportions, while little differences were observed in δ15N values of AA. Technically, using raw shells without chemical pretreatment and using a bleaching protocol is suitable for raw‐OM and intra‐OM, respectively, to obtain the bulk and AA‐specific δ15N data. These findings indicate that raw‐OM is sufficient for bulk and AA‐specific δ15N analyses in modern bivalve shells for environmental reconstructions and provide a framework to study the fossil shells. Key Points Organic phases in modern bivalve shells carry different nitrogen isotope compositions The amino acid composition differed between total organics and intra‐organics, resulting in different nitrogen isotope data

• Publisher: John Wiley & Sons, Inc,Wiley
• Subject: Amino acids / Archives / bivalve shells / Bivalvia / Bleaching / Chemical composition / Cleaning / Fossils / Fractionation / Hydrogen peroxide / Isotope composition / Mollusks / Nitrogen / nitrogen isotope composition / Nitrogen isotopes / Organic matter / pretreatment protocols / Relative abundance / Shells / Solvents / Stable isotopes