A novel trophic archive: Practical considerations of compound-specific amino acid δ15N analysis of carbonate-bound organic matter in bivalve shells (Arctica islandica)

Compound-specific stable nitrogen isotope (δ15N) analysis of amino acids (CSIA-AA) can be used to refine the trophic position (TP) of organisms and determine the ecosystem δ15N baseline. It serves as a powerful tool to understand trophic dynamics and the nitrogen biogeochemistry of ecosystems. Extending this information back in time could be achieved by applying CSIA-AA to bivalve shell carbonate-bound organics (CBOM) in conjunction with growth pattern analysis (bivalve sclerochronology). However, due to the low content of AA in shells, the measurement of δ15N values of individual AA (δ15NAA values) is extremely challenging. Furthermore, a standardized pretreatment procedure is currently not available. In this study, we firstly present a refined analytical technique for calibrating δ15NAA data obtained by gas chromatography-combustion-isotope ratio mass spectrometry (GC-C-IRMS), considering (1) the effect of low signals on δ15NAA values, (2) AA-specific drift in δ15NAA values during analytical sequences, (3) the use of internal standard material(s), and (4) scale-normalization. Using the new approach, we achieve an analytical precision of 0.21 and 0.51 ‰ for six derivatized AA standards and 0.31 to 1.06 ‰ for 13 derivatized AA from shell powder. Secondly, a critical step of sample pretreatment, i.e., demineralization method, is addressed by comparing three demineralization methods. The cation-exchange resin (CEX resin) method showed minimal methodologically induced fractionation for 10 AA of the shell and is hence considered suitable for CSIA-AA. Lastly, using the refined analytical technique and sample pretreatment, we report the first high-resolution sclerochronologal δ15NAA series of an Arctica islandica specimen. Relatively small sample sizes (50–100 mg) from the inner portion of outer shell layer (iOSL) allow for a temporal resolution of 1–3 years per sample. The δ15NAA data from the iOSL show a lower TP of the studied bivalve in the ontogenetic older portion (age 8–14: 1.5 ± 0.1) than the younger portion (age 1–7: 2.1 ± 0.1), suggesting an ontogenetic change of its food sources from benthic particles to pelagic phytoplankton. Our findings also propose a novel, high-resolution proxy (i.e., δ15NPhe in CBOM from the ontogenetic older shell portion) for the ecosystem nitrogen isotopic baseline. This study opens promising potentials to reconstruct high-resolution changes of the origin and fate of nitrogen in past aquatic food webs.