Amino acid δ13C and δ15N patterns from sediment trap time series and deep-sea corals: Implications for biogeochemical and ecological reconstructions in paleoarchives

Abstract

Recent work using compound-specific stable isotopes of amino acids (CSI-AA) in proteinaceous deep-sea corals opens a new realm of high-fidelity reconstruction for biogeochemical and ecological changes in the ocean. However, underlying these CSI-AA paleoceanographic applications are a series of fundamental assumptions, which hold first that baseline-proxy AA isotope values fixed at the base of food webs represent integrated δ13C and δ15N values of primary production, and second they are unaltered during subsequent export and incorporation from particles into corals. We explored long-term δ13C and δ15N CSI-AA data on a sediment trap time series together with contemporaneous, geographically close deep-sea bamboo corals (Isidella sp.) in the California margin, directly testing these assumptions for the first time. Our data show that isotope values of essential (δ13CEAA) and source AAs (δ15NPhe) in sinking particles quantitatively track bulk δ13C and δ15N values of export production. These CSI-AA baseline proxies varied independently of carbon flux, trophic position (TPCSI-AA) and microbial alteration, suggesting that they were well preserved in the sinking particles consumed by corals. Paired comparisons between sinking particles and corals revealed minor elevations of δ13CEAA (by ∼2‰) and δ15NPhe (by ∼1‰) in available coral specimens. We hypothesize that the difference in δ13CEAA is due to the geographic offset in δ13C values of primary production expected between the (more offshore) sediment trap site and (more onshore) coral specimens, whereas the δ15NPhe offset is likely related to expected minor trophic fractionation. Using empirical models derived from the sediment trap time series, we demonstrate for the first time that CSI-AA in proteinaceous deep-sea corals can reconstruct known bulk δ15N values of export production, source nitrogen δ15N values, and exported TPCSI-AA values with very good fidelity. Together, these findings represent a major advance in our understanding of AA isotope behavior in modern and paleoarchives, and can be used to underpin the rapidly evolving use of CSI-AA-based tools in multiple paleoceanographic studies and archives.