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

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 delta C-13 and delta N-15 values of primary production, and second they are unaltered during subsequent export and incorporation from particles into corals. We explored long-term d13C and delta(15N) 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 (delta C-13(EAA)) and source AAs (delta N-15(Phe)) in sinking particles quantitatively track bulk delta C-13 and delta N-15 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 delta C-13(EAA) (by similar to 2 parts per thousand) and delta N-15(Phe) (by similar to 1 parts per thousand) in available coral specimens. We hypothesize that the difference in delta 13C(EAA) is due to the geographic offset in delta C-13 values of primary production expected between the (more offshore) sediment trap site and (more onshore) coral specimens, whereas the delta N-15(Phe) 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 delta N-15 values of export production, source nitrogen delta N-15 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. (C) 2020 Elsevier Ltd. All rights reserved.

Automated summary

Recent research on compound-specific stable isotopes of amino acids in proteinaceous deep-sea corals has opened up a new realm of high-fidelity reconstruction for biogeochemical and ecological changes in the ocean. By testing fundamental assumptions underlying these applications, it was found that baseline-proxy AA isotope values are fixed at the base of food webs and remain unaltered during subsequent export and incorporation from particles into corals. The study demonstrates for the first time that CSI-AA in proteinaceous deep-sea corals can accurately reconstruct known bulk delta N-15 values of export production, source nitrogen delta N-15 values, and exported TPCSI-AA values.