Developing bioproxies of past ocean ecosystem change through compound-specific stable isotope analysis of proteinaceous deep-sea corals.

Abstract:

Our understanding of current and future ocean conditions is framed by our ability to reconstruct past changes in ecosystem structure and function recorded in paleoarchives. One such archive, proteinaceous deep-sea corals, act as “living sediment traps” with the potential to greatly improve our ability to reconstruct long-term, high-resolution biogeochemical records of export production. Compound-specific stable isotope analysis (CSIA) of individual amino acids (AAs) in deep-sea corals has provided highly detailed new tools to reconstruct changes in both plankton community composition and sources of nitrogen. However, to realize the full potential of CSIA in deep-sea corals, it is critical to better understand the link between the biogeochemical signatures of deep-sea coral polyp tissue and diagenetically resistant proteinaceous skeletal material. We conducted the first detailed comparison of δ¹³C and δ¹⁵N values for individual AAs between tissue and skeleton for three deep-sea coral genera (Primnoa, Isidella, and Kulamanamana). For δ¹³C values, we found minimal offsets in both essential and non-essential AAs across genera, strongly supporting coral skeleton AA fingerprinting as a new tool to reconstruct plankton community structure. Similarly, there was no significant offset in source AA δ¹⁵N values between tissue and skeleton, supporting the use of Phe δ¹⁵N as a proxy for baseline nitrogen sources. However, and rather unexpectedly, we found that the d¹⁵N values of the trophic AA group were consistently 3-4‰ lighter in skeleton than polyp tissue for all three genera. We hypothesize that this may reflect a partitioning of either N flux or pathways associated with AA transamination between polyp and skeleton tissues. This offset leads to an underestimate of trophic position using current CSIA-based calculations. Overall, our work strongly supports the applicability of CSIA in proteinaceous deep-sea corals to reconstruct past changes in biogeochemical cycling and plankton community dynamics. However, it also indicates that a new correction factor will be required to reconstruct accurate records of change in plankton trophic structure.