Tracing Isotopic Signatures of a Deep-Sea Coral from the North-Central California Margin

Deep-sea corals can provide high-resolution records of ocean climate and exported primary production. Bamboo corals, a gorgonian deep-sea coral, can be utilized both as a deep water (calcite) and surface water (gorgonin) archive since the gorgonin geochemistry reflects a surface-derived food source. As such, understanding isotopic fractionation between source material, coral polyps and gorgonin is key to interpreting these important biogeochemical archives. Here we analyzed suspended particulate organic matter collected June through August of 2019 from the surface waters of Cordell Bank National Marine Sanctuary (38.23˚N, 123.29˚W) for phytoplankton identification and assemblages, chlorophyll-a content, and δ¹³C and δ¹⁵N signatures. We compared this to the δ¹³C and δ¹⁵N signatures of bamboo coral polyp tissue and the recent gorgonin growth from previously collected corals in this same region. We found the bamboo coral organic node showed a distinct 3‰ δ¹³C offset when compared to polyp tissue, and minimal offset in respect to δ¹⁵N (<0.15‰). We further interpret past changes in isotopic composition using multiple samples from the same coral archive spanning the last century, based upon a bomb-radiocarbon chronology.