Stable isotopic composition of deep-sea gorgonian corals Primnoa spp.: a new archive of surface processes

The deep-sea gorgonian coral Primnoa spp. live in the Atlantic and Pacific Oceans at depths of 65 to 3200 m. They have an arborescent growth form with a skeletal axis composed of annual rings made from calcite and gorgonin. Lifespans may exceed several hundreds of years. It has been suggested that isotope profiles from the gorgonin fraction of the skeleton could be used to reconstruct long-term, annual-scale variations in surface productivity. We tested assumptions about the trophic level, intra-and inter-colony isotopic reproducibility, and preservation of isotopic signatures in a suite of modern and fossil specimens. Measurements of gorgonin 15 N indicate that Primnoa spp. feed mainly on zooplankton and/or sinking particulate organic matter (POM sink ), and not on suspended POM (POM susp ) or dissolved organic carbon (DOC). Gorgonin 13 C and 15 N in specimens from NE Pacific shelf waters, NW Atlantic slope waters, the Sea of Japan, and a South Pacific (Southern Ocean sector) seamount were strongly correlated with surface apparent oxygen utilization (AOU; the best available measure of surface productivity), demonstrating coupling between skeletal isotopic ratios and biophysical processes in surface water. Time-series isotopic profiles from different sections along the same colony, and different colonies inhabiting the same area were identical for 13 C, while 15 N profiles were less reproducible. Similarity in C:N, 13 C and 15 N between modern and fossil specimens suggest that isotopic signatures are preserved over millennial timescales. These results support the use of Primnoa spp. as historical recorders of surface water processes such as biological productivity and the isotopic composition of source nutrients.