Radiocarbon evidence for annual growth rings in the deep-sea octocoral Primnoa resedaeformis

The deep-sea gorgonian octocoral Primnoa resedaeformis is distributed throughout the Atlantic and Pacific Oceans at depths of 65 to 3200 m. It has a 2-part skeleton of calcite and gorgonin. Towards the inside of the axial skeleton gorgonin and calcite are deposited in concentric growth rings, similar to tree rings. Colonies were collected from the NE Channel (NW Atlantic Ocean, southwest of Nova Scotia, Canada) from depths of 250 to 475 m. Radiocarbon was measured in individual rings isolated from sections of each colony, after dissolution of calcite. Each delta super(14)C measurement was paired with a ring age determined by 3 amateur ring counters. The precision of ring counts averaged better than +/-2 yr. Accurate reconstruction of 20th century bomb-radiocarbon showed that (1) the growth rings are formed annually, (2) the gorgonin is derived from surface particulate organic matter (POM) and (3) useful environmental data are recorded in the organic endoskeletons of deep-sea octocorals. These results support the use of P. resedaeformis as a long- term, high resolution monitor of ocean surface conditions, particularly in temperate and boreal environments where proxy data are lacking.