Tropical Atlantic bamboo corals bomb radiocarbon and nitrogen isotopes

Deep-sea bamboo corals (family Isididae) have been used as archives for reconstructing changes in the past ocean. However, uncertainties remain regarding the interpretation of geochemical signals from their organic nodes, specifically the water depth of the signals recorded by the coral. Here we explore this question by measuring 14C and 15N compositions of six bamboo corals collected from the central and eastern tropical Atlantic between 700 m and 2000 m water depth. By comparing coral 14C to measured seawater data, regional shallow-water coral records and climate-model outputs, we find contrasting results. Our bamboo coral 14C results from the eastern tropical Atlantic support previous studies that suggest organic node carbon is sourced primarily from the upper mixed layer of the ocean. By contrast, bamboo corals from the oligotrophic central Atlantic suggests that organic node radiocarbon records the Δ14C of the deep chlorophyll maximum layer rather than the surface mixed layer. Combined with δ15N data, this observation suggests that sinking and/or ambient zooplankton supported by phytoplankton from the deep chlorophyll maximum layer can contribute a significant proportion of the diet of bamboo corals. These results suggest that the age models of bamboo corals residing beneath oligotrophic regions should not be determined from mixed layer Δ14C. Radiocarbon analyses were conducted at the Bristol Radiocarbon Accelerator Mass Spectrometry (BRAMS) Facility at the University of Bristol following the procedure of Knowles et al. (2019, doi:10.1017/RDC.2019.28). δ15N were gently ground to homogenise, weighed out into 0.5 - 1 mg aliquots into 3.5 × 5 mm tin capsules (part #041060) from Costech Analytical Technologies, Inc and sent out for the δ15N analyses to the Center for Stable Isotope Biogeochemistry (CSIB), University of California Berkeley.