Reinterpreting radiocarbon records in bamboo corals – New insights from the tropical North Atlantic

Abstract

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 radiocarbon (14C) and nitrogen (15N) isotopic compositions of the organic nodes in 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 between the two regions. Our bamboo coral 14C results from the eastern tropical Atlantic support previous studies that suggest organic node carbon is sourced primarily from the mixed layer of the ocean. By contrast, the 14C of bamboo coral organic nodes from the oligotrophic central Atlantic better correlates with the 14C content of the subsurface deep chlorophyll maximum layer rather than the surface mixed layer. Combined with nitrogen isotope data, this observation suggests that sinking and/or ambient zooplankton feeding on phytoplankton from the deep chlorophyll maximum layer can contribute a significant proportion of the diet of bamboo corals. These results suggest that the carbon source for bamboo corals organic nodes may not always reside in the mixed layer, especially in oligotrophic regions, which has implications for 14C-based age model development in bamboo corals.