Carbon stable isotope record in the coral species Siderastrea stellata: A link to the Suess Effect in the tropical South Atlantic Ocean

Coral skeletons are natural archives whose geochemical signatures provide insights into the tropical ocean history beyond the instrumental record. Carbon stable isotopes from coral skeletons (δ13Ccoral) have been used as a proxy for multiple variables on a seasonal basis. Long-term changes in coral δ13C could be related to the changing isotopic composition of the dissolved inorganic carbon (δ13CDIC). δ13CDIC in turn reflects changes in the δ13C of atmospheric CO2, which in the modern Earth system is governed primarily by anthropogenic injection of CO2 into the atmosphere - a process known as the Suess Effect. Here we report three δ13C coral-based records of Siderastrea stellata from the tropical South Atlantic. U-series dating for the colonies 12SFB-1, 13SS-1 and 13SS-2 suggests these corals lived 13, 57 and 65 years, respectively. Short-term δ13C variations in their skeletal aragonite are dominated by interannual variation. All three δ13C records additionally exhibit an overall decreasing trend, with a depletion of about −0.0243 ± 0.0057‰·yr−1 (12SFB-1), −0.0208 ± 0.0007‰·yr−1 (13SS-1) and −0.0214 ± 0.0013‰·yr−1 (13SS-2). These rates of the coral records from Rocas Atoll are similar to the reported trend for the δ13C of atmospheric CO2 over the years 1960-1990 (−0.023 to −0.029‰·yr−1), and to the decreasing rates of global δ13CDIC. Our findings suggest that multiple δ13C coral-based records are required for confidently identifying local changes in the δ13CDIC of the ocean. This information, in turn, can be used to infer changes in the δ13C of the atmospheric CO2 composition and provide valuable information about recent changes on the carbon biogeochemical cycle during the Anthropocene epoch.