Post-Bomb Radiocarbon Records of Surface Corals from the Tropical Atlantic Ocean

Abstract

Delta-14C records are reported for post-bomb corals from three sites in the tropical Atlantic Ocean, In corals from 18 degrees S in the Brazil Current, Delta-14C values increased from ca. -58 per mil in the early 1950s to +138 per mil by 1974, then decreased to 110 per mil by 1982. Shorter records from 8 degrees S off Brazil and from the Cape Verde Islands (17 degrees N) showed initially higher Delta-14C values before 1965 than those at 18 degrees S, but showed lower rates of increase of Delta-14C during the early 1960s. There is general agreement between the coral results and Delta-14C of dissolved inorganic carbon (DIC) measured in seawater previously for locations in the tropical Atlantic Ocean. Delta-14C values at our tropical ocean sites increased at a slower rate than those observed previously in the temperate North Atlantic (Florida and Bermuda), owing to the latter's proximity to the bomb 14C input source in the northern hemisphere. Model results show that from 1960-1980 the Cape Verde coral and selected DIC Delta-14C values from the North Equatorial Current agree with that calculated for the North Atlantic based on an isopycnal mixing model with a constant water mass renewal rate between surface and subsurface waters. This is in contrast to Delta-14C values in Bermuda corals that showed higher post-bomb values than those predicted using a constant water mass renewal rate, hence indicating that ventilation in the western north Atlantic Ocean had decreased by a factor of 3 during the 1960s and 1870s (Druffel 1989).