On the radiocarbon record in banded corals: Exchange parameters and net transport of 14CO2 between atmosphere and surface ocean

Abstract

We have made radiocarbon measurements of banded hermatypic corals from Florida, Belize, and the Galapagos Islands. Interpretation is presented here of these previously reported results. These measurements represent the 14C/12C ratios in dissolved inorganic carbon (DIOC) in the surface ocean waters of the Gulf Stream and the Peru Current at the time of coral ring formation. A depletion in radiocarbon concentration was observed in coral rings that grew from A.D. 1900–1952. It was caused by dilution of existing 14C levels with dead CO2 from fossil fuel burning (the Suess effect, or Se). The observed depletion of radiocarbon was greater in corals from the Gulf Stream (−11‰) than in corals from the Peru Current (−6‰). A similar trend was observed in the distribution of bomb‐produced 14C in corals that had grown during the years following A.D. 1952. The concentration of bomb‐produced radiocarbon was much higher in corals from temperate regions (Florida, Belize, Hawaiian Islands) than in corals from tropical regions (Galapagos Islands and Canton Island). A linear relationship appears to exist between the preanthropogenic Δ14C values and the Se values measured in the individual corals, because the 14C in corals is derived from two different carbon reservoirs: (1) the atmosphere and (2) the subsurface ocean. A linear relationship is also observed between the preanthropogenic Δ14C values and the concentration of bomb‐produced 14C in the individual corals during A.D. 1973. The apparent radiocarbon ages of the surface waters in temperate and tropical oceans during the preanthropogenic period range from about 280 to 520 years B.P. (−40 to −69‰). At all investigated locations, it is likely that waters at subsurface depths have the same apparent radiocarbon age of about 670 years B.P. From the change of oceanic Δ14C in the surface ocean during post‐bomb times, the approximate annual rate of net input of 14CO2 to the ocean waters is calculated to be about 8% of the prevailing 14C difference between atmosphere and ocean. From this input and from preanthropogenic Δ14C values found at each location, it can be seen that vertical mixing of water in the Peru Current is about 3 times greater than that in the Gulf Stream.