Radiocarbon in corals: A tracer of past changes in the carbon cycle and climate

Full text: Radiocarbon in annual and monthly bands of coralline aragonite reflects the 14C/12C ratio of the dissolved inorganic carbon (DIC) in surface ocean water that surrounds the coral. The growth rates of massive corals vary from 0.4 to 2.5 cm/yr, and many species grow to several meters in diameter. Therefore, time histories of radiocarbon can be reconstructed for periods of several centuries from corals that grew in tropical and subtropical locations (30N to 30S). Radiocarbon in the surface ocean is controlled mainly by physical processes such as upwelling and lateral transport, which are in turn controlled by climatic variables such as wind speed. Thus, past records of climate are obtained from high precision radiocarbon measurements of banded corals. Radiocarbon time histories for the Pacific and Atlantic Oceans reveal interannual, decadal and centennial variability of past climate. The exchange rate of CO2 between the atmosphere and surface ocean (90 Gt C/yr) is large compared to the net transport of anthropogenic CO2 into the ocean (∼2 Gt C/yr. About one-third of this net CO2 emission is centered in the mid- and eastern tropical Pacific due to enhanced upwelling of subsurface waters that contain high pCO2 values. Thus, variability of the upwelling rate in this region of the world ocean could significantly change the net transport of anthropogenic CO2 into the ocean. Changes observed in surface ocean radiocarbon during the past few centuries will be evaluated in term of their implications for past changes in the carbon cycle. (author)