Radiocarbon (
14C) is one of the most important tracers for the global ocean circulation because of its half-life (5,730 years) comparable to the time span taken by surface-ocean water to circulate to the ocean bottom and back (1,000-2,000 years).
14C is produced naturally in the upper atmosphere, quickly combined with oxygen to form radioactive carbon dioxide (
14CO
2), and incorporated into the ocean, soil, vegetation, etc. Since the atmosphere circulates very rapidly, the global distribution of atmospheric
14C is almost uniform. On the other hand,
14C concentration in the ocean varies markedly according to depth, region, and water mass because of the timescale of the global ocean circulation (1,000-2,000 years), local water-mass movements, etc. Atmospheric testing of nuclear bombs performed in the latter 1950s and early 1960s increased atmospheric
14C concentration by 70-100%. The bomb-produced
14C was incorporated into the surface ocean via air-sea CO
2 exchange, and surface water was greatly enriched in
14C relative to deeper water. This provided a favorable opportunity to investigate the vertical mixing between surface water and deeper water. Hermatypic corals secrete CaCO
3 skeletons in the tropical/subtropical surface ocean, with some species forming annual growth bands in their skeletons and occasionally growing to form gigantic colonies containing hundreds of years of coral growth.
14C analysis of annually-banded coral skeletons, which started in the 1970s, has provided a lot of information about past sea-surface
14C concentration in the tropics and subtropics, contributing to our understanding of vertical mixing and horizontal advection in the ocean. Here, I enunciate the rationale for
14C analysis of coral annual bands, review the results obtained so far, and discuss the usefulness of this method for the study of ocean environment.
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