サンゴ骨格年輪の^<14>C分析による海洋環境研究(<特集>サンゴ年輪と低緯度の海洋環境)
スポンサーリンク
概要
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Radiocarbon (^<14>C) 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). ^<14>C is produced naturally in the upper atmosphere, quickly combined with oxygen to form radioactive carbon dioxide (^<14>CO_2), and incorporated into the ocean, soil, vegetation, etc. Since the atmosphere circulates very rapidly, the global distribution of atmospheric ^<14>C is almost uniform. On the other hand, ^<14>C 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 ^<14>C concentration by 70-100%. The bomb-produced ^<14>C was incorporated into the surface ocean via air-sea CO_2 exchange, and surface water was greatly enriched in ^<14>C 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. ^<14>C analysis of annually-banded coral skeletons, which started in the 1970s, has provided a lot of information about past sea-surface ^<14>C 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 ^<14>C analysis of coral annual bands, review the results obtained so far, and discuss the usefulness of this method for the study of ocean environment.
- 2004-12-27