炭素14による海洋循環研究

抄録

Radiocarbon (¹⁴C) as a tracer for (1) the turnover time of the deep ocean and (2) CO₂ gas exchange between the atmosphere and surface ocean is reviewed. Just after the discovery of radiocarbon in the late 1940s, oceanographers realized that the distribution of radiocarbon in the ocean could reveal the turnover time of the deep ocean, which was debated for a long time. GEOSECS (Geochemical Ocean Section Study) in the 1970s achieved success in more than two thousand measurements of radiocarbon around the world ocean. And the radiocarbon data showed that the turnover time of the deep ocean was estimated to be on centennial timescales (500–1000 years). The radiocarbon from GEOSECS also revealed the global mean of the CO₂ exchange at the sea surface. The mean exchange rates estimated from the natural- and bomb-produced radiocarbon agreed well. However, the exchange rate derived from the GEOSECS radiocarbon data was found to be overestimated by about 25% by more radiocarbon data from WOCE (World Ocean Circulation Experiment) in the 1990s. As a result, the empirical equation for the relationship between the gas exchange rate, which depends on the radiocarbon budget in the ocean, and wind speed has been revised downward too.