I/I and ΔC records in a modern coral from Rowley Shoals off northwestern Australia reflect the 20th-century human nuclear activities and ocean/atmosphere circulations.

Radionuclides produced by 20th-century human nuclear activities from 1945 (e.g., atmospheric nuclear explosions and nuclear-fuel reprocessing) made significant impacts on earth's surface environments. Long-lived shallow-water corals living in tropical/subtropical seas incorporate the anthropogenically-produced radionuclides, including I and C, into their skeletons, and provide time series records of the impacts of nuclear activities. Here, we present I/I and ΔC time series records of an annually-banded modern coral skeleton from Rowley Shoals, off the northwestern coast of Australia, in the far eastern Indian Ocean. The I/I and ΔC records, covering the period 1930s-1990s, exhibit distinct increases caused by the nuclear activities, and their increasing profiles are clearly different from each other. The first distinct I/I increase occurs from 1955 to 1959, followed by a decrease in 1960-1963. The increase is probably due to US atmospheric nuclear explosions in Bikini and Eniwetok Atolls in 1954, 1956 and 1958. The I produced in those nuclear tests would be transported by the North Equatorial Current, a portion of which passes through the Indonesian Throughflow and then reaches Rowley Shoals. This initial increase from 1955 is, however, absent in the ΔC record, which shows a distinct increase from 1959 and its peak around the mid-1970s, followed by a gradual decrease. This absence and the 4-year-delayed ΔC increase are likely due to dilution of explosion-produced C with natural carbon (by seawater mixing and air-sea gas exchange) being much more intense than that of explosion-produced I with natural iodine (by the same processes), suggesting that the I/I ratio is a more conservative anthropogenic tracer in surface ocean waters, as compared to ΔC. The second I/I increase is contemporaneous with a rapid ΔC increase during 1964-1967, followed by a rapid I/I decrease in 1968-1969; the increases can be ascribed to very large atmospheric nuclear explosions conducted in the former Soviet Union in 1961-1962. The third I/I increase appears between 1969/1970 and 1992, which can be attributed to airborne I released from nuclear-fuel reprocessing facilities in Europe, the former Soviet Union and the US. The coral I/I and ΔC time series records, combined with previous studies, enhance our understanding of the behavior of anthropogenic I and C in the global ocean and atmosphere.