We report here measurements of bomb-test 90Sr activity in the CaCO3 skeletons of banded head forming corals collected from nine locations in the tropical Pacific and Indian Oceans. Density variations in skeletal carbonate demarcate annual growth bands and allow one to section individual years. Measurements of 90Sr activity in the annual bands reconstruct the activity of the water in which the coral grew. Our oldest records date to the early years of the nuclear era and record not only fallout deposition from the major U.S. and Soviet tests of 1958-1962, but also the huge, and largely unappreciated, localized inputs from the U.S. tests at Eniwetok and Bikini atolls during 1952-1958. In the 1960's the 90Sr activity in Indian Ocean surface water was twice as high as activity levels in the South Pacific at comparable latitudes. We suggest that substantial amounts of northern hemisphere fallout moved west and south into the Indian Ocean via passages through the Indonesian archipelago. Equatorial Pacific 90Sr levels have remained relatively constant from the mid 1960's through the end of the 1970's in spite of 90Sr decay, reflecting a large-scale transfer of water between the temperate and tropical North Pacific. Activity levels at Fanning Is. (4°N, 160°W) appear to vary in conjunction with the 3-4 year El Niño cycle. © 1985 Elsevier Science Publishers B.V.
