Bomb-test 90Sr in Pacific and Indian Ocean surface water as recorded by banded corals

Abstract

Earth and Planetary Science Letters, 74 (1985) 306-314 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands Bomb-test in Pacific and Indian Ocean surface water as recorded by banded corals J.R. T o g g w e i l e r 1 and Susan T r u m b o r e 2 t Geophysical Fluid Dynamics Program, Princeton Unirersit), Princeton, NJ 08542 (U.S.A.) 2 Lamont-Doherty Geological Obsert, atory of Columbia Unieersity Palisades, N Y 10964 (U.S.A.) Received November 14, 1984; revised version received May 13, 1985 We report here measurements of bomb-test 9~ activity in the CaCO 3 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 9~ 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 9~ 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 9~ levels have remained relatively constant from the mid 1960 s through the end of the 1970 s in spite of 9~ decay, reflecting a large-scale transfer of water between the temperate and tropical North Pacific. Activity levels at Fanning Is. (4~ 160~ appear to vary in conjunction with the 3-4 year El Niho cycle. I. Introduction Knutson et al. [1] demonstrated that apparent changes in density observed in X-radiographs of cross sectioned coral heads were related to sea- sonal changes in growing environment. Moore and Krishnaswami [2], Nozaki et al. [3], Druffel and Linnick [4], and Druffel [5,6] have subsequently used the X-ray technique to section coral heads from many locations for the purpose of measuring the t4C activity Of individual growth years. These measurements have been Used to document changes in the 14C activity of ocean surface water caused by both nuclear bomb tests [2-5] and the burning of zac-free fossil fuels [4,5], and to track climato- logical effects over the last several hundred years We report here measurements of bomb-test 9~ in banded coral skeletons from nine locations in the tropical Pacific and Indian Oceans. 9~ is a relatively long-lived fission product (half life = 28 years) of the atmospheric bomb tests conducted during the 1950's and early 1960's. It has no natural sources. Following a large nuclear test 0012-821X/85/$03.30 9 1985 Elsevier Science Publishers B.V. radioactive debris is injected into the stratosphere where it is dispersed around the globe. It mixes down to the troposphere over a one- or two-year period where it is rapidly scavenged by hydrologi- cal and chemical processes in the lower atmo- sphere [7]. 9~ falling on land is generally retained within soils, while that falling on the ocean follows the ocean currents as a passive tracer of the ocean's circulation. The distribution of bomb-produced tritium in the ocean has been widely used to study circulation and mixing processes in the ocean [8-11]. Much less use has been made of 9~ for a number of reasons; the large voliame of water required (50-100 liters), the difficulty in separat- ing Sr from seawater, and a low signal to noise ratio regarding background and blank radioactivi- ties. As an ocean tracer, 9~ has a singular virtue: the 9~ to Ca ratio incorporated into coral skele- tons is the same as the 9~ to Ca ratio in seawater. Corals average out seasonal variations in the 9~ content of surface water and spare one the trouble of separating Sr from seawater. By measuring the 9~ activity of individual coral growth bands, one