Fallout plutonium and natural radionuclides in annual bands of the coral Montastrea annularis, St. Croix, U.S. Virgin Islands

We have investigated the banded coral Montastrea annularis as a recorder of the history of fallout Pu in surface seawater. Thirty annual growth bands, representing growth during ‘coral years’ 1951–1980, were subsampled from M. annularis collected at St. Croix, U.S. Virgin Islands. ‘Coral years’ begin and end in late summer-early autumn and are designated for the calendar year in which they end. ^239,240Pu was finite in coral years 1954–1980, and the coral Pu record is very simply related to the fallout history of ⁹⁰Sr. Peaks in coral Pu in coral years 1959 and 1964 correspond to fallout peaks in 1959 and 1963, respectively. Peak broadening and time lags in the coral Pu record, as compared to the ⁹⁰Sr fallout record are consistent with retention of fallout Pu in surface seawater for about two years (characteristic removal time) during the period of major fallout, and possibly longer thereafter. The simplicity of the coral Pu record and its close correspondence with fallout history suggest that ^239,240Pu was incorporated into the coral skeleton with constant discrimination relative to Ca; the effects of speciation and oxidation state upon Pu incorporation are presently unknown.

To aid the Pu interpretation Ca, Mg, Sr, Na and natural radionuclides (²³⁸U, ²²⁸Ra, ²³²Th and ²¹⁰Pb) were also determined in the annual bands. In small samples (0.5 g) Ca, Mg and Na show correlated variations which could be due to seasonal variability in uptake. The ²³⁸U and ²²⁸Ra records were generally consistent with uptake, at constant discrimination, from surface-water reservoirs of nearly constant concentration, although one sample showed probable diagenetic addition of U. ²³²Th was not detected with certainty; this implies that terrigenous particles were not consistently entrapped within the coral skeleton. Interpretation of ²¹⁰Pb was difficult because ²²⁶Ra was not measured.

Montastrea annularis preserves a record of fallout Pu. To make this record useful it must be considered in the broadest possible geochemical context.