Anomalous levels of 90Sr and 239,240Pu in Florida corals: Evidence of coastal processes

Abstract

Strontium-90, a radionuclide whose primary source is fallout from nuclear weapons testing, serves as a tritium-like tracer of ocean circulation. The historical record of 90Sr activities in the annual bands of island corals have been shown by other investigators to reflect the 90Sr concentration in surface waters at those site. Strontium-90 activities measured in annual bands in Montastrea annularis from the Florida Keys are 30–120% higher than those in corresponding peak activity years (1960–1965) of a Bermuda coral ( Diploria). The Bermuda 90Sr activity record reflects the fallout source only, whereas the additional 90Sr activity in the Florida Keys is expected to reflect a coastal runoff source as well as the fallout. The coastal circulation patterns off the northern and western edge of the Florida Current further act to concentrate and prolong the exposure of the runoff 90Sr to the corals. Six measured 239,240Pu activities in the Florida coral are 30% of 239,240Pu activities in island coral records previously reported. Since Pu is expected to be scavenged by particles in coastal waters, this decrease in 239,240Pu substantiates the importance of coastal influences in the Florida 90Sr record. Strontium-90 activities measured in subannual coral bands from 1973 to 1974 reflect seasonal changes in the 90Sr concentrations in the surface layer of the coastal waters. This may reflect Loop Current intrusion events. The seasonal and long-term coral 90Sr data presented in this paper suggests that coastal 90Sr coral time series may be very useful for documenting coastal circulation patterns.