55Fe Concentration and Specific Activities in North Pacific Marine Organisms

Marine organisms have been shown to have up to 1000-fold higher specific activities of iron-55 than that in seawater. It was also shown that iron-55 specific activities increase 10 to 30-fold in salmon between mid and northern latitudes. These observations are currently explained on the basis of a different chemical form for iron-55 in seawater and greater dilution of the iron-55 by stable iron at mid-latitudes than at the northern latitudes. The half-time for iron-55 concentrations in mature harvested salmon at all latitudes is approximately 10 months, which is comparable to the half-time for movement of stratospheric debris to the earth´s surface. This observation precluded the possibility that early tropospheric deposition of iron-55 from the 1961-62 Russian nuclear tests contributed significantly to the long-term concentrations of iron-55 in marine biota.