55 cts = observed gross # of counts (in Xp or Sp) error [S] - calibration error associated with concentrations of S. When more than one aliquot of a dissolved sediment sample was analyzed, the isotopic concentration reported is that derived by weighing each concentration measured by its associated counting error (as above) and computing a weighted mean concentration and error (Stevenson, 1966). The concentrations for al] the isotopes of plutonium, uranium and 2105, (vei 210 Pb) reported are those measured at counting date. 2105, has a physical half-life short enough Of these, only ( ty = 22.26 yr.) that the concentration at the separation date would be appreciably different from the concentration at the sample collection date. That the 21056 concentrations reported are interpreted as being only about 6% lower than 2105), concentrations at the collection date was derived from the following reasoning. About three and one-half *'po half-lives ( ty = 138 d) had elapsed between the °!po separation (and counting) date and the sample collection date. Any unsupported 34a "' "Po originally present in the sample could have been present at only about ‘2. of its original unsupported concentration. Any 2105), produced by the t.clear testing program would have achieved equilibrium with 2105, Tong before ‘ese samples were collected. Therefore, it seems reasonable to assume that “ya large natural disequilibrium between 21055 and 7!9po would lead to sig“Scant concentrations of unsupported 2105, or excess 210 Pb in these samples tt chemical separation date. If such disequilibrium existed, it would “ue to be reflected in the water column. Large natural 21055 - 2105, dis- “-"ltriums are not observed today, even near continental land masses (Schel1 ".,1972), If one then assumes that the *!po concentrations measured in “ett samples were those in equilibrium with 2155, the reported 21°po concen“t255 are 6% about 5% lower (due to physical decay of the 210 Pb) than the = Concentrations in the sediments at the date of sample collection.