SEASONAL CYCLING OF TRACE-ELEMENTS CS-137, BE-7, AND PU-239+240 IN LAKE-MICHIGAN

Trace elements, fallout Cs-137(t1/2 = 30.2 years) and cosmogenic Be-7 (t1/2 = 53.4 days) were measured in trap samples collected from two sites in southern Lake Michigan at selected times during the period from 1982 through 1987. Concentrations of Cs-137 in trap material were virtually constant from top to bottom during the period when the lake was vertically well-mixed. Fluxes of Cs-137 were more than two orders of magnitude higher than atmospheric loading. With the development of the thermocyline, concentrations of the isotope diminished to undetectable levels at a rate indicating a 1-month residence time of resuspended components in the epilimnion. Declines in mass flux (10 to 0.2 g m-2 d-1) and trace element concentrations (some by 30 x) also occurred at this time. Epilimnetic concentration of Be-7 increased from a prestratification value of about 30 dpm/g to a maximum of about 120 dpm/g in July. During calcite formation in August, the concentration rapidly declined mainly as a result of enhanced particle settling rates. Throughout the period of stratification, a maximum in concentration and flux of Be-7 persisted in trap material between 40 and 60 m depth but only about 15% of the atmospheric flux was transferred to the benthic boundary region below 60 m. In near-bottom trap materials, the Cs-137 concentration was reduced at the onset of stratification by addition of radiocesium-deficient authigenic materials. By November, concentrations of Cs-137 had returned to prestratification levels through remineralization of these materials. A decrease of 22% in the seasonally invariant ratio of Cs-137/K-40 between 1982 and 1986 indicated a decay-corrected removal time of about 20 years, which is consistent with the rate of decrease of Pu concentrations in the well-mixed water column measured over more than a decade. In benthic trap samples, Be-7 concentration decreased exponentially throughout the stratified period, with a time constant comparable to the half-life of the isotope. The resuspendable sediment pool, evidently isolated from atmospheric loading during stratification, was "recharged" with Be-7 following late fall overtum. Box and advection-diffusion models describing the storage and vertical distribution of Be-7 during the winter-mixing period successfully account for observations, implying an inverse relation between water depth and winter recharging of the resuspendable pool. A plutonium-calibrated box model for the seasonal cycle of tracers in surface waters describes the main features of the variation of Pu isotopes, Cs-137 and Be-7.