Seasonal cycling of trace elements 137Cs, 7Be, and 239+240Pu in Lake Michigan

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Abstract
Trace elements, fallout 137Cs (t½=30.2 years) and cosmogenic 7Be (t½=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 137Cs in trap material were virtually constant from top to bottom during the period when the lake was vertically well‐mixed. Fluxes of 137Cs were more than two orders of magnitude higher than atmospheric loading. With the development of the thermocline, 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 7Be 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 7Be 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 137Cs concentration was reduced at the onset of stratification by addition of radiocesium‐deficient authigenic materials. By November, concentrations of 137Cs had returned to prestratification levels through remineralization of these materials. A decrease of 22% in the seasonally invariant ratio of 137Cs/ 40K 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, 7Be 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 7Be following late fall overturn. Box and advection‐diffusion models describing the storage and vertical distribution of 7Be 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, 137Cs and 7Be.