Post-depositional mobility of
137Cs,
239+240Pu and
210Pb was assessed in six small lake basins by comparing sedimentary nuclide profiles with their known fallout history. Laminae couplets, when present, were determined to be varves because the
137Cs and
239+240Pu 1963 fallout peaks are present in laminae couplets corresponding to years 1962–1964. There is no evidence of mobility of
210Pb, because 1) mass accumulation rates based on
210Pb agree with those based on
137Cs and
239+240Pu peak depths and with those based on varve counts, and 2)
210Pb ages agree with varve ages. Significant mobility of
137Cs is evident from the penetration of
137Cs to depths 15–20 cm deeper than
239+240Pu. Deep penetration of
137Cs in spite of a sharp gradient below the peak is interpreted by a numerical model to suggest that
137Cs is present in two distinct forms in these sediments, 67–82% as an immobile form and 18–33% reversibly adsorbed with a K
d
of approximately 5000. The profiles can be interpreted equally well assuming a small portionof the total
137Cs was present as an extremely mobile phase (K
d
5000) in the months to years following peak fallout, slowly becoming more strongly adsorbed. High NH
4
+
concentrations in porewaters may enhance diffusion of the mobile form of
137Cs, but not of the immobile form of
137Cs that defines the sharp gradient. Mobility of
137Cs is likely also enhanced by the low clay content and the high porosity of these sediments. Thus the first detection of
137Cs in the sediments cannot automatically be assumed to correspond to a date of 1952 (initial testing of thermonuclear weapons), although the depth of the peak can be assumed to correspond to 1963 (the year of maximum fallout from testing of thermonuclear weapons).
239+240Pu is a more reliable sediment chronometer than
137Cs because it is significantly less mobile.This is the sixth of a series of papers to be published by this journal following the 20
th
anniversary of the first application of
210Pb dating of lake sediments. Dr P.G. Appleby is guest editing this series.
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