Evaluating the mobility of137Cs,239+240Pu and210Pb from their distributions in laminated lake sediments |
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| Authors: | John Crusius Robert F Anderson |
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| Institution: | (1) Dept. of Oceanography, University of British Columbia, V6T 1Z4 Vancouver, B.C., Canada;(2) Lamont-Doherty Earth Observatory of Columbia University Palisades, 10964, N.Y., USA |
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| Abstract: | Post-depositional mobility of137Cs,239+240Pu and210Pb 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 the137Cs and239+240Pu 1963 fallout peaks are present in laminae couplets corresponding to years 1962–1964. There is no evidence of mobility of210Pb, because 1) mass accumulation rates based on210Pb agree with those based on137Cs and239+240Pu peak depths and with those based on varve counts, and 2)210Pb ages agree with varve ages. Significant mobility of137Cs is evident from the penetration of137Cs to depths 15–20 cm deeper than239+240Pu. Deep penetration of137Cs in spite of a sharp gradient below the peak is interpreted by a numerical model to suggest that137Cs 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 total137Cs 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 of137Cs, but not of the immobile form of137Cs that defines the sharp gradient. Mobility of137Cs is likely also enhanced by the low clay content and the high porosity of these sediments. Thus the first detection of137Cs 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 than137Cs 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 of210Pb dating of lake sediments. Dr P.G. Appleby is guest editing this series. |
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| Keywords: | radionuclides 210Pb mobility varves chronology |
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