Mathematics – Logic
Scientific paper
Mar 1975
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1975gecoa..39..285r&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 39, Issue 3, pp.285-304
Mathematics
Logic
59
Scientific paper
This paper describes the use of 210 Pb and 137 Cs radioactivity measurements to determine the rates of sedimentation in the Great Lakes. Cores from eight locations in Lake Michigan were chosen for examination to cover as wide as possible a range of sedimentation rates and representative sedimentary environments. The surficial 210 Pb activity in the sediments varies between 7 and 23 pCi/g dry wt and its profile in each core shows the expected exponential decrease with depth consistent with the assumption of uniform sedimentation rate over the last hundred years and secular equilibrium between supported 210 Pb and 226 Ra (0.5-1.0 pCi/g dry wt). Companion measurements of 137 Cs indicate that the coring technique satisfactorily recovered the uppermost levels of the deposit and that the mobility of both radionuclides within the sediment is probably small. Based on the limited number of cores analyzed to date, it appears that modern sedimentation rates are not very different from average rates for the last 7000 yr. The excess 210 Pb appears to originate primarily from atmospheric fallout, but a further inventory of the 210 Pb distribution over the lake bottom must be made to properly assess the significance of other sources. The spatial distributions of both 137 Cs and 210 Pb at certain stations suggest that the mode of transport of these radionuclides are comparable and involve attachment to settling particles. A mathematical model is developed which accounts for the observed limited mobility of both 210 Pb and 137 Cs in several of the cores in terms of post-depositional redistribution by physical or biological mixing processes.
Edgington David N.
Robbins John A.
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