210Pb and 137Cs chronology of sediments from small, shallow Arctic lakes

Abstract

Arctic environmental conditions present special difficulties with the use of nuclides 210Pb and 137Cs for measuring lake sediment chronology. Fluxes of these nuclides are lower in the Arctic than in mid-latitudes, shortening the history that can be resolved. Ice prevents atmospheric input of nuclides to sediment during much of the year and can cause redistribution of sediment in shallow lakes. Results from analyses of seven sediment cores from two lakes on the Belcher Islands in southeastern Hudson Bay show that sediment redistribution by ice is a major process in one of the lakes. In order to measure an undisturbed sedimentation rate, it is necessary that a total input or flux of 137Cs and 210Pb be measured or estimated. Total unsupported 210Pb in the area, 14.8 dpm/cm 2, is the average of measurements in three cores from two lakes that have consistent total 137Cs inventories closest to the estimated 137Cs flux. Cores from undisturbed sites have slower sedimentation rates, and 137Cs appears in layers dated before the early 1950s when it would have first appeared in sediment. 137Cs cannot be used for chronological evaluation of these cores. Mixing of surface sediment layers and lower dating resolution than in other cores are responsible for the apparent downward movement of this nuclide. 210Pb measurements of undisturbed sedimentation rates in the lakes range from 0.0044 to 0.0180 g/cm 2/a. The sedimentation rate in a lake used for raw sewage oxidation doubled after the beginning of sewage input. Rates at sites where sediment is redistributed range from 0.0259 to 0.0309 g/cm 2/a with similarly higher total 137Cs and unsupported 210Pb values. Dates of sediment layers measured using the constant rate of supply model are verified by 210Pb profile discontinuities corresponding to major storms in 1958 and in 1940.