Geochronology of lake sediments

Abstract

This paper describes partially successful attempts to measure rates of sedimentation in lakes using radiometric techniques. We show that 210Pb activity precipitated from the atmosphere is ideal for dating lake sediments of ⩽ 100 yr age. In fresh waters, lead is quickly removed from solution on to particulate matter so that unsupported 210Pb activity in sediments is essentially that due to overhead fallout from the atmosphere. In three lake sediments, the atmospheric 210Pb activity lies between 5–50 dpm/g sediment dry weight, compared to the radium supported 210Pb activity of about 2 dpm/g. 55Fe activity has been measured in the same cores dated by the 210Pb method. Its activity is easily measurable in lakes and forms a method of pinpointing the sediment strata accumulated in 1963–1964, the peak years of its fallout. The concentration of 137Cs has also been measured in a core, which however was not dated by other methods. Analogous to 55Fe, 137Cs activity is also useful for identifying the sediment layer accumulated during 1963–1964, although it seems that diffusion appreciably changes its depth distribution. 32Si activity has been measured in sediments from two lakes. Its activity is also measurable and promises to be useful as a dating method for sediments of ⩽ 2000 yr age.