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 thatunsupported²¹⁰Pb 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.