Lecture-7-Radio-isotopes Dating of Lakes Sediments.

Abstract

Systematic Studies of sedimentation rate variations are essential for interpretation of a variety of lake sediment interaction processes and also important in the study of the kinetics of transfer between the lake sediment reservoirs. Mass accumulation rates of minerals, but also organic debris, nutrient elements, and trace metals in sediments and their temporal variations require accurate sedimentation rate measurements. Studies of temporal variations in lacustrine sedimentation and the relation of such variations to climatic and/or anthropogenically induced changes in lakes, require knowledge of sedimentation rates and their temporal variations over time scale generally ranging from a few years to tens of thousands of years. Various methods have been adopted for estimation of rate of sedimentation. In the latter part of the last century, the common technique used, in estimation of sedimentation rate is based on the study of rapid rise of ragweed pollen (Ambrosia) due to extensive forest cutting and soil cultivation by many (Bortleson and Lee, 1972; Craig, 1972; Kemp et al., 1974; and Maher, 1977). However, the recent dating techniques involve natural radio-isotope lead-210 estimation (ICrishnaswami et al., 1971; Koide et al., 1973) and the bomb fallout nuclide Cesium-137 (Pennington et al., 1973). The relatively short lived isotopes (22.3 years for Pb21° and 30 years for CS137 are ideally suited for lake sediments whose rates of sedimentation are on the order of a few millimeters per year (for details see Krishnaswami and La!, 1978).