Cesium-137 redistribution in the sediments of the playa, Lake Tyrrell, northwestern Victoria. II. Patterns of cesium-137 and pollen redistribution

Abstract

Near-surface sediments of the northern basin of the playa, Lake Tyrrell, were analysed for the fallout radioisotope 137Cs (to 20 cm depth) and pollen grains (to 1 cm) and the salt-crust depth at each sampling site was measured. 137Cs areal concentrations (MBq km −2) were found to have a highly significant ( p < 0.001) negative correlation with salt-crust thickness and a highly significant ( p < 0.001) positive correlation with pollen concentrations (no. cm −3). Areal concentrations of 137Cs and pollen were generally depleted in the southwest of the sampled area where salt crust thickness was greatest (≈10 cm), and enriched in the northeast where salt crust was absent. A number of possible mechanisms affecting the distribution of 137Cs and pollen on the lake floor have been identified. 137Cs may be adsorbed on fine-grained sedimentary particles and/or absorbed by algae or bacteria. The transport of these particles, suspended in brine bodies blown across the lake floor by wind and deposited on the sediment surface as the brine evaporates, results in the northerly redistribution of 137Cs. Regional pollen, which acts as a fine sedimentary particle, is similarly affected. The salt crust, in the southwest of the basin, was seen as a barrier to the penetration of 137Cs-labelled particles and pollen grains to the sediment surface and, with the seasonal dissolution of the halite and redistribution of brine, as a source for these materials. The northeast area of the basin acts as a sink for these materials. Local erosion of top soil on the lake margins, and subsequent reworking, may contribute additional material. These redistributed materials eventually become part of the sedimentary sequence, and consideration should be given to redistribution mechanisms before selection of suitable core sites, and when interpreting palaeoenvironmental data from salt lakes. Recommended strategies are outlined.