Concentrazione di stronzio nei sedimenti lacustri: anomalie nei laghi svizzeri

Abstract

Sr contents in sediments of Lake Gerzensee and Lake Loclat show high variations between the Oldest Dryas and the present. In general, the Sr content is low (250 ppm); however, high values (2500 ppm) occur during the Bolling in Lake Gerzensee and during the Allerød in Lake Loclat. Since there is no evidence of an allochtonous source of Sr, increasing contents in sediments result from a geochemical process governed by th interaction and superimposition of several factors: a general warming of the climate during the Bølling, chalk precipitation due to algal development and the change in the catchment area soil cover (from tundra to forest). The combination of climatic, physico-chemical, and biological changes constitutes a cycle of dissolution and precipitation of carbonate contributing to Sr increase in sediments. The shift in the Sr anomaly between Lake Garzensee (Bølling) and Lake Loclat (Allerød) is explained by a dilution of Sr by detrital carbonate input and a change in the water system of Lake Loclat. The latter was connected to a large fake (Lake of Neuchàtel) during the Oldest Dryas-Bolling period, whereas Lake Garzensee was in a closed system. In a large lake, the sedimentary record is disturbed by a number of factors, such as the hydrologic regime, a long residence time of waters, as well as river discharge and input of detrital materials. From the mid-Bølling onward, the connection between Lake Loclat and Lake Neuchàtel ceased to exist. Since then Lake Loclat has evolved in a similar way to Lake Garzensee.