A 15,000-year Isotopic Record from Lake Zürich of Deglaciation and Climatic Change in Switzerland

Abstract

Ostracod and pelecypod carbonate preserved since the latest Pleistocene in the bottom sediments of Lake Zürich was probably formed under constant temperature conditions. Temporal shifts in the oxygen-isotype ratios for those carbonates therefore correspond to shifts in the isotopic character of the net lake inflow. Alpine deglaciation, which released isotopically light meltwaters, commenced prior to ca. 15,000 yr B.P. and was essentially completed in northern Switzerland by 12,4000 ± 250 yr B.P., well before substantially increased atmospheric heat was available from either increased insolation or an increased atmospheric CO2 content. The early Holocene climate for the region was characterized by minor short-term fluctuations in mean annual air temperatures and an overall amelioration of up to 2°C. Contemporaneously precipitated authigenic and biogenic carbonates have recorded partitioning of carbon isotopes between surface and bottom waters during photosynthetic production and subsequent oxidation of organic matter in the lake. The degree of that partitioning provides an index of the relative production rates, which were low in the late Pleistocene meltwater lake but considerably enhanced during the Holocene.