A modern snapshot of the isotopic composition of lacustrine biogenic carbonates – records of seasonal water temperature variability

Carbonate shells and encrustations from lacustrine organisms provide proxy records of past environmental and climatic changes. The carbon isotopic composition ([delta].sup.13 C) of such carbonates depends on the [delta].sup.13 C of dissolved inorganic carbon (DIC). Their oxygen isotopic composition ([delta].sup.18 O) is controlled by the [delta].sup.18 O of the lake water and by water temperature during carbonate precipitation. Lake water [delta].sup.18 O, in turn, reflects the [delta].sup.18 O of atmospheric precipitation in the catchment area, water residence time and mixing, and evaporation. A paleoclimatic interpretation of carbonate isotope records requires a site-specific calibration based on an understanding of these local conditions.