Limnol. Oceanogr., 44(2), 1999, 320–333

Stable isotope analyses of discrete seasonal layers from a 108-yr annually laminated freeze-core from Baldeggersee, a small, eutrophic lake in central Switzerland, provide information on the climatological and environmental factors, including lake eutrophication, that control oxygen and carbon isotopic composition of epilimnic biologically induced calcite precipitate. During the last 100 yr, Baldeggersee has undergone major increases in productivity and eutrophication in response to nutrient loading from agriculture and industrialization in the lake’s watershed. Calibration of the isotopic signal in Baldeggersee to historical limnological data quantitatively links evidence of isotopic depletion in the sedimented calcite to trophic state of the lake. d18O values from the spring/summer ‘‘light’’ sediment layers steadily diverged to more depleted values in response to historical eutrophication: measured d18O values were up to 21.5‰ more negative than calculated equilibrium d18O values. Evidence for 13C depletion in the calcite, relative to equilibrium values, is more difficult to ascertain because of an overall dominance of isotopic enrichment in the dissolved inorganic pool as productivity in Baldeggersee increases. A positive association exists between the degree of oxygen18 depletion and the calcite crystal size. Thus, large amorphous calcite grains can be used as a proxy for recognizing apparent isotopic nonequilibrium in sediment sequences from highly productive lacustrine environments from all geologic time scales. In contrast to the light layers, the oxygen isotopic composition of the calcite in the late summer/fall ‘‘dark’’ sediment layers is unaffected by the apparent isotope nonequilibrium. Oxygen and carbon isotope values from the dark laminae in the Baldeggersee sediment therefore provide environmental and climatological proxies that can be calibrated with known environmental and regional climate data for the last century. Stable oxygen (d18O) and carbon (d13C) isotope measurements from lacustrine, biologically induced, authigenic calcite precipitate have been used to reconstruct a variety of paleoenvironmental and paleoclimatic changes. Oxygen isotope records in lake sediments have been shown to be accurate paleoprecipitation archives, providing information on rapid continental climate change (Stuiver 1970; Siegenthaler and Eicher 1986; McKenzie and Hollander 1993). Variations in d13C values have been successfully used to detect significant changes in the rate or amount of surface-water pro1 Present address: Department of Geological Sciences, University of Michigan, 2534 C.C. Little Building, Ann Arbor, Michigan