Stable isotope response to lake eutrophication: Calibration of a high-resolution lacustrine sequence from Baldeggersee, Switzerland

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 f actors, including lake eutrophication, that control oxygen and carbon isoto pic composition of epilimnic biologically induced calcite precipitate. During the last 100 yr, Baldeggersee has undergone major increases in produ ctivity and eutrophication in response to nutrient loading from agriculture and industrialization in the lake's watershed. Calibration of the isotopic signal in Baldeggersec to historical limnological data quantitatively link s evidence of isotopic depletion in the sedimented calcite to trophic state of the lake. delta(18)O values from the spring/summer "light" sediment lay ers steadily diverged to more depleted values in response to historical eut rophication: measured delta(18)O values were up to -1.5 parts per thousand more negative than calculated equilibrium delta(18)O values. Evidence for C -13 depletion in the calcite, relative to equilibrium values, is more diffi cult 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 oxygen 18 depletion and t he calcite crystal size. Thus, large amorphous calcite grains can be used a s a proxy for recognizing apparent isotopic nonequilibrium in sediment sequ ences from highly productive lacustrine environments from all geologic time scales. In contrast to the light layers, the oxygen isotopic composition of the cal cite in the late summer/fall "dark" sediment layers is unaffected by the ap parent isotope nonequilibrium. Oxygen and carbon isotope values from the da rk 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.