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

摘要

Stable isotope analyses of discrete seasonal layers from a 108-yr annually laminated freeze-core from Baldeg- gersee, 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 evidencc of isotopic depletion in the sedimented calcite to trophic state of the lalce. delt 18O values from the spring/summer ``light'' sediment layers steadily diverged to more depleted values in response to historical eutrophication: measured delta 18O values were up to - 1.5/1000 more negative than calculated equilibrium delta 18O 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 oxygen- 18 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,