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Teranes, JL, Bernasconi SM.  2005.  Factors controlling δ13C values of sedimentary carbon in hypertrophic Baldeggersee, Switzerland, and implications for interpreting isotope excursions in lake sedimentary records. Limnology and Oceanography. 50:914-922. AbstractWebsite

We use stable carbon isotope values (delta(13)C) in sedimented organic matter and carbonate as proxy indicators of productivity changes in a highly eutrophic to hypertrophic lake. Stable isotope data from a seasonally sampled sediment core recovered from Baldeggersee, Switzerland were compared with direct and inferred total phosphorus (P) concentrations. Carbon isotope values of sedimentary organic matter (delta(13)C(org)) in both seasonal varves and over the past 100 yr are not linearly related to total P concentrations, nor do they mirror observed trends in carbonate isotope values (delta(13)C(CaCO3)). Baldeggersee delta(13)C(org) values are influenced by both variations in the relative inputs of eukaryotic biomass, which becomes enriched in C-13 with increasing primary productivity, and the contribution of microbial biomass produced in the expanding anoxic bottom waters, which is typically very depleted in C-13. We also examined the fractionation within the organic matter-CO2-CaCO3 system, calculated as epsilon(total organic carbor(TOC)) = 10(3){[(delta(13)C(CaCO3) + 1,000)/(delta(13)C(org) + 1,000)] - 1}. Thresholds within the measured epsilon(TOC) values represent Baldeggersee trophic status more accurately than delta(13)C(org) or delta(13)C(CaCO3) values alone. In alkaline lakes with endogenic carbonate precipitates, STOC values can facilitate accurate interpretation of values and trends in organic carbon isotope data, and can help to better assess trophic history and lake remediation efforts in lake systems that have been heavily affected by cultural eutrophication.

Teranes, JL, McKenzie JA, Lotter AF.  1999.  Stable isotope response to lake eutrophication: Calibration of a high-resolution lacustrine sequence from Baldeggersee, Switzerland. Limnology and Oceanography. 44:320-333. AbstractWebsite

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 Baldeggersec to historical limnological data quantitatively links evidence of isotopic depletion in the sedimented calcite to trophic state of the lake. delta(18)O values from the spring/summer "light" sediment layers steadily diverged to more depleted values in response to historical eutrophication: 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 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, 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.