Factors controlling d13C values of sedimentary carbon in hypertrophic Baldeggersee, Switzerland, and implications for interpreting isotope excursions in lake sedimentary records

Teranes, Jane L., Stefano M. Bernasconi

Limnol. Oceanogr., 50(3), 2005, 914-922 | DOI: 10.4319/lo.2005.50.3.0914

ABSTRACT: We use stable carbon isotope values (δ13C) 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 (δ13Corg) 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 (δ13CCaCO3). Baldeggersee δ13Corg values are influenced by both variations in the relative inputs of eukaryotic biomass, which becomes enriched in 13C with increasing primary productivity, and the contribution of microbial biomass produced in the expanding anoxic bottom waters, which is typically very depleted in 13C. We also examined the fractionation within the organic matter-CO2-CaCO3 system, calculated as εtotal organic carbon(TOC) = 103{[(δ13CCaCO3 + 1,000)/(δ13Corg + 1,000)] - 1}. Thresholds within the measured εTOC values represent Baldeggersee CaCO3 trophic status more accurately than δ13Corg or δ13CCaCO3 values alone. In alkaline lakes with endogenic carbonate precipitates, εTOC 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.

Article Links

Please Note