Early diagenesis of organic matter in lake sediments: a stable carbon isotope study of pore waters

Abstract

The isotopic composition of organic carbon ( δ 13C) in the upper 8 cm of sediment in Mohonk Lake, a freshwater lake in New York State, U.S.A., was measured to be −27 ± 1‰ (relative to PDB) with a slight trend toward enrichment of 13C down the sediment column. No palaeoenvironmental signal can be inferred from these data due to largescale seasonal changes in the PT of autochthonous organic carbon. The sediment pore waters were analyzed for dissolved inorganic carbon (DIC), the isotopic composition of the DIC ( δ 13C DIC) and total alkalinity (ΣAlk). The depth profiles reveal steep gradients in each of these species in both the shallow (littoral zone) and deep sediments. ΣAlk and DIC increased by a factor of three from bottom water concentrations to 5 cm below the sediment-water interface and δ 13C DIC increased by ∼ 20‰ at 8-cm depth. ΣAlk and DIC concentrations in the littoral sediments also increased by a factor of 5 by 10-cm depth with a δ 13C DIC increase of + 15%o to that depth. The positive enrichment in 13C must be due to methane fermentation which produces as a by-product CO 2 that is 13C enriched. A mass-balance calculation showed that ∼66% of DIC produced within the deep sediments and 55% within the littoral sediments was from CO 2 generated via methanogenesis plus methane oxidation. The ratio of breakdown rate of organic matter to sedimentation rate calculated from a diagenetic model of the carbonate species was 0.8 and 0.25 cm −1 for the deep and littoral sediments, respectively. Simple diagenetic models fitted to the dissolved carbonate species do not reproduce the observed carbon-isotope profiles. 13C-poor DIC derived from anaerobic oxidation of methane is proposed as the likely cause of the lower than expected δ 13C-values.