Biogeochemical influence on carbon isotope signature in boreal lake sediments

Abstract

The sources of dissolved inorganic carbon (DIC) were determined in Lake 658 at the Experimental Lakes Area in northwest Ontario, Canada. The study covered a period from June to October 2001 in five different locations in the lake. The pore water chemistry (concentration of DIC and dissolved organic carbon (DOC), pH, total alkalinity (TA), and isotopic composition of DIC (δ¹³C_DIC)) were determined on a monthly basis. Additionally, isotopic composition of the sedimentary organic carbon (δ¹³C_org) was measured. The carbon dynamics in the sediment was simulated by a diagenetic model, which accounts for basic processes controlling the concentration of DIC, while the model to describe the isotope data must include other processes such as oxidation of methane within the sediments. In the sediment pore water the concentration of DIC represents approx. 20% of the total carbon and its isotopic composition reflects the combination of organic carbon degradation using various electron acceptors and methanogenesis. The production of methane, which forms via acetate fermentation and is partially oxidized in the upper layer of the sediment, was especially pronounced in the shallow littoral zones of the lake.