Stable carbon isotope composition and concentrations of CO2 and CH4 in the deep catotelm of a peat bog

Vertical profiles of concentration and C-isotopic composition of dissolved methane and carbon dioxide were observed over 26 months in the catotelm of a deep (6.5 m) peat bog in Switzerland. The dissolved concentrations of these gases increase with depth while CO₂ predominates over CH₄ (CO₂ ca. 5 times CH₄). This pattern can be reproduced by a reaction-advection-ebullition model, where CO₂ and CH₄ are formed in a ratio of 1:1. The less soluble methane is preferentially lost via outgassing (bubbles). The isotopic fractionation between CO₂ and CH₄ also increases with depth, with α_C values ranging from 1.045 to 1.075. The isotopic composition of the gases traces the passage of respiration-derived CO₂ (from the near surface) through a shallow zone with methanogenesis of low isotopic fractionation (splitting of fermentation-derived acetate). This solution then moves through the catotelm, where methanogenesis occurs by CO₂ reduction (large isotopic fractionation). In the upper part of the catotelm the C-13-depleted respiration-derived CO₂ pool buffers the isotopic composition of CO₂; the δ¹³C of CO₂ increases only slowly. At the same time strongly depleted CH₄ is formed as CO₂ reduction consumes the depleted CO₂. In the lower part of the catotelm, the respiration-derived CO₂ and shallow CH₄ become less important and CO₂ reduction is the dominant source of CO₂ and CH₄. Now, the δ¹³C values of both gases increase until equilibrium is reached with respect to the isotopic composition of the substrate. Thus, the δ¹³C values of methane reach a minimum at intermediate depth, and the deep methane has δ¹³C values comparable to shallow methane. A simple mixing model for the isotopic evolution is suggested. Only minor changes of the observed patterns of methanogenesis (in terms of concentration and isotopic composition) occur over the seasons. The most pronounced of these is a slightly higher rate of acetate splitting in spring.