Stable carbon isotope composition and concentrations of CO 2 and CH 4 in the deep catotelm of a peat bog

Abstract

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 CO 2 predominates over CH 4 (CO 2 ca. 5 times CH 4). This pattern can be reproduced by a reaction–advection–ebullition model, where CO 2 and CH 4 are formed in a ratio of 1:1. The less soluble methane is preferentially lost via outgassing (bubbles). The isotopic fractionation between CO 2 and CH 4 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 CO 2 (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 CO 2 reduction (large isotopic fractionation). In the upper part of the catotelm the C-13-depleted respiration-derived CO 2 pool buffers the isotopic composition of CO 2; the δ 13C of CO 2 increases only slowly. At the same time strongly depleted CH 4 is formed as CO 2 reduction consumes the depleted CO 2. In the lower part of the catotelm, the respiration-derived CO 2 and shallow CH 4 become less important and CO 2 reduction is the dominant source of CO 2 and CH 4. Now, the δ 13C values of both gases increase until equilibrium is reached with respect to the isotopic composition of the substrate. Thus, the δ 13C values of methane reach a minimum at intermediate depth, and the deep methane has δ 13C 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.