Carbon isotope signature of CH4 and CO2 as a tool to unravel diagenetic pathways in lake sediments

English Abstract:

In aquatic sediments, methane (CH₄), a potent greenhouse gas, is produced through the microbial breakdown of organic matter (OM) once sulfate is depleted, and can be consumed by aerobic and anaerobic oxidation (methanotrophy). It is well known that methanotrophy as well as methanogenesis, i.e. acetate fermentation (acetoclasty) and CO₂ reduction with H₂ (hydrogenotrophy), are processes that modify the carbon isotopic composition (δ¹³C) of CH₄ and CO₂ contained in the sediments. To unravel methane diagenetic pathways in the sediments of an oligotrophic and seasonally anoxic Canadian Shield lake, we determined high-resolution vertical profiles of dissolved CH₄ and CO₂, as well as their δ¹³C. The concentration profiles of CH₄ and CO₂ were modeled, using a one-dimensional transport-reaction equation, to constrain the depth-intervals (zones) where these species are produced or consumed in the first 25 cm of the sedimentary column, and to estimate their net production/consumption rates in each of the zones. The comparison of CH₄ and CO₂ net reaction rates enables us to constrain, in each zone, rates for acetoclasty, hydrogenotrophy, methanotrophy and OM fermentation. Then, fitting the measured δ¹³C profiles with those simulated by a steady-state transport-reaction model allows us to clarify the relative contribution of acetoclasty and hydrogenotrophy. The model takes into account the constrained rates and established isotope fractionation factors reported in the literature for all the above-mentioned processes. We conclude that nearly 100% of CH₄ was produced by hydrogenotrophy following the non-fractionating fermentation of humic substances to CO₂ and H₂.