Observation of methanogenesis and potential iron-dependent anaerobic oxidation of methane in old lake sediments, a study of two boreal forest lakes.

Abstract: Organic and inorganic carbon can enter inland waters in different ways, and often a considerable amount of this carbon is coming from terrestrial input. Once this terrestrial carbon enters a lake, the carbon may be degraded, mineralized or eventually buried in the sediment. Below the oxic zone of the sediment carbon may be used by archaea to produce methane (CH4). The CH4 can then diffuse up in the sediment and escape to the bottom waters, or the CH4 can be oxidized by bacteria using oxygen as an oxidant. There is also an anoxic process to oxidize CH4 (anaerobic oxidation of methane: AOM), using sulfate (SO4) and by recent findings also ferric iron (Fe(III)) as electron acceptors. In this study the main questions of interest were if CH4 is produced in deep (i.e. old) lake sediments and if CH4 is oxidized anaerobically using Fe(III). Two Swedish boreal forest lakes were studied, sediment profiles of CH4 were conducted in the field (down to 60 cm). Collected sediments were sliced anoxically at different depths and then analyzed for ferrous iron (Fe(II)), Fe(III) and SO4. Sediment from different depths was also incubated anoxic in order to test if CH4 production depends on sediment age. The results show that methanogenic activity occurs by degrading old carbon in deep boreal forest lake sediments, and that a certain part of this might then be oxidized anaerobically. However, all cores exposed a general trend of increasing CH4 concentrations with sediment depth, indicating that CH4 production in old sediment layers is greater than AOM. AOM could therefore only act as a partial sink for CH4 in anoxic deep sediments.