Methane in Lakes: Variability in Stable Carbon Isotopic Composition and the Potential Importance of Groundwater Input

Abstract

Methane (CH4) is an important component of the carbon (C) cycling in lakes. CH4 production enables carbon in sediments to be either reintroduced to the food web via CH4 oxidation or emitted as a greenhouse gas making lakes one of the largest natural sources of atmospheric CH4. Large stable carbon isotopic fractionation during CH4 oxidation makes changes in 13C:12C ratio (δ13C) a powerful and widely used tool to determine the extent to which lake CH4 is oxidized, rather than emitted. This relies on correct δ13C values of original CH4 sources, the variability of which has rarely been investigated systematically in lakes. In this study, we measured δ13C in CH4 bubbles in littoral sediments and in CH4 dissolved in the anoxic hypolimnion of six boreal lakes with different characteristics. The results indicate that δ13C of CH4 sources is consistently higher (less 13C depletion) in littoral sediments than in deep waters across boreal and subarctic lakes. Variability in organic matter substrates across depths is a potential explanation. In one of the studied lakes available data from nearby soils showed correspondence between δ13C-CH4 in groundwater and deep lake water, and input from the catchment of CH4via groundwater exceeded atmospheric CH4 emissions tenfold over a period of 1 month. It indicates that lateral hydrological transport of CH4 can explain the observed δ13C-CH4 patterns and be important for lake CH4 cycling. Our results have important consequences for modelling and process assessments relative to lake CH4 using δ13C, including for CH4 oxidation, which is a key regulator of lake CH4 emissions.

The Effects of Water Column Dissolved Oxygen Concentrations on Lake Methane Emissions—Results From a Whole‐Lake Oxygenation Experiment

“…The resupply of deep CH 4 to the surface waters ceased upon stratification, and because the oxygenation experiment did not influence DO levels above 4.5 m depth (and there were always DO present there anyway), we found no credible mechanisms by which the oxygenation could have caused the initial REF-EXP flux difference or influenced its development. The observed δ 13 C-CH 4 values, used in the mass balance calculations, were within the ranges previously reported in northern lake studies regarding δ 13 C-CH 4 in anoxic waters near the sediments (Schenk et al, 2021;Thalasso et al, 2020), and in the DO-rich epilimnion (Schenk et al, 2021;Thottathil et al, 2019). Accordingly, the results of f MOX were also similar to previous studies, for example, Bastviken et al (2002).…”

“…The final degradation step under such conditions is methanogenesis by methanogenic Archaea (Rudd & Hamilton, 1978; Zeikus & Winfrey, 1976). This process occurs within lake sediments and is considered to contribute most of the lake CH 4 , although groundwater input may also add some CH 4 (Dabrowski et al., 2020; Einarsdottir et al., 2017; Lecher et al., 2017; Olid et al., 2022; Schenk et al., 2021), and there is an ongoing debate regarding the extent of oxic CH 4 production in oxygenated surface lake water (Günthel et al., 2019; Peeters et al., 2019). Some of the CH 4 produced in lakes is emitted to the atmosphere through ebullition, that is, rapid and episodic release of bubbles forming in the sediments where CH 4 production creates an oversaturation in CH 4 (Aben et al., 2017; Bastviken et al., 2004; Wik et al., 2013).…”

Methane and CO2 production in the wetland Lake Podpeč (Slovenia)

“…The data are comparable to the latest studies on boreal lakes and wetlands (Thompson et al 2016;Schenk et al 2021). However, in the study performed by Schenk et al (2021), isotopic ratios of CH 4 in deeper sediments were consistent with mixing/transition between CH 4 production pathways, indicating a higher contribution of the CO 2 reduction pathway. The study further indicates that the isotopic composition of CH 4 sources is consistently higher in littoral sediments than in deep waters across boreal and subarctic lakes.…”

“…Methanogenesis produced CH 4 with δ 13 C CH4 and δ 2 H CH4 values corresponding predominantly to acetotrophic methanogenesis (Whiticar 1999). The data are comparable to the latest studies on boreal lakes and wetlands (Thompson et al 2016;Schenk et al 2021). However, in the study performed by Schenk et al (2021), isotopic ratios of CH 4 in deeper sediments were consistent with mixing/transition between CH 4 production pathways, indicating a higher contribution of the CO 2 reduction pathway.…”

Revisiting Archived Rye Grains Discovered at the Neolithic Site Cuneşti (Romania)

“…On the other hand, the rye grain used as sample for 14 C dating could have suffered from an unidentified process which caused a depletion of the heavy isotopes of carbon. One possible explanation is that the decomposition of the tissue by methanogenic bacteria (Schenk et al 2021) may have occurred in an anaerobic environment, such as during flooding or due to the preservation of the sample itself. Also, the decomposition process could alter the 14 C composition by adding younger carbon.…”