Page 145 - Microbial methane cycling in a warming world From biosphere to atmosphere Michiel H in t Zandt

archaea) have been detected in permafrost habitats, including drained lakes, permafrost soils, and deep submarine permafrost (Kao-Kniffin et al. 2015; Shcherbakova et al. 2016; Winkel et al. 2018). Sequence-based studies suggest that ANME-2a,b are of marine origin and are introduced into the lakes during lagoon formation. Sulfate-dependent AOM can form an important additional CH4 filter in these lakes, and more research is needed to quantify their role in the mitigation of CH4 emission.
Thermokarst lakes are important contributors to the global GHG budget
Thermokarst processes do not only accelerate thaw, but also stimulate the release of carbon, nitrogen, and other nutrients from deeper permafrost layers (Bowden 2010). In turn, microbial activity can further accelerate the release of nutrients and greenhouse gases. Thermokarst lake waters are supersaturated with CO2 and CH4, indicating a constant input and turnover of terrestrial carbon stocks which is corroborated by dissolved organic matter signatures (Laurion et al. 2010; Roiha, Laurion and Rautio 2015; Deshpande et al. 2016). Both CO2 and CH4 are emitted into the atmosphere through diffusion and ebullition (Fig. 2) (Zimov et al. 1997, 2006; Sepulveda-Jauregui et al. 2015; Matveev, Laurion and Vincent 2018). Ebullition is suggested to be the major emission pathway of thermokarst lakes with an expected emission of 4.1 ± 2.2 Tg CH4 y−1 which equals 17-26% of the 24.2 ± 1-.5 Tg CH4 yr-1 that is emitted from all northern lakes (Zimov et al. 1997; Walter Anthony et al. 2006; Walter Anthony, Smith and Stuart Chapin 2007; Bastviken et al. 2011; Wik et al. 2013, 2016; Matveev, Laurion and Vincent 2018).
However, a study in Québec on lithalsas found that diffusion rates can exceed ebullition (Matveev, Laurion and Vincent 2018). In addition, ice-bubble storage fluxes comprise a third emission pathway that occurs when seasonally ice-trapped bubbles are released upon thaw in the spring (Sepulveda-Jauregui et al. 2015). These bubble fluxes can account for 10% of CH4 emissions, as observed in Yedoma thermokarst lakes in Alaska (Fig. 2) (Heslop et al. 2015). Although increased permafrost thaw leads to greater gross primary production, large thaw sites in Alaska were found to be a net source of GHGs (Vogel et al. 2009; Hayes et al. 2014). Increased primary production is therefore unlikely to offset net increases in GHG release from thawing permafrost.
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