Page 136 - Microbial methane cycling in a warming world From biosphere to atmosphere Michiel H in t Zandt

Chapter 6. Roles of thermokarst lakes in a warming world
2006; Strauss et al. 2017). These deposits are found in Alaska and Siberia, and they originate from the late Pleistocene (Kanevskiy et al. 2011; Grosse et al. 2013; Schirrmeister et al. 2013). The ice-rich Yedoma deposits remained unglaciated during the last ice age and contain organic carbon deposits from alluvial plains, hillslopes and polygonal lowlands (Strauss et al. 2012). Yedoma landscapes are highly sensitive to climate change as observed in Western Siberia, Alaska, and Québec (Payette et al. 2004; Osterkamp 2007; Shirokova et al. 2013).In addition, global warming projections estimate a loss of all Yedoma by the end of the 21st century (Lawrence and Slater 2005; Zimov et al. 2006). Gradual thawing of the ice-rich permafrost leads to the formation of thermokarst, of which thermokarst lakes are a characteristic feature
(Osterkamp et al. 2000; Zimov et al. 2006).
Nowadays, thermokarst landscapes are estimated to cover 20-40% of the northern
permafrost regions, including the Yukon delta, the Alaska north slope and the coastal regions of the Kara Sea, the Laptev Sea, and the East Siberian Sea (Osterkamp et al. 2009; Schuur et al. 2015; Olefeldt et al. 2016). Thermokarst lakes also occur with lower densities on the Qinghai-Tibetan Plateau, which covers 8% of the global permafrost surface (Mu et al. 2016a).
Thermokarst lakes accelerate permafrost thaw and emit old carbon stocks into the atmosphere
Thermokarst lakes play a significant role in the current climate by functioning as point sources of GHG emissions in comparison with surrounding soils and sediments (Zimov et al. 2006; Matveev et al. 2016). Thermokarst lake expansion therefore results in an increase in GHG emissions. Observation-based modeling predicts the largest methane (CH4) emission rates of 50 Tg yr-1 from newly thawed permafrost around 2050 compared with current rates of around 1 Tg yr-1 (Schneider von Deimling et al. 2015b; Dean et al. 2018). This peak coincides with the expected highest expansion of thermokarst landscapes (Schneider von Deimling et al. 2015b).
The expansion of thermokarst landscapes is accelerated by the physical phenomenon of rapid heat conduction through thawed water bodies leading to abrupt thaw beneath and around the thermokarst lakes. Modeled lake sediment temperatures are about 10°C higher than mean annual air temperatures (Jorgenson et al. 2010), leading to vertical and radial expansion of thermokarst lakes. Therefore, heat conduction in thermokarst lakes significantly contributes to early (<100 years) thaw progression (Plug and West 2009; Wellman, Voss and Walvoord
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