Page 137 - Microbial methane cycling in a warming world From biosphere to atmosphere Michiel H in t Zandt

2013). However, the heat conduction effects are different for shallow and deeper thermokarst lakes.
Shallow bedfast ice lakes completely freeze over during winter, whereas deeper floating ice lakes have a year-round unfrozen sediment (Brewer 1958; Arp et al. 2011). Within the Arctic coastal plain of northern Alaska many bedfast ice lakes have transitioned to floating ice lakes since the 1980s (Arp et al. 2012). This has direct consequences for cumulative heat storage in lake sediments. For floating ice lakes in Alaska, layers of unfrozen ground, called taliks, form underneath the lakes (Ling and Zhang 2003; West and Plug 2008). Similar observations were made for Siberian thermokarst lakes where field measurements showed an increase in thaw depth below lakes of 5-10 cm yr-1 (Brouchkov et al. 2004). This thaw progression is generally rapid in the first years and slows down over time (Ling and Zhang 2003). Overall, taliks facilitate rapid thaw beneath lakes and cause deeper carbon stocks to become bioavailable (Fig. 1) (Jorgenson, Shur and Pullman 2006; Rowland, Travis and Wilson 2011; Sannel and Kuhry 2011; Walter Anthony et al. 2018). Besides vertical growth, thermokarst lakes also undergo horizontal expansion (Fig. 1). The collapse of surrounding Yedoma increases lake surface area, whereas slumping of adjacent sediment introduces organic material into the lake (West and Plug 2008).
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Stage I Stage II
Active Layer Vegetation Thermokarst Lake Formation
Stage III
Stage IV
Ice Wedge Permafrost
Tali
k
Figure 1. The process of thermokarst lake formation in ice-rich Yedoma permafrost. Stage I: permafrost stage, Yedoma with major ice wedges; stage II: thaw processes lead to thermokarst lake formation; stage III & IV: thaw progression results in horizontal and vertical lake expansion. Figure based on data from (Walter Anthony et al. 2014; Bouchard et al. 2017).
The undecomposed organic matter from permafrost is highly reactive upon thaw (Ewing et al. 2015; Mueller et al. 2015; Schneider von Deimling et al. 2015b; Weiss et al. 2016). Freshly
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