Page 101 - Microbial methane cycling in a warming world From biosphere to atmosphere Michiel H in t Zandt

Introduction
The expansion and submersion of Northern latitude peatlands plays a key role in global methane (CH4) and carbon (C) cycles (e.g. Charman et al. 2013; Morris et al. 2018). Globally, peatlands serve as long-term carbon sinks (Gorham 1991; Clymo, Turunen and Tolonen 1998) that store more carbon than the world’s forests combined, despite covering only 3% of the world’s surface land area (Xu et al. 2018). At the time of the Last Glacial Maximum, peatlands worldwide stored 600,000 Tg C (Yu et al. 2010). This estimate is based on assumptions of peat layer thickness and depth at the ocean basin scale, but few in situ observations of peat deposit properties are available to verify these assumptions.
Methane is globally the second most prevalent greenhouse gas, with emissions to the atmosphere amounting to 550-594 Tg CH4 each year (Saunois et al. 2019). Continental shelves and deltas are important sinks within the global carbon cycle (Saunois et al. 2019; Oppo, De Siena and Kemp 2020) and are responsible for 80-85% of oceanic carbon sequestration (Muller- Karger et al. 2005). Shelf regions contribute ~75% of global ocean CH4 flux to the atmosphere, with estimates of seep from oceanic shelves into bottom waters ranging between 6–12 Tg CH4 yr−1 (Weber, Wiseman and Kock 2019), and 16-48 Tg CH4 yr-1 (Judd et al. 2002). Reducing the uncertainties in these estimates requires further work at both regional and global scales (Saunois et al. 2019; Oppo, De Siena and Kemp 2020). The high CH4 concentrations in surface waters of continental shelves are due to CH4 inputs from estuaries and sea floor sediments, where methanogenesis is fuelled by high organic matter sedimentation (Carr et al. 2018; Zhuang et al. 2018). Methane entering the water column from the sea floor is released either as bubbles or by pore water diffusion and can be of either biogenic or thermogenic origin. Variations in atmospheric CH4 are due, in part, to the changing extent of peatlands over glacial interglacial periods (Frolking and Roulet 2007).
Triggered by post-glacial sea-level rise and, consequently, rising ground water, peatlands in the area between the Netherlands, the United Kingdom and Denmark, now the North Sea, developed by the process of paludification in the Late Pleistocene and Early Holocene (Fig. 1A). Large parts of the tectonically subsiding North Sea basin become dry during glacials and flooded during interglacials (Hijma et al. 2012). During the last ice age, ice sheets reaching as far south as the Doggerbank area were subjected to strong glacio-isostatic adjustment (GIA)
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