Page 124 - Microbial methane cycling in a warming world From biosphere to atmosphere Michiel H in t Zandt

Chapter 5. Early Holocene carbon storage and microbial activity in North Sea peats
representative of the range of CH4 concentrations present in southern and mid-North Sea basal- peat deposits. The findings confirm the long-held hypothesis that methane CH4 is stored in the southern North Sea basal-peat deposits formed during the Late Pleistocene and Early Holocene (Judd et al. 1997; Missiaen et al. 2002). These findings may indicate that basal-peats in other locations, particularly those from a similar period, have the potential to function as CH4 storage facilities and address an important gap in the inventory of global marine carbon and CH4 budgets.
A newly measured carbon store
The study area spans 43,158 km2, approximately ten percent of present-day European peatlands (Xu et al. 2018), and larger than the surface area of the Netherlands. The total carbon stored in these submerged basal-peat deposits is estimated to be 741 Tg C, corresponding to an average of 0.017 Tg C km-2. Present-day Northern peatlands have been estimated to store 547,000 Tg C, over a surface area of 4,000,000 km2 (Yu et al. 2010). The calculated amount of total carbon stored in North Sea basal-peat per km2 is lower due to the decomposition of carbon over time.
The CH4 budget of North Sea basal-peat deposits
Due to unattributable changes in atmospheric CH4 concentrations in the last decade, quantification of the global CH4 budget has been a focal point of discussion in the literature, and wetland emissions are the largest source of uncertainty (Saunois et al. 2019). In the present study, we estimated the volume of submerged basal-peat deposits in the mid- and southern North Sea basin and the corresponding stored CH4.
At all investigated sites, the methane concentrations in the basal-peat deposits were above the background concentrations in bottom water as well as previously reported background concentrations of shallow sediments. Due to a lack of published data, it is not possible to compare the CH4 concentrations measured here with those of other basal-peat deposits. Therefore, we compare the results of this study with the local water column and non-peat sediments from the same basin as well as from basins in other parts of the world.
It is unclear whether the methane originates from within the peat deposits, and the contribution of methanogenic microorganisms is difficult to quantify. Although the concentration of CH4 harboured within the peat deposits was low at most sites, the presence of
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