Using hydrologic measurements to investigate free-phase gas ebullition in a Maine peatland, USA

Abstract

Abstract. Northern peatlands cover more than 350 million ha and are an important source of methane (CH 4 ) and other biogenic gases contributing to climate change. Free-phase gas (FPG) accumulation and episodic release has recently been recognized as an important mechanism for biogenic gas flux from peatlands. It is likely that gas production and groundwater flow are interconnected in peatlands: groundwater flow influences gas production by regulating geochemical conditions and nutrient supply available for methanogenesis, while FPG influences groundwater flow through a reduction in peat permeability and by creating excess pore water pressures. Water samples collected from three well sites at Caribou Bog, Maine, show substantial dissolved CH 4 (5-16 mg L −1 ) in peat waters below 2 m depth and an increase in concentrations with depth. This suggests production and storage of CH 4 in deep peat that may be episodically released as FPG. Two min increment pressure transducer data reveal approximately 5 cm fluctuations in hydraulic head from both deep and shallow peat that are believed to be indicative of FPG release. FPG release persists up to 24 h during decreasing atmospheric pressure and a rising water table. Preferential flow is seen towards an area of relatively lower hydraulic head associated with the esker and pool system. Increased CH 4 concentrations are also found at the depth of the esker crest, suggesting that the high permeability esker is acting as a conduit for groundwater flow, driving a downward transport of labile carbon, resulting in higher rates of CH 4 production.

The effect of sampling effort on estimates of methane ebullition from peat

“…The last of these processes-ebullition-can show considerable spatial and temporal variability [Christensen et al, 2003;Tokida et al, 2007;Stamp et al, 2013] which can present challenges when attempting to establish the strength of CH 4 sources in different types of peatlands. Over a few meters, peatlands can display pronounced spatial variability in vegetation composition [Bubier et al, 1995;Pelletier et al, 2007], the position of the water table [Bubier et al, 1993;Laine et al, 2007;Bon et al, 2014;Chen and Slater, 2015], near-surface peat temperature [Bubier et al, 1995], microtopography [Belyea and Clymo, 2001] rates of decomposition and peat properties more generally [Belyea, 1996;Moore et al, 2007;Baird et al, 2016]. Each of these factors may have an effect on where and when ebullition occurs, and the spatial variability of ebullition may partly reflect the spatial patterns of these factors.…”

Ebullition of methane from peatlands: Does peat act as a signal shredder?

“…However, they were unable to isolate the environmental variable(s) responsible for diurnal cycles, although seasonal cycles were probably related to overall CH 4 production as mediated by peat temperature and the production of labile substrates. Ebullition may also be characterized by noncyclical "spikes" in flux (episodic ebullition) that have been linked to processes that alter bubble volume and mobility such as short-term (hourly) changes in atmospheric pressure [Tokida et al, 2007[Tokida et al, , 2009Comas et al, 2011] or longer-term (days to weeks) variations in water table position [Glaser et al, 2004;Bon et al, 2014]. increases in pressure can also cause ebullition Wright, 2012, 2014;Klapstein et al, 2014].…”

HIMMELI v1.0: HelsinkI Model of MEthane buiLd-up and emIssion for peatlands

“…Gas ebullition occurs, in principle, when the concentration of a dissolved gas reaches saturation, but in practice CH 4 ebullition has been observed in wetlands already with concentrations below saturation (Baird et al, 2004;Kellner et al, 2006;Waddington et al, 2009;Bon et al, 2014). Other gases increase the gas pressure and soil particles and impurities lower the energy barrier for gas nucleation.…”