Seasonal changes in peatland surface elevation recorded at GPS stations in the Red Lake Peatlands, northern Minnesota, USA

Abstract

[1] Northern peatlands appear to hold large volumes of free-phase gas (e.g., CH 4 and CO 2 ), which has been detected by surface deformations, pore pressure profiles, and electromagnetic surveys. Determining the gas content and its impact in peat is challenging because gas storage depends on both the elastic properties of the peat matrix and the buoyant forces exerted by pore fluids. We therefore used a viscoelastic deformation model to estimate these variables by adjusting model runs to reproduce observed changes in peat surface elevation within a 1300 km 2 peatland. A local GPS network documented significant changes in surface elevations throughout the year with the greatest vertical displacements associated with rapid changes in peat water content and unloadings due to melting of the winter snowpack. These changes were coherent with changes in water table elevation and also abnormal pore pressure changes measured by nests of instrumented piezometers. The deformation model reproduced these changes when the gas content was adjusted to 10% of peat volume, and Young's modulus was varied between 5 and 100 kPa as the peat profile shifted from tension to compression. In contrast, the model predicted little peat deformation when the gas content was 3% or lower. These model simulations are consistent with previous estimates of gas volume in northern peatlands and suggest an upper limit of gas storage controlled by the elastic moduli of the peat fabric.

Climatic drivers for multidecadal shifts in solute transport and methane production zones within a large peat basin

“…Overall, dissolved methane concentrations increased with depth, from 1990 to 2008 with concentrations in the deeper peat (1–4 m) exceeding those in the near‐surface horizons (0–1 m) by 2 to 4 times [ Romanowicz et al , , ; Glaser et al , ; Chasar , ; Corbett , ]. Rates of methanogenesis were sufficiently high during this period to produce large volumes of free‐phase gas within the deeper peat [ Rosenberry et al , ; Glaser et al , ; Parsekian et al , , ; Reeve et al , ] that episodically escaped through ruptures in hydraulic confining layers [ Rosenberry et al , ; Glaser et al , ; Reeve et al , ].…”

Peat porewaters have contrasting geochemical fingerprints for groundwater recharge and discharge due to matrix diffusion in a large, northern bog-fen complex

“…The Red Lake Peatland slopes to the north at low relief (~0.2 m km -1 ) from the Bog Crest to Troy Creek, into which the entire Red Lake II watershed drains. Individual landform elevations are transient due to the rise and fall of the peat surface (cm-scale oscillations) in response to subsurface methane accumulation and ebullition (Glaser et al, 2004;Reeve et al, 2013 (Figure 2). There was negligible antecedent precipitation in August prior to our sampling dates.…”

“…Since the severe regional droughts of the 1930's, precipitation rates have gradually increased in northwestern Minnesota, with decadal-scale dry/wet cycles transitioning into the wettest time period on record from 1991 to the present (http://www.ncdc.noaa.gov/cag/; Minnesota Climate Region 1). Vertical hydraulic gradient reversals from upwards flow during dry times to downwards flow during wet times occur in the GLAP on seasonal and multi-annual timescales across the GLAP (Romanowicz et al, 1993;Glaser et al, 1997;Reeve et al, 2013).…”

Impacts of Groundwater Drainage on Peatland Subsidence and Its Ecological Implications on an Atlantic Raised Bog

“…While subsidence appears to be continuing at a near constant rate (4–6 mm/year), the 2017 digital terrain model survey was conducted in a summer period, where ground levels are expected to be lower than in winter periods, which is when all other surveys took place. Thus, these rates cannot be considered absolute due to the dynamic nature of ground level fluctuation associated with peatland environments with ARB conditions (the oscillation of the peat surface at Clara is <10 cm in ARB areas but can be >30 cm; e.g., Reeve et al, ; Howie & Hebda , ); however, the consistent downward trend in ground surface levels is strongly indicative of an underlying drainage process inducing peat settlement…”

“…While subsidence appears to be continuing at a near constant rate (4-6 mm/year), the 2017 digital terrain model survey was conducted in a summer period, where ground levels are expected to be lower than in winter periods, which is when all other surveys took place. Thus, these rates cannot be considered absolute due to the dynamic nature of ground level fluctuation associated with peatland environments with ARB conditions (the oscillation of the peat surface at Clara is <10 cm in ARB areas but can be >30 cm; e.g., Reeve et al, 2013;Howie & Hebda , 2018); however, the consistent downward trend in ground surface levels is strongly indicative of an underlying drainage process inducing peat settlement Subsidence of peat soils occurs mainly by oxidation above the water table or by compression below (Schothorst, 1977;Schlotzhauer & Price, 1999); bio-oxidation processes are generally reported as the primary mechanism driving subsidence of peat soils. At Clara, the bog system still contains a significant area of ARB (though this is degrading, with losses >40% reported since 1991; NPWS, 2017) and a water table close to the ground surface, suggesting the water table regulates itself to shifting ground levels.…”