Controls on below ground free phase gas dynamics in a Northern peatland inferred from field-scale electrical resistivity tomography (ERT)

English Abstract:

Northern peatlands act as sources of methane and both sources and sinks of carbon dioxide, but the mechanisms that control subsurface free phase gas (FPG) cycling within peatlands, and therefore estimates of past, present, and future gas flux remain unclear. In particular, the spatiotemporal variability of FPG production and release within peatlands is uncertain. While chamber-based measurements provide a direct estimate of FPG flux, these methods measure only at the surface over a small footprint (< 1 m^2). We collected one hundred and twenty-seven 3D electrical resistivity tomography (ERT) datasets during July and August of 2013 in Caribou Bog, Maine to investigate changes in FPG at a unique spatiotemporal scale. Our setup consisted of 72 electrodes placed in the surface of the peat in a 28 by 10 m array configured to sample from the entire peat volume (7 m deep). Water levels, soil temperature, atmospheric pressure, and other environmental parameters were simultaneously acquired at a minimum of 1 hour sampling interval. Methane fluxes were also measured using a flow-through gas chamber and fast methane analyzer located within the ERT array over a limited time period. Although gas flux cannot be directly quantified through ERT measurements, we assume that resistivity and FPG content are directly related due to the well-known dependence of resistivity on water content and use information from our other sensors to support this assumption. Our results indicate that water table variation exhibits the strongest control on FPG changes, but that soil temperature and atmospheric pressure are also somewhat correlated with FPG changes throughout the peat. In the shallow and middle peat (0.2 to 4.4 m depth), we witness evidence for mobility-driven ebullition in responses to changes in atmospheric and/or hydrostatic pressure. In the deep peat (> 4.4 m) we infer that buoyancy-driven migration of FPG to upper layers or the atmosphere is the dominant mechanism resulting from changes in atmospheric and hydrostatic pressure.