The Biophysical Role of Water and Ice Within Permafrost Nearing Collapse: Insights From Novel Geophysical Observations

Abstract

The impact of permafrost thaw on hydrologic, thermal, and biotic processes remains uncertain, in part due to limitations in subsurface measurement capabilities. To better understand subsurface processes in thermokarst environments, we collocated geophysical and biogeochemical instruments along a thaw gradient between forested permafrost and collapse‐scar bogs at the Alaska Peatland Experiment site near Fairbanks, Alaska. Ambient seismic noise monitoring provided continuous high‐temporal resolution measurements of water and ice saturation changes. Maps of seismic velocity change identified areas of large summertime velocity reductions nearest the youngest bog, indicating potential thaw and expansion at the bog margin. These results corresponded well with complementary borehole nuclear magnetic resonance measurements of unfrozen water content with depth, which showed permafrost soils nearest the bog edges contained the largest amount of unfrozen water along the study transect, up to 25% by volume. In situ measurements of methane within permafrost soils revealed high concentrations at these bog‐edge locations, up to 30% soil gas. Supra‐permafrost talik zones were observed at the bog margins, indicating talik formation and perennial liquid water may drive lateral bog expansion and enhanced permafrost carbon losses preceding thaw. Comparison of seismic monitoring with wintertime surface carbon dioxide fluxes revealed differential responses depending on time and proximity to the bogs, capturing the controlling influence of subsurface water and ice on microbial activity and surficial emissions. This study demonstrates a multidisciplinary approach for gaining new understanding of how subsurface physical properties influence greenhouse gas production, emissions, and thermokarst development.

Permafrost microbial communities and functional genes are structured by latitudinal and soil geochemical gradients

“…Once entrained, frozen conditions are an environmental filter as a barrier to the import of resources, water, and microbial immigrants. But this filter also varies because microorganisms may remain active, especially in permafrost where temperatures approach 0 °C [ 66 ], transforming the community and the resources surrounding it [ 4 , 9 , 67 , 68 ]. The longer microorganisms are entrained in permafrost, the further these compositional and functional transformations can occur.…”

Tundra fire increases the likelihood of methane hotspot formation in the Yukon–Kuskokwim Delta, Alaska, USA

“…When ice-rich tundra with abundant near-surface groundice burns, vegetation and surface organic soil horizons are removed, deepening the seasonal active layer, leading to rapid thaw subsidence (thermokarst) and alteration of soil hydrology (Jones et al 2015, Michaelides et al 2019. In poorly draining areas, thermokarst promotes saturated, oxygen-poor conditions, favoring the conversion of freshly thawed permafrost carbon into greater proportions of CH 4 emissions, as opposed to CO 2 (James et al 2021, Walter Anthony et al 2021. Previous work linked this mechanism to extreme CH 4 emission hotspots in lakes and adjacent terrestrial environments across broad regions where thermokarst is most prevalent (Elder et al 2021, Walter Anthony et al 2021.…”

Watching the Cryosphere Thaw: Seismic Monitoring of Permafrost Degradation Using Distributed Acoustic Sensing During a Controlled Heating Experiment

“…The stiffening effect of the ice fraction on the porous soil frame controls seismic response to permafrost thermal state. Due to this sensitivity, seismic methods, including both active and passive, can be utilized to characterize and monitor subsurface permafrost systems; prior studies have leveraged coda wave interferometry techniques to track subtle daily, monthly, and seasonal changes in seismic velocity ( δv / v ) in such environments (James et al., 2017, 2021; Lindner et al., 2021; Steinmann et al., 2021). Horizontal‐to‐vertical spectral ratio (HVSR) methods, which connect time variations of the dominant (resonance) frequency with seasonal subsurface changes above and/or within permafrost, have also been explored (Köhler & Weidle, 2019; Kula et al., 2018).…”