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1. Permafrost Mapping with Electrical Resistivity Tomography: A Case Study in Two Wetland Systems in Interior Alaska

   https://doi.org/10.2113/JEEG19-091  

   Conaway, Christopher H. ; Johnson, Cordell D. ; Lorenson, Thomas D. ; Turetsky, Merritt ; Euskirchen, Eugénie ; Waldrop, Mark P. ; Swarzenski, Peter W. ( June 2020 , Journal of Environmental and Engineering Geophysics)

   null (Ed.)

   Surface-based 2D electrical resistivity tomography (ERT) surveys were used to characterize permafrost distribution at wetland sites on the alluvial plain north of the Tanana River, 20 km southwest of Fairbanks, Alaska, in June and September 2014. The sites were part of an ecologically-sensitive research area characterizing biogeochemical response of this region to warming and permafrost thaw, and the site contained landscape features characteristic of interior Alaska, including thermokarst bog, forested permafrost plateau, and a rich fen. The results show how vegetation reflects shallow (0–10 m depth) permafrost distribution. Additionally, we saw shallow (0–3 m depth) low resistivity areas in forested permafrost plateau potentially indicating the presence of increased unfrozen water content as a precursor to ground instability and thaw. Time-lapse study from June to September suggested a depth of seasonal influence extending several meters below the active layer, potentially as a result of changes in unfrozen water content. A comparison of several electrode geometries (dipole-dipole, extended dipole-dipole, Wenner-Schlumberger) showed that for depths of interest to our study (0–10 m) results were similar, but data acquisition time with dipole-dipole was the shortest, making it our preferred geometry. The results show the utility of ERT surveys to characterize permafrost distribution at these sites, and how vegetation reflects shallow permafrost distribution. These results are valuable information for ecologically sensitive areas where ground-truthing can cause excessive disturbance. ERT data can be used to characterize the exact subsurface geometry of permafrost such that over time an understanding of changing permafrost conditions can be made in great detail. Characterizing the depth of thaw and thermal influence from the surface in these areas also provides important information as an indication of the depth to which carbon storage and microbially-mediated carbon processing may be affected. 

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2. Repeated Permafrost Formation and Degradation in Boreal Peatland Ecosystems in Relation to Climate Extremes, Fire, Ecological Shifts, and a Geomorphic Legacy

   https://doi.org/10.3390/atmos13081170  

   Jorgenson, Mark Torre ; Kanevskiy, Mikhail ; Roland, Carl ; Hill, Kenneth ; Schirokauer, David ; Stehn, Sarah ; Schroeder, Britta ; Shur, Yuri ( August 2022 , Atmosphere)

   Permafrost formation and degradation creates a highly patchy mosaic of boreal peatland ecosystems in Alaska driven by climate, fire, and ecological changes. To assess the biophysical factors affecting permafrost dynamics, we monitored permafrost and ecological conditions in central Alaska from 2005 to 2021 by measuring weather, land cover, topography, thaw depths, hydrology, soil properties, soil thermal regimes, and vegetation cover between burned (1990 fire) and unburned terrain. Climate data show large variations among years with occasional, extremely warm–wet summers and cold–snowless winters that affect permafrost stability. Microtopography and thaw depth surveys revealed both permafrost degradation and aggradation. Thaw depths were deeper in post-fire scrub compared to unburned black spruce and increased moderately during the last year, but analysis of historical imagery (1954–2019) revealed no increase in thermokarst rates due to fire. Recent permafrost formation was observed in older bogs due to an extremely cold–snowless winter in 2007. Soil sampling found peat extended to depths of 1.5–2.8 m with basal radiocarbon dates of ~5–7 ka bp, newly accumulating post-thermokarst peat, and evidence of repeated episodes of permafrost formation and degradation. Soil surface temperatures in post-fire scrub bogs were ~1 °C warmer than in undisturbed black spruce bogs, and thermokarst bogs and lakes were 3–5 °C warmer than black spruce bogs. Vegetation showed modest change after fire and large transformations after thermokarst. We conclude that extreme seasonal weather, ecological succession, fire, and a legacy of earlier geomorphic processes all affect the repeated formation and degradation of permafrost, and thus create a highly patchy mosaic of ecotypes resulting from widely varying ecological trajectories within boreal peatland ecosystems. 

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

   https://doi.org/10.1029/2021JF006104  

   James, S. R. ; Minsley, B. J. ; McFarland, J. W. ; Euskirchen, E. S. ; Edgar, C. W. ; Waldrop, M. P. ( June 2021 , Journal of Geophysical Research: Earth Surface)

   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.

    

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4. Permafrost thaw depth measurements at beaver pond locations in Northwestern Alaska, 2022-2026

   https://doi.org/10.18739/A2DN3ZX65  

   Tape, Ken ; Glass, Thomas ; Jones, Benjamin ( January 2023 , NSF Arctic Data Center)

   This dataset contains permafrost thaw depth measurements at Alaskan beaver ponds collected as part of the Arctic Beaver Observation Network and NSF ANS #1850578. The Arctic Beaver Observation Network is a 5-year project (2021-2026) funded by the National Science Foundation. The natural science part of the project uses remote sensing to observe the progress and impacts of beaver engineering in the Arctic, starting in Alaska and extending into Canada and Eurasia. The project also establishes field sites at tundra beaver ponds to study the implications of beaver engineering on hydrology and permafrost, as well as pond evolution documented using Unmanned Aerial Systems (UAS). Remote sensing work will map beaver ponds over time. Field measurements at tundra beaver ponds are made in August and late March. Data generated by field measurements include water level and temperature from pressure-transducers, subsurface imaging from ground-penetrating radar, sonar measurements for beaver pond bathymetry, tabular data associated with water quality measurements, and ice thickness and water depth (in winter). Data is also posted from UAS surveys: annual visible and multi-spectral surveys, as well as snow depth. This dataset comprises thaw depth measurements along transects near beaver ponds, to document permafrost impacts over time. 

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5. Projecting Permafrost Thaw of Sub‐Arctic Tundra With a Thermodynamic Model Calibrated to Site Measurements

   https://doi.org/10.1029/2020JG006218  

   Garnello, A. ; Marchenko, S. ; Nicolsky, D. ; Romanovsky, V. ; Ledman, J. ; Celis, G. ; Schädel, C. ; Luo, Y. ; Schuur, E. A. G. ( June 2021 , Journal of Geophysical Research: Biogeosciences)

   Northern circumpolar permafrost thaw affects global carbon cycling, as large amounts of stored soil carbon becomes accessible to microbial breakdown under a warming climate. The magnitude of carbon release is linked to the extent of permafrost thaw, which is locally variable and controlled by soil thermodynamics. Soil thermodynamic properties, such as thermal diffusivity, govern the reactivity of the soil‐atmosphere thermal gradient, and are controlled by soil composition and drainage. In order to project permafrost thaw for an Alaskan tundra experimental site, we used seven years of site data to calibrate a soil thermodynamic model using a data assimilation technique. The model reproduced seasonal and interannual temperature dynamics for shallow (5–40 cm) and deep soil layers (2–4 m), and simulations of seasonal thaw depth closely matched observed data. The model was then used to project permafrost thaw at the site to the year 2100 using climate forcing data for three future climate scenarios (RCP 4.5, 6.0, and 8.5). Minimal permafrost thawing occurred until mean annual air temperatures rose above the freezing point, after which we measured over a 1 m increase in thaw depth for every 1 °C rise in mean annual air temperature. Under no projected warming scenario was permafrost remaining in the upper 3 m of soil by 2100. We demonstrated an effective data assimilation method that optimizes parameterization of a soil thermodynamic model. The sensitivity of local permafrost to climate warming illustrates the vulnerability of sub‐Arctic tundra ecosystems to significant and rapid soil thawing.

    

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