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1. Methane production controls in a young thermokarst lake formed by abrupt permafrost thaw

   https://doi.org/10.1111/gcb.16151  

   Pellerin, André ; Lotem, Noam ; Walter Anthony, Katey ; Eliani Russak, Efrat ; Hasson, Nicholas ; Røy, Hans ; Chanton, Jeffrey P. ; Sivan, Orit ( March 2022 , Global Change Biology)

   Methane (CH₄) release to the atmosphere from thawing permafrost contributes significantly to global CH₄emissions. However, constraining the effects of thaw that control the production and emission of CH₄is needed to anticipate future Arctic emissions. Here are presented robust rate measurements of CH₄production and cycling in a region of rapidly degrading permafrost. Big Trail Lake, located in central Alaska, is a young, actively expanding thermokarst lake. The lake was investigated by taking two 1 m cores of sediment from different regions. Two independent methods of measuring microbial CH₄ production, long term (CH₄accumulation) and short term (¹⁴C tracer), produced similar average rates of 11 ± 3.5 and 9 ± 3.6 nmol cm⁻³ d⁻¹, respectively. The rates had small variations between the different lithological units, indicating homogeneous CH₄production despite heterogeneous lithology in the surface ~1 m of sediment. To estimate the total CH₄production, the CH₄production rates were multiplied through the 10–15 m deep talik (thaw bulb). This estimate suggests that CH₄ production is higher than emission by a maximum factor of ~2, which is less than previous estimates. Stable and radioactive carbon isotope measurements showed that 50% of dissolved CH₄in the first meter was produced further below. Interestingly, labeled¹⁴C incubations with 2‐¹⁴C acetate and¹⁴C CO₂indicate that variations in the pathway used by microbes to produce CH₄depends on the age and type of organic matter in the sediment, but did not appear to influence the rates at which CH₄ was produced. This study demonstrates that at least half of the CH₄produced by microbial breakdown of organic matter in actively expanding thermokarst is emitted to the atmosphere, and that the majority of this CH₄is produced in the deep sediment.

    

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2. Decadal-scale hotspot methane ebullition within lakes following abrupt permafrost thaw

   https://doi.org/10.1088/1748-9326/abc848  

   Walter Anthony, K. M. ; Lindgren, P. ; Hanke, P. ; Engram, M. ; Anthony, P. ; Daanen, R. P. ; Bondurant, A. ; Liljedahl, A. K. ; Lenz, J. ; Grosse, G. ; et al ( February 2021 , Environmental Research Letters)

   Thermokarst lakes accelerate deep permafrost thaw and the mobilization of previously frozen soil organic carbon. This leads to microbial decomposition and large releases of carbon dioxide (CO₂) and methane (CH₄) that enhance climate warming. However, the time scale of permafrost-carbon emissions following thaw is not well known but is important for understanding how abrupt permafrost thaw impacts climate feedback. We combined field measurements and radiocarbon dating of CH₄ebullition with (a) an assessment of lake area changes delineated from high-resolution (1–2.5 m) optical imagery and (b) geophysical measurements of thaw bulbs (taliks) to determine the spatiotemporal dynamics of hotspot-seep CH₄ebullition in interior Alaska thermokarst lakes. Hotspot seeps are characterized as point-sources of high ebullition that release¹⁴C-depleted CH₄from deep (up to tens of meters) within lake thaw bulbs year-round. Thermokarst lakes, initiated by a variety of factors, doubled in number and increased 37.5% in area from 1949 to 2009 as climate warmed. Approximately 80% of contemporary CH₄hotspot seeps were associated with this recent thermokarst activity, occurring where 60 years of abrupt thaw took place as a result of new and expanded lake areas. Hotspot occurrence diminished with distance from thermokarst lake margins. We attribute older¹⁴C ages of CH₄released from hotspot seeps in older, expanding thermokarst lakes (¹⁴C_CH420 079 ± 1227 years BP, mean ± standard error (s.e.m.) years) to deeper taliks (thaw bulbs) compared to younger¹⁴C_CH4in new lakes (¹⁴C_CH48526 ± 741 years BP) with shallower taliks. We find that smaller, non-hotspot ebullition seeps have younger¹⁴C ages (expanding lakes 7473 ± 1762 years; new lakes 4742 ± 803 years) and that their emissions span a larger historic range. These observations provide a first-order constraint on the magnitude and decadal-scale duration of CH₄-hotspot seep emissions following formation of thermokarst lakes as climate warms.

    

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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. Anaerobic oxidation of methane does not attenuate methane emissions from thermokarst lakes

   https://doi.org/10.1002/lno.12349  

   Lotem, Noam ; Pellerin, André ; Anthony, Katey Walter ; Gafni, Almog ; Boyko, Valeria ; Sivan, Orit ( April 2023 , Limnology and Oceanography)

   The ongoing global temperature rise enhances permafrost thaw in the Arctic, allowing Pleistocene‐aged frozen soil organic matter to become available for microbial degradation and production of greenhouse gases, particularly methane. Here, we examined the extent and mechanism of anaerobic oxidation of methane (AOM) in the sediments of four interior Alaska thermokarst lakes, which formed and continue to expand as a result of ice‐rich permafrost thaw. In cores of surface (~ 1 m) lake sediments we quantified methane production (methanogenesis) and AOM rates using anaerobic incubation experiments in low (4°C) and high (16°C) temperatures. Methanogenesis rates were measured by the accumulation of methane over ~ 90 d, whereas AOM rates were measured by adding labeled‐¹³CH₄and measuring the produced dissolved inorganic¹³C. Our results demonstrate that while methanogenesis was vigorous in these anoxic sediments, AOM was lower by two orders of magnitude. In almost all sediment depths and temperatures, AOM rates remained less than 2% of the methanogenesis rates. Experimental evidence indicates that the AOM is strongly related to methanogens, as the addition of a methanogens' inhibitor prevented AOM. Variety of electron acceptor additions did not stimulate AOM, and methanotrophs were scarcely detected. These observations suggest that the AOM signals in the incubation experiments might be a result of enzymatic reversibility (“back‐flux”) during CH₄production, rather than thermodynamically favorable AOM. Regardless of the mechanism, the quantitative results show that near surface (< 1‐m) thermokarst sediments in interior Alaska have little to no buffer mechanisms capable of attenuating methane production in a warming climate.

    

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5. Recent warming reverses forty-year decline in catastrophic lake drainage and hastens gradual lake drainage across northern Alaska

   https://doi.org/10.1088/1748-9326/ac3602  

   Lara, Mark J. ; Chen, Yaping ; Jones, Benjamin M. ( November 2021 , Environmental Research Letters)

   Lakes represent as much as ∼25% of the total land surface area in lowland permafrost regions. Though decreasing lake area has become a widespread phenomenon in permafrost regions, our ability to forecast future patterns of lake drainage spanning gradients of space and time remain limited. Here, we modeled the drivers of gradual (steady declining lake area) and catastrophic (temporally abrupt decrease in lake area) lake drainage using 45 years of Landsat observations (i.e. 1975–2019) across 32 690 lakes spanning climate and environmental gradients across northern Alaska. We mapped lake area using supervised support vector machine classifiers and object based image analyses using five-year Landsat image composites spanning 388 968 km². Drivers of lake drainage were determined with boosted regression tree models, using both static (e.g. lake morphology, proximity to drainage gradient) and dynamic predictor variables (e.g. temperature, precipitation, wildfire). Over the past 45 years, gradual drainage decreased lake area between 10% and 16%, but rates varied over time as the 1990s recorded the highest rates of gradual lake area losses associated with warm periods. Interestingly, the number of catastrophically drained lakes progressively decreased at a rate of ∼37% decade⁻¹from 1975–1979 (102–273 lakes draining year⁻¹) to 2010–2014 (3–8 lakes draining year⁻¹). However this 40 year negative trend was reversed during the most recent time-period (2015–2019), with observations of catastrophic drainage among the highest on record (i.e. 100–250 lakes draining year⁻¹), the majority of which occurred in northwestern Alaska. Gradual drainage processes were driven by lake morphology, summer air and lake temperature, snow cover, active layer depth, and the thermokarst lake settlement index (R²_adj= 0.42, CV = 0.35,p< 0.0001), whereas, catastrophic drainage was driven by the thawing season length, total precipitation, permafrost thickness, and lake temperature (R²_adj= 0.75, CV = 0.67,p< 0.0001). Models forecast a continued decline in lake area across northern Alaska by 15%–21% by 2050. However these estimates are conservative, as the anticipated amplitude of future climate change were well-beyond historical variability and thus insufficient to forecast abrupt ‘catastrophic’ drainage processes. Results highlight the urgency to understand the potential ecological responses and feedbacks linked with ongoing Arctic landscape reorganization.

    

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