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1. Synthetic aperture radar (SAR) detects large gas seeps in Alaska lakes

   https://doi.org/10.1088/1748-9326/ad2b2a  

   Engram, Melanie; Anthony, Katey Walter (March 2024, Environmental Research Letters)

   Abstract Reservoirs of¹⁴C-depleted methane (CH₄), a potent greenhouse gas, residing beneath permafrost are vulnerable to escape where permafrost thaw creates open-talik conduits. However, little is known about the magnitude and variability of this methane source or its response to climate change. Remote-sensing detection of large gas seeps would be useful for establishing a baseline understanding of sub-permafrost methane seepage, as well as for monitoring these seeps over time. Here we explored synthetic aperture radar’s (SAR) response to large sub-permafrost gas seeps in an interior Alaskan lake. In SAR scenes from 1992 to 2011, we observed high perennial SAR L-band backscatter (σ⁰) from a ∼90 m-wide feature in the winter ice of interior Alaska’s North Blair Lake (NBL). Spring and fall optical imagery showed holes in the ice at the same location as the SAR anomaly. Through field work we (1) confirmed gas bubbling at this location from a large pockmark in the lakebed, (2) measured flux at the location of densest bubbles (1713 ± 290 mg CH₄m⁻²d⁻¹), and (3) determined the bubbles’ methane mixing ratio (6.6%), radiocarbon age (18 470 ± 50 years BP), and δ¹³C_CH4values (−44.5 ± 0.1‰), which together may represent a mixture of sources and processes. We performed a first order comparison of SARσ⁰from the NBL seep and other known sub-permafrost methane seeps with diverse ice/water interface shapes in order to evaluate the variability of SAR signals from a variety of seep types. Results from single-polarized intensity and polarimetric L-band SAR decompositions as well as dual-polarized C-band SAR are presented with the aim to find the optimal SAR imaging parameters to detect large methane seeps in frozen lakes. Our study indicates the potential for SAR remote sensing to be used to detect and monitor large, sub-permafrost gas seeps in Arctic and sub-Arctic lakes. 

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2. Detecting the permafrost carbon feedback: talik formation and increased cold-season respiration as precursors to sink-to-source transitions

   https://doi.org/10.5194/tc-12-123-2018  

   Parazoo, Nicholas C.; Koven, Charles D.; Lawrence, David M.; Romanovsky, Vladimir; Miller, Charles E. (January 2018, The Cryosphere)

   Abstract. Thaw and release of permafrost carbon (C) due to climate change is likely tooffset increased vegetation C uptake in northern high-latitude (NHL)terrestrial ecosystems. Models project that this permafrost C feedback mayact as a slow leak, in which case detection and attribution of the feedbackmay be difficult. The formation of talik, a subsurface layer of perenniallythawed soil, can accelerate permafrost degradation and soil respiration,ultimately shifting the C balance of permafrost-affected ecosystems fromlong-term C sinks to long-term C sources. It is imperative to understand andcharacterize mechanistic links between talik, permafrost thaw, andrespiration of deep soil C to detect and quantify the permafrost C feedback.Here, we use the Community Land Model (CLM) version 4.5, a permafrost andbiogeochemistry model, in comparison to long-term deep borehole data alongNorth American and Siberian transects, to investigate thaw-driven C sourcesin NHL (>55^∘N) from 2000 to 2300. Widespread talik at depth isprojected across most of the NHL permafrost region(14million km²) by 2300, 6.2million km² of which isprojected to become a long-term C source, emitting 10Pg C by 2100,50Pg C by 2200, and 120Pg C by 2300, with few signs ofslowing. Roughly half of the projected C source region is in predominantlywarm sub-Arctic permafrost following talik onset. This region emits only20Pg C by 2300, but the CLM4.5 estimate may be biased low by notaccounting for deep C in yedoma. Accelerated decomposition of deep soilC following talik onset shifts the ecosystem C balance away from surfacedominant processes (photosynthesis and litter respiration), butsink-to-source transition dates are delayed by 20–200 years by highecosystem productivity, such that talik peaks early (∼2050s, although boreholedata suggest sooner) and C source transition peaks late(∼2150–2200). The remaining C source region in cold northern Arcticpermafrost, which shifts to a net source early (late 21st century), emits5 times more C (95Pg C) by 2300, and prior to talik formation dueto the high decomposition rates of shallow, young C in organic-rich soilscoupled with low productivity. Our results provide important clues signalingimminent talik onset and C source transition, including (1) late cold-season(January–February) soil warming at depth (∼2m),(2) increasing cold-season emissions (November–April), and (3) enhancedrespiration of deep, old C in warm permafrost and young, shallow C in organic-rich cold permafrost soils. Our results suggest a mosaic of processes thatgovern carbon source-to-sink transitions at high latitudes and emphasize theurgency of monitoring soil thermal profiles, organic C age and content, cold-season CO₂ emissions, andatmospheric ¹⁴CO₂ as key indicatorsof the permafrost C feedback. 

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3. Soil respiration strongly offsets carbon uptake in Alaska and Northwest Canada

   https://doi.org/10.1088/1748-9326/ac1222  

   Watts, Jennifer D.; Natali, Susan M.; Minions, Christina; Risk, Dave; Arndt, Kyle; Zona, Donatella; Euskirchen, Eugénie S.; Rocha, Adrian V.; Sonnentag, Oliver; Helbig, Manuel; et al (August 2021, Environmental Research Letters)

   Abstract Soil respiration (i.e. from soils and roots) provides one of the largest global fluxes of carbon dioxide (CO₂) to the atmosphere and is likely to increase with warming, yet the magnitude of soil respiration from rapidly thawing Arctic-boreal regions is not well understood. To address this knowledge gap, we first compiled a new CO₂flux database for permafrost-affected tundra and boreal ecosystems in Alaska and Northwest Canada. We then used the CO₂database, multi-sensor satellite imagery, and random forest models to assess the regional magnitude of soil respiration. The flux database includes a new Soil Respiration Station network of chamber-based fluxes, and fluxes from eddy covariance towers. Our site-level data, spanning September 2016 to August 2017, revealed that the largest soil respiration emissions occurred during the summer (June–August) and that summer fluxes were higher in boreal sites (1.87 ± 0.67 g CO₂–C m⁻²d⁻¹) relative to tundra (0.94 ± 0.4 g CO₂–C m⁻²d⁻¹). We also observed considerable emissions (boreal: 0.24 ± 0.2 g CO₂–C m⁻²d⁻¹; tundra: 0.18 ± 0.16 g CO₂–C m⁻²d⁻¹) from soils during the winter (November–March) despite frozen surface conditions. Our model estimates indicated an annual region-wide loss from soil respiration of 591 ± 120 Tg CO₂–C during the 2016–2017 period. Summer months contributed to 58% of the regional soil respiration, winter months contributed to 15%, and the shoulder months contributed to 27%. In total, soil respiration offset 54% of annual gross primary productivity (GPP) across the study domain. We also found that in tundra environments, transitional tundra/boreal ecotones, and in landscapes recently affected by fire, soil respiration often exceeded GPP, resulting in a net annual source of CO₂to the atmosphere. As this region continues to warm, soil respiration may increasingly offset GPP, further amplifying global climate change. 

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4. Permafrost Carbon: Progress on Understanding Stocks and Fluxes Across Northern Terrestrial Ecosystems

   https://doi.org/10.1029/2023JG007638  

   Treat, Claire C.; Virkkala, Anna‐Maria; Burke, Eleanor; Bruhwiler, Lori; Chatterjee, Abhishek; Fisher, Joshua B.; Hashemi, Josh; Parmentier, Frans‐Jan W.; Rogers, Brendan M.; Westermann, Sebastian; et al (February 2024, Journal of Geophysical Research: Biogeosciences)

   Abstract Significant progress in permafrost carbon science made over the past decades include the identification of vast permafrost carbon stocks, the development of new pan‐Arctic permafrost maps, an increase in terrestrial measurement sites for CO₂and methane fluxes, and important factors affecting carbon cycling, including vegetation changes, periods of soil freezing and thawing, wildfire, and other disturbance events. Process‐based modeling studies now include key elements of permafrost carbon cycling and advances in statistical modeling and inverse modeling enhance understanding of permafrost region C budgets. By combining existing data syntheses and model outputs, the permafrost region is likely a wetland methane source and small terrestrial ecosystem CO₂sink with lower net CO₂uptake toward higher latitudes, excluding wildfire emissions. For 2002–2014, the strongest CO₂sink was located in western Canada (median: −52 g C m⁻² y⁻¹) and smallest sinks in Alaska, Canadian tundra, and Siberian tundra (medians: −5 to −9 g C m⁻² y⁻¹). Eurasian regions had the largest median wetland methane fluxes (16–18 g CH₄m⁻² y⁻¹). Quantifying the regional scale carbon balance remains challenging because of high spatial and temporal variability and relatively low density of observations. More accurate permafrost region carbon fluxes require: (a) the development of better maps characterizing wetlands and dynamics of vegetation and disturbances, including abrupt permafrost thaw; (b) the establishment of new year‐round CO₂and methane flux sites in underrepresented areas; and (c) improved models that better represent important permafrost carbon cycle dynamics, including non‐growing season emissions and disturbance effects. 

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5. First pan-Arctic assessment of dissolved organic carbon in lakes of the permafrost region

   https://doi.org/10.5194/bg-18-3917-2021  

   Stolpmann, Lydia; Coch, Caroline; Morgenstern, Anne; Boike, Julia; Fritz, Michael; Herzschuh, Ulrike; Stoof-Leichsenring, Kathleen; Dvornikov, Yury; Heim, Birgit; Lenz, Josefine; et al (January 2021, Biogeosciences)

   null (Ed.)

   Abstract. Lakes in permafrost regions are dynamic landscapecomponents and play an important role for climate change feedbacks. Lakeprocesses such as mineralization and flocculation of dissolved organiccarbon (DOC), one of the main carbon fractions in lakes, contribute to thegreenhouse effect and are part of the global carbon cycle. These processesare in the focus of climate research, but studies so far are limited to specificstudy regions. In our synthesis, we analyzed 2167 water samples from 1833lakes across the Arctic in permafrost regions of Alaska, Canada, Greenland,and Siberia to provide first pan-Arctic insights for linkages between DOCconcentrations and the environment. Using published data and unpublisheddatasets from the author team, we report regional DOC differences linked tolatitude, permafrost zones, ecoregions, geology, near-surface soil organiccarbon contents, and ground ice classification of each lake region. The lakeDOC concentrations in our dataset range from 0 to1130 mg L−1 (10.8 mg L−1 median DOC concentration). Regarding thepermafrost regions of our synthesis, we found median lake DOC concentrationsof 12.4 mg L−1 (Siberia), 12.3 mg L−1 (Alaska),10.3 mg L−1 (Greenland), and 4.5 mg L−1 (Canada). Our synthesisshows a significant relationship between lake DOC concentration and lakeecoregion. We found higher lake DOC concentrations at boreal permafrostsites compared to tundra sites. We found significantly higher DOCconcentrations in lakes in regions with ice-rich syngenetic permafrostdeposits (yedoma) compared to non-yedoma lakes and a weak but significantrelationship between soil organic carbon content and lake DOC concentrationas well as between ground ice content and lake DOC. Our pan-Arctic datasetshows that the DOC concentration of a lake depends on its environmentalproperties, especially on permafrost extent and ecoregion, as well asvegetation, which is the most important driver of lake DOC in this study.This new dataset will be fundamental to quantify a pan-Arctic lake DOC poolfor estimations of the impact of lake DOC on the global carbon cycle andclimate change. 

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