Small water bodies (i.e., ponds; <0.01 km2) play an important role in Earth System processes, including carbon cycling and emissions of methane. Detection and monitoring of ponds using satellite ...imagery has been extremely difficult and many water maps are biased toward lakes (>0.01 km2). We leverage high‐resolution (3 m) optical satellite imagery from Planet Labs and deep learning methods to map seasonal changes in pond and lake areal extent across four regions in Alaska. Our water maps indicate that changes in open water extent over the snow‐free season are especially pronounced in ponds. To investigate potential impacts of seasonal changes in pond area on carbon emissions, we provide a case study of open water methane emission budgets using the new water maps. Our approach has widespread applications for water resources, habitat and land cover change assessments, wildlife management, risk assessments, and other biogeochemical modeling efforts.
Plain Language Summary
Small water bodies (<0.01 km2) are an important driver of many Earth system processes. Despite their importance, many existing water mapping products have difficulty detecting these small water features and their seasonal changes in surface area. We used deep learning and high‐resolution (3 m) satellite imagery to map and monitor seasonal changes in the areal extent of lakes and small ponds across four regions in Alaska. The resulting water maps accounted for considerably more water coverage than existing products. The maps also effectively tracked widespread seasonal changes in pond and lake area that were not previously identified. This demonstrates the importance of monitoring surface water at high spatial resolutions and across seasons.
Key Points
Deep learning and 3 m resolution satellite imagery from Planet Labs can detect and track ponds and lakes >0.0001 km2
Total surface area for ponds (<0.01 km2) in boreal forest and tundra environments can vary by 20%–40% throughout an individual season
Ponds can contribute to a broad range (8%–37%) of total methane emissions from lakes and ponds in northern boreal forest and tundra
Methane (CH4) emissions from thawing permafrost amplify a climate warming feedback. However, upscaling of site‐level CH4 observations across diverse Arctic landscapes remains highly uncertain, ...compromising accuracy of current pan‐Arctic CH4 budgets and confidence in model forecasts. We report a 30,000‐km2 survey at 25‐m2 resolution (~1 billion observations) of CH4 hotspot patterns across Alaska and northwestern Canada using airborne imaging spectroscopy. Hotspots covered 0.2% of the surveyed area, concentrated in the wetland‐upland ecotone, and followed a two‐component power law as a function of distance from standing water. Hotspots decreased sharply over the first 40 m from standing water (y = 0.21×−0.649, R2 = 0.97), mirroring in situ flux observations. Beyond 40 m, CH4 hotspots diminished gradually over hundreds of meters (y = 0.004×−0.164, R2 = 0.99). This emergent property quantifies the distribution of strong methanogenic zones from site to regional scales, vastly improving metrics for scaling ground‐based CH4 inventories and validation of land models.
Plain Language Summary
Understanding Arctic methane emissions is crucial to forecasting the region's impact on global climate. Ongoing efforts suffer large uncertainties when upscaling emissions since direct observations rarely cover scales relevant to both process‐level (fine‐scale) biogeochemistry and land models that operate on much larger scales. We bridge these scale gaps via high‐resolution airborne detection of methane hotspots (25‐m2 pixels) across a 30,000‐km2 study domain. We quantified a key spatial property of Arctic methane emissions: their power law dependence on distance to nearest standing water. From the ground, we verified that wide‐ranging methane fluxes follow the same spatial power law pattern as domain‐wide hotspots. These conclusions can improve scaling of emissions in Arctic land models and potentially reduce the disparity between ground‐based and atmospheric emission budgeting.
Key Points
Airborne imaging spectrometry of broad regions in the North American Arctic has observed roughly 2 million methane emission hotspots
Hotspots follow two spatial power laws, where 40 m from water is a threshold for occurrence and a novel parameter for emissions upscaling
Wide‐ranging CH4 fluxes at two thermokarst lakes mirror the patterns of remotely sensed hotspots, validating the remotely derived power laws
Abstract
Rapid warming in Arctic tundra may lead to drier soils in summer and greater lightning ignition rates, likely culminating in enhanced wildfire risk. Increased wildfire frequency and ...intensity leads to greater conversion of permafrost carbon to greenhouse gas emissions. Here, we quantify the effect of recent tundra fires on the creation of methane (CH
4
) emission hotspots, a fingerprint of the permafrost carbon feedback. We utilized high-resolution (∼25 m
2
pixels) and broad coverage (1780 km
2
) airborne imaging spectroscopy and maps of historical wildfire-burned areas to determine whether CH
4
hotspots were more likely in areas burned within the last 50 years in the Yukon–Kuskokwim Delta, Alaska, USA. Our observations provide a unique observational constraint on CH
4
dynamics, allowing us to map CH
4
hotspots in relation to individual burn events, burn scar perimeters, and proximity to water. We find that CH
4
hotspots are roughly 29% more likely on average in tundra that burned within the last 50 years compared to unburned areas and that this effect is nearly tripled along burn scar perimeters that are delineated by surface water features. Our results indicate that the changes following tundra fire favor the complex environmental conditions needed to generate CH
4
emission hotspots. We conclude that enhanced CH
4
emissions following tundra fire represent a positive feedback that will accelerate climate warming, tundra fire occurrence, and future permafrost carbon loss to the atmosphere.
Abstract
Northern high-latitude lakes are critical sites for carbon processing and serve as potential conduits for the emission of permafrost-derived carbon and greenhouse gases. However, the fate ...and emission pathways of permafrost carbon in these systems remain uncertain. Here, we used the natural abundance of radiocarbon to identify and trace the predominant sources of methane, carbon dioxide, dissolved inorganic and organic carbon in nine lakes within the Yukon Flats National Wildlife Refuge in interior Alaska, a discontinuous permafrost region with high landscape heterogeneity and susceptibility to climate, permafrost, and hydrological changes. We find that although Yukon Flats lakes primarily process young carbon (modern to 1290 ± 60 years before present), permafrost-derived carbon is present in some of the sampled lakes and contributes, at most, 30 ± 10% of the dissolved carbon in lake surface waters. Apportionment of young carbon and legacy carbon (carbon with radiocarbon age ⩾5000 years before present) is decoupled among the dissolved inorganic and organic carbon species, with methane showing a stronger legacy signature. Our observations suggest that permafrost-thaw-related transport of carbon through Yukon Flats lacustrine ecosystems and into the atmosphere is small, and likely regulated by surficial sediments, permafrost distribution, wildfire occurrence, or masked by contemporary carbon processes. The heterogeneity of lakes across our study area and northern landscapes more broadly cautions against using any one region (e.g. Yedoma permafrost lakes) to upscale their contribution across the pan-Arctic.
Beavers have established themselves as a key component of low arctic ecosystems over the past several decades. Beavers are widely recognized as ecosystem engineers, but their effects on ...permafrost-dominated landscapes in the Arctic remain unclear. In this study, we document the occurrence, reconstruct the timing, and highlight the effects of beaver activity on a small creek valley confined by ice-rich permafrost on the Seward Peninsula, Alaska using multi-dimensional remote sensing analysis of satellite (Landsat-8, Sentinel-2, Planet CubeSat, and DigitalGlobe Inc./MAXAR) and unmanned aircraft systems (UAS) imagery. Beaver activity along the study reach of Swan Lake Creek appeared between 2006 and 2011 with the construction of three dams. Between 2011 and 2017, beaver dam numbers increased, with the peak occurring in 2017 (n = 9). Between 2017 and 2019, the number of dams decreased (n = 6), while the average length of the dams increased from 20 to 33 m. Between 4 and 20 August 2019, following a nine-day period of record rainfall (>125 mm), the well-established dam system failed, triggering the formation of a beaver-induced permafrost degradation feature. During the decade of beaver occupation between 2011 and 2021, the creek valley widened from 33 to 180 m (~450% increase) and the length of the stream channel network increased from ~0.6 km to more than 1.9 km (220% increase) as a result of beaver engineering and beaver-induced permafrost degradation. Comparing vegetation (NDVI) and snow (NDSI) derived indices from Sentinel-2 time-series data acquired between 2017 and 2021 for the beaver-induced permafrost degradation feature and a nearby unaffected control site, showed that peak growing season NDVI was lowered by 23% and that it extended the length of the snow-cover period by 19 days following the permafrost disturbance. Our analysis of multi-dimensional remote sensing data highlights several unique aspects of beaver engineering impacts on ice-rich permafrost landscapes. Our detailed reconstruction of the beaver-induced permafrost degradation event may also prove useful for identifying degradation of ice-rich permafrost in optical time-series datasets across regional scales. Future field- and remote sensing-based observations of this site, and others like it, will provide valuable information for the NSF-funded Arctic Beaver Observation Network (A-BON) and the third phase of the NASA Arctic-Boreal Vulnerability Experiment (ABoVE) Field Campaign.
Despite the global significance of the subsurface biosphere, the degree to which it depends on surface organic carbon (OC) is still poorly understood. Here, we compare stable and radiogenic carbon ...isotope compositions of microbial phospholipid fatty acids (PLFAs) with those of in situ potential microbial C sources to assess the major C sources for subsurface microorganisms in biogeochemical distinct shallow aquifers (Critical Zone Exploratory, Thuringia Germany). Despite the presence of younger OC, the microbes assimilated 14C‐free OC to varying degrees; ~31% in groundwater within the oxic zone, ~47% in an iron reduction zone, and ~70% in a sulfate reduction/anammox zone. The persistence of trace amounts of mature and partially biodegraded hydrocarbons suggested that autochthonous petroleum‐derived hydrocarbons were a potential 14C‐free C source for heterotrophs in the oxic zone. In this zone, Δ14C values of dissolved inorganic carbon (−366 ± 18‰) and 11MeC16:0 (−283 ± 32‰), an important component in autotrophic nitrite oxidizers, were similar enough to indicate that autotrophy is an important additional C fixation pathway. In anoxic zones, methane as an important C source was unlikely since the 13C‐fractionations between the PLFAs and CH4 were inconsistent with kinetic isotope effects associated with methanotrophy. In the sulfate reduction/anammox zone, the strong 14C‐depletion of 10MeC16:0 (−942 ± 22‰), a PLFA common in sulfate reducers, indicated that those bacteria were likely to play a critical part in 14C‐free sedimentary OC cycling. Results indicated that the 14C‐content of microbial biomass in shallow sedimentary aquifers results from complex interactions between abundance and bioavailability of naturally occurring OC, hydrogeology, and specific microbial metabolisms.
Key Points
Microbial lipids (PLFAs) and C sources in aquifers with different subsurface‐surface relationships were analyzed for their 14C‐content
Subsurface microbes incorporated 14C‐free C in proportions depending on the physiological strategies of the microorganism community
Autotrophic nitrite oxidation occurred in oxic zone, whereas heterotrophy on sedimentary C dominated in sulfate reduction/anammox zone
Despite the global significance of the subsurface biosphere, the degree to which it depends on surface organic carbon (OC) is still poorly understood. Here, we compare stable and radiogenic carbon ...isotope compositions of microbial phospholipid fatty acids (PLFAs) with those of in situ potential microbial C sources to assess the major C sources for subsurface microorganisms in biogeochemical distinct shallow aquifers (Critical Zone Exploratory, Thuringia Germany). Despite the presence of younger OC, the microbes assimilated
C-free OC to varying degrees; ~31% in groundwater within the oxic zone, ~47% in an iron reduction zone, and ~70% in a sulfate reduction/anammox zone. The persistence of trace amounts of mature and partially biodegraded hydrocarbons suggested that autochthonous petroleum-derived hydrocarbons were a potential
C-free C source for heterotrophs in the oxic zone. In this zone, Δ
C values of dissolved inorganic carbon (-366 ± 18‰) and 11MeC16:0 (-283 ± 32‰), an important component in autotrophic nitrite oxidizers, were similar enough to indicate that autotrophy is an important additional C fixation pathway. In anoxic zones, methane as an important C source was unlikely since the
C-fractionations between the PLFAs and CH
were inconsistent with kinetic isotope effects associated with methanotrophy. In the sulfate reduction/anammox zone, the strong
C-depletion of 10MeC16:0 (-942 ± 22‰), a PLFA common in sulfate reducers, indicated that those bacteria were likely to play a critical part in
C-free sedimentary OC cycling. Results indicated that the
C-content of microbial biomass in shallow sedimentary aquifers results from complex interactions between abundance and bioavailability of naturally occurring OC, hydrogeology, and specific microbial metabolisms.
Carbon dioxide and methane emissions are the two primary anthropogenic climate-forcing agents and an important source of uncertainty in the global carbon budget. Uncertainties are further magnified ...when emissions occur at fine spatial scales (<1 km), making attribution challenging. We present the first observations from NASA’s Earth Surface Mineral Dust Source Investigation (EMIT) imaging spectrometer showing quantification and attribution of fine-scale methane (0.3 to 73 tonnes CH
4
hour
−1
) and carbon dioxide sources (1571 to 3511 tonnes CO
2
hour
−1
) spanning the oil and gas, waste, and energy sectors. For selected countries observed during the first 30 days of EMIT operations, methane emissions varied at a regional scale, with the largest total emissions observed for Turkmenistan (731 ± 148 tonnes CH
4
hour
−1
). These results highlight the contributions of current and planned point source imagers in closing global carbon budgets.
First glimpse of methane and carbon dioxide emissions observed with the Earth Surface Mineral Dust Source Investigation (EMIT).
Methane (CH4) emissions from climate‐sensitive ecosystems within the northern permafrost region represent a potentially large but highly uncertain source, with current estimates spanning a factor of ...seven (11–75 Tg CH4 yr−1). Accelerating permafrost thaw threatens significant increases in pan‐Arctic CH4 emissions, amplifying the permafrost carbon feedback. We used airborne imaging spectroscopy with meter‐scale spatial resolution and broad coverage to identify a previously undiscovered CH4 emission hotspot adjacent to a thermokarst lake in interior Alaska. Hotspot emissions were confined to <1% of the 10 ha lake study area. Ground‐based chamber measurements confirmed average daily fluxes from the hotspot of 1,170 mg CH4 m−2 d−1, with extreme daily maxima up to 24,200 mg CH4 m−2 d−1. Ground‐based geophysical measurements revealed thawed permafrost directly beneath the CH4 hotspot, extending to a depth of ∼15 m, indicating that the intense CH4 emissions likely originated from recently thawed permafrost. Hotspot emissions accounted for ∼40% of total diffusive CH4 emissions from the lake study site. Combining study site findings with hotspot statistics from our 70,000 km2 airborne survey across Alaska and northwestern Canada, we estimate that pan‐Arctic terrestrial thermokarst hotspots currently emit 1.1 (0.1–5.2) Tg CH4 yr−1, or roughly 4% of the annual pan‐Arctic wetland budget from just 0.01% of the northern permafrost land area. Our results suggest that significant proportions of pan‐Arctic CH4 emissions originate from disproportionately small areas of previously undetermined thermokarst emissions hotspots, and that pan‐Arctic CH4 emissions may increase non‐linearly as thermokarst processes increase under a warming climate.
Plain Language Summary
We conducted high‐resolution airborne surveys of near‐surface methane (CH4, a powerful greenhouse gas) anomalies in permafrost ecosystems in Alaska and northwestern Canada as part of NASA's Arctic Boreal Vulnerability Experiment (ABoVE). These measurements provided fine‐scale resolution for the remote detection of CH4 emission hotspots from natural Arctic environments. Repeated flights over Big Trail Lake near Fairbanks, AK revealed a previously undiscovered CH4 hotspot at this intensive study site. Ground‐based measurements confirmed extremely high surface‐to‐atmosphere emissions at this location, on the shore of a permafrost‐thaw pond that formed after 1963. Geophysical surveys confirmed the presence of thawed permafrost underneath the hotspot, extending to a depth ∼15 m. We hypothesize that recent permafrost thaw and subsidence made soils with highly decomposable organic carbon available for microbial metabolism, conversion into CH4, and enhanced emission to the atmosphere. Extrapolating our observed hotspot fluxes across the pan‐Arctic, we estimate that thermokarst CH4 hotspots constitute less than 0.01% of the pan‐Arctic land area, but contribute roughly 4% of annual pan‐Arctic wetland emissions. We further hypothesize that Arctic CH4 emissions may grow significantly in the future with anticipated increases in thermokarst across the permafrost landscape.
Key Points
Repeat airborne spectral imaging geolocated a thermokarst methane (CH4) hotspot with ground‐validated emissions >10 g CH4 m−2 d−1
Hotspot CH4 emissions arose from <1% of our 10 ha thermokarst lake study area but comprised ∼40% of the total diffusive emissions
Ground‐based and airborne observations suggest thermokarst hotspots emit roughly 1.1 Tg CH4 yr−1 or 4% of pan‐Arctic wetland CH4 emissions
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