9:15 AM - 9:30 AM
[ACG42-02] Localized thaw subsidence around Beaver Creek, Yukon: Detected by InSAR image analysis and field validation
Keywords:Permafrost, Wildfire, Polygon, Thermokarst, ALOS-2, InSAR
Permafrost stores about twice as much carbon as is in the atmosphere, and there is concern that microbial decomposition of stored organic carbon will promote the release of CO2 and CH4 into the atmosphere, thereby providing positive feedback on global warming (Schuur et al., 2022). However, Earth System Models (ESMs) in the latest IPCC AR6 report overlooked permafrost thawing processes due to insufficient observation data and did not consider abrupt thawing or wildfire-vegetation-permafrost interactions (Schädel et al., 2024). Therefore, there is a need for broad satellite observations of permafrost thawing processes, including abrupt thawing.
Our study focused on Beaver Creek (BC), Yukon Territory, Canada, particularly on the post-wildfire area in southern BC (28 km² burned between July and September 2019) and polygon terrain in northern BC. We conducted an Interferometric Synthetic Aperture Radar (InSAR) image analysis with the JAXA's L-band SAR satellite ALOS-2/PALSAR-2 data. Interannual ground deformation was analyzed using StripMap 10m (SM3) data archived since 2015. In the preliminary analysis, thaw subsidence around BC was spatially heterogeneous, and the coherence was locally low (Yanagiya et al., JpGU2023). Therefore, we conducted high-spatial resolution observations with StripMap 3m (SM1) data in the summer of 2023. Furthermore, we conducted field observations in the late summers of 2022 and 2023 as part of the international research project "PRISMARCTYC." The research objective of PRISMARCTYC is to understand the impacts of permafrost thaw on soil, water fluxes, and the carbon cycle. We carried out thaw depth measurements, GNSS measurements, and drone aerial photography in subsidence areas detected by InSAR images.
On the northern slope of the fire scar, we detected annual subsidence signals of about 6 cm from 2019 to 2023 and a seasonal subsidence signal of up to 15 cm from June to September 2023. The spatial distribution of the subsidence signal on the slope corresponded to the area surrounded by moraines on the surficial geology map. The average thaw depth at the control site adjacent to the fire scar was 45 cm in 2023, while the thaw depth on the transect along the slope ranged from 60 to 111 cm. The thaw depth reached its maximum value in the area of maximum thaw subsidence. These results suggest that the 2019 fire triggered localized melting of ice lenses or subsurface ground ice in the sedimentary layer distributed between moraines. Additionally, a more heterogeneous seasonal subsidence signal was detected in the central part of the fire scar, where hummocky moraines are distributed. The heterogeneous subsidence signal was inconsistent with the spatial pattern of dNBR, an index of burn severity calculated from Sentinel-2 data. However, it was generally consistent with an index of topographic undulation. Although SM3 data did not detect significant annual ground deformation in this area, this might be attributed to resolution limitations. We will analyze annual deformation with SM1 data and verify in the following field observation.
In the polygon terrain site of northern BC, ground deformation within the polygon could not be detected due to the low coherence caused by inundation. However, we detected a seasonal subsidence signal of up to 16 cm near the polygon from June to September 2023. In contrast to the fire scar, surface vegetation, topography, and surficial geology distribution were generally uniform inside and outside the maximum subsidence area. We will further collaborate with the electrical resistivity survey and coring teams to elucidate the subsurface structures in the maximum subsidence area and their relation to the expansion process of the polygon terrain.
Our study focused on Beaver Creek (BC), Yukon Territory, Canada, particularly on the post-wildfire area in southern BC (28 km² burned between July and September 2019) and polygon terrain in northern BC. We conducted an Interferometric Synthetic Aperture Radar (InSAR) image analysis with the JAXA's L-band SAR satellite ALOS-2/PALSAR-2 data. Interannual ground deformation was analyzed using StripMap 10m (SM3) data archived since 2015. In the preliminary analysis, thaw subsidence around BC was spatially heterogeneous, and the coherence was locally low (Yanagiya et al., JpGU2023). Therefore, we conducted high-spatial resolution observations with StripMap 3m (SM1) data in the summer of 2023. Furthermore, we conducted field observations in the late summers of 2022 and 2023 as part of the international research project "PRISMARCTYC." The research objective of PRISMARCTYC is to understand the impacts of permafrost thaw on soil, water fluxes, and the carbon cycle. We carried out thaw depth measurements, GNSS measurements, and drone aerial photography in subsidence areas detected by InSAR images.
On the northern slope of the fire scar, we detected annual subsidence signals of about 6 cm from 2019 to 2023 and a seasonal subsidence signal of up to 15 cm from June to September 2023. The spatial distribution of the subsidence signal on the slope corresponded to the area surrounded by moraines on the surficial geology map. The average thaw depth at the control site adjacent to the fire scar was 45 cm in 2023, while the thaw depth on the transect along the slope ranged from 60 to 111 cm. The thaw depth reached its maximum value in the area of maximum thaw subsidence. These results suggest that the 2019 fire triggered localized melting of ice lenses or subsurface ground ice in the sedimentary layer distributed between moraines. Additionally, a more heterogeneous seasonal subsidence signal was detected in the central part of the fire scar, where hummocky moraines are distributed. The heterogeneous subsidence signal was inconsistent with the spatial pattern of dNBR, an index of burn severity calculated from Sentinel-2 data. However, it was generally consistent with an index of topographic undulation. Although SM3 data did not detect significant annual ground deformation in this area, this might be attributed to resolution limitations. We will analyze annual deformation with SM1 data and verify in the following field observation.
In the polygon terrain site of northern BC, ground deformation within the polygon could not be detected due to the low coherence caused by inundation. However, we detected a seasonal subsidence signal of up to 16 cm near the polygon from June to September 2023. In contrast to the fire scar, surface vegetation, topography, and surficial geology distribution were generally uniform inside and outside the maximum subsidence area. We will further collaborate with the electrical resistivity survey and coring teams to elucidate the subsurface structures in the maximum subsidence area and their relation to the expansion process of the polygon terrain.