11:30 〜 11:45
[BCG05-08] Methane fluxes associated with groundwater springs in continuous permafrost of central Yakutia, Northern Eurasia
キーワード：methane, groundwater springs, permafrost groundwater
Groundwater is a known and significant source of methane. The major methane pathways to the atmosphere, associated with groundwater, include outgassing during pumping and shale gas deposits development, emission from submarine groundwater discharge zones and groundwater-fed lakes. In permafrost-affected regions, particularly in continuous permafrost, the pathways of upward methane migration are limited and confined to “geocryologic windows”, non-frozen ground sections under large lakes, or associated with intra- and sub-permafrost groundwater springs. Still, methane fluxes associated with permafrost groundwater in deep aquifers remain a poorly studied of the global methane cycle. This abstract presents first data on methane fluxes associated with continuous permafrost groundwater measured in central Yakutia, Northern Eurasia.
Our fieldwork was carried out annually from 2021 to 2023, along the right bank of the middle Lena River near Yakutsk, where several important groundwater springs, i.e., Buluus, Ulakhan-Taryn and Sullar, among others, have been previously described and were studied in detail. This region is underlain continuous permafrost with thickness reaching from 300 to 700 m, that hosts several complexes of permafrost groundwaters. The area is unevenly studied in terms of geocryology, though several boreholes encountered intra-permafrost taliks with base at depths from 10-20 m to over 100 m, in the southern part of the studied region. Groundwater springs are mostly present as either spring channels or sand boils.
Methane fluxes were measured using dark closed floating chambers. A 0.01 m3 circular chamber with 0.08 m2 covered area, coated with reflective film, was placed at the water surface; gas mixture samples were taken at regular intervals with plastic syringes and transferred to glass vials pre-filled with NaCl solution. A single CH4 flux measurement took from 15 to 20 mins, and at least three fluxes were measured at each observation point. In the laboratory, methane concentration in samples was measured by gas chromatography with flame ionization detector; in post-processing, individual methane fluxes were calculated from the linear approximation slope. In 2023, the lowest CH4 fluxes were observed at the Buluus Spring, where it ranges from near-zero to 6.0 mgC.m-2.h-1 which is significantly lower compared to other studied springs: Ulakhan-Taryn, from 170 to 233 mgC.m-2.h-1, and Sullar, 330 mgC.m-2.h-1. Isotopic 13C-CH4 data support microbiological origin of methane emitted from these two latter springs, and atmospheric origin for the Buluus spring. Elevated methane concentrations were also observed in the air close to Ulakhan-Taryn and Sullar springs, 4.5 and 5.6 ppm respectively, but not around the Buluus spring.
Observed CH4 fluxes increase northward, but whether it is related to changes in local geology, permafrost conditions or other features, remain unknown. Our results are consistent with published data on significant supra-permafrost groundwater discharge in the Buluus spring.The Sullar spring was sampled annually from 2021 to 2023; we observed a ten-fold increase in methane flux from this spring, from 39 mgC.m-2.h-1 in 2021, to 234 mgC.m-2.h-1 in 2022 and 330 mgC.m-2.h-1 in 2023, as mentioned above. Observed CH4 fluxes are not found to be related neither with dissolved CH4 content nor with other environmental parameters (water temperature, barometric pressure). It is also unclear if seasonality played any role in this increase; observations were made in summer 2021 and 2022, but in early autumn 2023. The preliminary data from the 2024 field campaign will be presented at the meeting.
Our fieldwork was carried out annually from 2021 to 2023, along the right bank of the middle Lena River near Yakutsk, where several important groundwater springs, i.e., Buluus, Ulakhan-Taryn and Sullar, among others, have been previously described and were studied in detail. This region is underlain continuous permafrost with thickness reaching from 300 to 700 m, that hosts several complexes of permafrost groundwaters. The area is unevenly studied in terms of geocryology, though several boreholes encountered intra-permafrost taliks with base at depths from 10-20 m to over 100 m, in the southern part of the studied region. Groundwater springs are mostly present as either spring channels or sand boils.
Methane fluxes were measured using dark closed floating chambers. A 0.01 m3 circular chamber with 0.08 m2 covered area, coated with reflective film, was placed at the water surface; gas mixture samples were taken at regular intervals with plastic syringes and transferred to glass vials pre-filled with NaCl solution. A single CH4 flux measurement took from 15 to 20 mins, and at least three fluxes were measured at each observation point. In the laboratory, methane concentration in samples was measured by gas chromatography with flame ionization detector; in post-processing, individual methane fluxes were calculated from the linear approximation slope. In 2023, the lowest CH4 fluxes were observed at the Buluus Spring, where it ranges from near-zero to 6.0 mgC.m-2.h-1 which is significantly lower compared to other studied springs: Ulakhan-Taryn, from 170 to 233 mgC.m-2.h-1, and Sullar, 330 mgC.m-2.h-1. Isotopic 13C-CH4 data support microbiological origin of methane emitted from these two latter springs, and atmospheric origin for the Buluus spring. Elevated methane concentrations were also observed in the air close to Ulakhan-Taryn and Sullar springs, 4.5 and 5.6 ppm respectively, but not around the Buluus spring.
Observed CH4 fluxes increase northward, but whether it is related to changes in local geology, permafrost conditions or other features, remain unknown. Our results are consistent with published data on significant supra-permafrost groundwater discharge in the Buluus spring.The Sullar spring was sampled annually from 2021 to 2023; we observed a ten-fold increase in methane flux from this spring, from 39 mgC.m-2.h-1 in 2021, to 234 mgC.m-2.h-1 in 2022 and 330 mgC.m-2.h-1 in 2023, as mentioned above. Observed CH4 fluxes are not found to be related neither with dissolved CH4 content nor with other environmental parameters (water temperature, barometric pressure). It is also unclear if seasonality played any role in this increase; observations were made in summer 2021 and 2022, but in early autumn 2023. The preliminary data from the 2024 field campaign will be presented at the meeting.