4:10 PM - 4:30 PM
[BGM02-08] Origin of deep methane from pull-apart basin on the Itoigawa-Shizuoka Tectonic Line: Impacts between deep carbon and surface ecosystem in Lake Suwa
★Invited Papers
Keywords:Methane-specific stable carbon and radiocarbon profiles, Active seep gas, Dissolved inorganic carbon, Pull-apart basin
Lake Suwa, located at the intersection of the Itoigawa-Shizuoka Tectonic Line (ISTL) and the Median Tectonic Line (MTL), is the most active fault lake in Japan. Lake Suwa is shallow, with an average depth of 4.3 meters and a maximum depth of 6.3 meters. Since the lake has a large catchment area (512 m2) and 31 inflowing rivers, it has a sedimentary layer more than 370 m thick with a high annual sedimentation rate (∼1 cm/yr). The benthic sediments of Lake Suwa are rich in organic matter (TOC ∼5.5 wt%) and contain methane-rich bubbles, which are produced by benthic microorganisms, including methanogenic archaea [1]. Furthermore, a natural gas field, which is dissolved in a deep aquifer, is formed in the deep sedimentary layer, and several active seep sites are observed in the lake [1, 2]. In addition, there are several hot spring wells (Kamisuwa and Shimosuwa hot springs) along with the active fault group on the northern shore of the lake. Helium isotope ratio analysis of gases associated with hot spring water indicated that the gas associated with the hot spring water around Lake Suwa included a contribution by gas originating from the mantle [3]. These hot spring wells are also scattered throughout Lake Suwa and are an influential factor in the physical heterogeneity in the formation of lake icing during the severe winter season [4].
In summary, there are three origins of methane emission in Lake Suwa: 1) from the surface hydrosphere, 2) from deep sedimentary layers, and 3) from the mantle. In this study, we are investigating the interaction of these methane sources on the hydrosphere ecosystem using radiocarbon measurement. Methane and carbon dioxide collected at seep sites, where gas inflow was regularly observed in Lake Suwa, were 14C-depleted (Δ14CCH4 = −989.8 ±0.3‰, Δ14CCO2 = −951.2 ±1.0‰). The results clearly indicated that the origin of methane is different from that of methane released from the benthic sediments (Δ14CCH4 = +21.7 ±3.8‰). The Δ14C values of dissolved inorganic carbon (DIC) in surface lake water collected at seep sites (Δ14CDIC = −103.1 to −630.6‰) and near the lakeshore (Δ14CDIC = −100.9 ±3.5‰) suggested that the influence of deep carbon occurred not only around the seep sites but also at the lakeshore. Using the application of the new analytical method [5, 6], we confirmed that the signal of deep carbon is transmitted to algae and fish through DIC, and succeeded in quantitatively evaluating the picture of the utilization of deep-derived carbon including methane as a carbon source in the lake ecosystem.
This study was supported in part by the Japan Society for the Promotion of Science (JSPS) with the joint research between the JAMSTEC and Shinshu University.
References:
[1] Urai et al. (2021) Detection of planktonic coenzyme factor 430 in a freshwater lake: small-scale analysis for probing archaeal methanogenesis. Prog Earth Planet Sci, 8, 62.
[2] Iwata et al. (2020) Temporal and spatial variations in methane emissions from the littoral zone of a shallow mid-latitude lake with steady methane bubble emission areas. Agric For Meteorol 295, 108184.
[3] Umeda et al. (2013) Release of mantle and crustal helium from a fault following an inland earthquake. Appl Geochem 37, 134.
[4] Urai et al. (2022) Origin of Deep Methane from Active Faults along the Itoigawa-Shizuoka Tectonic Line between the Eurasian and North American Plates: 13C/12C & 14C/12C Methane Profiles from a Pull-Apart Basin at Lake Suwa. (Submitted).
[5] Kawagucci et al. (2020) Radiocarbon content of carbon dioxide and methane in hydrothermal fluids of Okinawa Trough vents. Geochemical Journal 54, 129-138.
[6] Yokoyama et al. (2019) A single stage Accelerator Mass Spectrometry at the Atmosphere and Ocean Research Institute, The University of Tokyo. Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms 455, 311-316.
In summary, there are three origins of methane emission in Lake Suwa: 1) from the surface hydrosphere, 2) from deep sedimentary layers, and 3) from the mantle. In this study, we are investigating the interaction of these methane sources on the hydrosphere ecosystem using radiocarbon measurement. Methane and carbon dioxide collected at seep sites, where gas inflow was regularly observed in Lake Suwa, were 14C-depleted (Δ14CCH4 = −989.8 ±0.3‰, Δ14CCO2 = −951.2 ±1.0‰). The results clearly indicated that the origin of methane is different from that of methane released from the benthic sediments (Δ14CCH4 = +21.7 ±3.8‰). The Δ14C values of dissolved inorganic carbon (DIC) in surface lake water collected at seep sites (Δ14CDIC = −103.1 to −630.6‰) and near the lakeshore (Δ14CDIC = −100.9 ±3.5‰) suggested that the influence of deep carbon occurred not only around the seep sites but also at the lakeshore. Using the application of the new analytical method [5, 6], we confirmed that the signal of deep carbon is transmitted to algae and fish through DIC, and succeeded in quantitatively evaluating the picture of the utilization of deep-derived carbon including methane as a carbon source in the lake ecosystem.
This study was supported in part by the Japan Society for the Promotion of Science (JSPS) with the joint research between the JAMSTEC and Shinshu University.
References:
[1] Urai et al. (2021) Detection of planktonic coenzyme factor 430 in a freshwater lake: small-scale analysis for probing archaeal methanogenesis. Prog Earth Planet Sci, 8, 62.
[2] Iwata et al. (2020) Temporal and spatial variations in methane emissions from the littoral zone of a shallow mid-latitude lake with steady methane bubble emission areas. Agric For Meteorol 295, 108184.
[3] Umeda et al. (2013) Release of mantle and crustal helium from a fault following an inland earthquake. Appl Geochem 37, 134.
[4] Urai et al. (2022) Origin of Deep Methane from Active Faults along the Itoigawa-Shizuoka Tectonic Line between the Eurasian and North American Plates: 13C/12C & 14C/12C Methane Profiles from a Pull-Apart Basin at Lake Suwa. (Submitted).
[5] Kawagucci et al. (2020) Radiocarbon content of carbon dioxide and methane in hydrothermal fluids of Okinawa Trough vents. Geochemical Journal 54, 129-138.
[6] Yokoyama et al. (2019) A single stage Accelerator Mass Spectrometry at the Atmosphere and Ocean Research Institute, The University of Tokyo. Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms 455, 311-316.