11:45 AM - 12:00 PM
[BCG06-08] Impacts between deep carbon and hydrosphere in a fault lake (Lake Suwa) : 14C-isoscape approach
Keywords:Radiocarbon measurements, Pull-apart basin, Dissolved inorganic carbon, methane
Lake Suwa, located at the intersection of the Itoigawa-Shizuoka Tectonic Line (ISTL) and the Median Tectonic Line (MTL), is the most representative fault lake in Japan with the unique geological feature of a pull-apart sedimentary basin at the Eurasian plate and North American plate boundary. Since the lake has a large catchment area (512 km2) and 31 inflowing rivers, it has a sedimentary layer more than 370 m thick with a high annual sedimentation rate (∼1 cm/yr). The sedimentary layer has a natural dissolved-in-water field, and several active seep sites are observed in the lake[1]. During the winter season, the entire surface of the lake is frozen over, but active seep sites do not freeze (a hole in a pot) [1].
The main component of the seeping gas is methane (90.1%), with carbon dioxide (7.0%) and nitrogen (0.5%). Based on the carbon isotope ratio of methane (δ13C = -65.6‰) and the methane/ethane ratio (2.2×104), the methane in the outgassing is assumed to be the microbial origin. For the radiocarbon measurements, methane and carbon dioxide in the seeping gas were 14C-dead (Δ14CCH4 = -989.8 ±0.3 ‰, Δ14CCO2 = -951.2 ±1.0 ‰), indicating their origin in the deep subsurface, isolated from the atmosphere. The value of dissolved inorganic carbon (DIC) in the surface lake water sampled near the spring site was Δ14CDIC = -630.6 ±1.7 ‰, indicating that deep-derived carbon is incorporated into the lake water through mineralization and dissolution [1].
In this presentation, we report on a radiocarbon isoscape analysis of the entire lake system for the impact of deep carbon derived from seeping gases. We collected surface lake and river water at 12 locations and analyzed them as previously reported [2,3]. The radiocarbon values ranged from Δ14CDIC = -459.7 to -97.0 ‰, with Δ14CDIC = -127.3 ‰ in the Tenryu River, the only outflow river. Since the Δ14CDIC values for the four main inflow rivers ranged from Δ14CDIC = -91.8 to -60.8 ‰, this difference is assumed to be the effect of deep carbon contained in the seeping gas. For the entire lake, the values in the southern part of Suwa Lake tend to be more 14C-depleted than those in the northern part. This may reflect the possibility that the upwelling flux of deep carbon is larger in the southern part of Lake Suwa. We will conduct its spatial infrared imaging map to compare with the radiocarbon isoscape. Focusing on the entire ecosystem, starting from the primary aquatic producers [4], the values of Wakasagi (Δ14C = -97.9 to -89.0‰) and largemouth bass (Δ14C = -97.7 to -82.0‰) in the lake were intermediate between those of lake water and river water. These results suggest that Wakasagi and largemouth bass prey on diets derived from terrestrial and riverine sources in addition to prey resources produced in the lake.
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.
[1] 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. ACS Earth and Space Chemistry, 6, 1689-1697.
[2] 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.
[3] Urai et al. (2021) Origin of deep methane associated with a unique community of microorganisms in an organic- and Iodine-rich aquifer. ACS Earth and Space Chemistry, 5, 1-11.
[4] Urai et al. (2021) Detection of planktonic coenzyme factor 430 in a freshwater lake: Small-scale analysis for probing archaeal methanogenesis. Progress in Earth and Planetary Science, 8, Article number: 62. doi: 10.1186/s40645-021-00450-7.
The main component of the seeping gas is methane (90.1%), with carbon dioxide (7.0%) and nitrogen (0.5%). Based on the carbon isotope ratio of methane (δ13C = -65.6‰) and the methane/ethane ratio (2.2×104), the methane in the outgassing is assumed to be the microbial origin. For the radiocarbon measurements, methane and carbon dioxide in the seeping gas were 14C-dead (Δ14CCH4 = -989.8 ±0.3 ‰, Δ14CCO2 = -951.2 ±1.0 ‰), indicating their origin in the deep subsurface, isolated from the atmosphere. The value of dissolved inorganic carbon (DIC) in the surface lake water sampled near the spring site was Δ14CDIC = -630.6 ±1.7 ‰, indicating that deep-derived carbon is incorporated into the lake water through mineralization and dissolution [1].
In this presentation, we report on a radiocarbon isoscape analysis of the entire lake system for the impact of deep carbon derived from seeping gases. We collected surface lake and river water at 12 locations and analyzed them as previously reported [2,3]. The radiocarbon values ranged from Δ14CDIC = -459.7 to -97.0 ‰, with Δ14CDIC = -127.3 ‰ in the Tenryu River, the only outflow river. Since the Δ14CDIC values for the four main inflow rivers ranged from Δ14CDIC = -91.8 to -60.8 ‰, this difference is assumed to be the effect of deep carbon contained in the seeping gas. For the entire lake, the values in the southern part of Suwa Lake tend to be more 14C-depleted than those in the northern part. This may reflect the possibility that the upwelling flux of deep carbon is larger in the southern part of Lake Suwa. We will conduct its spatial infrared imaging map to compare with the radiocarbon isoscape. Focusing on the entire ecosystem, starting from the primary aquatic producers [4], the values of Wakasagi (Δ14C = -97.9 to -89.0‰) and largemouth bass (Δ14C = -97.7 to -82.0‰) in the lake were intermediate between those of lake water and river water. These results suggest that Wakasagi and largemouth bass prey on diets derived from terrestrial and riverine sources in addition to prey resources produced in the lake.
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.
[1] 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. ACS Earth and Space Chemistry, 6, 1689-1697.
[2] 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.
[3] Urai et al. (2021) Origin of deep methane associated with a unique community of microorganisms in an organic- and Iodine-rich aquifer. ACS Earth and Space Chemistry, 5, 1-11.
[4] Urai et al. (2021) Detection of planktonic coenzyme factor 430 in a freshwater lake: Small-scale analysis for probing archaeal methanogenesis. Progress in Earth and Planetary Science, 8, Article number: 62. doi: 10.1186/s40645-021-00450-7.