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[MIS11-06] Anomalous Sulfate Ion Concentration in Porewater of Surface Sediments off Noroshi, Noto Peninsula
Keywords:Noto Peninsula earthquake, Fluid, Sulfate reduction, Methane flux, Earthquake-induced sediments
Introduction
The 2024 Noto Peninsula earthquake, which occurred on 1 January, has been pointed out to be related to "fluids" distributed in the crust, which have caused earthquake swarms in the past. Many active faults are located around the epicentre area, and the influence of fluid migration along these faults could be observed after a major earthquake. Additionally, major earthquakes can leave event deposits due to landslide and tsunamis triggered by the earthquake. In this study, surface sediments were collected from the seafloor around the epicentre of the Noto Peninsula earthquake, three months after the event. This study aims to investigate the migration of deep fluids and changes in surface sedimentary environment by analyzing chemical and isotopic compositions of pore water extracted from the sediment samples.
Materials and methods
Surface sediments were collected using piston corers and multiple corers from 10 locations around the epicentre off the Noto Peninsula, during the KH-24-E1 cruise conducted by R/V Hakuho Maru from 4-16 March 2024. Sediments sampled by the multiple corer (MC) ranged in length from 6 cm to 30 cm. Pore water was immediately extracted on board from the collected sediments using a interstitial water squeezer or Rhizon Sampler. The major dissolved ion concentrations, dissolved inorganic carbon (DIC) concentrations, and the carbon isotopic composition of DIC (δ13C-DIC) were measured in these pore water samples.
Results and Discussion
Of the MC samples collected from 10 sites, only in MC05 (37°32.79'N, 137°25.97'E, 438 m depth) off Noroshi, Noto Peninsula, showed a rapid decrease in SO42- concentrations near the surface, becoming nearly depleted around 10 cm below the seafloor. Other samples maintained SO42- concentrations of approximately 28 mM, similar to seawater, with no depth-related change.
In MC05, as SO42- concentrations decreased, DIC concentrations increased, while δ13C-DIC and Ca2+ concentrations decreased. These changes indicate sulfate reduction through anaerobic oxidation of organic matter and methane. The sulfate–methane transition zone (SMTZ), where SO42- becomes depleted, marks the shift from sulfate reduction to methanogenesis, resulting in increased methane concentrations below. The Ca2+ decrease is attributed to calcium carbonate precipitation from increased alkalinity due to sulfate reduction. The DIC-to-SO42- ratio suggests a slight dominance of methane oxidation, with both organic matter oxidation and anaerobic oxidation of methane contributing to DIC production.
A lamina structure was observed in the top 1 cm of MC05, likely from sediment deposited rapidly after the 2024 Noto Peninsula earthquake. This sudden sedimentation, rich in organic matter, may have accelerated sulfate reduction by cutting off oxygen supply from seawater. The sedimentation rate estimated from the SO42- gradient was 4.5 cm·yr-1, closely matching the 5.3 cm·yr-1 rate calculated from the 1 cm seismic deposit accumulated over 69 days, supporting the hypothesis that rapid earthquake-induced deposition caused the sulfate decrease.
Assuming the SO42- reduction is entirely due to methane oxidation, the methane flux from below was estimated at 1.7 x 10-1 mmol·cm-2·yr-1-about 100 times higher than Blake Ridge's flux (up to 1.8 x 10-3 mmol·cm-2·yr-1). Even accounting for organic matter oxidation, the methane flux is about an order of magnitude higher than Blake Ridge, suggesting the influence of methane-rich fluids from deep subsurface in MC05.
Conclusion
Of the MC samples collected at 10 sites off the Noto Peninsula, only MC05 off Noroshi showed a rapid decrease in SO42- concentration. This rapid decrease in SO42- concentration may be attributed to either rapid sediment deposition from seismic shaking or the upward migration of methane-rich fluids from deep underground. However, which of these two factors is responsible remains unclear at this stage. To address this, measurements of organic carbon content in the sediments are planned.
The 2024 Noto Peninsula earthquake, which occurred on 1 January, has been pointed out to be related to "fluids" distributed in the crust, which have caused earthquake swarms in the past. Many active faults are located around the epicentre area, and the influence of fluid migration along these faults could be observed after a major earthquake. Additionally, major earthquakes can leave event deposits due to landslide and tsunamis triggered by the earthquake. In this study, surface sediments were collected from the seafloor around the epicentre of the Noto Peninsula earthquake, three months after the event. This study aims to investigate the migration of deep fluids and changes in surface sedimentary environment by analyzing chemical and isotopic compositions of pore water extracted from the sediment samples.
Materials and methods
Surface sediments were collected using piston corers and multiple corers from 10 locations around the epicentre off the Noto Peninsula, during the KH-24-E1 cruise conducted by R/V Hakuho Maru from 4-16 March 2024. Sediments sampled by the multiple corer (MC) ranged in length from 6 cm to 30 cm. Pore water was immediately extracted on board from the collected sediments using a interstitial water squeezer or Rhizon Sampler. The major dissolved ion concentrations, dissolved inorganic carbon (DIC) concentrations, and the carbon isotopic composition of DIC (δ13C-DIC) were measured in these pore water samples.
Results and Discussion
Of the MC samples collected from 10 sites, only in MC05 (37°32.79'N, 137°25.97'E, 438 m depth) off Noroshi, Noto Peninsula, showed a rapid decrease in SO42- concentrations near the surface, becoming nearly depleted around 10 cm below the seafloor. Other samples maintained SO42- concentrations of approximately 28 mM, similar to seawater, with no depth-related change.
In MC05, as SO42- concentrations decreased, DIC concentrations increased, while δ13C-DIC and Ca2+ concentrations decreased. These changes indicate sulfate reduction through anaerobic oxidation of organic matter and methane. The sulfate–methane transition zone (SMTZ), where SO42- becomes depleted, marks the shift from sulfate reduction to methanogenesis, resulting in increased methane concentrations below. The Ca2+ decrease is attributed to calcium carbonate precipitation from increased alkalinity due to sulfate reduction. The DIC-to-SO42- ratio suggests a slight dominance of methane oxidation, with both organic matter oxidation and anaerobic oxidation of methane contributing to DIC production.
A lamina structure was observed in the top 1 cm of MC05, likely from sediment deposited rapidly after the 2024 Noto Peninsula earthquake. This sudden sedimentation, rich in organic matter, may have accelerated sulfate reduction by cutting off oxygen supply from seawater. The sedimentation rate estimated from the SO42- gradient was 4.5 cm·yr-1, closely matching the 5.3 cm·yr-1 rate calculated from the 1 cm seismic deposit accumulated over 69 days, supporting the hypothesis that rapid earthquake-induced deposition caused the sulfate decrease.
Assuming the SO42- reduction is entirely due to methane oxidation, the methane flux from below was estimated at 1.7 x 10-1 mmol·cm-2·yr-1-about 100 times higher than Blake Ridge's flux (up to 1.8 x 10-3 mmol·cm-2·yr-1). Even accounting for organic matter oxidation, the methane flux is about an order of magnitude higher than Blake Ridge, suggesting the influence of methane-rich fluids from deep subsurface in MC05.
Conclusion
Of the MC samples collected at 10 sites off the Noto Peninsula, only MC05 off Noroshi showed a rapid decrease in SO42- concentration. This rapid decrease in SO42- concentration may be attributed to either rapid sediment deposition from seismic shaking or the upward migration of methane-rich fluids from deep underground. However, which of these two factors is responsible remains unclear at this stage. To address this, measurements of organic carbon content in the sediments are planned.