2:45 PM - 3:00 PM
[MIS20-04] Depth of origin of boron in surface pore fluids around the epicenter of the Noto Peninsula earthquake
Keywords:Noto Peninsula earthquake, Fluid, boron, Depth of Origin
Introduction
The Noto Peninsula earthquake that occurred on January 1, 2024 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 distributed on the seafloor around the epicenter, and the influence of fluid movement along these faults may be observed immediately after a major earthquake. In this study, we collected surface sediments from the seafloor around the epicenter of the Noto Peninsula earthquake in March 2024, soon after the earthquake, and examined the chemical and isotopic composition of the pore water contained in them to discuss the involvement of deep fluids.
Geological Background
The Noto Peninsula is located on the west side of the Itoigawa-Shizuoka Tectonic Line, where the North American Plate and the Eurasian Plate have collided since the beginning of the Pliocene to Pleistocene. Many active faults have developed along the coastline along the northern coast of the Noto Peninsula, and swarm earthquakes have been frequently observed around these faults. Among them, a magnitude 6-7 earthquake occurred at the western end of the Noto Peninsula in 2007, and at the eastern end in 2024 and 1993, respectively.
Sampling and Analysis Methods
From March 4 to 16, 2024, surface sediments were collected from two locations on the seafloor around the epicenter area off the Noto Peninsula (west end: PC05, east end: PC08) using a 4-m-long piston core. Porewater was extracted from the collected surface sediments immediately on board the vessel using a hydraulic press. Porewater samples were analyzed for total carbonate and ammonia concentrations on board the vessel. The porewater samples brought back to the ship were measured for major element concentrations using an ion chromatograph, trace element concentrations using an inductively coupled plasma mass spectrometer, and boron isotope ratios using a multi-connector inductively coupled plasma mass spectrometer.
Results and Discussion
The chemical composition of PC05 pore water was similar to that of seawater. On the other hand, Ca2+, SO42-, and boron isotope ratios in the porewater of PC08 decreased with depth and total carbonate, ammonia, and boron concentrations increased with depth compared to seawater. Since carbonate precipitates on the reducing seafloor, the ratio of ammonia concentration to total carbonate, corrected for the total carbonate consumed by carbonate precipitation, was higher in the porewater of PC08 than in the seafloor sediments. This suggests that excess total carbonate is supplied by the anaerobic oxidation of methane in PC08. The boron concentrations and isotopic ratios in the pore water of PC08 suggest that PC08 is supplied with boron from depth, which has a lower isotopic ratio than that of seawater. The equilibrium temperature with clay minerals was estimated to be about 80°C using a geothermometer based on isotopic fractionation with clay minerals. Using the average geothermal gradient in the Japan Sea (85°C/km), it is suggested that the boron was in equilibrium with clay minerals around 1 km below the seafloor. This suggests that the boron in the surface sediments of PC08 originated at a depth of about 1 km below the seafloor.
The Noto Peninsula earthquake that occurred on January 1, 2024 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 distributed on the seafloor around the epicenter, and the influence of fluid movement along these faults may be observed immediately after a major earthquake. In this study, we collected surface sediments from the seafloor around the epicenter of the Noto Peninsula earthquake in March 2024, soon after the earthquake, and examined the chemical and isotopic composition of the pore water contained in them to discuss the involvement of deep fluids.
Geological Background
The Noto Peninsula is located on the west side of the Itoigawa-Shizuoka Tectonic Line, where the North American Plate and the Eurasian Plate have collided since the beginning of the Pliocene to Pleistocene. Many active faults have developed along the coastline along the northern coast of the Noto Peninsula, and swarm earthquakes have been frequently observed around these faults. Among them, a magnitude 6-7 earthquake occurred at the western end of the Noto Peninsula in 2007, and at the eastern end in 2024 and 1993, respectively.
Sampling and Analysis Methods
From March 4 to 16, 2024, surface sediments were collected from two locations on the seafloor around the epicenter area off the Noto Peninsula (west end: PC05, east end: PC08) using a 4-m-long piston core. Porewater was extracted from the collected surface sediments immediately on board the vessel using a hydraulic press. Porewater samples were analyzed for total carbonate and ammonia concentrations on board the vessel. The porewater samples brought back to the ship were measured for major element concentrations using an ion chromatograph, trace element concentrations using an inductively coupled plasma mass spectrometer, and boron isotope ratios using a multi-connector inductively coupled plasma mass spectrometer.
Results and Discussion
The chemical composition of PC05 pore water was similar to that of seawater. On the other hand, Ca2+, SO42-, and boron isotope ratios in the porewater of PC08 decreased with depth and total carbonate, ammonia, and boron concentrations increased with depth compared to seawater. Since carbonate precipitates on the reducing seafloor, the ratio of ammonia concentration to total carbonate, corrected for the total carbonate consumed by carbonate precipitation, was higher in the porewater of PC08 than in the seafloor sediments. This suggests that excess total carbonate is supplied by the anaerobic oxidation of methane in PC08. The boron concentrations and isotopic ratios in the pore water of PC08 suggest that PC08 is supplied with boron from depth, which has a lower isotopic ratio than that of seawater. The equilibrium temperature with clay minerals was estimated to be about 80°C using a geothermometer based on isotopic fractionation with clay minerals. Using the average geothermal gradient in the Japan Sea (85°C/km), it is suggested that the boron was in equilibrium with clay minerals around 1 km below the seafloor. This suggests that the boron in the surface sediments of PC08 originated at a depth of about 1 km below the seafloor.