14:00 〜 14:15
[SCG52-02] 日本海溝アウターライズにおける間隙水の特性とその起源
★招待講演
キーワード：日本海溝、アウターライズ、間隙水地球化学、水－岩石相互作用
In the outer rise region on the seaward side of the Japan trench, normal faults that break the crust developed due to plate bending associated with subduction, forming a horst and graben structure. Large-scale outer rise earthquakes are known to occur in conjunction with trench-type earthquakes. However, no large-scale outer rise earthquake has occurred more than 10 years after the 2011 Tohoku-oki earthquake, the largest trench-type earthquake in Japanese history. Besides its urgency, the fluid distribution in the outer rise region is considered to be a determining factor in the subsequent water and temperature structure of the subduction zone. The previous studies proposed that persistent brittle bend-faulting across the entire outer trench-slope might provide high-permeability pathways for intensive fluid transport into the lithosphere, so it is extremely important to evaluate the characteristics and origins of the porewater in the area. Helium isotope (3He/4He) ratio in sediment pore water sampled in the outer rise of Japan trench have indicated supply of mantle-derived fluids, which suggests a large-scale fluid circulation. To further understand the detailed pathways of deep fluids circulation, multi-chemical composition of pore water in sediments is efficacious since it's driven by water-rock interactions and the addition of deep fluids.
Therefore, we collected sediment samples from the vicinity of outer trench-slope of the Japan trench during the R/V Hakuho-Maru KH-20-8, KH-22-6 and Shinsei-Maru KS-23-6 cruises to understand origins of porewater by measuring chemical compositions. We measured the major cation, anion and nutrients concentrations, and oxygen and hydrogen stable isotope ratios of pore water squeezed from 10 cores. As a result, the porewater samples on the seaward slope of the Japan Trench were characterized by two types: one with enriched Ca and low values of δ18O compared to seawater, and the other with low SO42- and high NH4+ and PO43- concentrations. The former type porewater was thought to be produced due to alteration of volcanogenic materials in sediment by water-rock interactions. And the latter type porewater was thought to be produced by enhanced organic matter degradation. Both procedures suggest supply of more altered/degraded fluids from downward at some sites. Sites closer to the trench axis tend to exhibit the latter porewater feature, which may be related to differences in permeability and organic matter fluxes among different faults.
In addition to chemistry data, data obtained by microbiota analysis conducted in the same sediment layer will be given in this presentation for detailed discussion.
Therefore, we collected sediment samples from the vicinity of outer trench-slope of the Japan trench during the R/V Hakuho-Maru KH-20-8, KH-22-6 and Shinsei-Maru KS-23-6 cruises to understand origins of porewater by measuring chemical compositions. We measured the major cation, anion and nutrients concentrations, and oxygen and hydrogen stable isotope ratios of pore water squeezed from 10 cores. As a result, the porewater samples on the seaward slope of the Japan Trench were characterized by two types: one with enriched Ca and low values of δ18O compared to seawater, and the other with low SO42- and high NH4+ and PO43- concentrations. The former type porewater was thought to be produced due to alteration of volcanogenic materials in sediment by water-rock interactions. And the latter type porewater was thought to be produced by enhanced organic matter degradation. Both procedures suggest supply of more altered/degraded fluids from downward at some sites. Sites closer to the trench axis tend to exhibit the latter porewater feature, which may be related to differences in permeability and organic matter fluxes among different faults.
In addition to chemistry data, data obtained by microbiota analysis conducted in the same sediment layer will be given in this presentation for detailed discussion.