Abstract:We collected bottom water samples and unconsolidated sediment samples for noble-2 gas analysis in the water column as well as in the pore water phase along the Nankai 3 Trough to assess the sources and the fluxes of seafloor fluid emission at the local/regional 4 scale.
5The results indicate that the terrigenic He emission is rather variable in terms 6 1
“…One might argue that isotopic values from these ~ 24- to 86-cm-long gravity cores at sites PC1, PC2, and PC8 simply reflect that the pore fluids of the seafloor sediment are equilibrated with seawater. Indeed, there are several studies in the Okinawa Trough 30 , forearc region of the Japan trench 43 , and the Nankai Trough 44 reporting that pore fluids at depths of several tens of centimetres are not equilibrated with seawater, implying that those isotopic values at sites PC1, PC2, and PC8 also are not likely to be equilibrated with seawater. Figure 4 Schematic drawing of hydrothermal circulation across the Moho in the outer slope of the Japan trench.…”
Plate bending-related normal faults (i.e. bend-faults) develop at the outer trench-slope of the oceanic plate incoming into the subduction zone. Numerous geophysical studies and numerical simulations suggest that bend-faults play a key role by providing pathways for seawater to flow into the oceanic crust and the upper mantle, thereby promoting hydration of the oceanic plate. However, deep penetration of seawater along bend-faults remains controversial because fluids that have percolated down into the mantle are difficult to detect. This report presents anomalously high helium isotope (3He/4He) ratios in sediment pore water and seismic reflection data which suggest fluid infiltration into the upper mantle and subsequent outflow through bend-faults across the outer slope of the Japan trench. The 3He/4He and 4He/20Ne ratios at sites near-trench bend-faults, which are close to the isotopic ratios of bottom seawater, are almost constant with depth, supporting local seawater inflow. Our findings provide the first reported evidence for a potentially large-scale active hydrothermal circulation system through bend-faults across the Moho (crust-mantle boundary) in and out of the oceanic lithospheric mantle.
“…One might argue that isotopic values from these ~ 24- to 86-cm-long gravity cores at sites PC1, PC2, and PC8 simply reflect that the pore fluids of the seafloor sediment are equilibrated with seawater. Indeed, there are several studies in the Okinawa Trough 30 , forearc region of the Japan trench 43 , and the Nankai Trough 44 reporting that pore fluids at depths of several tens of centimetres are not equilibrated with seawater, implying that those isotopic values at sites PC1, PC2, and PC8 also are not likely to be equilibrated with seawater. Figure 4 Schematic drawing of hydrothermal circulation across the Moho in the outer slope of the Japan trench.…”
Plate bending-related normal faults (i.e. bend-faults) develop at the outer trench-slope of the oceanic plate incoming into the subduction zone. Numerous geophysical studies and numerical simulations suggest that bend-faults play a key role by providing pathways for seawater to flow into the oceanic crust and the upper mantle, thereby promoting hydration of the oceanic plate. However, deep penetration of seawater along bend-faults remains controversial because fluids that have percolated down into the mantle are difficult to detect. This report presents anomalously high helium isotope (3He/4He) ratios in sediment pore water and seismic reflection data which suggest fluid infiltration into the upper mantle and subsequent outflow through bend-faults across the outer slope of the Japan trench. The 3He/4He and 4He/20Ne ratios at sites near-trench bend-faults, which are close to the isotopic ratios of bottom seawater, are almost constant with depth, supporting local seawater inflow. Our findings provide the first reported evidence for a potentially large-scale active hydrothermal circulation system through bend-faults across the Moho (crust-mantle boundary) in and out of the oceanic lithospheric mantle.
“…Noble gas analysis in the pore fluids of ocean sediments has already been employed successfully to identify a mantle helium component in the sediments, e.g. close to the Pacific-Australian subduction zone off the coast of New Zealand (Tomonaga et al, 2013), along the Nankai Trough close to Japan (Tomonaga et al, 2020) or in the vicinity of an active hydrothermal system in the Guaymas Basin, Gulf of California (Horstmann et al, 2021).…”
Section: Methodsmentioning
confidence: 99%
“…Our data suggest, that at the Azores hydrothermal fluids emanate over larger areas of the ocean floor through wide-spread pathways in the pelagic sediment body (non-localized transport). At the Nankai Trough off-shore of Japan diffusion-controlled He transport at the ocean floor has already been observed at sampling sites where a general 3 He enrichment was observed in the ocean bottom water, so only the analysis of He concentrations in the sediment pore water could allow the identification of fluid emission (Tomonaga et al, 2020). Additionally, the upward transport of hydrothermal fluids through the sediment body could be attenuated around the Azores, due to ash layers from volcanic eruptions in the ocean sediments, which have been observed during the M141/1 cruise (Schmidt et al, 2020;Chang et al, 2021), and which might play a role in inhibiting a detectable input of hydrothermal fluids into the overlying ocean water.…”
“…1) and suggested that high temperature (> 80 °C) fluid is present in sediment layers at depth, and further suggested that basinwide fluid circulation from the plate interface to the seafloor may occur through active and inactive splay faults below the basin, as well as stratigraphic fluid migration within the basin. Recently Tomonaga et al (2020) suggest that mantlederived fluid is directly emitted from seafloor close to the C0002 drilled hole, based on noble gas analysis.…”
Section: Fluid Migration Through Kumano Basin Mvsmentioning
confidence: 99%
“…Considering that there are several fault systems around KMV#14 (KBEFZ, normal faults, etc.) and that fluid must be supplied to KMV#14, it is possible that KMV#14 is connected to these fault systems (e.g., Tomonaga et al 2020).…”
Section: Subseafloor Fluid Migration and Active Fluid/ Sediment Emissmentioning
We investigated a mud volcano (MV) in a fault zone located at the southern edge of the Kumano Basin, the largest forearc basin along the Nankai Trough. Existing seismic sections show a truncated bottom-simulating reflection by a conduit below a topographic high, indicating the presence of an MV. New shipboard acoustic observations show that the fluid may be seeping through the seafloor, which in turn indicates that there are sufficient fluids for larger scale fluid migration in this area. Autonomous underwater vehicle-based high resolution acoustic observations and pH measurements indicate that soft sediment covers most of the MV and surrounding seafloor and that mud and small amounts of "high-backscattered materials" are sprinkled within the crater and around the MV. The MV type is different from those in the landward part of the Kumano Basin: the southern MV is smaller in size, has a steeper slope angle than the those in the northern Kumano Basin, and is located in a fault zone. The characteristics of this 14th Kumano Basin MV suggest that it is an expression of the larger scale fluid and sediment migration along the southern edge of the Kumano Basin, which may provide information regarding fluid and sediment migration along fault systems in the Nankai Trough accretionary prism.
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