The combination of CH4 venting and gas hydrate occurrence on the seafloor was observed by a remotely operated vehicle (ROV) offshore Joetsu, on the eastern margin of the Japan Sea. The intensity and location of venting varied within a few days, suggesting the potential for short-term gas hydrate formation and rapid geochemical changes. To investigate temporal variations in gas hydrate dynamics, we deployed a long-term in situ water sampling system (OsmoSampler), consisting of a pump and a sampling coil, next to the vent to collect interstitial water continuously for a year. However, the volume recovered from the sample was only a quarter of the expected amount. The presence of gas hydrate was confirmed in the OsmoSampler during its recovery by ROV. This unexpected event was attributed to a CH4 intrusion into the system-either by direct intake through the tip of sampling coil or by a structurally vulnerable connections-followed by gas hydrate formation with the prefilled distilled water. The resulting blockage reduced the rate of sampling. Despite this technical limitation, OsmoSampler geochemical data revealed dynamic variations. Cl- concentrations sporadically fluctuated from 50 to 900 mM repeatedly until 40 m of coil length, indicating alternately saline and fresh conditions likely due to repeated cycles of gas hydrate growth and dissociation. At 40 m, a distinct geochemical transition-marked by an increase in Cl- and SO4 2- concentrations to 440 mM and 26 mM, respectively, and a sharp decline in CH4 to 0 mM-closely resembled to the composition of surface interstitial water. This suggests that seawater infiltration into the sampling system was likely triggered by the collapse of the gas hydrate due to buoyancy. The topographic depression observed by the ROV during the recovery supports this interpretation. These findings provide new insights into short-term gas hydrate instability near the seafloor and the sudden release of CH4 from sediments into the overlying water column.

A recently acquired multidisciplinary dataset comprising acoustic surveys (high-resolution sub-bottom profiles, multi-beam bathymetry, and broad band mid-water echo sounder), geochemistry (gas chemical and isotopic composition, porewater chemistry), and sedimentology (core lithology and X-ray CT) in the area of the Landsort deep (450 m of depth), south of Stockholm Archipelago, revealed the existence of an extensive (20 km2) region of the seafloor where massive gas release is occurring in the form of multiple bubble streams. This new discovery represents a major seafloor methane release site in Europe and is comparable in area to other large sites worldwide such as the ones in Svalbard and in the South Atlantic Ocean associated with gas hydrate provinces. The gas is formed mostly by methane of microbial origin. Surprisingly, bubbles rise 100’s of meters above the seafloor and reach surface waters above the halocline/oxycline at around 80 m of depth. Some bubbles appear to reach the sea-air interface and their potential methane contribution to the atmosphere is under investigation. Another surprising observation is the absence of major seafloor features like pockmarks in the gas release area. The reasons for the seafloor methane release in the Landsort deep are still not entirely clear, but our preliminary acoustic and sedimentological data suggest that bottom currents may have acted to facilitate the accumulation of organic-rich sediments in a thick drift deposit during the Holocene and the modern warm period (latest 100 years). Our data further suggest that the high sedimentation rate in the drift deposit continuously supplies fresh organic matter that is quickly buried below a thin sulphate reduction zone, fueling vigorous methanogenesis and abundant methane formation. Similar methane release sites might be discovered in other known large drift deposits in the Baltic Sea.

Subduction input (sediment before subduction)-located seaward of the trench is one of the largest iodine budgets on the earth's surface. It is responsible for the deep iodine source in the landward of the trench where the iodine flux is significantly high. However, the distribution in the subduction input is poorly understood, contrary to the subducted sediment (sediment after subduction) landward of the trench. We determined iodine concentration and I-129/I-127 ratio of the interstitial water from the seafloor to the basement continuously at a subduction input site similar to 250 km southwest of the Sunda Trench for the first time to understand the iodine distribution. In the study site, the iodine concentration increased with depth linearly to similar to 100 mu M at 1400 mbsf. Iodine isotope ratios (I-129/I-127) remained constant as low as similar to 400 x 10(-15) from 400 to 1400 mbsf, suggesting that the iodine distribution was mainly controlled by old iodine-rich fluid (low I-129/I-127 ratio and high iodine concentration) supplied along the basement and by mixing with seawater (high I-129/I-127 ratio and low iodine concentration). The linear iodine gradient was changed at similar to 200 and similar to 1200 mbsf, where the methane concentration rapidly increased and total organic carbon decreased. This indicates that young iodine (low I-129/I-127 ratio) was released from the organic materials in the sediment into the interstitial water at these depths. This is the first observation of in situ iodine/methane addition to the interstitial water associated with the organic decomposition. The iodine concentration and I-129/I-127 ratio indicated that iodine in the subduction input was either derived from the in situ sediment or allochthonous fluid transported from subducted sediment due to differences in physical properties and permeability. This allochthonous iodine transportation to the subduction input may broaden the concept of the iodine cycling in the subduction system, including the sediments after and before subduction.

Concretionary carbonates in deep-sea methane seep fields are formed as a result of microbial methane degradation, called anaerobic oxidation of methane (AOM). Recently, active microorganisms, including anaerobic methanotrophic archaea, were discovered from methane seep-associated carbonate outcroppings on the seafloor. However sedimentary buried carbonate nodules are a hitherto unknown microbial habitat. In this study, we investigated the microbial community structures in two carbonate nodules collected from a high methane flux site in a gas hydrate field off the Oki islands in the Sea of Japan. The nodules were formed around sulfate-methane interfaces (SMI) corresponding to 0.7 and 2.2 m below the seafloor. Based on a geochemical analysis, light carbon isotopic values ranging from −54.91‰ to −37.32‰ were found from the nodules collected at the shallow SMI depth, which were attributed to the high contributions of AOM-induced carbonate precipitation. Signatures of methanotrophic archaeal populations within the sedimentary buried nodule were detected based on microbial community composition analyses and quantitative real-time PCR targeted 16S rRNA, and functional genes for AOM. These results suggest that the buried carbonate nodule currently develops AOM-related microbial communities, and grows depending on the continued AOM under high methane flux conditions.

Highly active gas venting and outcrops of gas hydrates were recently found at the Umitaka Spur, on the eastern margin of the Japan Sea, through seafloor observations using a remotely operated vehicle (ROV). This was followed by sampling by using conventional coring techniques. In this study, we deployed a long-term osmotic fluid sampling system (OsmoSampler) to record geochemical changes in the shallow sedimentary environment over time. The OsmoSampler collected interstitial water at 30 cm below seafloor (cmbsf), providing us with a continuous daily record of the concentrations of SO42-, Cl-, CH4, and C2H6 dissolved in interstitial waters over one year. General mean concentrations of SO42- and Cl- repeatedly showed synchronous increases and decreases over both long-term (i.e. dozens of days) and short-term (i.e. 3-5 days) periods. Such fluctuations relative to background seawater concentrations were likely caused by saline and fresh water generation due to gas hydrate formation and dissolution. The CH4 concentration was significantly higher (> 1 mM) during large fluctuations in SO42- and Cl- over the first 3 months, which was attributable to gas-venting activity. This high venting activity of CH4 could have promoted the rapid, large-scale formation of gas hydrate. The CH4 concentration decreased gradually after the first 3 months, and was accompanied by a period of relatively low SO42- and Cl- concentrations due to the predominance of gas hydrate dissolution around the sampling site. In later months, the C2H6 concentration approached similar to 1 mu M and was accompanied by occasional, positive CH4 spikes, reflecting the release of trapped gases associated with the dissolution of hydrates. These processes are essentially controlled by the fluctuations in gas concentrations along the gas migration path, and are well-characterized by shallow interstitial water geochemistry.

Gas plume in the water column is often observed over the shallow gas hydrate occurrences in the eastern margin of the Japan Sea. It has been concerned that the ground strength will be reduced by the gas-rich fluids rising in the shallow layer of sediments. In this study, mechanical test, physical test, and chemical analysis were conducted on the sediment cores collected from the shallow gas hydrate and gas plume fields in off Joetsu and off Akita/Yamagata. There are no differences in physical properties of the each cores. The mechanical properties were low and dissolved CH 4 concentration was high at the gas plume site of off Joetsu area. As the sedimentation rate increased, the sediment strength tended to decrease at gas plume-free site of off Akita/Yamagata area. There was no relationship between dissolved CH 4 concentration and sediment strength in either area. These results suggest that the sediment strength at the gas plume site is low in situ.

Plate-boundary fault rupture during the 2004 Sumatra-Andaman subduction earthquake extended closer to the trench than expected, increasing earthquake and tsunami size. International Ocean Discovery Program Expedition 362 sampled incoming sediments offshore northern Sumatra, revealing recent release of fresh water within the deep sediments. Thermal modeling links this freshening to amorphous silica dehydration driven by rapid burial-induced temperature increases in the past 9 million years. Complete dehydration of silicates is expected before plate subduction, contrasting with prevailing models for subduction seismogenesis calling for fluid production during subduction. Shallow slip offshore Sumatra appears driven by diagenetic strengthening of deeply buried fault-forming sediments, contrasting with weakening proposed for the shallow Tohoku-Oki 2011 rupture, but our results are applicable to other thickly sedimented subduction zones including those with limited earthquake records.

A holistic view of the Bengal–Nicobar Fan system requires sampling the full sedimentary section of the Nicobar Fan, which was achieved for the first time by International Ocean Discovery Program (IODP) Expedition 362 west of North Sumatra. We identified a distinct rise in sediment accumulation rate (SAR) beginning ∼9.5 Ma and reaching 250–350 m/Myr in the 9.5–2 Ma interval, which equal or far exceed rates on the Bengal Fan at similar latitudes. This marked rise in SAR and a constant Himalayan-derived provenance necessitates a major restructuring of sediment routing in the Bengal–Nicobar submarine fan. This coincides with the inversion of the Eastern Himalayan Shillong Plateau and encroachment of the west-propagating Indo–Burmese wedge, which reduced continental accommodation space and increased sediment supply directly to the fan. Our results challenge a commonly held view that changes in sediment flux seen in the Bengal–Nicobar submarine fan were caused by discrete tectonic or climatic events acting on the Himalayan–Tibetan Plateau. Instead, an interplay of tectonic and climatic processes caused the fan system to develop by punctuated changes rather than gradual progradation.

The Mw 9.2 Sumatra earthquake in 2004 resulted in unexpectedly shallow megathrust slip, which amplified the earthquake magnitude and caused a devastating tsunami. At two sites (Sites U1480 and U1481) International Ocean Discovery Program (IODP) Expedition 362 cored the input sediment with R/V JOIDES Resolution ~250 km seaward of the Sumatra subduction zone to groundtruth the material properties that contributed to the unexpectedly shallow seismogenic slip and a distinctive forearc prism structure of the North Sumatra subduction zone. The recovered sediment comprise a Late Cretaceous to Miocene abyssal-plain environment facies consisting of mixed tuffaceous and pelagic sediments and a series of intercalated pelagic and igneous materials, which is overlain by a thick sequence of siliciclastic sediments (mostly siliciclastic mud, siliciclastic sand and calcareous mud) of the Nicobar fan. Here we present preliminary results from shipboard geochemical analyses of interstitial waters. The sulfate-methane transition zone (SMTZ) exists at 120 mbsf. Concentrations of ammonium and phosphate have positive peaks above the SMTZ, which reflect the remineralization of organic matter. Low alkalinity and calcium concentration below the SMTZ indicate carbonate precipitation. Release of silica and cations (K, Ca, Na, Al) to the interstitial water are indicative of volcaniclastic ash alteration in the upper 20 mbsf. A subsequent depletion of potassium below 400 mbsf to values as low as 1 mM suggest zeolite formation, consistent with observations in the recovered sediment. The high sulfate concentration of 15 mM in pelagic sediment at 1403 mbsf may reflect a presence of sulfate-rich fluid in the basement aquifer. Ongoing post-expedition analyses of interstitial water geochemistry will provide additional insights into fluid-rock interactions and fluid flow processes which will shed light on the evolving properties of the sediment incoming to the North Sumatra subduction zone.

Gas hydrate in deep-sea sediments is a potential energy resource, and has been the focus of extensive drilling research. However, direct evaluation of the amount of the gas hydrate in marine sediments has been difficult because the gas hydrate in recovered sediment cores is at least partly dissociated due to the drop in pressure and increase in temperature during onboard recovery. In this study, we apply a new method based on oxygen isotopic composition of the H2O fraction of both hydrate and mud sub-samples (δ18OH and δ18OM, respectively) in order to evaluate the volume percentage of gas hydrate in core sections collected from the Japan Sea off Joetsu and Oki, which contain different fabrics of the hydrate within hemipelagic mud. We measured isotopic composition of CO2 equilibrated with H2O of the sub-samples of a small size (typically 0.3cm3) carefully separated from the core sediments and sealed in glass vials. The volume percentage of gas hydrate (H in %) was determined using porosity of the mud sub-samples and oxygen isotopic composition of the bulk pore water (δ18OPW) squeezed from a certain length of core sediment (typically 20cm) including dissociated hydrate. 28 out of the 29 examined core sections indicate the relation in the isotopic values of the three components, δ18OH>δ18OPW>δ18OM, as expected from isotopic fractionation that enriches 18O in the hydrate component. Evaluated H-values of the 28 sections ranged from 1.0% to 95.4% and, for most of the section, the H-value was clearly larger than the value estimated by the hydrate distribution on core images. Our new method can, in a simple manner, correct for the underestimation of hydrate amount caused as a result of dissociation during core handling. Our oxygen isotopic data of the hydrate and mud sub-samples fits poorly with the isotopic evolutional curve that assumes Rayleigh fractionation in a closed system. This implies that the pore water isotopic composition may have been homogenized by diffusion and advection of less 18O-depleted pore water from the surrounding sediments. Presence of micro-scale hydrate in the mud matrix was suspected for some sections from the Joetsu site, which present a small difference between δ18OH and δ18OM as well as high CH2/CO2 ratios in headspace gas. We suggest that this method, if carried out with careful and quick onboard sampling, is appropriate for the estimation of gas hydrate as an energy resource based on the amount of hydrate present in marine mud.

The characteristic seafloor topography associated with gas hydrates in shallow sediments was reported in the seafloor of the SE margin of Tsushima Basin and Oki Trough, Japan. Interstitial water and seawater collected from these areas during the UT14 cruise were analyzed for characterizing the fluid geochemistry responsible for methane migration toward the seafloor and formation of hydrates. In the eastern margin of Tsushima Basin, high concentrations of sulfate and alkalinity in interstitial water reflect very shallow SMI depths (~1.7mbsf), strong methane fluxes, and methane generation due to the decomposition of organic matters in shallow sediments. The low concentrations of silicate dissolved in seawater indicate that the buried old organic matters are responsible for the formation and distribution of gas hydrates near the seafloor. MIS09-P11 Japan Geoscience Union Meeting 2016

On the Umitaka Spur of the Japan Sea, methane venting on the seafloor has been observed by ROV Hyper Dolphin, its strength and location change in short period. These changes effect on chemical and ecological environment of pore water and shallow sediments, continuous observation of these fluctuations are key to understand the dynamics of gas hydrate system near the seafloor. We have applied an osmotic fluid sampling system (OsmoSampler) from September 2013 to October 2014 to collect pore waters near the seafloor continuously and show the variations of gas and fluid geochemistry associated with the activity of gas venting. Although the concentration of sulfate is lower than that of seawater, it fluctuates for days accompanying small variation of methane concentration. The concentrations of chloride and other major ions also fluctuate near the seawater value. These changes of gas and fluid geochemistry may reflect the change of methane flux and following formation/dissociation of gas hydrates.