戸丸 仁

Hydrate Ridge off the coast of Oregon, USA, is a prime example for gas hydrate occurrences in active margin settings. It is part of the Cascadia Margin and was the focus of Ocean Drilling Program (ODP) Leg 204, which successfully recovered fluids from nine sites from the southern part of the ridge. Iodide concentrations in pore fluids associated with gas hydrates are strongly enhanced, by factors up to 5000 compared to seawater, which allows the use of this biophilic element as tracer for organic source regions. We applied the cosmogenic isotope I-129 (T-1/2= 15.7 Ma) system to determine the age of the organic source formation responsible for the iodide enrichment. In all sites at ODP Leg 204, I-129/I ratios were found to decrease with depth to values around 250 x 10(-15), corresponding to minimum ages of 40 Ma, but in several sites, maxima in the I-129/I ratios point to the local addition of young iodide. The results indicate that a large amount of iodide was derived from deep accreted sediments of Eocene age, and that additional source regions provide iodide of Late Miocene age. The presence of old iodide in the pore waters suggests that fluid pathways are open to allow transport over large distances into the gas hydrate fields. The strong correlation between iodide and methane in hydrate fields coupled with the similarity in transport parameters in aqueous solutions suggests that a large fraction of methane in gas hydrates also has old sources and is transported into the present locations from source regions of Eocene age. (C) 2007 Elsevier B.V. All rights reserved.

During Integrated Ocean Drilling Program Expedition 315, coring at two planned riser drilling sites was conducted. For Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) Stage 2, 3.5 km riser drilling is planned at Site C0001. We cored at this site to 458 m core depth below seafloor (CSF IODP Method A: core expansion lengths overlap [are not scaled]) and cut 59 cores (31 with the hydraulic piston coring system [HPCS], 2 with the extended shoe coring system [ESCS], and 26 with the rotary core barrel [RCB]) from five holes covering a slope basin (Unit I) and the top 250 m of the underlying accretionary prism (Unit II). The slope basin is composed mainly of Quaternary to late Pliocene silty clay and clayey silt with intercalations of volcanic ash. The boundary between Units I and II, identified at 207.17 m CSF, is an unconformity characterized by a thick sand layer. Unit II is composed of more consolidated muddominated sediments of late Pliocene to late Miocene age. Structural style and stress state vary widely across a highly deformed zone at 220 m CSF. A normal fault indicating northeast-southeast extension is dominant above this zone however, a few thrust faults dipping at 50° were encountered just above the deformed zone. These thrust faults are consistent with the northwest-southeast shortening subparallel to the direction of plate convergence. On the other hand, many thrust and strike-slip faults as well as a normal fault are found below the 220 m CSF deformed zone. The geometry and kinematics of planar structures display great variation. Fault plane solutions computed from normal and thrust faults are consistent with northeast-southwest extension and northwest-southeast shortening, respectively. A total of 48 whole-round samples were taken for interstitial geochemistry. Obtained data show meaningful trends for most elements, and potential contamination of drilling fluid is taken into consideration however, changing trends do not necessarily correspond to unit boundaries. Methane and ethane concentrations and their ratio (C1/C2) decrease with depth to 100 m CSF and remain constant to the base of Unit I. The increase of methane concentrations and C1/C2 ratios in Unit II indicate the contribution of biogenic methane. Total organic carbon and calcium carbonate decrease monotonously to the base of Unit I and remain low throughout Unit II. Physical properties also show a clear break at the boundary between Units I and II. Porosity decreases downhole within each unit however, there is a gap across the unit boundary. Thermal conductivity is almost constant throughout Unit I and decreases with depth in Unit II. Downhole temperature was measured with the advanced piston corer temperature tool (APCT3) at seven depths to 170.98 m CSF and yielded a generally linear downhole temperature increase, with a gradient of 0.042°C/m. For NanTroSEIZE Stage 3, 6 km riser drilling is planned at Site C0002. We drilled to 1057 m CSF, cored the intervals from seafloor to 204 m CSF and 475 to 1057 m CSF, and cut 86 cores (18 with the HPCS, 2 with the ESCS, and 66 with the RCB) from three holes. We penetrated the basal unconformity of the Kumano forearc basin at ∼936 m CSF and cored another 120 m into the accretionary prism. The forearc basin sequence was divided into two units based on lithofacies these units corresponded to Units II and III defined by LWD, respectively. Both units are dominated by mud and mudstone however, the Unit I contains more sand and silt intercalation and has a much faster sedimentation rate. The age ranges from Quaternary to late Miocene. Underlying accretionary prism materials contain more lithified and deformed sediments. Only one nannofossil event, of late Miocene age, was determined for Unit IV hence, no significant gap was detected across the unconformity. Faults and shear zones are clustered at certain depths around 700, 920-950, and 1000-1050 m CSF. Three deformation phases were recognized by fault analyses. The earliest phase is a thrust fault (and possibly a strike-slip fault) and exhibits northwest-southeast shortening. Two phases of normal faulting occurred subsequent to thrusting. The first is recorded in shear zones and indicates northeast-southwest extension. The second is recorded in normal faults and indicates north-south extension, consistent with the present stress direction acquired from LWD results. A total of 31 whole-round samples were taken for interstitial water analyses. Changes in concentration for most elements seem to be controlled by unit boundaries. A downward increase of ethane and concomitant decrease of C1/C 2 ratios in Unit IV suggest some contribution of thermogenic hydrocarbons. Physical properties show complex trends with depth. Downhole temperature was measured at eight depths to 159.0 m CSF and showed an almost linear downhole increase with a gradient of 0.043°C/m, identical to that found at Site C0001.

The I-129 geochronology of marine pore water is useful for the understanding of the origin of methane in gas hydrates because of the close association between I and marine organic materials responsible for methane generation. We report I-129/I ratios in pore waters from three deep cores in the eastern Nankai Trough gas hydrate field, two located on the outer ridge and one in the forearc basin. As in previous studies of gas hydrate fields, I ages of pore water are consistently older than those of the host sediments. For the first time, however, the results demonstrate that the potential I source formations vary considerably across the forearc setting: While I at the basin site reaches ages close to 50 Ma, all I ages at the two ridge sites are <32 Ma. The latter two sites also demonstrate the influence of I from formations younger than 10 Ma. The results suggest that I and, by association, methane on the outer ridge are derived mainly from Miocene to Pliocene forearc sediments through the active faults, while the main source for the forearc basin is the old accretionary wedge related to an earlier subduction configuration of Eocene age, which acts as a backstop in the current subduction system.

The isotopic composition (delta D) and delta O-18) and chloride concentration (Cl-) of pore waters from the northern Cascadia continental margin offshore Vancouver Island were measured to characterize the relations between the water flow regime and the distribution, formation and dissociation of gas hydrates. The delta D values of pore waters in gas hydrate-bearing sediments are slightly higher (similar to 1 parts per thousand) than those of seawater as the result of gas hydrate dissociation during core recovery and handling. Within the seismic blanking zone, the delta D values were slightly lower (similar to-1 parts per thousand) than values measured from sites outside the blanking area (0 parts per thousand). We attribute these differences to 1) distillation of D-rich water during hydrate formation in the center of the blanking zone and 2) limited migration of pore water between inside and outside of the blanking zone due to different fluid fluxes. In contrast, the delta O-18 values and Cl- concentrations do not show significant spatial variation due to decreased isotopic fractionation of oxygen and small fraction of chloride relative to hydrogen isotope during gas hydrate formation. The delta D value of pore water, therefore, appears to be a sensitive indicator of gas hydrate occurrence. We estimate that gas hydrate occupied at least 2.0 to 6.3% of sediment pore space using delta D distribution in this area. (C) 2007 Elsevier B.V. All rights reserved.

We report here results of a study on the influence of sample mass on isotope ratios and accuracy of I-129/I determinations in the AMS system at PRIME Lab, Purdue U. Iodine from four samples, two coal-bed methane brines, one surface water sample and a blank, was extracted and precipitated as AgI, following established methods. The resulting samples were subdivided into four sequences of targets, containing between 1.5 and 0.1 mg of AgI. Beam currents for samples with masses above 0.3 mg did not show a dependence on mass, but fell off strongly for smaller masses. The resulting isotope ratios were within the instrumental error limits and did not vary with sample mass, but accuracy decreased for samples with masses below 0.3 mg. The results demonstrate that the presence of 5000 I-129 atoms in the target is sufficient for making a successful AMS determination, a level considerably lower than for other methods used in mass spectrometry. The ability of producing reliable I-129/I ratios from targets as small as 0.1 mg of AgI enlarges considerably the range of applications possible for this isotopic system. (c) 2007 Elsevier B.V. All rights reserved.

Presented here are halogen concentrations (Cl, Br and I) in pore waters and sediments from three deep cores in gas hydrate fields of the Nankai Trough area. The three cores were drilled between 1999 and 2004 in different geologic regions of the northeastern Nankai Trough hydrate zone. Iodine concentrations in all three cores increase rapidly with depth from seawater concentrations (0.00043 mmol/L) to values of up to 0.45 mmol/L. The chemical form of I was identified as I-, in accordance with the anaerobic conditions in marine sediments below the SO4 reduction depth. The increase in I is accompanied by a parallel, although lesser increase in Br concentrations, while Cl concentrations are close to seawater values throughout most of the profiles. Large concentration fluctuations of the three halogens in pore waters were found close to the lower boundary of the hydrate stability zone, related to processes of formation and dissociation of hydrates in this zone. Generally low concentrations of I and Br in sediments and the lack of correlation between sediment and pore water profiles speak against derivation of I and Br from local sediments and suggest transport of halogen rich fluids into the gas hydrate fields. Differences in the concentration profiles between the three cores indicate that modes of transportation shifted from an essentially vertical pattern in a sedimentary basin location to more horizontal patterns in accretionary ridge settings. Because of the close association between organic material and I and the similarity of transport behavior for I- and CH4, the results suggest that the CH4 in the gas hydrates also was transported by aqueous fluids from older sediments into the present layers. (c) 2007 Elsevier Ltd. All rights reserved.

The authors report here halogen concentrations in pore waters and sediments collected from the Mallik 5L-38 gas hydrate production research well, a permafrost location in the Mackenzie Delta, Northwest Territories, Canada. Iodine and Br are commonly enriched in waters associated with CH4, reflecting the close association between these halogens and source organic materials. Pore waters collected from the Mallik well show I enrichment, by one order of magnitude above that of seawater, particularly in sandy layers below the gas hydrate stability zone (GHSZ). Although Cl and Br concentrations increase with depth similar to the I profile, they remain below seawater values. The increase in I concentrations observed below the GHSZ suggests that I-rich fluids responsible for the accumulation of CH4 in gas hydrates are preferentially transported through the sandy permeable layers below the GHSZ. The Br and I concentrations and I/Br ratios in Mallik are considerably lower than those in marine gas hydrate locations, demonstrating a terrestrial nature for the organic materials responsible for the CH4 at the Mallik site. Halogen systematics in Mallik suggest that they are the result of mixing between seawater, freshwater and an I-rich source fluid. The comparison between I/Br ratios in pore waters and sediments speaks against the origin of the source fluids within the host formations of gas hydrates, a finding compatible with the results from a limited set of I-129/I ratios determined in pore waters, which gives a minimum age of 29 Ma for the source material, i.e. at the lower end of the age range of the host formations. The likely scenario for the gas hydrate formation in Mallik is the derivation of CH4 together with I from the terrestrial source materials in formations other than the host layers through sandy permeable layers into the present gas hydrate zones. (c) 2006 Elsevier Ltd. All rights reserved.

Fluid migration in subduction zones is one of the key phenomena to understand the global mass transfer system. While active volcanoes provide the most recognizable conduits for fluid flow in active margins, the existence of a large number of active fluid seepages demonstrates that other forms of fluid release are also important in subduction zone settings. The authors collected fluid samples from springs and wells across the forearc area in Kyushu, a southwestern island of Japan, covering hot spring activities associated with active volcanism and the Median Tectonic Line (MTL), a major fault system present in the southwestern part of Japan. In order to determine sources of these fluids, halogen concentrations as well as I-129/I and Cl-36/Cl ratios were measured in samples from several locations. While Cl concentrations of the forearc fluids in Kyushu range between seawater and meteoric water value, I concentrations are considerably higher than seawater value. Fluids in the Miyazaki area are much higher in I, and somewhat higher in Br, than waters in the Oita area, which is closely associated with the MTL. The differences between those two areas are also pronounced in I-129/I ratios, which range between 800 and 900 x 10(-15) in the Oita area and between 100 and 360 x 10(-15) in the Miyazaki area. The I-129/I ratios obtained from the Oita area are compatible with an I derivation from subducting marine sediments, similar to findings from an earlier investigation of fluids collected from Satsuma-Iwojima, an active volcano south of Kyushu Island. In the Miyazaki area, on the other hand, I ages are too old to be derived from currently subducting marine sediments and point to a derivation from old organic-rich materials in the upper plate of the forearc region. The results demonstrate the presence of very different fluid systems in the forearc area of Kyushu: old CH4-rich fluids dominate in the seaward side of the forearc, while fluids close to the MTL and the Quaternary Volcanic Front demonstrate derivations from subducting marine sediments. The latter fluids in the MTL area probably are transported through the fractures associated with the fault activities, suggesting that this fault system reaches the transition zone between upper and lower plates in this region. (c) 2006 Elsevier Ltd. All rights reserved.

A gas hydrate field with highly active venting of methane was recently found near Sado Island in the eastern Japan Sea. Piston cores were collected from active venting sites and nearby locations in the Umitaka Spur-Joetsu Knoll area during two cruises in 2004 (UT04) and 2005 (KY05-08). We report here halogen concentrations and I-129/I ratios in pore waters associated with gas hydrates from these expeditions. The strongly biophilic behavior of I and, to a lesser degree, of Br together with the presence of the long-lived iodine radioisotope (I-129) allow evaluation of potential source materials for methane in gas hydrate systems. Depth profiles of all three halogens, particularly the very rapid downward increases of Br and I concentrations, strongly suggest input of deep fluids enriched in Br and I, but the profiles also display the effects of gas hydrate formation and dissociation. Although the I-129/I ratios are modified by I-129 from seawater and sediments at shallow depth, likely ratios of the deep fluids are estimated to be between 400 x 10(-15) and 600 x 10(-15), equivalent to a Late Oligocene to Early Miocene age. Ages in the active methane venting sites typically are closer to the old end of this range than those in the reference sites. This age range suggests that the methane associated with venting and gas hydrate formation in this area is derived from organic materials accumulated during the initial opening of the Japan Sea. The Umitaka Spur-Joetsu Knoll gas hydrate field demonstrates the movement of deep fluids associated with the release of significant amounts of methane from the seafloor, processes which might be important components of mass transfer and carbon cycle in the shallow geosphere. (c) 2006 Elsevier B.V. All rights reserved.

Umitaka Spur, situated on an unusual collisional plate boundary along the eastern margin of the Japan Sea, hosts gas seeps, pock-marks, collapse structures, and gas hydrates. Piston cores were recovered from this ridge to understand carbon cycling, pore fluid gradients and authigenic mineralization above a methane-charged system. We present the chemistry of fluids and solids from three cores adjacent to seep locations. High fluxes of CH4 and alkalinity transport carbon from a deep zone of methanogenesis toward the seafloor. Methane, however, reacts with SO42- across a shallow sulfate-methane transition (SMT), which generates additional alkalinity and HS-. A fraction of these CH4 oxidation products form authigenic carbonate and pyrite. These minerals are not readily apparent from visual inspection of split cores, because they exist as micritic coatings on microfossils or as framboidal pyrite. They are, however, readily observed in chemical analyses as peaks of "labile" Ca, Sr, Ba or S in sediment at or near the SMT. Carbon inputs and outputs nicely balance across the SMT in all three cores if one considers four relevant fluxes: loss of alkalinity to the seafloor, addition of methane from below, addition of alkalinity from below, and carbonate precipitation. Importantly, in all cores, the magnitude of the fluxes decreases in this order. Although some carbon rising from depth forms authigenic carbonate, most (> 80%) escapes to the ocean as alkalinity. Nonetheless, authigenic fronts in sediment on Umitaka Spur are a significant reservoir of inorganic carbon. Given calculated pore fluid fluxes for Ca and Sr, the fronts require tens of thousands of years to form, suggesting that the present state and loss of carbon represent long-lived processes. (C) 2007 Elsevier Ltd. All rights reserved.

Because gas hydrate is preferentially enriched in the heavy water isotopes, the delta O-18 and delta D values of pore waters collected from gas hydrate-bearing sediment can provide information on the abundance and mechanisms of gas hydrate formation. Pore waters sampled from deep-seated (40 to 125 mbsf) gas hydrate deposits in Hydrate Ridge during ODP Leg 204 show depletion in dissolved Cl- and enrichments in O-18 and D due to gas hydrate destabilization during core recovery. The oxygen and hydrogen isotopic fractionation factors (alpha(O) = 1.0025 and alpha(H) = 1.022) estimated from an extensive data set (n = 30 samples) correspond to experimentally determined values. In contrast, pore waters from shallow samples (<25 mbsf) at the ridge summit (n = 32) are highly enriched in dissolved Cl- and depleted in O-18 and D, consistent with formation of massive gas hydrate deposits at rates faster than those at which these anomalies would be removed by advection or diffusion. The water isotopic fractionation factors in the brine are significantly lower than those experimentally determined, with alpha(O) of 1.0010 (average value of 1.0012) and alpha(H) of 1.008 (average value of 1.008). We discuss several factors that may be causing this anomalous fractionation and suggest that low gas occupancy in hydrate lattice (high hydration number) may be responsible for the observed small fractionation. If this were the case, the oxygen and hydrogen fractionation may serve as an indicator of hydration number during formation of gas hydrate in natural systems.

In: Tréhu, A.M., Bohrmann, G., Torres, M.E., Colwell, F.S. (Eds.)

In: Tréhu, A.M., Bohrmann, G., Torres, M.E., Colwell, F.S. (Eds.)

In: Tréhu, A.M., Bohrmann, G., Torres, M.E., Colwell, F.S. (Eds.)

In: Dallimore, S.R., Collett, T.S. (Eds.), Scientific Results from the Mallik 2002 Gas Hydrate Production Research Well Program, Mackenzie Delta, Northwest Territories, Canada

In: Dallimore, S.R., Collett, T.S. (Eds.), Scientific Results from the Mallik 2002 Gas Hydrate Production Research Well Program, Mackenzie Delta, Northwest Territories, Canada

In: Dallimore, S.R., Collett, T.S. (Eds.), Scientific Results from the Mallik 2002 Gas Hydrate Production Research Well Program, Mackenzie Delta, Northwest Territories, Canada

Drilling in the Cascadia accretionary complex enable us to evaluate the contribution of dehydration reactions and gas hydrate dissociation to pore water freshening. The observed freshening with depth and distance from the prism toe is consistent with enhanced conversion of smectite to illite, driven by increase in temperature and age of accreted sediments. Although they contain gas hydrate - as evidenced by discrete low chloride spikes - the westernmost sites drilled on Hydrate Ridge show no freshening trend with depth. Strontium data reveal that all the melange samples contain deep fluids modified by reaction with the subducting oceanic crust. Thus we infer that, at the westernmost sites, accretion is too recent for the sediments to have undergone significant illitization. Our data demonstrate that a smooth decrease in dissolved chloride with depth cannot generally be used to infer the presence or to estimate the amount of gas hydrate in accretionary margins.

At the summit of Hydrate Ridge (ODP Sites 1249 and 1250), pore fluids are highly enriched in dissolved chloride (up to 1370 mM) in a zone that extends from near the sediment surface (similar to1 mbsf) to depths of 25 5 mbsf. Below this depth, brines give way to chloride values approaching seawater concentrations with lower chloride anomalies superimposed on baseline values. We developed a one dimensional, non-steady state, transport reaction model to simulate the observed chloride enrichment at Site 1249. Our model shows that in order to reach the observed high chloride values, methane must be transported in the gas phase from the depth of the BSR to the seafloor. Methane transport exclusively in the dissolved phase is not enough to form methane hydrate at the rates needed to generate the observed chloride enrichment. Methane transport in the gas phase is consistent with geophysical and logging data, estimates of gas pressure beneath the BSR, and observations of bubble plumes at the seafloor. In order to reproduce the observed chloride and gas hydrate distributions, the model requires an enhanced rate of hydrate formation in near surface sediments, which we implement through depth-dependent kinetic constants. We argue that this is justified by changes in geomechanical properties of the sediment. At depths shallower than 25 mbsf the force of crystallization can overcome effective overburden stress, and hydrate growth proceeds by particle displacement, thus minimizing capillary inhibition effects. Our calculations indicate the hydrates in the upper sediments of the ridge summit are probably younger than 1500 years, although the age is difficult to constrain. Independent estimates based on seafloor observations at this site yield gas hydrate formation rates at the ridge crest on the order of 10(2) mol m(-2) year(-1). These rates are several orders of magnitude higher than those estimated for Site 997 on the Blake Ridge. (C) 2004 Elsevier B.V. All rights reserved.

Large uncertainties about the energy resource potential and role in global climate change of gas hydrates result from uncertainty about how much hydrate is contained in marine sediments. During Leg 204 of the Ocean Drilling Program (ODP) to the accretionary complex of the Cascadia subduction zone, we sampled the gas hydrate stability zone (GHSZ) from the seafloor to its base in contrasting geological settings defined by a 3D seismic survey. By integrating results from different methods, including several new techniques developed for Leg 204, we overcome the problem of spatial under-sampling inherent in robust methods traditionally used for estimating the hydrate content of cores and obtain a high-resolution, quantitative estimate of the total amount and spatial variability of gas hydrate in this structural system. We conclude that high gas hydrate content (30-40% of pore space or 20-26% of total volume) is restricted to the upper tens of meters below the seafloor near the summit of the structure, where vigorous fluid venting occurs. Elsewhere, the average gas hydrate content of the sediments in the gas hydrate stability zone is generally < 2% of the pore space, although this estimate may increase by a factor of 2 when patchy zones of locally higher gas hydrate content are included in the calculation. These patchy zones are structurally and stratigraphically controlled, contain up to 20% hydrate in the pore space when averaged over zones similar to 10 m thick, and may occur in up to similar to 20% of the region imaged by 3D seismic data. This heterogeneous gas hydrate distribution is an important constraint on models of gas hydrate formation in marine sediments and the response of the sediments to tectonic and environmental change. Published by Elsevier B.V.

Standard scientific operations on Ocean Drilling Program (ODP) Leg 204 documented a horizon of massive gas hydrate and highly saline pore water similar to 0-20 m below the southern summit of Hydrate Ridge offshore Oregon. The sediment zone lies near active seafloor gas venting, raising the possibility that free gas co-exists with gas hydrate in shallow subsurface layers where pore waters have become too saline to precipitate additional gas hydrate. Here we discuss a unique experiment that addresses this important concept. A 1-m-long pressurized core was retrieved from similar to 14 m below sea floor at Site 1249 and slowly degassed at similar to 0degreesC in the laboratory over similar to 178 h to determine in situ salinity and gas concentrations in the interval of massive gas hydrate. The core released similar to 95 l of gas (predominantly methane), by far the greatest gas volume ever measured for a I m core at ambient shipboard pressure and temperature conditions. Geochemical mass-balance calculations and the pressure of initial gas release (4.2 MPa) both imply that pore waters had an in situ salinity approaching or exceeding 105 g kg(-1), the approximate salinity required for a gas hydrate-free gas-brine system. Relatively high concentrations of propane and higher hydrocarbon gases at the start of core degassing also suggest the presence of in situ free gas. Gas hydrate, free gas and brine likely co-exist in shallow sediment of Hydrate Ridge. Near-seafloor brines, produced when rapid gas hydrate crystallization extracts large quantities of water, impact the distribution and cycling of gas and gas hydrate in this region and perhaps elsewhere. (C) 2004 Elsevier B.V All rights reserved.

Research and training vehicle Umitaka-maru sailed to the methane seep area on a small ridge in the eastern margin of the Sea of Japan on July to August 2004 to survey the ocean floor methane hydrate and related acoustic signatures of methane plumes using a quantitative echo sounder [1]. We carried out high-resolution mapping of methane plumes using a quantitative echo sounder with positioning data from GPS and also measured averaged echo intensity from the methane plumes both in every 100m range and everyone minute by the echo integrator. We obtained a number of interesting results from the present echo-sounder survey. We registered 36 plumes on echogram, ranging about 100m in diameter and 200m to 700m in height, reaching up to 300m to 600m below sea level and measured the integrated volume backscattering strength (SV) of each methane plume. The strongest SV, -33dB, of the plumes was stronger than SV of fish school. Averaged SV tend to show the highest values around the bottom and the middle of plumes, whereas the SVs are relatively low at the top of plumes. We recovered several fist-sized chunks of methane hydrate by piston coring at the area where we observed methane plumes. Methane hydrate was recovered throughout two meters-long piston core interval, indicating thick hydrate deposits in shallow sediments near the methane plumes. A follow - up project, we are planning to measure SV of methane bubbles and methane hydrate floating in water columns through an experimental study in a large water tanks.

The Nankai Trough runs along the Japanese Islands, where extensive BSRs have been recognized in its forearc basins. High resolution seismic surveys and site-survey wells undertaken by the MITI have revealed the gas hydrate distribution at a depth of about 290 mbsf. The MITI Nankai Trough wells were drilled in late 1999 and early 2000. The highlights were successful retrievals of abundant gas hydrate-bearing cores in a variety of sediments from the main hole and the post survey well-2, keeping the cored gas hydrate stable, and the obtaining of continuous well log data in the gas hydrate-dominant intervals from the main hole, the post survey well-1 and the post survey well-3. Gas-hydrate dominant layers were identified at the depth interval from 205 to 268 mbsf. Pore-space hydrate, very small in size, was recognized mostly filling intergranular pores of sandy sediments. Anomalous chloride contents in extracted pore water, core temperature depression, core observations as well as visible gas hydrates confirmed the presence of pore-space hydrates within moderate to thick sand layers. Gas hydrate-bearing sandy strata typically were 10 cm to a meter thick with porosities of about 40 %. Gas hydrate saturations in most hydrate-dominant layers were quite high, up to 90 % pore saturation. All the gas hydrate-bearing cores were subjected to X-ray CT imagery measurements for observation of undisturbed sedimentary textures and gas-hydrate occurrences before being subjected to other analyses, such as (1) petrophysical properties, (2) biostratigraphy, (3) geochemistry, (4) microbiology and (5) gas hydrate characteristics.

Interstitial water expelled from gas hydrate-bearing and -free sediments in the Nankai Trough are analyzed in terms of Cl-, SO42-, delta(18)O and deltaD. The baselines for the Cl- concentration and delta(18)O value are close to seawater values (530 mM and 0 parts per thousand), indicating that the interstitial water is of seawater origin. The deltaD values decrease with depth, implying isotopic exchange of hydrogen between upwelling biogenic methane depleted in D and interstitial water. The Cl- concentrations in gas hydrate-bearing sediments are anomalously low, while the delta(18)O and deltaD values are both high, suggesting that the water forming these gas hydrates was poor in Cl- and enriched in O-18 and D during gas hydrate formation. Calculation of the gas whydrate saturations using Cl- and delta(18)O anomalies gives results of up to 80 % in sand, and shows that the delta(18)O baseline is not consistent with the Cl- baseline. The delta(18)O baseline increases by +1 parts per thousand in gas hydrate-free clay and silt. This is considered to be caused by clustering of water molecules after gas hydrate dissociation in response to the upward migration of the base of gas hydrate stability, as indicated by the presence of a double bottom-simulating reflector at this site. The water clusters enriched in O-18 are responsible for the increase in the delta(18)O baseline with normal Cl-. The abrupt shallowing of the base of gas hydrate stability may induce the dissociation of gas hydrates and the accumulation of gases in the new stability zone, representing a geological process that increases gas hydrate saturation.

Interstitial waters extracted from the sediment cores from the exploration wells, "BH-1" and "MITI Nankai Trough", drilled similar to60 km off Omaezaki Peninsula in the eastern Nankai Trough, were analyzed for the chloride and sulfate concentrations to examine the depth profiles and occurrence of subsurface gas hydrates. Cored intervals from the seafloor to 310 mbsf were divided into Unit 1 (similar to70 mbsf, predominated by mud), Unit 2 (70-150 mbsf, mud with thin ash beds), Unit 3 (150-250+ mbsf, mud with thin ash and sand), and Unit 4 (275-310 mbsf, predominated by mud). The baseline level for Cl- concentrations was 540 mM, whereas low chloride anomalies (103 to 223 mM) were identified at around 207 mbsf (zone A), 234-240 mbsf (zone B), and 258265 mbsf (zone C) in Unit 3. Gas hydrate saturation (Sh %) of sediment pores was calculated to be 60 % (zone A) to 80 % (zones B and C) in sands whereas only a few percent in clay and silt. The total amount of gas hydrates in hydrate-bearing sands was estimated to be 8 to 10 m(3) of solid gas hydrate per m(2), or 1.48 km(3) CH4 per 1 km(2). High saturation zones (A, B and C) were consistent with anomaly zones recognized in sonic and resistivity logs. 2D and high-resolution seismic studies revealed two BSRs in the study area. Strong BSRs (BSR-1) at similar to263 mbsf were correlated to the boundary between gas hydrate-bearing sands (zone C) and the shallower low velocity zone, while the lower BSRs (BSR-2) at similar to289 mbsf corresponded to the top of the deeper low velocity zone of the sonic log. Tectonic uplift of the study area is thought to have caused the upward migration of BGHS. That is, BSR-1 corresponds to the new BGHS and BSR-2 to the old BGHS. Relic gas hydrates and free gas may survive in the interval between BSR-1 and BSR-2, and below BSR-2, respectively. Direct measurements of the formation temperature for the top 170 m interval yield a geothermal gradient of similar to4.3 degreesC/ 100 m. Extrapolation of this gradient down to the base of gas hydrate stability yields a theoretical BGHS at similar to230 mbsf, surprisingly similar to35 m shallower than the base of gas hydrate-bearing sands (zone C) and BSR-1. As with the double BSRs, another tectonic uplift may explain the BGHS at unreasonably shallow depths. Alternatively, linear extrapolation of the geothermal gradient down to the hydrate-bearing zones may not be appropriate if the gradient changes below the depths that were measured. Recognition of double BSRs (263 and 289 mbsf) and probable new BGHS (similar to230 mbsf) in the exploration wells implies that the BGHS has gradually migrated upward. Tectonically induced processes are thought to have enhanced dense and massive accumulation of gas hydrate deposits through effective methane recycling and condensation. To test the hypothetical models for the accumulation of gas hydrates in Nankai accretionary prism, we strongly propose to measure the equilibrium temperatures for the entire depth range down to the free gas zone below predicted BGHS and to reconstruct the water depths and uplift history of hydrate-bearing area.

The recognition of finely disseminated gas hydrate in deep marine sediments heavily depends on various indirect techniques because this mineral quickly decomposes upon recovery from in situ pressure a;.,id temperature conditions. Here, we discuss molecular properties of closely spaced gas voids (formed as a result of core recovery) and gas hydrates from an area of relatively low gas flux at the flanks of the southern Hydrate Ridge offshore Oregon (ODP Sites 1244, 1245 and 1247). Within the gas hydrate occurrence zone (GHOZ), the concentration of ethane (C-2) and propane (C-3) in adjacent gas voids shows large variability. Sampled gas hydrates are enriched in C-2 relative to void gases but do not contain C-3. We suggest that the observed variations in the composition of void gases is a result of molecular fractionation during crystallization of structure I gas hydrate that contains C-2 but excludes C-3 from its crystal lattice. This hypothesis is used to identify discrete intervals of finely disseminated gas hydrate in cored sediments. Variations in gas composition help better constrain gas hydrate distribution near the top of the GHOZ along with variations in pore water chemistry and core temperature. Sediments near the base of the gas hydrate stability zone are relatively enriched in C2+ hydrocarbon Eases. Complex and poorly understood geological and geochemical processes in these deeper sediments make the identification of gas hydrate based on molecular properties of void gases more ambiguous. The proposed technique appears to be a useful tool to better understand the distribution of gas hydrate in marine sediments and ultimately the role of gas hydrate in the global carbon cycle. (C) 2004 Elsevier Ltd. All rights reserved.

The Nankai hydrate field, Japan, is an example of gas-hydrate deposits associated with an active subduction zone. In order to determine the origin of gas hydrates in this area, I-129/I ratios together with halogen concentrations were measured in a set of pore-water samples collected from two boreholes in the Nankai hydrate field. Iodine concentrations are between 100 and 230 muM, i.e., strongly enriched compared to seawater, while Cl concentrations were found to be close to that of seawater. Except for one sample, I-129/I ratios are between 180 and 520 x 10(-15), giving minimum ages between 24 and 48 Ma. Because these ages are considerably older than present host sediments (< 2 Ma) and subducting marine sediments (< 21 Ma) in this area, iodine (and, by association, methane in the gas hydrates) must have been derived from source formations located in the continental side of the subduction zone. The results do not support derivation of gas hydrates from present host sediments or currently subducting sediments, but could be related to release and long-time recycling of fluids from marine formations of early Tertiary age.

Contributed to the section of “Inorganic Geochemistry”