Search

A multibeam echosounder survey was conducted (deeper than ca. 300 m water depth, total area: ca. 1.8 × 103 km2) in the southern basin of Lake Baikal, Russia, in June 2015, 2016, and 2017. Characteristic morphology of the lake floor was mapped on the ancient Tankhoy stratum, covered with present sediment, by high-resolution bathymetry. Sediment core sampling operations were conducted in August 2015 and August 2016 at a characteristic mound-like landform named Kedr (after the Kedrovaya River). Sub-surface gas hydrates (GH), containing not only microbial but also thermogenic gases (Hachikubo et al. 2016), were retrieved. Core lithology and sub-bottom profiler survey suggest that Kedr is a mud volcano (MV). Hydrogen and oxygen isotopic anomalies were observed in the sediment pore waters. This suggests the water results from clay mineral dehydration, which is the first observation of this process in Lake Baikal sediment pore water. The thermogenic gases, mud breccia, and water from clay mineral dehydration suggest potential ascending gas as well as water from greater depths, presumably from the ancient Tankhoy stratum under the Kedr MV.

One-dimensional (1-D) ice flow models are used to construct the age scales at the Dome C and Dome Fuji drilling sites (East Antarctica). The poorly constrained glaciological parameters at each site are recovered by fitting independent age markers identified within each core. We reconstruct past accumulation rates, that are larger than those modelled using the classical vapour saturation pressure relationship during glacial periods by up to a factor 1.5. During the Early Holocene, changes in reconstructed accumulation are not linearly related to changes in ice isotopic composition. A simple model of past elevation changes is developed and shows an amplitude variation of 110–120 m at both sites. We suggest that there is basal melting at Dome C (0.56±0.19 mm/yr). The reconstructed velocity profile is highly non-linear at both sites, which suggests complex ice flow effects. This induces a non-linear thinning function in both drilling sites, which is also characterized by bumps corresponding to variations in ice thickness with time.

First data are reported on a new manifestation of carbonate-barite mineralization found at a site of methane emanations on the western slope of the Kuril Basin, Sea of Okhotsk. Morphological types of barite, aragonite, and low-magnesian calcite are considered in detail; the results of carbon and oxygen isotope study of carbonate concretions and crusts are presented. It is shown that the barite was formed in sediments owing to the diffusion infiltration of the barium-rich fluids through sedimentary succession. The component and isotope compositions of gases are determined and the relatively elevated content of heavy hydrocarbons is revealed. It was assumed that the relatively heavy isotope composition of carbonates is caused by the influence of fluid released from deep sedimentary horizons owing to the dehydration of clay minerals during post- sedimentation transformations. Obtained data show that the origin of carbonate-barite mineralization is related to the migration of hydrocarbons (mainly methane) and barium-bearing cold gas-fluid flows, which were derived not only from near-surface reservoirs but also from deeper-seated sources.

The sudden release of methane from seas due to ocean warming and/or sea level drop, leading to extensive mass wasting at continental margins, has been suggested as a possible cause of global climate change. In the northeastern continental slope of the Sakhalin Island (Sea of Okhotsk), numerous gas hydrate-related manifestations have been reported, including hydroacoustic anomaly (gas flare) in the water column, pockmarks and mounds on the seafloor, seepage structures and bottom-simulating reflectors (BSRs). The gas hydrate found at 385 mbsl represents the shallowest occurrence ever recorded in the Okhotsk Sea. In this study, we modeled the gas hydrate stability zone (GHSZ) using methane gas composition, water temperature and geothermal gradient to see if it is consistent with the observed depth of the BSR. An important distinction can be made between the seafloor containing seepage features and normal seafloor in terms of their thermal structure. The depth of the BSR matches well with the base of GHSZ estimated from the background heat flow (geothermal gradient). A large slope failure feature is found in the northern Sakhalin continental slope. We explore the possibility that this failure was caused by gas hydrate dissociation, based on the past climate change history and inference from the GHSZ calculation. Prediction of the natural landslide is difficult; however, new stratigraphic evidence from subbottom profiles suggests that the landslide occurred at 20 ka which is roughly consistent with the late stage of the Last Glacial Maximum.

Assessments of the molecular and isotopic composition of hydrate-bound and dissolved gases in pore water were conducted during the multi-phase gas hydrate project (MHP-09) cruise VER09-03 to the southern basin of Lake Baikal in September 2009. To avoid changes in gas composition during core sampling and transport, various headspace methods were investigated aimed at preserving the dissolved gases in pore water. When distilled water was added to the sediment samples, the concentrations of carbon dioxide and oxygen decreased because of dissolution into the water and/or microbial consumption. When the headspace was not flushed with inert gases, trace levels of hydrogen and ethylene were detected. The findings suggest that best preparation is achieved by flushing the headspace with helium, and adding a saturated aqueous solution of sodium chloride. This improved headspace method served to examine the molecular and isotopic compositions of gas samples retrieved at several new sites in the southern basin. Methane was the major component, and the proportion of ethane ranged widely from 0.0009 to 1.67 mol% of the total hydrocarbon gases. The proportions of propane and higher hydrocarbons were small or less than their detection limits. The carbon isotope signatures suggest that microbial-sourced methane and ethane were dominant in the Peschanka study area, whereas ethane was of thermogenic origin at all other study sites in the southern basin of Lake Baikal.

The Sakhalin Slope Gas Hydrate Project (SSGH) is an international collaborative effort by scientists from Japan, Korea, and Russia to investigate natural gas hydrates (GHs) that have accumulated on the continental slope off Sakhalin Island, Okhotsk Sea. From 2009 to 2011, field operations of the SSGH-09, -10, and -11 projects were conducted. GH-bearing and -free sediment cores were retrieved using steel hydro- and gravity corers. The concentrations of sulfate ions in sediment pore waters were measured to investigate sulfate concentration–depth profiles. Seventeen cores showed linear depth profiles of sulfate concentrations. In contrast, eight cores and two cores showed concave-up and -down profiles plausibly explained by sudden increase and decrease in methane flux from below, respectively, presumably caused by the formation of gas hydrate adjacent to the core sampling sites.

Multidisciplinary surveys were conducted to investigate gas seepage and gas hydrate accumulation on the northeastern Sakhalin continental slope (NESS), Sea of Okhotsk, during joint Korean–Russian–Japanese expeditions conducted from 2003 to 2007 (CHAOS and SSGH projects). One hundred sixty-one gas seeps were detected in a 2000 km 2 area of the NESS (between 53°45′N and 54°45′N). Active gas seeps in a gas hydrate province on the NESS were evident from features in the water column, on the seafloor, and in the subsurface: well-defined hydroacoustic anomalies (gas flares), side-scan sonar structures with high backscatter intensity (seepage structures), bathymetric structures (pockmarks and mounds), gas- and gas-hydrate-related seismic features (bottom-simulating reflectors, gas chimneys, high-amplitude reflectors, and acoustic blanking), high methane concentrations in seawater, and gas hydrates in sediment near the seafloor. These expressions were generally spatially related; a gas flare would be associated with a seepage structure (mound), below which a gas chimney was present. The spatial distribution of gas seeps on the NESS is controlled by four types of geological structures: faults, the shelf break, seafloor canyons, and submarine slides. Gas chimneys that produced enhanced reflection on high-resolution seismic profiles are interpreted as active pathways for upward gas migration to the seafloor. The chimneys and gas flares are good indicators of active seepage.

International audience; We report on the isotopic composition of dissolved inorganic carbon (DIC) in pore-water samples recovered by gravity coring from near-bottom sediments at gas hydrate-bearing mud volcanoes/gas flares (Malenky, Peschanka, Peschanka 2, Goloustnoe, and Irkutsk) in the Southern Basin of Lake Baikal. The δ13C values of DIC become heavier with increasing subbottom depth, and vary between −9.5 and +21.4‰ PDB. Enrichment of DIC in 13C indicates active methane generation in anaerobic environ- ments near the lake bottom. These data confirm our previous assumption that crystallization of carbonates (siderites) in subsurface sediments is a result of methane generation. Types of methanogenesis (microbial methyl- type fermentation versus CO2-reduction) were revealed by determining the offset of δ13C between dissolved CH4 and CO2, and also by using δ13C and δD values of dissolved methane present in the pore waters. Results show that both mechanisms are most likely responsible for methane generation at the investigated locations.

Hydrate-bearing sediment cores were retrieved from recently discovered seepage sites located offshore Sakhalin Island in the Sea of Okhotsk. We obtained samples of natural gas hydrates and dissolved gas in pore water using a headspace gas method for determining their molecular and isotopic compositions. Molecular composition ratios C1/C2+ from all the seepage sites were in the range of 1,500–50,000, while δ13C and δD values of methane ranged from −66.0 to −63.2‰ VPDB and −204.6 to −196.7‰ VSMOW, respectively. These results indicate that the methane was produced by microbial reduction of CO2. δ13C values of ethane and propane (i.e., −40.8 to −27.4‰ VPDB and −41.3 to −30.6‰ VPDB, respectively) showed that small amounts of thermogenic gas were mixed with microbial methane. We also analyzed the isotopic difference between hydrate-bound and dissolved gases, and discovered that the magnitude by which the δD hydrate gas was smaller than that of dissolved gas was in the range 4.3–16.6‰, while there were no differences in δ13C values. Based on isotopic fractionation of guest gas during the formation of gas hydrate, we conclude that the current gas in the pore water is the source of the gas hydrate at the VNIIOkeangeologia and Giselle Flare sites, but not the source of the gas hydrate at the Hieroglyph and KOPRI sites.

Differences in seepage activity among three gas-seepage structures including hydrate-bearing sites at the Derugin Basin, NE Sakhalin Island, Russia were investigated. Chemical analyses of pore-water geochemistry, water-content distribution and stable isotopes were conducted to describe the complicated geochemical seepage environments involving a flux of free-gas and/or gas-saturated water. Traces of deep ascending fluid were not found in the hydrate-containing Hieroglyph seep, but were suggested in the lower parts of cores from the CHAOS and Kitami seeps based on the presence of abnormally heavy deuterium.

New hydrate-bearing seepage structures off Sakhalin in the Sea of Okhotsk were investigated from 2003 to 2006 within the framework of the CHAOS project. We obtained samples of natural gas hydrate and measured the molecular and isotopic compositions of hydrate-bound gas. Methane δ13C and δD were in the range of -67 to -63‰ and -207 to -193‰, respectively. These results indicate a microbial origin produced by CO2 reduction according to Whiticar's diagram. Because ethane δ13C showed a thermogenic origin, hydrate-bound gas contains a small amount of thermogenic gas. The hydration numbers of the samples were estimated as 6.19 ± 0.02 using the Raman spectra of the C–H stretching mode and a thermodynamic calculation. Heat flow calorimetry revealed that the values for dissociation heat of the samples were 18.1 ± 0.3 kJ mol-1 (from hydrate phase to gas and ice phases) and 55.4 ± 0.4 kJ mol-1 (from hydrate phase to gas and water phases), which agree well with the values in of literature.

We report on the structural properties of natural gas hydrate crystals from the Sea of Okhotsk. Using powder X-ray diffraction (PXRD), it was determined that sediments from four locations contained type I gas hydrate, which encage mostly methane (96–98%) and a small amount of carbon dioxide. For all hydrates, the lattice constant was estimated to be 11.85 A ˚ at 113 K, which approximately equals that of pure methane hydrate. The result is in good agreement with the structure of artificially synthesized methane + carbon dioxide mixed-gas hydrates. These results suggest that the lattice constant of the natural gas hydrate does not change due to a change of CO 2 gas content. In addition, the thermal expansion of the sampled hydrate was measured for the temperature range of 83–173 K, and the resulting density of the hydrate crystal at 273 K was estimated to be 0.92 g / cm 3 . These results are essential for applying natural gas hydrates as an alternative natural fuel resources.

Experimental investigations were conducted to understand the formation process of CO_2 hydrate the chimney structure by using a gas bubble emission technique in water within a pressure cell. The detailed process was video-recorded and analyzed to study the initiation and growth behavior of hydrate chimney while the cell pressure was increased and gas supply rate decreased gradually with time. In the initial stage of chimney growth, a hydrate crystal started to form in a cup shape at the gas nozzle and ascended together with gas bubbles due to mechanical weakness of the hydrate/nozzle contact. Growth of hydrate chimney occurred with supercooling of 3K(overpressure of 0.60MPa) or more, and continued until the top end was closed completely by hydrate.

Guest–host interac-tions play a crucial role in the crystal structure, and thusclathrate hydrates are thermodynamically stable only whenguestmoleculesareencagedinthehostcages.Therelationbetween structural type of clathrate hydrate and guestmolecule is categorized according to the size of the guestmolecule.

To better understand how ice fabric evolves in polar ice sheets, we use X-ray diffraction to measure ice crystal orientations. X-ray measurement equipment which can measure the orientation of the c axis and a axis of each crystal in a thin section with high measurement accuracy was developed. In this study, we present a-axes orientation distribution of the deep part of the GRIP (Greenland summit) ice core. At some depths, we find an anisotropic distribution of a-axes orientation. Long-term uniaxial compression tests are also carried out on the GRIP ice core to investigate the ice fabric evolution process. The c-axis orientation distribution develops into a stronger single maximum as the strain increases up to about 20% strain. We find that the a axes of each grain also tend to cluster close to nearly a mutual direction. We discuss the development process of ice fabrics, taking into consideration the distribution of the a-axis orientations. It is suggested that these fabrics may be attributed to a local simple shear deformation in the deep part of an ice sheet.

Three different approaches to ice-core age dating are employed to develop a depth-age relationship at Dome F: (1) correlation of the ice-core isotope record to the geophysical metronome(Milankovich surface temperature cycle) inferred from the deep borehole temperature profile at Vostok,(2) importing a known chronology from another(Devils Hole) paleoclimatic signal, and(3) direct ice sheet flow modeling. Inverse Monte Carlo sampling is used to constrain the accumulation rate reconstruction and ice flow simulations in order to find the best-fit glaciological time scale matched with the two other chronologies. General uncertainty of the different age estimates varies from 2 to 6kyr on average and reaches 6-14kyr at maximum. Whatever the causes of this discrepancy might be, they are thought to be of different origins, and the age errors are assumed to be independent. Thus, the average time scale for the Dome F ice core down to a depth of 2500m(ice age of 335kyr) is deduced consistently with all three age-depth relationships within the standard deviation limits of ±3.3kyr, and its accuracy is estimated as 1.4kyr on average. The constrained ice-sheet flow model allows extrapolation of the ice age-depth curve further to the glacier bottom and predicts the ages at depths of 2800, 3000, and 3050m to be 615±70, 1560±531, and 2985±1568kyr, respectively.

The processes of formation and dissociation of gas hydrates were investigated by monitoring pressure and temperature variations in a pressure cell in order to understand the kinetic behavior of gas hydrate and the controlling factors for the phase transition of gas hydrate below freezing. Gas hydrates were made from guest gases (CH4, CO2 and their mixed-gas) and fine ice powder. We found that formation and dissociation speeds of gas hydrates were not controlled by temperature and pressure conditions alone. The results of this study suggested that pressure levels at the formation of mixed-gas hydrate determine the transient equilibrium pressure itself.

Long-term changes of snow-accumulation rate in Antarctica are a major uncertainty in our understanding of past climate. Because the visible strata in polar ice are due to variations in the sizes and concentrations of air inclusions and microparticles, the scattered light intensity from an ice core yields valuable information on the stratification, which is likely to provide estimates of the annual accumulation rates. Identification of each layer is therefore necessary, and we developed an optical scanner apparatus to record detailed visible strata of ice cores. The apparatus records the two-dimensional distribution of light-scattering intensity along ice-core samples and produces an image of the whole ice-core sample by an image analysis process. These images showed that ice from Dome Fuji ice core contained a large number of layers. Volcanic layers were also well identified. We processed the scattering intensity on the enhanced intensity images to produce an intensity profile. This profile showed that the period of the intensity variations is consistent with a core-dating model applied to the Dome Fuji ice core. We also found that the intensity peaks are closely correlated to peaks in Ca2+ ion concentrations. Thus, our scanning method is a promising approach to measuring annual-layer thickness and, as a result, may be used to infer past accumulation rates in Antarctica.

Geochemical properties of gas hydrate accumulation associated with an active gas vent on the continental slope offshore northeast Sakhalin Island in the Sea of Okhotsk have been investigated. The pore water chemistry data suggest that the gas hydrates (GHs) were formed in an environment of upward-migrating fluid combined with a mechanism of pore water segregation. The upward infiltration of water enriched mainly by Cl– and K+ species appears to occur on the background of earlier diagenesis processes within the gas vent sediments. The GHs were formed from water with chlorinity ranging from 530 to 570 mM. The 18O and D of GH water varied from –1.4 to –1.8 ‰ and from –13 to –18 ‰, respectively, representing a mix of seawater and infiltrating fluid water. A complex interaction of pore water, water of ascending fluid and segregated pore water during hydrate formation is also supported by water content measurements and observed gas hydrate structure. The direction of segregated water is opposite to upward fluid migration. Decreasing activity of the gas vent is inferred by comparing the present top of the recovered hydrate layer with previous observations.

The Antarctic ice sheet preserves paleo-climate information in the form of physical and chemical stratigraphy. A deep ice core was continuously drilled down to a depth of 2,503 m at Dome Fuji station, East Dronning Maud Land, Antarctica, during the 1993-97 JARE inland operations. Oxygen isotope measurements were conducted on 7 to 50 cm-long ice core samples selected from the entire core depth. A time scale for the Dome Fuji core is calculated from past accumulation rates and an ice flow model. Past accumulation rates were converted from oxygen isotope values by using an empirical equation obtained in the Dome Fuji area. A steady-state flow model was preciously developed for a time scale calculation of the Summit ice core, Greenland. Using reference depth points from volcanic signals and annual layer thickness values measured on the Dome Fuji core allows for tuning of the calculated time scale. A depth-age profile was obtained for the past 320 kyr. The obtained paleo-temperature profile shows the characteristics of the past three glacial and interglacial periods. The power spectrum of ƒÂ18O change over an interval of 320 kyr reveals three dominant cycles. The paleo-temperature profile coincides quite well with the Vostok ice core data in general but not in detail, suggesting that further studies are needed both for chronological investigations and a multi-factor, cross-correlation analysis between deep ice cores for climatological understanding.

Mechanical anisotropy of ice core samples has been observed in various uniaxial compression tests. The c-axis orientation distribution is the primary influence on the mechanical behavior of ice cores. A strong single-maximum fabric pattern is observed in the deep parts of the ice sheet. In this region, polycrystalline ice is very hard along the vertical axis; however, it easily shears along the horizontal plane. Thus, by acquiring the distribution of c-axis orientations throughout the ice sheet, the mechanical anisotropy of ice sheet flow behavior can be understood. Analysis of fabric measurements on the Dye 3, GRIP, and Dome F ice cores suggests that the c-axis orientation distribution depends primarily on vertical strain. Therefore, if the ice thickness at some point in the ice sheet is known, it should be possible to predict the distribution of c-axis orientations at that depth. Uniaxial compression tests were carried out along various directions of the Dye 3, GRIP, and Dome F ice cores. A contour map of mechanical anisotropy was then made to relate the compression direction to the vertical strain. This clarified the flow enhancement factor in every compression direction at a given vertical strain.

Oxgen isotope ratio and chemistry profiles were compared to find the corresponding interstadials during the Wisconsin Ice Age between the GRIP (Greenland) and Dome Fuji (Antarctica) deep ice core data for the past one hundred and sixty thousand years. Eight interstadials in GRIPδ18O profile were found to correspond to those in Dome Fuji δ18O profile. Eleven interstadials in GRIPδ18O profile were found to correspond to those in Dome Fuji chemistry (calcium, nitrate and sulfate) profile, which is better suited for the purpose of interstadial search than the δ18O profile at Dome Fuji. The Eemian interglacial period at Dome Fuji seems to be much shorter and more stable than that in the GRIP profile. Three major periods having higher contents of calcium, nitrate and sulfate appear at Dome Fuji, ranging (1) between interstadials number 1 and 8, (2) between interstadials number 17 and 19, and (3) before the Eemian, which correspond to relatively cold and stable periods in the GRIP δ18O profile. These findings promise a favorable outcome from more detailed bipolar comparison in the future for an understanding of climatic linkage conditions and the driving forces between northern and southern hemispheres.

Recent results of physical analyses of the Dome Fuji ice core are summarized with special attention to new methods introduced in the present studies. Microphysical processes which affect the ice core records are reviewed to better understand the paleoclimatic and paleoenvironmental signals stored.

Dating of ice cores is of important but is difficult for an ice core where there is low snow accumulation, and also for the deep part because seasonal chemical and isotopic signals are not easily preserved due to vapor migration after snow deposition and molecular diffusion in the deep part of ice sheet. In this paper, an attempt to reveal annual layer thickness is conducted on the basis of precise number density measurement of air bubbles and air hydrates. The annual layer thickness from air bubbles and hydrates agrees well with a calculated value within 10-15% at all depths of the 2500 m deep core. The obtained thickness in the interglacial period according to Eemian period in the Greenland ice core was half of the calculated value.

Cloudy band stratigraphy was observed in the GRIP and NGRIP, Greenland ice cores. Gray value profiles obtained from the photographic recording were analyzed to extract annual layer signals. Simple counting of gray value peaks is effective enough when annual layer thickness is relatively small (smaller than approximately 20 mm), but smoothing of the gray value profile is needed when annual layer thickness is larger. Smoothing can be done by adopting a running mean over a range of half of annual layer thickness estimated from ice flow modeling. A comparison of the DEP profile with the gray value profile revealed that the DEP profile seems to reflect the seasonal variation in general, but not exactly.

Surface hollows appearing after microtome shaving were observed on poly-and single crystal specimens under an optical microscope in a cold room laboratory. Characteristic shell-shaped hollows appeared with their major axis perpendicular to the shaving direction. Fewer hollows appeared at specimen surfaces when the basal plane of the ice crystal was parallel or perpendicular to the specimen surface. The results observed could be easily interpreted if cleavage cracks form parallel or perpendicular to the basal plane near the blade edge during the microtome shaving process.

Many tiny cracks are observed near the surface of deep ice cores drilled from polar ice sheets. These cracks probably form during the ice core drilling process at hydrostatic pressure, which increases linearly with depth. We studied crack formation in ice experimentally by deforming ice single crystals using a uniaxial compression apparatus with controlled hydrostatic pressure. The uniaxial compressive stress was applied to ice samples under a hydrostatic pressure of 20 MPa at -18℃. A constant strain rate was set at ∿(10)^s^. The c-axis orientations of the ice single crystals were parallel to the compression axis. The compression test results show that cracks formed in ice samples at a stress level of over 15MPa after 0.5% compressive strain, except for samples that deformed non-uniformly. These cracks grew along basal and parallel planes to the c-axis. This result suggests that the tiny cracks at the surface of ice cores could originate in local high stress just under the tip of the cutter.

Deep ice core drilling was carried out at Dome Fuji, Antarctica in 1995 and 1996 from the bottom of the casing installed in 1993 and reached 2503.52m in December 1996. We used a JARE type electromechanical drill with a core barrel of 2.2m length. Total numbers of ice corings and chip collections were 1369 and 837 respectively. The mean coring depths per run and per day were 1.75m and 8.21m, respectively. Quality of ice cores was perfect throughout the whole depth, even in the brittle zone. We report the outline of the system, coring performance, and troubles encountered.

From 1993 to 1998,members of the Dome F project group carried out ice coring at Dome F station. They recovered a 2503-m-long ice core. Specific tasks for the ice core processing included (1) core storage, (2) cutting and packing the ice samples for transportation from the station to Japan, and (3) performing initial ice analyses that can be done at the station. In this report we outline the operation and describe our experiences related to the ice core processing to aid similar processing in the future.

The GISP deep drill became stuck in 1981,but was free the following year. The NGRIP/EPICA deep drill has suffered from two big setbacks : The drill is stuck both at NGRIP in Greenland and at Dome C in Antarctica. Both events occurred in a period with routine drilling and high productivity. The reasons for the two events are believed to be different, but the chosen bore-hole liquid seems to be problematic. The densifier can adhere to the surface of the ice cuttings, making fine ice cuttings to sink in the liquid, in spite of a liquid density of 935kg/m^3. In spite of changed procedures and modified constructions, the drill became stuck again at NGRIP. It was freed using glycol, making use of both the temperature and temperature gradient in the hole.

A high-resolution study of c-axis fabrics has been performed on the NorthGRIP deep ice core from North Greenland. A newly developed automatic ice-fabric analyzer has been used to measure c-axis orientations on vertical thin sections at 142 different depths between 100 and 2920 m in the core. Detailed comparison studies show that this new method produces results that are in full accordance with those obtained from conventional manual measurements. Fabric development in ice of the NorthGRIP core can be explained by rotation of c axes, controlled by the prevailing stress systems. In the upper 800 mthe c axes appear to distribute uniformly, but in the depth interval 900–2500m they tend to cluster around a vertical plane. This vertical girdle pattern is strengthened with depth, forming the first clear evidence of this fabric type in a Greenland deep ice core. Such fabric development has previously been observed in the Vostok ice core from East Antarctica, where converging flow is believed to occur. the most likely interpretation of the NorthGRIP girdle fabric is that the c axes are rotating away from a horizontal-tension axis across the main ice divide, which runs north-northwest–south-southeast through the NorthGRIP drilling site. This is supported by information available from surface velocity measurements. Within the girdle fabric, increased concentration of c axes parallel to the core axis is observed with increasing depth, indicating combined effects of vertical compression and horizontal tension on the fabric development. from about 2500 m depth, the girdle-type fabric starts to give way to a strong vertical single-maximum fabric, which persists to 2920m depth, where drilling was terminated 150 mabovebedrock in the year 2000. the single maximum seems to suggest that bed-parallel simple shear is exerting a strong influence on the fabric in the lowest part of the ice sheet down to 2920 m.

The geophysical metronome (Milankovitch components of the past surface temperature variations) and the isotope–temperature transfer function deduced from the borehole temperature profile at Vostok station, Antarctica, are applied to date the 2500 mdeep ice core from Dome Fuji station, Antarctica, and to reconstruct paleoclimatic conditions at the drilling site on the basis of the local δ18O isotope record. Special attention is paid to consistency of this depth–age relation with the mass-balance reconstruction and predictions of ice-flow modeling. the present-day ice mass-balance rate at Dome Fuji is estimated as 3.2 cm a–1. the ice age at the borehole bottom (590m above the bedrock) is around 335 ± 4.5 kyr and may reach 2000 kyr at about 3000 mdepth.The difference in the ice-sheet surface temperatures between Holocene optimum and Last Glacial Maximum is found to be 17.8˚C at the temporal isotope/temperature slope, about 30% lower than the modern geographical estimates. A good agreement between modeled and measured (preliminary data) borehole temperatures is obtained at the geothermal flux 0.059 Wm–2 and ice-fusion temperature (–2˚C) at the ice–rock interface with minimum (zero) melt rates.

Deep ice cores from polar ice sheets form a proxy data bank for global climate variations. Japanese researchers have been involved in international core drilling and analysis projects in Greenland since 1989. One of the major outcomes of the GRIP project is the possible existence of rapid climatic oscillation during the Eemian interglacial period. However. physical property observations suggest that this apparent oscillation could have resulted from complicated ice flow near the bed as revealed by disturbed cloudy band features. A new ice core project, NGRIP, is expected to resolve the above question.

210-year records of dust concentration, electrical conductivity, acidity (pH), non-sea salt sulfate (nss sulfate) and nitrate concentrations were obtained for an ice core from Site-J, Greenland. The ice core was well dated by counting annual cycles of the δ^O profile and by tuning with time markers established for tritium peaks and the Laki 1783 eruption signal. Dust records in both ice cores from Site-J and Crete suggest that dust storms occurred in 1818,1900,1919,1936 and 1943 over Greenland. Electrical conductivity shows remarkable peaks probably due to volcanic eruptions with VEI larger than 4. The anthropogenic effect on precipitation chemistry started in ca. 1860,40 years earlier than the time previously reported. Nss sulfate increased remarkably, much more than nitrate, because of the rapid increase in solid fossil fuel combustion, but by the early 1970's, the nitrate concentration exceeded the nss sulfate concentration due to the intense increase in liquid fossil fuel combustion since the 1950's. The decrease in sulfate and nitrate concentrations started in the early 1970's. This is attributed to the spread of suppression facilities for anthropogenic pollutants emitted by the combustion of solid and liquid fossil fuels.

Field studies of air bubble and clathrate inclusions in the NGRIP ice core reveal two stages of clathrate formation. In the transition zone translucent primary clathrates with rough surfaces and irregular shapes, formed from individual bubbles, split up and form smaller specimens. Deeper down in the transition zone (~1100 m), clathrate metamorphosis sets in, giving rise to the formation of transparent secondary clathrates, with smooth and regular, often poly-hedral or faceted shapes. Rod-like crystals of great length (~2 mm) often show signs of separation into smaller units that fragment into smaller specimens. These findings imply that the simplified picture of one bubble forming one clathrate has to be modified.

Studying freezing and thawing mechanisms is important to understand rock slope failures in cold regions. This study investigated crack development in rock specimens by freezing and thawing action. Rock specimens were prepared from welded tuff. A freezing test was conducted in a temperature-controlled chamber where the temperature varied from + 5°C to –18°C. The frozen rock specimens were thawed in distilled water. The test lasted 3.5 hours for each freeze-thaw cycle, including 2 hours for freezing and 1.5 hours for thawing. The P-wave velocity, acoustic emission, and porosity were measured, and observations on the change in appearance of the outer surface and section of the specimen were made. The results obtained are summarized as follows: (1) cracks in rock developed during the freezing of pore water; (2) the initial cracks in rock caused by freezing and thawing action formed at a weak position of the surface layer and then propagated to the inside of the specimen; (3) the patterns of cracks produced by freezing and thawing cycles depended on the rock structure, such as its geometry, porosity and pore diameter; and (4) the rate of crack development was affected by the degree of water saturation and mechanical properties of the rock.

During traverses in East Dronning Maud Land, which includes Dome Fuji Station (77°19′01″S, 39°42′12″E, elevation 3810m a.s.l.), many glaciological observations have been conducted by the Japanese Antarctic Research Expedition. Among the quantities observed are annual net accumulation (A_ : kg m^), stable oxygen isotope composition (δ^O : ‰) in the surface snow and mean annual surface temperature distribution. The characteristics of these quantities have been analyzed, and the following equation relating them was obtained : A_=1.104×10^5exp (0.1462δ^O). In addition, a relation between δ^O and surface temperature has been obtained. These relations suggest a possibility that core chronology and surface temperature variations can be reproduced from the δ^O values in the 2500m-deep ice core drilled at Dome Fuji Station.

Basic chemical analyses of a 2503m deep ice core obtained at Dome Fuji Station, Antarctica in 1995-1996 were carried out to determine stable oxygen and hydrogen isotope ratios, major chemical compositions and microparticle concentrations. We here describe the analytical procedure and the results as well as the determination of working time scale. The ice core covers the past 340ka which includes three glacial-interglacial cycles clearly shown by the stable oxygen isotope profile. Major chemical compositions and microparticle concentrations larger than 0.52μm show high concentrations in glacials and low in interglacials.

Physical analyses of the Dome Fuji ice core have been carried out to reveal fundamental properties of the core : (1) Stratigraphy, (2) Bulk density by a volumetric method, (3) Bulk density by an x-ray transmission method, (4) Total gas content, (5) Permeability and bubble volume, (6) Distribution of air-bubbles, (7) Distribution of clathrate hydrates, (8) Raman spectral N_2/O_2 ratios of air-bubbles and clathrate hydrates, (9) Ice fabrics, (10) Ice grain size, (11) Laser scattering tomography, (12) DC-ECM and AC-ECM, (13) Mechanical test, and (14) Crystalline structures of ice and clathrate hydrate. In the present paper the main results obtained so far are summarized.

Air bubbles trapped near the surface of an ice sheet are transformed into air hydrates below a certain depth Their volume and number varies partly with environment and climate. Air bubbles and hydrates at 120-2200 m depth in the Dome Fuji (Dome F) ice core were examined with a microscope. This depth range covers the Holocene/Last Glacial/Last Interglacial/Previous Glacial periods. No air bubbles were seen below about 1100 m depth, and air hydrates began to appear from about 600 m. The observed number of air bubbles and hydrates was similar to that found in the Vostok ice core. For the ice covering the Last Glacial Maximum period, however the hydrate concentration in the Dome F core is about half that of the Vostok core. Reference to snow metamorphism and packing does not explain this finding.

Uniaxial compression tests were performed on samples of the Greenland Ice Gore Project (GRIP) deep ice core, both in the field and later in a cold-room laboratory, in order to understand the ice-flow behavior of large ice sheets. Experiments were conducted under conditions of constant strain rate (type A) and constant load (type B). Fifty-four uniaxial-compression test specimens from 1327-2922 m were selected. Each test specimen (25 mm x 25 mm x 90 mm) was prepared with its uniaxial stress axis inclined 45° from the core axis in order to examine the flow behavior of strong single-maximum ice-core samples with basal planes parallel to the horizontal plane of the ice sheet. The ice-flow enhancement factors show a gradual increase with depth down to approximately 2000 m. These results can be interpreted in terms of an increase in the fourth-order Schmid factor. Below 2000 m depth, the flow-enhancement factor increases to about 20-30 with a relatively high variability When the Schmid factor was > 0.46, the enhancement factor obtained was higher than expected from the .-axis concentrations measured. The higher values of flow-enhancement factor were obtained from specimens with a cloudy band structure. It was revealed that cloudy bands affect ice-deformation processes, but the details remain unclear.

An X-ray transmission method has been developed to obtain a continuous profile of bulk densities of ice cores. Intensities of X-rays transmitted through an ice-core sample were continuously measured by an X-ray detector during translation of the sample across the X-ray beam. A thick section of an ice core with a constant thickness was prepared by band-sawing followed by microtome planing. The X-ray intensity profile obtained was converted to a density profile using a calibration curve for X-ray absorption vs ice thickness. Using this method, spatial resolution of the density profile was down to 1 mm. X-ray radiographs were also taken on a two-dimensional detector imaging plate, in order to observe layer structures of the ice cores.The method was applied to Dome Fuji (Antarctica) ice cores from the surface to 110 m depth. From the density profile obtained we calculated the power spectrum of the density variation by the discrete Fourier transform, and obtained several peaks at different frequencies. The center period in the spectrum was close to the annual accumulation thickness at the drill site.