Crater-like Depressions Research Articles

The effect of current and lambda on white-etch-crack failures

White etching cracks (WECs) have been associated with premature failure of wind turbine roller bearings. Various drivers for the generation of WECs have been identified such as loading conditions, slip, steel quality, lubrication, hydrogen embrittlement, corrosion fatigue cracking, and stray electrical currents passing through the surface. In this work, a benchtop test rig utilizing a three-ring-on-roller test configuration was used to investigate the effect of electrical current and operation in different lubricating regimes, defined by lambda (λ), on high-quality bearing steel samples tested in a commercially available power transmission EP gear lubricant. It was observed that there is an inverse correlation between the magnitude of electric current applied to the ring/roller system and time-to-failure. Higher current magnitudes lead to shorter time-to-failure than lower current magnitudes, with macropitting as the main failure mode. Sub-surface investigation revealed the presence of WECs in all cases. For the same current magnitude, tests conducted in boundary and mixed lubrication regimes showed that time-to-failure increased as λ increased, and the tests resulted in WEC related macropits, whereas tests conducted in near-hydrodynamic regime resulted in surface damage with no macropit. It was also noted that a shift toward near-hydrodynamic lubrication resulted in a distinct surface distress on the roller surface. Furthermore, there seems to be a transition in the mixed regime during which the surface distress occurred. The damage on the surface of the test samples resembled non-spatially, periodic, groove-like corrugations and, in some cases, crater-like depressions. Sub-surface imaging, performed by sequential sectioning, revealed the presence of WECs in all cases, and broad, branching cracks that were more prevalent under the more severe boundary conditions.

The Amami Rift: Clarifying the roles of rifting and volcanism in the central Ryukyu Arc

Arc volcanism in the Ryukyu subduction system shows an along-arc variation characterized by the development of subaerial volcanoes in the northern arc and submarine ones in the southern arc. The central arc is a transitional area in arc volcanism, but its characteristics and tectonic controls remain largely unknown. This paper presents detailed new data on the tectonic and volcanic features of the Amami Calderas in the central Ryukyu Arc based on high-resolution bathymetry and a seismic reflection profile. Ship-based bathymetry reveals that ENE–WSW to E–W trending normal faults form a local rift structure which is ~60 km long, ~10 km wide and ~ 400 m deep. Small volcanic edifices and crater-like depressions are scattered in and around the rift. A number of bubble plumes were detected by shipboard multibeam sonar in the rift. AUV near-bottom mapping confirmed positive temperature anomalies near the bubble plumes, indicative of hydrothermal venting. Our observations demonstrate that the previously named Amami Calderas are not volcanic in origin, but are instead tectonic, and we therefore re-name these features the Amami Rift. The Amami Rift is the northernmost active rift along the central Ryukyu Arc and its unique location provides new insights into the interaction of rifting and arc volcanism. Yokoate-jima Island is a large volcanic edifice close to the Amami Rift whose surrounding seafloor shows no distinctive rift structures and few diffuse lineaments. In contrast, the Amami Rift consists of small scattered volcanic edifices and a prominent focused rift. This sharp volcano-tectonic contrast between Yokoate-jima Island and Amami Rift suggests that focused rifting makes arc volcanism scattered rather than focused in the area, with relatively small volcanic edifices. Another notable feature of the Amami Rift and surrounding area is that volcanic edifices, indicators of hydrothermal activity and bubble plumes are aligned not parallel to the rift, but in the cross-arc direction, indicating the presence of cross-arc volcanism. Cross-arc volcanism has been recently reported in the northern Ryukyu Arc, but this is the first time it has been observed in the central arc. Our observations combined with previous studies suggest a combination of cross-arc volcanism and crustal extension characterizes the spatial distribution of active magmatism in the central Ryukyu Arc.

Morphology, Formation, and Activity of Three Different Pockmark Systems in Peri-Alpine Lake Thun, Switzerland

Pockmarks are crater-like depressions formed by upward fluid flow (gas and/or liquid) through the unconsolidated sediment column on the floor of oceans and lakes. While pockmarks are well described in the marine realm, they have essentially been overlooked in lacustrine settings, likely due to a lack in economic interest to apply high-resolution hydroacoustic techniques in lakes. A swath-bathymetry survey on Lake Thun, Switzerland, revealed the existence of three pockmark systems. One pockmark (110 m in diameter) was discovered near a big karst system at Beatenberg at a water depth of ~217 m. Its activity is probably associated with episodic groundwater seepage induced by earthquakes, floods and snowmelt. At another site, Daerligen, we detected at ~60 m water depth the presence of multiple smaller pockmarks (~1.5 to 10 m in diameter) that seem to be active, continuously liberating CH4 gas by bubble ebullition. The CH4 displayed a biogenic carbon isotopic signature, however, the exact origin of the gas remains unknown. The third site, Tannmoos (~35 m water depth), comprises two large pockmarks (20–43 m in diameter) connected to a karst system in gypsum-carrying bedrock. One of these pockmarks is constituted of several unit pockmarks (e.g., sub-pockmarks; 0.3 to 0.8 m in diameter). While strong evidence is still lacking, we suggest that groundwater discharge occasionally occurs through these unit pockmarks during periods of intense precipitation. Hence, this study reveals the existence of three pockmark systems of variable morphology and mechanisms of formation within the same lake, reflecting different hydrological and biogeochemical regimes. Moreover, it underscores the potential importance of pockmarks in influencing hydrological and CH4 budgets in lakes. Clearly more work on quantifying seasonal fluxes of groundwater and CH4 release via lacustrine pockmarks is required, and it needs to be seen whether the observations made in Lake Thun are universal and apply also to many other lacustrine environments worldwide.

Marine Gas Hydrate Geohazard Assessment on the European Continental Margins. The Impact of Critical Knowledge Gaps

This paper presents a geohazard assessment along the European continental margins and adjacent areas. This assessment is understood in the framework of the seafloor’s susceptibility to (i.e., likelihood of) being affected by the presence of hydrate deposits and the subsequent hazardous dissociation processes (liquefaction, explosion, collapse, crater-like depressions or submarine landslides). Geological and geophysical evidence and indicators of marine gas hydrates in the theoretical gas hydrate stability zone (GHSZ) were taken into account as the main factors controlling the susceptibility calculation. Svalbald, the Barents Sea, the mid-Norwegian margin-northwest British Islands, the Gulf of Cádiz, the eastern Mediterranean and the Black Sea have the highest susceptibility. Seafloor areas outside the theoretical GHSZ were excluded from this geohazard assessment. The uncertainty analysis of the susceptibility inference shows extensive seafloor areas with no data and a very low density of data that are defined as critical knowledge gaps.

The Late-Quaternary deposits of the Piedra Buena Terrace (Patagonian continental slope, SW Atlantic): An example of interaction between bottom currents and seafloor morphology

The Piedra Buena Terrace (PBT), situated in the Patagonian continental slope, is swept by the vigorous flow of Antarctic-sourced bottom water masses, and intersected by a considerable submarine canyon system. It thus represents a world-class natural laboratory to investigate the sedimentary dynamics of deep-ocean settings influenced by both, along-slope, and down-slope sedimentary processes. In the framework of the Pampa Azul project, the PBT was surveyed with swath bathymetry and a high-resolution parametric sub-bottom seismic profiler and sampled at several locations with a 7 m long gravity corer. Through the integration of the acoustic data, grain size results from a sediment core and two AMS 14C ages this work attempts to: (1) describe for the first time the detailed geomorphology of a poorly known region of the Patagonian continental slope; (2) depict the principal sub-surface seismic units, illustrating their internal structure, geometry, and distribution; (3) establish a correlation between sub-surface seismic facies and sediment deposits using the first age constraints for this region; (4) propose a model with the depositional history of the PBT, focusing on the interaction between along-slope and down-slope processes and seafloor morphology. The acquired bathymetric data imaged with unprecedent detail the geomorphology of the southern part of the PBT revealing the presence of a field of kilometric-sized, sub-circular, crater-like depressions, and a submarine canyon. The analyzed seismic sub-bottom profiles showed that the shallow sedimentary structure of the study area is represented by patchy-mounded deposits characterized by three distinctive features: cyclic stacks of two seismic facies (FA and FB), mounded shape, and preferential location northwards the morphological depressions and the submarine canyon. Integration of a seismic sub-bottom profile with one sediment core indicates that the alternation of seismic facies reflects variations in the hydrodynamic conditions. FA is related to a high to moderate energy medium with input of terrigenous material and FB is related to a low energy environment with a limited contribution of sediments from the continent. The patchy distribution, mounded shape and preferential location are associated with hydrodynamic processes occurring between the geomorphological features and the north-flowing Antarctic bottom currents, which result in the deposition of mounded contourites in the northern flank of depressions and hybrid along-slope down-slope deposits northwards a submarine canyon. The 14C ages constrain the chronological framework of these sediments to the Late-Pleistocene. This is in contrast with previous investigations that have suggested much older ages (Late Miocene) for the sub-surface sediments of PBT. Finally, a conceptual model is here proposed to illustrate the Late Quaternary sediment depositional history of the PBT where the interaction between north-flowing bottom contouritic currents and seafloor relief played an important role in the deposition and preservation of sediments in this area.

Morphological, molecular and phylogenetic characterization of a new Chloeia (Annelida: Amphinomidae) from a pockmark field

Pockmarks are crater-like depressions in seabed sediments commonly found along continental margins and generally associated with episodic fluid seepage. In this study, we describe a new species of Chloeia (Amphinomidae) gathered from a pockmark field, located off the southeastern Brazilian continental margin (~750 m depth), SW Atlantic. The complete mitochondrial genome is given, together with a multi-gene phylogenetic analysis including our new species and other amphinomids. The new species is similar to Chloeia bimaculata, C. violacea and C. kudenovi in having the longest neuropodial cirri on the second chaetiger. Chloeia pocicola sp. nov. differs from these congeners mainly by the combination of its dorsal pigmentation pattern, eyes, notopodial cirri, and neuropodial chaetae. Our phylogenetic hypothesis suggests Chloeia as a sister group of Archinome, which departs from previous studies, that instead suggests the clade Chloeia-Notopygos. This study increases our knowledge on the biodiversity of deep-sea pockmark environments, and contribute to the systematics of Amphinomidae, which is generally well represented in shallow waters, but also occurs in deep waters, including chemosynthetic environments.

The longevity of pockmarks – A case study from a shallow water body in northern Denmark

Pockmarks or crater-like depressions formed due to fluid venting, is a well-known geological phenomenon. On a global scale, pockmarks expel a significant amount of fluids and greenhouse gasses to the marine environment and the atmosphere. Despite pockmarks being well-studied, the longevity and history of activity are rarely addressed. In this study, we analyse more than twenty nearshore seafloor depressions of varying morphology and size in the shallow waters of the western Limfjord in northern Denmark. By using high-resolution seismo-acoustic and sediment core data, we characterize the morphology and sediments of the pockmarks. We further use 14C-dating and pore water analyses to address the timing and activity of the depressions. Our results show that the depressions coincide with submarine groundwater discharge areas and hence represent freshwater pockmarks. The pockmarks most likely formed initially due to two eruptive events of thermogenic light oil/gas ca. 2000 and 900 yrs. BP, both triggered by relative sea-level fall. Freshwater seepage continues to sustain and develop the pockmark morphology and we show that pockmarks grow by collapse of pockmark walls and amalgamation of discrete erosive seepage sites at the bottom. The episodic expulsions of thermogenic fluids and the continuous seepage of freshwater probably prolonged the longevity of the studied pockmarks and further demonstrates the complex nature of pockmark formation and activity. While thermogenic and biogenic gas escape to the surface, on the global scale, is relevant for the greenhouse gas budget, the outflow of groundwater which may or may not be polluted is relevant with respect to the geochemistry, biota and productivity of the receiving marine environment especially in shallow water settings.

Acoustic and biological characteristics of seafloor depressions in the North Yellow Sea Basin of China: Active fluid seepage in shallow water seafloor

High resolution multibeam bathymetry revealed the occurrence of numerous craterlike depressions, so-called pockmarks, in the sea floor of the North Yellow Sea Basin (NYSB). To investigate whether these pockmarks are related to any type of fluid flow or gas seepage from subsurface stratum, acoustic survey and box samplings were conducted in a selected area containing several typical pockmarks. This allowed for an integrated analysis of the morphology, distribution, and subsurface strata as well as a direct comparison of seafloor characteristics among different areas inside and outside of a pockmark. Additionally, surface sediments were analyzed for particle structure and grain size. Based on the combination of multibeam bathymetry and backscatter intensity data, 282 seafloor depressions were accurately identified. Moreover, the formation of pockmark can be attributed to existence of gas chimneys and other shallow gases revealed by shallow seismic profile records in the subsurface strata. Sediments inside and outside the pockmark showed remarkable discrepancies in terms of grain size, and benthic fauna. Additionally, strong internal reflection area was observed inside the depressions where benthic habitats are abundant and sediments contain strawberry pyrite and carbonate cementation. Side scan sonar also recorded sediment plumes in the water column, which indicates that the pockmarks are still active. Several “pearl-string-like” accumulations of pockmark were found in the study area, which well agree with the distribution of buried channels and lakes. These results thus provide new insights in interpreting shallow water fluid seepage, with implications for our current knowledge of overall sedimentation process in offshore continental shelves.

Cocconeis cascadensis, a new monoraphid diatom from mountain streams in Northern California, USA

A new diatom species, Cocconeis cascadensis, is reported from streams in the Southern Cascade Mountain Range in California (USA). Light and scanning electron microscopy reveals morphological novelties related to the structure of the areolae on the sternum valve, which are surrounded externally by crater-like depressions, as well as apically elongated bumps in the submarginal hyaline area of the raphe valve. The valve morphology of the coarsely striated taxa, Cocconeis placentula var. lineata, C. pseudolineata and C. neodiminuta from the studied streams, and C. disculus from freshwaters in Europe, is presented for comparison. The newly described species possesses a unique combination of morphological features, representing an intermediary between the more complex valve ultrastructure of marine Cocconeis taxa and the freshwater members of the C. placentula species complex, with simple slit-like areolae. Cocconeis cascadensis inhabits cold-water mountain streams with low conductivity, low nutrients and low dissolved organic carbon concentrations.

Seafloor fluid seeps on Kimki Ridge, offshore southern California: Links to active strike-slip faulting

The Kimki Ridge fluid seeps are located in western Catalina Basin about 60km southwest of the southern California mainland and at a water depth of approximately 1100m. Multichannel seismic reflection profiles collected by the U.S. Geological Survey (USGS) in 2014 show acoustic transparency within the Kimki Ridge, suggesting the possibility of fluid seeps and possible sub-seafloor fluid pathways. Subsequent multibeam bathymetric and backscatter intensity data collected during a cooperative University of Washington/USGS cruise in early 2016 show subtle seafloor buildups with high acoustic backscatter (reflectivity) in three places along Kimki Ridge, supporting the existence of fluid seepage. A Remotely Operated Vehicle (ROV) dive, conducted as part of the Nautilus Exploration Program, took place in August 2016 to confirm the presence of these previously unknown seeps and document their characteristics as well as those of any associated biological communities. Two of the three seeps were explored by ROV, and showed abundant evidence of fluid seepage, including characteristic algal mats, chemosynthetic clams, and authigenic carbonate formation. The seeps are comprised of carbonate buildups 1–3m thick and 300–500m across. Within these areas, we interpret broad crater-like depressions 30–50m across and 1–2m deep to be individual seep vents. The seep areas appear to be broad zones of diffuse seepage that support chemosynthetic biologic communities; however, active venting was not observed. Geochemical analyses of rock samples collected from the seeps indicate microbially driven anaerobic oxidation of methane at or near the sediment water interface. Seismic-reflection profiles show chimney-like fluid pathways along the limbs and in the axis of the fold forming Kimki Ridge, and evidence of methane in shallow sediments can be traced into the adjacent Catalina Basin. A system of closely spaced faults located at the axis of the Kimki Ridge anticline may serve as pathways to allow fluid flow to the seafloor. Our data are consistent with other studies that suggest that transpression is an important component in the formation and localization of fluid seeps in a strike-slip setting, implying that seep formation may be a common occurrence at fault stepovers or transpressional bends in strike-slip systems.

Pockmarks on the Mendeleev Rise, central Arctic Ocean

Circular to semi-circular crater-like depressions in the seafloor are commonly referred to as pockmarks. Pockmarks were discovered in the 1960s on echo-sounding profiles from the Scotian Shelf, eastern Canada (King & MacLean 1970). While there is no consensus regarding their formation process, pockmarks are in general linked to the removal of soft surface sediments during fluid escape at the seafloor (Judd & Hovland 2009). In the Arctic Ocean, pockmarks are found in sizes ranging from 100 m in diameter, both on the continental shelf and in deeper-water settings. Within a limited area of the Mendeleev Rise crest at about 78°20′ N, pockmarks dominate the seafloor morphology in water depths around 800 m. The Mendeleev Rise comprises one of the major undersea features in the Arctic Ocean; it extends approximately 900 km along the 180° meridian from the continental slope west of Chukchi Borderland to about 84° N (Fig. 1a). Portions of the rise shallower than approximately 1000 m were subjected to ice grounding, except in the vicinity of 78° 20′ N where the 800 m deep crest contains no direct evidence of ice scouring (Jakobsson et al. 2014). Multibeam swath bathymetry collected with USCGC Healy in …

Crystal orientation dependence of surface modification in molybdenum mirror irradiated with helium ions

The crystal orientation dependence on a surface damage in molybdenum mirror has been investigated under the irradiation with 3keV helium ions to a fluence of 1022ions/m2. A strong correlation between the surface roughening and the grain orientation was confirmed from SEM–EBSD analysis. It was found that significant surface roughening including crater-like depressions is formed on the grain surfaces with crystalline planes close to (100), while relatively smooth surface remains on grains with the other directions. In contrast, the reflectivity measurement of the single crystal molybdenum with a spectroscopic ellipsometry showed smaller reduction in (100) sample than in (110) and (111) samples. The smallest helium desorption in (100) sample was also detected with thermal desorption spectroscopy in comparison with them in (110) and (111) samples, which indicates (110) and (111) samples include the large amount of defects acting as effective trapping site for He atoms. A cross-sectional TEM observation actually showed a relatively large bubbles at sub-surface regions of (110) sample. These results indicate that the optical reflectivity should be affected by not only a top surface profile but also internal radiation damages such as high density of helium bubbles.

Completely free and mobile intra-abdominal tumor

This section features outstanding photographs of clinical materials selected for their educational value or message, or possibly their rarity. The images are accompanied by brief case reports (limit 2 typed pages, 4 references). Our readers are invited to sumit items for consideration. A 65-year-old man presented with chronic intermittent abdominal pain and moderate weight loss.The patient had no significant medical history or previous surgeries. A round, mobile mass was detected in the lower abdomen during his examination and could be detected in different locations at different times during the examination (right lower quadrant and left lower quadrant). No abnormalities were found in his clinical laboratory results. A round mass approximately 9 cm in diameter was detected adjacent to the bladder during abdominal ultrasonography (USG). The mass showed similar features on computed tomography (CT) examination. Further, it was hyperdense and did not have an apparent connection to any adjacent structure (Fig A). The patient underwent a laparotomy that found that the mass was located among the intestinal loops.The mass was also found to be completely mobile, not connected to any organ, and was removed through an open incision. The patient’s postoperative course was uneventful, and he was discharged on postoperative day 5. The surgically removed mass was observed to be a smooth, 9 × 9 × 8 cm body with a shiny outer surface.The mass was very firm, white-gray in color, and had a dense appearance (Fig B). Additionally, there were crater-like depressions on the surface of the mass that were possibly the results of physical impressions. After cutting open the mass, the surface was found to be laminarly arranged in a circumferential fashion.The center of the lesion was composed of a calcified area, approximately 2 × 1 × 0.5 cm. (Fig C). The lesion lacked areas of necrosis or hemorrhage but had a homogenous, concentric appearance that was perceived during sectioning.Classic tissue processing, including formalin fixation, paraffin embedding, and hematoxylin and eosin staining, showed an almost acellular acidophilic, hyalinized tissue throughout the mass, except in the peripheral regions (Fig D). In the peripheral regions, a few cells with acidophilic cytoplasms and normochromatic, spindle-shaped nuclei were observed. Dystrophic calcification was seen focally in the central region of the mass. On the basis of these morphologic findings, an immunohistochemical examination for mesenchymal markers was conducted, but it did not show positive staining for any of the mesenchymal markers tested. The negative immunohistochemical results made the diagnosis of the origin of this tumor very difficult and subjective. This section features outstanding photographs of clinical materials selected for their educational value or message, or possibly their rarity. The images are accompanied by brief case reports (limit 2 typed pages, 4 references). Our readers are invited to sumit items for consideration. This section features outstanding photographs of clinical materials selected for their educational value or message, or possibly their rarity. The images are accompanied by brief case reports (limit 2 typed pages, 4 references). Our readers are invited to sumit items for consideration.

Characterization of microbial activity in pockmark fields of the SW-Barents Sea

Multibeam bathymetry revealed the occurrence of numerous craterlike depressions, so-called pockmarks, on the sea floor of the Hammerfest Basin and the Loppa High, south-western Barents Sea. To investigate whether these pockmarks are related to ongoing gas seepage, microbial processes associated with methane metabolism were analyzed using geochemical, biogeochemical and microbiological techniques. Gravity cores were collected along transects crossing individual pockmarks, allowing a direct comparison between different locations inside (assumed activity center), on the rim, and outside of a pockmark (reference sites). Concentrations of hydrocarbons in the sediment, particularly methane, were measured as headspace (free) gas, and in the occluded and adsorbed gas fraction. Down to a depth of 2.6m below sea floor (mbsf) sulfate reduction rates were quantified by radiotracer incubations. Concentrations of dissolved sulfate in the porewater were determined as well. Neither the sulfate profiles nor the gas measurements show any evidence of microbial activity or active fluid venting. Methane concentrations and sulfate reduction rates were extremely low or even below the detection limit. The results show that the observed sediment structures are most likely paleo-pockmarks, their formation probably occurred during the last deglaciation.

FSpatial and temporal dynamics of cellulose degradation and biofilm formation by Caldicellulosiruptor obsidiansis and Clostridium thermocellum.

Cellulose degradation is one of the major bottlenecks of a consolidated bioprocess that employs cellulolytic bacterial cells as catalysts to produce biofuels from cellulosic biomass. In this study, we investigated the spatial and temporal dynamics of cellulose degradation by Caldicellulosiruptfor obsidiansis, which does not produce cellulosomes, and Clostridium thermocellum, which does produce cellulosomes. Results showed that the degradation of either regenerated or natural cellulose was synchronized with biofilm formation, a process characterized by the formation and fusion of numerous crater-like depressions on the cellulose surface. In addition, the dynamics of biofilm formation were similar in both bacteria, regardless of cellulosome production. Only the areas of cellulose surface colonized by microbes were significantly degraded, highlighting the essential role of the cellulolytic biofilm in cellulose utilization. After initial attachment, the microbial biofilm structure remained thin, uniform and dense throughout the experiment. A cellular automaton model, constructed under the assumption that the attached cells divide and produce daughter cells that contribute to the hydrolysis of the adjacent cellulose, can largely simulate the observed process of biofilm formation and cellulose degradation. This study presents a model, based on direct observation, correlating cellulolytic biofilm formation with cellulose degradation.

GIS-based mapping for marine geohazards in seabed fluid leakage areas (Gulf of Cadiz, Spain)

This paper applies, for the first time in offshore deepwater, a method based on geographic information systems for seafloor susceptibility assessment as a first approach to marine geohazard mapping in fluid leakage areas (slope instabilities, gas escapes, seabed collapses, pockmarks, etc.). The assessment was carried out in a known seabed fluid-flow province located on the Iberian margin of the Gulf of Cadiz, Spain. The method (based on statistical bivariate analysis) creates a susceptibility map that defines the likelihood of occurrence of seafloor features related to fluid flow: crater-like depressions and submarine landslides. It is based on the statistical index (Wi) method (Van Westen in Statistical landslide hazard analysis. ILWIS 2.1 for Windows application guide. ITC Publication, Enschede, pp 73–84, 1997), in which Wi is a function of the cartographic density of seafloor features on “factor maps”. The factors selected monitor the seafloor’s capability to store and transfer hydrocarbon gases and gravitational instability triggers: geology-lithology, gas hydrate stability zone thickness (temperature, pressure–water depth and geothermal gradient), occurrence of diapirs, proximity to faults or lineaments, and slope angle of the seafloor. Results show that the occurrence of seafloor features related to fluid flow is highest where the factors “gas source and storage” and “pathways of fluid escape” converge. This means that they are particularly abundant over diapirs in contourite deposits, in the vicinity of faults, and inside theoretical gas hydrate stability fields thinned by warm undercurrents. Furthermore, the submarine landslides located on the Palaeozoic-Toarcian basement are not related to fluid leakage. This methodology provides helpful information for hazard mitigation in regional selection of potential drill sites, deep-water construction sites or pipeline routes. It is an easily applied and useful tool for taking the first step in risk assessment on a regional scale for vast areas where fluid leakage may be present, the geological model is known, and the geologically hazardous features have already been mapped.

Linking a linear pockmark train with a buried Palaeozoic structure: a case study from the St. Lawrence Estuary

The 15-km-long Matane pockmark train belongs to a series of NNE-striking alignments of pockmarks mapped on the seafloor of the St. Lawrence Estuary. It includes 109 pockmarks that show a complete transition from well-defined, relatively deep (up to 8.6 m), crater-like depressions to subtle, partly buried morphological features, suggesting that pockmarks have formed at different periods along the whole alignment and that the location of fluid release has changed through time. On seismic profiles, pockmarks are characterized by vertical seismic chimneys that root in the (fractured?) hinge zone of an open anticline within the autochthonous Palaeozoic rocks of the St. Lawrence Platform. In absence of a geochemical characterization of expelled gas, the relationship between the Matane pockmark train and the anticline in a domain characterized by mature source rocks is the strongest evidence for the genetic link between pockmarks and the release of gas from an active hydrocarbon system or a reservoir located in the Palaeozoic succession.

Gingival stippling in dogs: Clinical and structural characteristics

Diagnosis of periodontal disease, a frequent disorder in dogs, rests mainly upon the assessment of changes affecting the gingiva. Loss of gingival stippling, one of the early symptoms of periodontal disease in the human, could also indicate early periodontitis in the dog. The aim of this study was to determine the presence of gingival stippling in dogs, its location on alveolar processes, its association with age and inflammation, and its relation to the microstructure of the gingiva. The study concerns clinical, scanning electron and light microscopic examination of the gingiva of 48 dogs. Stippling, i.e. the granular appearance of the gingiva resembling irregularly distributed crater-like depressions of varying sizes, was present in adult but absent in young and senescent dogs. Stippling was most prominent at the level of the canine and the carnassial teeth. In contrast to the human gingiva, depressions and elevations of the stippled gingiva of the dog were not distributed uniformly. Presence of stippling indicates the presence of well developed mucosal layers and extensive interdigitation of epithelial rete pegs and papillary layer. Interaction of inelastic collagen fibres and a high hyaluronan content within papillary layer resulting in a high turgor of tissue due to the extensive water binding capacity of hyaluronic acid could also play a role in shaping of the gingival surface. The significance of gingival stippling in the diagnosis of periodontal disease in dogs is limited, as stippling occurs in healthy but also slightly inflamed attached gingiva of adult individuals.

日本海東縁，上越海盆の高メタンフラックス域におけるメタンハイドレートの成長と崩壊

A number of extensive methane plumes and active methane seeps associated with large blocks of methane hydrates exposed on the seafloor strongly indicate extremely high methane flux and large accumulations of methane hydrate in shallow sediments of the Umitaka spur and Joetsu knoll of the Joetsu basin 30 km off Joetsu city, Niigata Prefecture. Crater-like depressions, incised valleys, and large but inactive pockmarks also indicate methane activities over the spur and knoll. These features imply strong expulsions of methane gas or methane-bearing fluids, and perhaps lifting and floating-up of large volumes of methane hydrate to the sea surface. High heat flow, ∼100 mK/m, deposition of organic-rich strata, ∼1.0 to 1.5%TOC, and Pliocene-Quaternary inversion-tectonics along the eastern margin of the Japan Sea facilitate thermal maturation of organic matters, and generation and migration of light-hydrocarbons through fault conduits, and accumulation of large volumes of methane as methane hydrate in shallow sediments. Microbial methane generation has also contributed to reinforcing the methane flux of the Joetsu basin. Regional methane flux as observed by the depth of the sulfate-methane interface (SMI) is significantly high, < 1 m to 3 m, when compared to classic gas hydrate fields of Blake Ridge, 15 to 20 m, and Nankai trough, 3 to 15 m. δ13C of methane hydrate and seep gases are mostly within -30 to -50‰, the range of thermogenic methane, while dissolved methane of the interstitial waters a few kilometers away from seep sites are predominated by microbial with δ13C of -50 to -100‰. Seismic profiles have revealed fault-related, well-developed gas chimney structures, 0.2 to 3.5 km in diameter, on the spur and knoll. The structures are essential for conveying methane from deep-seated sources to shallow depths as well as for accumulating methane hydrate (gas chimney type deposits). The depth of BSR, which represents the base of gas hydrate stability (BGHS), on the spur and knoll is generally 0.20 to 0.23 seconds in two-way-travel time, whereas the BSRs in gas chimneys occur at 0.14 to 0.18 seconds, exhibiting a sharp pull-up structure. The apparent shallow BGHS is due to the accumulation of large volumes of high-velocity methane hydrate in gas chimneys. The depth to BGHS is estimated to be 115 m on an experimentally determined stability diagram, based on an observed thermal gradient of 100 mK/m. Then the velocity of the sediments on the Umitaka spur is calculated to be 1000 m/s, which is anomalously low compared to normal pelagic mud of 1600-1700 m/s. This exciting finding leads to the important implication that sediments of the Umitaka spur contain significant amounts of free gas, although the sediments are well within the stability field of methane hydrate. The reasons for the existence of free gas in the methane hydrate stability field are not fully explained, but we propose the following possible mechanisms for the unusual co-existence of methane hydrate and free-gas in clay-silt of the spur. (i) High salinity effect of residual waters, (ii) degassing from ascending fluids, (iii) bound water effect and deficiency of free-waters, and (iv) micro-pore effect of porous media. All of these processes relate to the development of gas hydrate deposits of the Umitaka spur.(View PDF for the rest of the abstract.)

Quasi-3-D model to describe topographic effects on non-spherical comet nucleus evolution

A new quasi three-dimensional code is presented and used to explore the thermal evolution of non-spherical-shape cometary nuclei. Calculations are done for spherical nuclei with different obliquities, spherical nuclei with crater-like depressions and spheroid-shape nuclei. The results obtained for both the gas and dust fluxes agree with previous simulations. Local dust crust formation can occur when the comet is located far from the Sun (around 3.5 AU), creating active and inactive areas on the surface. For a given set of parameters, the H 2O production rate is comparable to the one observed for comets. A pre–post-perihelion asymmetry exists for H 2O, CO 2 and CO production rates. It is shown that crater-like depressions can be erased within the lifetime of a comet and that spheroid-shape nuclei have a higher production rate than equal area spherical nuclei.