Mapping gas hydrate and fluid flow indicators and modeling gas hydrate stability zone (GHSZ) in the Ulleung Basin,East (Japan) Sea: Potential linkage between the occurrence of mass failures and gas hydrate dissociation

The Ulleung Basin, East (Japan) Sea, is well-known for the occurrence of submarine slope failures along its entire margins and associated mass-transport deposits (MTDs). Previous studies postulated that gas hydrates which broadly exist in the basin could be related with the failure process. In this study, we identified various features of slope failures on the margins, such as landslide scars, slide/slump bodies, glide planes and MTDs, from a regional multi-channel seismic dataset. Seismic indicators of gas hydrates and associated gas/fluid flow, such as the bottom-simulating reflector (BSR), seismic chimneys, pockmarks, and reflection anomalies, were re-compiled. The gas hydrate occurrence zone (GHOZ) within the slope sediments was defined from the BSR distribution. The BSR is more pronounced along the southwestern slope. Its minimal depth is about 100 m below seafloor (mbsf) at about 300 m below sea-level (mbsl). Gas/fluid flow and seepage structures were present on the seismic data as columnar acoustic-blanking zones varying in width and height from tens to hundreds of meters. They were classified into: (a) buried seismic chimneys (BSC), (b) chimneys with a mound (SCM), and (c) chimneys with a depression/pockmark (SCD) on the seafloor. Reflection anomalies, i.e., enhanced reflections below the BSR and hyperbolic reflections which could indicate the presence of gas, together with pockmarks which are not associated with seismic chimneys, and SCDs are predominant in the western-southwestern margin, while the BSR, BSCs and SCMs are widely distributed in the southern and southwestern margins. Calculation of the present-day gas-hydrate stability zone (GHSZ) shows that the base of the GHSZ (BGHSZ) pinches out at water depths ranging between 180 and 260 mbsl. The occurrence of the uppermost landslide scars which is below about 190 mbsl is close to the range of the GHSZ pinch-out. The depths of the BSR are typically greater than the depths of the BGHSZ on the basin margins which may imply that the GHOZ is not stable. Close correlation between the spatial distribution of landslides, seismic features of free gas, gas/fluid flow and expulsion and the GHSZ may suggest that excess pore-pressure caused by gas hydrate dissociation could have had a role in slope failures.     

Morphology of pockmarks along the western continental margin of India: Employing multibeam bathymetry and backscatter data

This study addresses the morphology of pockmarks along the western continental margin of India using multibeam bathymetry and backscatter data. Here, for the first time we have utilized the application of ArcGIS (Geographical Information System) for understanding the morphology of pockmarks from the western continental margin of India. The pockmarks observed in water depths of 145–330 m are circular, elliptical or elongated in plan-view, with an average length and width of 157 (±72) m and 83 (±19) m respectively. The average pockmark relief and perimeter are 1.9 (±0.9) m and 412 (±181) m, respectively. The pockmarks have average areas and volumes of 10 759 m² and 15 315 m³ respectively. Spatial separation that coincides with 210 m isobath divides the pockmarks into two groups with differing distributions and morphologies. These pockmarks originated from seepages of biogenic or thermogenic gas or from pore fluids from deeper sources, migrated vertically along the faults. Besides a possible structural control, the pockmark morphologies are also affected by bottom currents and/or by submarine slumping. The average acoustic backscatter strength from pockmark centre is higher (−35 dB) than the average backscatter of the total area (−40 dB), which suggests their possible linkage to the precipitation of diagenitic minerals from biodegradation of seepage material.     

Shallow plumbing systems inferred from spatial analysis of pockmark arrays

This study describes and analyses an extraordinary array of pockmarks at the modern seabed of the Lower Congo Basin (offshore Angola), in order to understand the fluid migration routes and shallow plumbing system of the area. The 3D seismic visualization of feeding conduits (pipes) allowed the identification of the source interval for the fluids expelled during pockmark formation. Spatial statistics are used to show the relationship between the underlying (polarised) polygonal fault (PPFs) patterns and seabed pockmarks distributions. Our results show PPFs control the linear arrangement of pockmarks and feeder pipes along fault strike, but faults do not act as conduits. Spatial statistics also revealed pockmark occurrence is not considered to be random, especially at short distances to nearest neighbours (<200 m) where anti-clustering distributions suggest the presence of an exclusion zone around each pockmark in which no other pockmark will form. The results of this study are relevant for the understanding of shallow fluid plumbing systems in offshore settings, with implications on our current knowledge of overall fluid flow systems in hydrocarbon-rich continental margins.     

Glacial and deglacial seafloor methane emissions from pockmarks on the northern flank of the Storegga Slide complex

The Storegga Slide complex is a multi-stage slope failure on the Norwegian continental margin where the most recent major event occurred 8.1 ka b.p. (calendar years before present). Its northern flank contains pockmark features that are commonly inferred to be related to the historical and modern venting of methane-bearing fluids. Three jumbo piston cores (JPC), one from a pockmark and two background cores at variable distances from this site (proximal, 5 km, and distal, 15 km) on the northern flank of the slide (806–1,524 m water depths), were sampled at 10 cm resolution to assess the geologic record of methane venting in the Nyegga pockmark field. Six down-core radiocarbon measurements on mixed planktonic foraminifer species reveal ages of 9.4–16.4 ka b.p. Bathymodiolus mussel shell horizons, indicators of methane-rich environments, have been dated at 15.8–17.6 and ~22 ka b.p. in the pockmark core. Stable isotope analyses on planktonic (Neogloboquadrina pachyderma sinistral) and benthic (Islandiella norcrossi, Melonis barleeanum) Foraminifera reveal δ¹⁸O values indicative of a clear glacial/deglacial transition (−1.5‰ shift in planktonic species). Both planktonic and benthic δ¹³C signatures record multiple excursions, interpreted to reflect the influence of methane in the environment; these δ¹³C excursions occur in the pockmark core and also in the distal background core. While authigenic calcite formation on the seafloor may play an important role in producing such excursions, these data together suggest the influence of methane seepage within the pockmark field over the past 25 ka, whereby seepage was particularly active between 13 and 15 ka. This is consistent with previously inferred regional increases in porewater pressure associated with glacial loading and higher sedimentation rates, which can cause gas hydrate and slope instability.     

Bathymetry of the Tonga Trench and Forearc: a map series

Four new bathymetric maps of the Tonga Trench and forearc between 14 °S and 27 °S display the important morphologic and structural features of this dynamic convergent margin. The maps document a number of important geologic features of the margin. Major normal faults and fault lineaments on the Tonga platform can be traced along and across the upper trench slope. Numerous submarine canyons incised in the landward slope of the trench mark the pathways of sediment transport from the platform to mid- and lower-slope basins. Discontinuities in the trench axis and changes in the morphology of the landward slope can be clearly documented and may be associated with the passage and subduction of the Louisville Ridge and other structures on the subducting Pacific Plate. Changes in the morphology of the forearc as convergence changes from normal in the south to highly-oblique in the north are clearly documented. The bathymetric compilations, gridded at 500- and 200-m resolutions and extending along 500 km of the landward trench slope and axis, provide complete coverage of the outer forearc from the latitude of the Louisville Ridge-Tonga Trench collision to the northern terminus of the Tonga Ridge. These maps should serve as a valuable reference for other sea-going programs in the region, particularly the Ocean Drilling Program (ODP) and the National Science Foundation MARGINS initiative.     

Origin of pockmarks and chimney structures on the flanks of the Storegga Slide,offshore Norway

Seafloor pockmarks and subsurface chimney structures are common on the Norwegian continental margin north of the Storegga Slide scar. Such features are generally inferred to be associated with fluid expulsion, and imply overpressures in the subsurface. Six long gravity and piston cores taken from the interior of three pockmarks were compared with four other cores taken from the same area but outside the pockmarks, in order to elucidate the origins and stratigraphy of these features and their possible association with the Storegga Slide event. Sulfate gradients in cores from within pockmarks are less steep than those in cores from outside the pockmarks, which indicates that the flux of methane to the seafloor is presently smaller within the pockmarks than in the adjacent undisturbed sediments. This suggests that these subsurface chimneys are not fluid flow conduits lined with gas hydrate. Methane-derived authigenic carbonates and Bathymodiolus shells obtained from a pockmark at >6.3 m below the seafloor indicate that methane was previously available to support a chemosynthetic community within the pockmark. AMS ¹⁴C measurements of planktonic Foraminifera overlying and interlayered with the shell-bearing sediment indicate that methane was present on the seafloor within the pockmark prior to 14 ka ¹⁴C years b.p., i.e., well before the last major Storegga Slide event (7.2 ka ¹⁴C years b.p., or 8.2 ka calendar years b.p.). These observations provide evidence that overpressured fluids existed within the continental margin sediments off Norway during the last major advance of Pleistocene glaciation.     

Characterization of seafloor pockmark seepage of hydrocarbons employing fractal: A case study from the western continental margin of India

Here we apply quantitative technique to describe the seafloor seepages based on the multi-beam backscatter and bathymetric investigations to characterize the pockmark morphology. The variable seafloor backscatter strength for coarser seafloor sediments are related to the diagenesis derived from biodegraded seepages. In this regard, box counting method is used to estimate ‘fractal dimension’ for backscatter imagery data of 398 blocks. These blocks are further sub-grouped into six classes depending on the spread of pockmark related seepages. The study area lies 102 km west off Marmagao along the central west coast of India which contains pre-dominantly (70%) gas-charged sediments. Comparison between the estimated self-similar fractals reveals that there is approximately 97% correlation between the box (D_box) and information (D_info) dimensions. Box dimension–derived fractal dimension values, suggest that the seepages are more along the fault trace in deeper waters, in comparison to sparsely distributed shallow water seepages. Besides, this poor seepage is confined within the smooth to moderately rough seafloor. It is established that the high backscatter strength along the upper slope of the pockmark region having higher fractal dimensions reflects multifractal behavior of seepage distribution. Entire area indicates patchy seepage patterns as supported by estimated fractal values showing intermittent fluctuations, which emphasizes non-linear behavior. Estimated self organizing criticality (SOC) parameters for six representative blocks reveal that the nature of pockmark, fault trace, sediment nature coupled with slumping of pockmark’s wall, sediment movement due to bottom currents are controlling the dynamic balance in the area seepage system. Further, our study emphasizing the multifractal behavior of seepage blocks, clearly depicts the drift in the seepage pattern.     

Slope transport of suspended particulate matter on the Aquitanian margin of the Bay of Biscay

Spatial and temporal characteristics of the water masses and the dispersion of the suspended particulate matter were investigated using current meter, hydrographic and nephelometric observations, gathered during the ECOFER experiment (1989–1991) in the Cap-Ferret Canyon on the Aquitanian margin of the Bay of Biscay. While characteristics of the deep water masses were stable from one year to another, large hydrographic change in the upper 500 m related to winter renewal induced by poleward advection of warm and saline water along the continental slope. The slope circulation and seasonal eddy activity appear as important dynamical mechanisms that control the entrainment and the dispersion of the suspended particulate matter from the neritic domain to the deep ocean. A predominantly northward along-slope current with occasional reversal characterizes this circulation. The nephelometric structures also showed seasonal changes in the overall suspended particulate matter content, but recurrent features, such as the presence of intermediate nepheloid layers at the shelf-break depth and various depths along the slope (∼500, 1000 and 2000 m), were observed. These nepheloid layers extended off the slope to about 10–30 km, but especially laterally along the slope. Their presence indicated that suspended particulate matter exchanges between the shelf and the slope occurred mainly in the head of the canyon and along the southern open slope. The intermediate nepheloid layers around 500 m depth detached from the slope particularly in regions where the bottom slope is close to critical for the M₂ internal tide.     

ROV study of a giant pockmark on the Gabon continental margin

A giant, 800-m wide pockmark, called Regab, was discovered along the Equatorial African margin at 3160-m water depth and was explored by remote operated vehicle (ROV) as part of the Zaiango (1998–2000) and Biozaire (2001–2003) projects carried out conjointly by TOTAL and a number of French research institutes. A microbathymetric map obtained using the ROV sensors shows that the pockmark actually consists of a cluster of smaller pockmarks aligned N70 along a 15-m deep depression. Methane was recorded all over the pockmark, the highest values along the axis of the depression where massive carbonate crusts and dense seep communities were also found. Several faunal species belong to the Vesicomyidae and Mytilidae bivalve families, as well as to Siboglinidae (Vestimentifera) tubeworms. Preliminary analyses confirm their association with symbiotic bacteria, thus documenting their dependence on fluid seeps. The pockmark appears to be related to an infilled channel, visible on the seismic data 300 m below the seafloor, which may act as a reservoir for biogenic fluids supplied to the trap from the surrounding sediments.     

珠江口表层水中主要常量离子的研究

９９４年９月采集了珠江口表层海水水样,并对其溶解态的Ｋ＋、Ｎａ＋、Ｃａ２＋、Ｍｇ２＋、ＳＯ２－４、Ｃｌ等化学要素进行了测定。结果表明,珠江口调查海域表层水中溶解态的Ｋ＋、Ｎａ＋、Ｃａ２＋、Ｍｇ２＋、ＳＯ２－４主要受陆源淡水与外海水物理混合稀释的影响,与Ｃｌ具有良好的直线正相关;Ｃａ２＋／Ｃｌ、ＳＯ２－４／Ｃｌ明显高于大洋水相应比值,Ｎａ＋／Ｃｌ、Ｍｇ２＋／Ｃｌ略高,而Ｋ＋／Ｃｌ比大洋比值低;在与中国其它各主要河口表层水对应Ｃｌ比较中,珠江口的ＳＯ２－４／Ｃｌ较高,而Ｃａ２＋／Ｃｌ,Ｍｇ２＋／Ｃｌ比值居低;上述各主要常量离子和Ｃｌ的等值线均沿自北向南走向递增。     

Seismic evidence and formation mechanism of gas hydrates in the Zhongjiannan Basin,Western margin of the South China Sea

An analysis of 3D seismic data from the Zhongjiannan Basin in the western margin of the South China Sea (SCS) reveals seismic evidence of gas hydrates and associated gases, including pockmarks, a bottom simulating reflector (BSR), enhanced reflection (ER), reverse polarity reflection (RPR), and a dim amplitude zone (DAZ). The BSR mainly surrounds Zhongjian Island, covering an area of 350 km² in this 3D survey area. The BSR area and pockmark area do not match each other; where there is a pockmark developed, there is no BSR. The gas hydrate layer builds upward from the base of the stability zone with a thickness of less than 100 m. A mature pockmark usually consists of an outside trough, a middle ridge, and one or more central pits, with a diameter of several kilometers and a depth of several hundreds of meters. The process of pockmark creation entails methane consumption. Dense faults in the study area efficiently transport fluid from large depths to the shallow layer, supporting the formation of gas hydrate and ultimately the pockmark.     

More than a century of bathymetric observations and present-day shallow sediment characterization in Belfast Bay, Maine, USA: implications for pockmark field longevity

Mechanisms and timescales responsible for pockmark formation and maintenance remain uncertain, especially in areas lacking extensive thermogenic fluid deposits (e.g., previously glaciated estuaries). This study characterizes seafloor activity in the Belfast Bay, Maine nearshore pockmark field using (1) three swath bathymetry datasets collected between 1999 and 2008, complemented by analyses of shallow box-core samples for radionuclide activity and undrained shear strength, and (2) historical bathymetric data (report and smooth sheets from 1872, 1947, 1948). In addition, because repeat swath bathymetry surveys are an emerging data source, we present a selected literature review of recent studies using such datasets for seafloor change analysis. This study is the first to apply the method to a pockmark field, and characterizes macro-scale (>5 m) evolution of tens of square kilometers of highly irregular seafloor. Presence/absence analysis yielded no change in pockmark frequency or distribution over a 9-year period (1999–2008). In that time pockmarks did not detectably enlarge, truncate, elongate, or combine. Historical data indicate that pockmark chains already existed in the 19th century. Despite the lack of macroscopic changes in the field, near-bed undrained shear-strength values of less than 7 kPa and scattered downcore ¹³⁷Cs signatures indicate a highly disturbed setting. Integrating these findings with independent geophysical and geochemical observations made in the pockmark field, it can be concluded that (1) large-scale sediment resuspension and dispersion related to pockmark formation and failure do not occur frequently within this field, and (2) pockmarks can persevere in a dynamic estuarine setting that exhibits minimal modern fluid venting. Although pockmarks are conventionally thought to be long-lived features maintained by a combination of fluid venting and minimal sediment accumulation, this suggests that other mechanisms may be equally active in maintaining such irregular seafloor morphology. One such mechanism could be upwelling within pockmarks induced by near-bed currents.     

High-resolution P-Cable 3D seismic imaging of gas chimney structures in gas hydrated sediments of an Arctic sediment drift

The newly developed P-Cable 3D seismic system allows for high-resolution seismic imaging to characterize upper geosphere geological features focusing on geofluid expressions (gas chimneys), shallow gas and gas hydrate reservoirs. Seismic imaging of a geofluid system of an Arctic sediment drift at the Vestnesa Ridge, offshore western Svalbard, provides significantly improved details of internal chimney structures from the seafloor to ∼500 m bsf (below seafloor). The chimneys connect to pockmarks at the seafloor and indicate focused fluid flow through gas hydrated sediments. The pockmarks are not buried and align at the ridge-crest pointing to recent, topography-controlled fluid discharge. Chimneys are fuelled by sources beneath the base of gas hydrate stability zone (GHSZ) that is evident at ∼160–170 m bsf as indicated by a bottom-simulating reflector (BSR). Conduit centres that are not vertically straight but shift laterally by up to 200 m as well as discontinuous internal chimney reflections indicate heterogeneous hydraulic fracturing of the sediments. Episodically active, pressure-driven focused fluid flow could explain the hydro-fracturing processes that control the plumbing system and lead to extensive pockmark formation at crest of the Vestnesa Ridge. High-amplitude anomalies in the upper 50 m of the chimney structures suggest formations of near-surface gas hydrates and/or authigenic carbonate precipitation. Acoustic anomalies, expressed as high amplitudes and amplitude blanking, are irregularly distributed throughout the deeper parts of the chimneys and provide evidence for the variability of hydrate and/or carbonate formation in space and time.     

Pockmarks and seafloor instability in the Olbia continental slope (northeastern Sardinian margin, Tyrrhenian Sea)

The seafloor morphology and the subsurface of the continental slope of the Olbia intraslope basin located along the eastern, passive Sardinian margin (Tyrrhenian Sea) has been mapped through the interpretation of high-resolution multibeam bathymetric data, coupled with air-gun and sparker seismic profiles. Two areas, corresponding to different physiographic domains, have been recognized along the Olbia continental slope. The upper slope domain, extending from 500 to 850 m water depth, exhibits a series of conical depressions, interpreted as pockmarks that are particularly frequent in seafloor sectors coincident with buried slope channels. In one case, they are aligned along a linear gully most likely reflecting the course of one of the abandoned channels. The location of the pockmarks thus highlights the importance of the distribution of lithologies within different sedimentary bodies in the subsurface in controlling fluid migration plumbing systems. A linear train of pockmarks is, however, present also away from the buried channels being related to a basement step, linked to a blind fault. Two bathymetric highs, interpreted as possible carbonate mounds, are found in connection with some of the pockmark fields. Although the genetic linkage of the carbonate mounds with seafloor fluid venting cannot be definitively substantiated by the lack of in situ measurements, the possibility of a close relationship is here proposed. The lower slope domain, from 850 m down to the base of the slope at 1,200 m water depth is characterized by a sudden gradient increase (from 2° to 6°) that is driven by the presence of the basin master fault that separates the continental slope from the basin plain. Here, a series of km-wide headwall scars due to mass wasting processes are evident. The landslides are characterized by rotated, relatively undeformed seismic strata, which sometimes evolve upslope into shallow-seated (less than 10 m), smaller scale failures and into headless chutes. Slope gradient may act as a major controlling factor on the seafloor instability along the Olbia continental slope; however, the association of landslides with pockmarks has been recognized in several continental slopes worldwide, thus the role of over-pressured fluids in triggering sediment failure in the Olbia slope can not be discarded. In the absence of direct ground truthing, the geological processes linked to subsurface structures and their seafloor expressions have been inferred through the comparison with similar settings where the interpretation of seafloor features from multibeam data has been substantiated with seafloor sampling and geochemical data.     

The spatial pattern and drainage cell characteristics of a pockmark field,Nile Deep Sea Fan

Over 25,300 seabed pockmarks were mapped from the Rosetta Channel region of the Western Nile Deep Sea Fan (NDSF) using concurrent High Resolution 2D, Chirp profiler and multibeam bathymetry data which spans the Holocene–Pleistocene period. Within the region, a pockmark field containing >13,800 pockmarks was analysed using spatial statistics to determine the distribution of pockmarks within the field. Pockmarks within the field are small (∼16 m diameter), shallow (∼0.5 m deep) circular depressions which formed within the last ∼ 6500 years. The fluid source for the field is identified as an accumulation/generation of gas beneath a hemipelagic seal c. 20–40 ms beneath the seabed. The position of the pockmarks is shown to be unrelated to the depth to the fluid source and an irregular high amplitude acoustic anomaly which is tentatively interpreted as a possible carbonate precipitate of biogenic microbial activity. Statistical spatial analysis of the field confirms the distribution of pockmarks is not random. An exclusion zone surrounding each individual pockmark is identified. The exclusion zone is a unique minimum radius around each pockmark which is not penetrated by any other pockmark. The exclusion zone works in unison with Self-Organised Criticality (SOC) in determining the spatial distribution of pockmarks within the field. The exclusion zone is interpreted as a pockmark “drainage cell”. A conceptual model for a pockmark drainage cell is proposed whereby pockmark formation dissipates a radius/area of fluid and overpressure, thereby preventing the formation of another pockmark within that cell. Consequently, pockmarks are observed to separate or produce anti-clustering tendencies within the field.     

Giant iceberg plow marks at more than 1 km water depth offshore West Greenland

During a recent (2006) cruise of RV ‘Dana’ high resolution side-scan sonar and sub-bottom profiling was carried out on selected shelf and slope transects offshore Disko Bugt, central West Greenland. Available commercial seismic data from the margin indicate here irregular seabed morphology with erosional features locally extending down to c. 1100 m water depth. The newly acquired side-scan sonar data reveal the presence of giant iceberg plow marks extending (sub)parallel to the slope between about 800 and 1085 m water depth. The height difference between bottom of the scours and rim crest is up to 40 m. The largest plow marks are about 750 m wide. To date no observed modern icebergs including those from Antarctica have drafts in excess of c. 500 m. Taking into account maximum glacial sea level lowering of c. 120 m, the paleo-iceberg keel depth was at least 950 m. Due to the presence of the relatively shallow (< 700 m) sill of Davis Strait to the south, calving of these paleo-icebergs is thought to have occurred from an ice margin in the Baffin Bay region (Jakobshavn Isbræ paleo-ice stream?). The depth of occurrence (> 1000 m) and dimensions of the plow marks are concluded to be exceptional, and comparable to glacial scouring features reported from the Arctic Ocean.     

The Rockall Bank Mass Flow: Collapse of a moated contourite drift onlapping the eastern flank of Rockall Bank,west of Ireland

The Rockall Bank Mass Flow (RBMF) is a large, multi-phase submarine slope failure and mass flow complex. It is located in an area where the Feni Drift impinges upon the eastern flank of the Rockall Bank in the NE Atlantic. A 6100 km² region of slope failure scarps, extending over a wide water depth range and with individual scarps reaching up to 22 km long and 150 m high, lies upslope of a series of mass flow lobes that cover at least 18,000 km² of the base of slope and floor of the Rockall Trough. The downslope lobe complex has a negative topographic relief along much of its northern boundary, being inset below the level of the undisplaced contourite drift at the base of slope. The southern margin is topographically more subtle but is marked by the sharp termination of sediment waves outside the lobe. Within the lobe complex the southern margin of the largest lobe shows a positive relief along its southern margin. The initial failure is suggested to have occurred along coherent layer-parallel detachment surfaces at depths of up to 100 m and this promoted initial downslope block sliding which in turn transformed into debris flows which moved out into the basin. The remains of a deep erosional moat linked to the onlapping contourite complex bisects the region of failed slope, and post-failure thermohaline currents have continued to modify the mass flow in this area. Differential sedimentation and erosion associated with the moat may have promoted slope instability. Following the major failure phase, continuous readjustments of the slope occurred and resulted in small-volume turbidites found in shallow gravity cores collected on the lobes. The short term trigger for the failure remains uncertain but earthquake events associated with a deep-seated tectonic lineament to the north of the mass flow may have been important. A Late Pleistocene age for the slope failure is likely. The RBMF is unusual in that it records large-scale collapse of a contourite body that impinged on a sediment-undersupplied slope system. Unlike many other large slope failure complexes along the NE Atlantic margin, the RBMF occurs in a region where there was little overloading by glacial sediment.     

Physical experimental study on the formation mechanism of pockmark by aeration

Abstract

As one of the common geological disasters on the seafloor, the occurrence of pockmarks often indicates the existence of submarine fluid. And the formation of the pockmarks is generally attributed to the soil carried away by the fluid. The research established physical model test and found: (1) The pockmark formed after aeration, and particle gradation changed in the center area of the pockmark where coarse particle (particle size/0.25?mm) increased obviously and fine particle (particle size ≤0.075?mm) decreased significantly; (2) Comparing the volume of the pockmark with the volume of the soil extracted by siphon, it can be found that the volume of the pockmark was much larger than the loss of soil, and the difference reached its peak when the inflation pressure was 20?kPa; (3) Comparing the cone resistance of the soil in the surface and boundary, it can be found that the resistance of the center and marginal soil was much bigger than the outside, and the cone resistance of the boundary was also improved greatly. The research considered that the formation of pockmark was not only related to the soil being carried away by the bottom current, but also the consolidation by vibration existed.     

Seismic and oceanographic evidence of present-day bottom-water dynamics in the Lousy Bank—Hatton Bank area,NE Atlantic

High-resolution multi-channel seismic records from the eastern margin of the Iceland Basin, Northeast Atlantic, are used to infer regional patterns of bottom water dynamics from seafloor morphology and distribution of the most recent geological units deposited along the margin. This information is combined with results from oceanographic and hydrodynamic measurements made in the area. The study area is located between Lousy Bank and Hatton Bank, where deep-water currents are forced around structural highs, leaving a complex pattern of topographically controlled sediment transport pathways. At the top and upper flank of Lousy Bank, George Bligh Bank and Hatton Bank topographic forcing leads to considerable acceleration of the northward flowing North Atlantic Current. At greater water depth, seismic facies indicative of bottom current action are found to be widespread. In addition, seafloor morphology displays moat features extending over large distances. The occurrence of these moats is confined to specific water depths in the range from 700–1400 m and from 1800 to 2200 m depth range. These depth ranges correspond to the basal depth stratum of the North Atlantic Current and the depth range of the Deep Northern Boundary Current, respectively. Geological evidence suggests maximum near-bottom current speed of about 0.5 m/s for these moat areas. Using oceanographic data we suggest that formation of the moats is not likely to be associated with the presence of a persistent high-speed contour current core, but probably originates from the occurrence of solibores and thus may be related to the internal wave field of the Iceland Basin possible linked to atmospheric pressure variation.