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1.
Pockmarks are observed worldwide along the continental margins and are inferred to be indicators of fluid expulsion. In the present study, we have analysed multibeam bathymetry and 2D/3D seismic data from the south-western Barents Sea, in relation to gas hydrate stability field and sediment type, to examine pockmark genesis. Seismic attributes of the sediments at and beneath the seafloor have been analysed to study the factors related to pockmark formation. The seabed depths in the study area are just outside the methane hydrate stability field, but the presence of higher order hydrocarbon gases such as ethane and/or propane in the expelled fluids may cause localised gas hydrate formation. The selective occurrence of pockmarks in regions of specific seabed sediment types indicates that their formation is more closely related to the type of seabed sediment than the source path of fluid venting such as faults. The presence of high acoustic backscatter amplitudes at the centre of the pockmarks indicates harder/coarser sediments, likely linked to removal of soft material. The pockmarks show high seismic reflection amplitudes along their fringes indicating deposition of carbonates precipitated from upwelling fluids. High seismic amplitude gas anomalies underlying the region away from the pockmarks indicate active fluid flow from hydrocarbon source rocks beneath, which is blocked by overlying less permeable formations. In areas of consolidated sediments, the upward flow is limited to open fault locations, while soft sediment areas allow diffused flow of fluids and hence formation of pockmarks over a wider region, through removal of fine-grained material.  相似文献   

2.
About a decade ago, a large field of pockmarks (individual features up to 30 m in diameter and <2 m deep) was discovered in water depths of 15–40 m in the Bay of Concarneau in southern Brittany along the French Atlantic coast, covering an overall area of 36 km2 and characterised by unusually high pockmark densities in places reaching 2,500 per square kilometre. As revealed by geophysical swath and subbottom profile data ground-truthed by sediment cores collected during two campaigns in 2005 and 2009, the confines of the pockmark field show a spectacular spatial association with those of a vast expanse of tube mats formed by a benthic community of the suspension-feeding amphipod Haploops nirae. The present study complements those findings with subbottom chirp profiles, seabed sonar imagery and ultrasonic backscatter data from the water column acquired in April 2011. Results show that pockmark distribution is influenced by the thickness of Holocene deposits covering an Oligocene palaeo-valley system. Two groups of pockmarks were identified: (1) a group of large (>10 m diameter), more widely scattered pockmarks deeply rooted (up to 8 ms two-way travel time, TWTT) in the Holocene palaeo-valley infills, and (2) a group of smaller, more densely spaced pockmarks shallowly rooted (up to 2 ms TWTT) in interfluve deposits. Pockmark pore water analyses revealed high methane concentrations peaking at ca. 400 μl/l at 22 and 30 cm core depth in silty sediments immediately above Haploops-bearing layers. Water column data indicate acoustic plumes above pockmarks, implying ongoing pockmark activity. Pockmark gas and/or fluid expulsion resulting in increased turbidity (resuspension of, amongst others, freshly settled phytoplankton) could at least partly account for the strong spatial association with the phytoplankton-feeding H. nirae in the Bay of Concarneau, exacerbating impacts of anthropogenically induced eutrophication and growing offshore trawling activities. Tidally driven hydraulic pumping in gas-charged pockmarks represents a good candidate as large-scale short-term triggering mechanism of pockmark activation, in addition to episodic regional seismic activity.  相似文献   

3.
Pockmarks off Big Sur, California   总被引：1，自引：0，他引：1  
A pockmark field was discovered during EM-300 multi-beam bathymetric surveys on the lower continental slope off the Big Sur coast of California. The field contains ∼1500 pockmarks which are between 130 and 260 m in diameter, and typically are 8-12 m deep located within a 560 km2 area. To investigate the origin of these features, piston cores were collected from both the interior and the flanks of the pockmarks, and remotely operated vehicle observation (ROV) video and sampling transects were conducted which passed through 19 of the pockmarks. The water column within and above the pockmarks was sampled for methane concentration. Piston cores and ROV collected push cores show that the pockmark field is composed of monotonous fine silts and clays and the cores within the pockmarks are indistinguishable from those outside the pockmarks. No evidence for either sediment winnowing or diagenetic alteration suggestive of fluid venting was obtained. 14C measurements of the organic carbon in the sediments indicate continuous sedimentation throughout the time resolution of the radiocarbon technique (∼45?000 yr BP), with a sedimentation rate of ∼10 cm per 1000 yr both within and between the pockmarks. Concentrations of methane, dissolved inorganic carbon, sulfate, chloride, and ammonium in pore water extracted from within the cores are generally similar in composition to seawater and show little change with depth, suggesting low biogeochemical activity. These pore water chemical gradients indicate that neither significant accumulations of gas are likely to exist in the shallow subsurface (∼100 m) nor is active fluid advection occurring within the sampled sediments. Taken together the data indicate that these pockmarks are more than 45?000 yr old, are presently inactive, and contain no indications of earlier fluid or gas venting events.  相似文献   

4.
《Marine Geology》2006,225(1-4):45-62
A systematic mapping program incorporating more than 5000 km of side scan sonar and seismic reflection tracklines in the western Gulf of Maine has identified more than 70 biogenic natural gas deposits, occupying 311 km2 in nearshore muddy embayments. Many of these embayments also contain pockmark fields, with some exhibiting geologically active characteristics including the observance of plumes of escaping fluids and sediment. Pockmarks, hemispherically shaped depressions of various size and depths, formed through fluid escape of gas and/or pore water, are sometimes found within or outside gas fields, although many gas fields lack pockmarks altogether. Although the origin of the natural gas remains unclear, if coastal environments at times of lower sea level were similar to the present, numerous lake, wetland, valley fill and estuarine sources of organic-rich material may have formed on the inner shelf. If these deposits survived transgression and remain buried, they are potential gas sources.Intensive mapping of the Belfast Bay pockmark field in 1998 produced the first nearly continuous side scan sonar mosaic of a Gulf of Maine pockmark field with a corresponding 3-dimensional geological model generated from seismic data. Statistical analysis of pockmark geometry, gas deposit loci, and subsurface evidence for gas-enhanced reflectors suggest that gas migration from deeper lateral sources along permeable subsurface strata may be the mechanism for pockmark formation in areas lacking gas-curtain seismic reflections. The coarse-grained transgressive ravinement unconformity between Pleistocene glacial-marine mud and Holocene mud may act as a conduit for distributing methane to the field's margins.  相似文献   

5.
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.  相似文献   

6.
北黄海海底麻坑群形态的定量研究及控制因素   总被引：1，自引：0，他引：1  
本文基于高分辨率多波束水深数据和反向散射强度数据,对北黄海海底麻坑群的形态参数进行定量研究,结合水深、地形坡度和后向散射强度的变化准确界定了麻坑的轮廓,识别出圆形、椭圆形、拉长型麻坑共282个,并在ArcGIS软件中对其形态参数进行了分析计算,麻坑的平均长轴1.36 km,短轴0.78 km,直径0.94 km,面积0.88 km2,平均周长3.82 km,长宽比1.83,深度0.3~2.5 m,平均面积密集度13%,麻坑的剖面形态有麻坑边缘凹陷、中部有明显凸起（W1型）,麻坑边缘凹陷、中部略凸起（W2型）,麻坑中部单纯凹陷（V型）,分别集中分布在麻坑群的北部、南部、西部。麻坑的平面规模大、深度小的原因与地层中形成麻坑的游离气体浓度较小有关,也可能受到了地震、海啸等外力的诱发。麻坑的长轴优势走向为ENE-WSW、NNE-SSW,底流对其形状的塑造起了较大作用,部分麻坑成串排列,形成串珠状的麻坑链,其排列方式受到海底古河道、古潟湖等沉积地层结构的控制。海底麻坑群发育区反向散射强度为-60~-71 dB,麻坑内部较麻坑外部平均高5 dB,可能为麻坑内部气体泄漏引起海底沉积物被剥蚀后残留下的粗颗粒物质或海底生物活动留下的遗迹导致的。  相似文献   

7.
南海礼乐盆地海底麻坑地貌及成因分析   总被引：1，自引：0，他引：1  
本文基于高分辨率多波束测深和浅地层剖面数据,首次对南海礼乐盆地南部坳陷海底麻坑进行了系统的识别研究。共识别出各类麻坑81个,其中麻坑直径最大约2.4 km,坑深最大约157 m。麻坑种类多样:按平面形态主要分为圆形、椭圆形、拉长形和新月形麻坑;按组合方式分为孤立麻坑、链状麻坑和复合麻坑;按直径分为正常麻坑和大型麻坑。区域内发育多条大型海底峡谷,峡谷侵蚀引起两侧地层稳定性降低,气体储层遭受破坏,泄露的气体沿断层或气烟囱等喷发出海底形成麻坑。而因麻坑生成时剥蚀的沉积物质与周围水体混合并逐渐发展成浊流,在一定程度上促进海底峡谷向下延伸。研究区内单个麻坑的平面形态最初为圆形或椭圆形,之后由于重力流和峡谷侵蚀的影响,逐渐发展成拉长形或新月形,麻坑之间也会发生组合形成复合麻坑。链状麻坑与冲沟的形成联系密切,沿垂直于等深线方向展布的链状麻坑在重力流的冲刷下,发育成底部平坦的麻坑冲沟。对比分析全球其他海域麻坑,发现海底麻坑尺寸与水深关系密切,在深水区域更容易发育大型麻坑。  相似文献   

8.
Integrating novel and published swath bathymetry (3,980 km2), as well as chirp and high-resolution 2D seismic reflection profiles (2,190 km), this study presents the mapping of 436 pockmarks at water depths varying widely between 370 and 1,020 m on either side of the Strait of Gibraltar. On the Atlantic side in the south-eastern Gulf of Cádiz near the Camarinal Sill, 198 newly discovered pockmarks occur in three well localized and separated fields: on the upper slope (n=14), in the main channel of the Mediterranean outflow water (MOW, n=160), and on the huge contourite levee of the MOW main channel (n=24) near the well-known TASYO field. These pockmarks vary in diameter from 60 to 919 m, and are sub-circular to irregularly elongated or lobate in shape. Their slope angles on average range from 3° to 25°. On the Mediterranean side of the strait on the Ceuta Drift of the western Alborán Basin, where pockmarks were already known to occur, 238 pockmarks were identified and grouped into three interconnected fields, i.e. a northern (n=34), a central (n=61) and a southern field (n=143). In the latter two fields the pockmarks are mainly sub-circular, ranging from 130 to 400 m in diameter with slope angles averaging 1.5° to 15°. In the northern sector, by contrast, they are elongated up to 1,430 m, probably reflecting MOW activity. Based on seismo-stratigraphic interpretation, it is inferred that most pockmarks formed during and shortly after the last glacial sea-level lowstand, as they are related to the final erosional discontinuity sealed by Holocene transgressive deposits. Combining these findings with other existing knowledge, it is proposed that pockmark formation on either side of the Strait of Gibraltar resulted from gas and/or sediment pore-water venting from overpressured shallow gas reservoirs entrapped in coarse-grained contourites of levee deposits and Pleistocene palaeochannel infillings. Venting was either triggered or promoted by hydraulic pumping associated with topographically forced internal waves. This mechanism is analogous to the long-known effect of tidal pumping on the dynamics of unit pockmarks observed along the Norwegian continental margin.  相似文献   

9.
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.  相似文献   

10.
Two pockmark fields, located along the coastal zone of the Patras and Corinth gulfs, Greece were surveyed in detail. The pockmark fields, which are 30 km apart, are formed in shallow waters at depths of 20–40 m and are about 0.5–1 km from the shoreline. The oceanographic data suggest that two different mechanisms were responsible for their formation. The pockmark field in the Patras Gulf appears to have been formed as a result of methane seepage from the seabed, whereas the field in the Corinth Gulf appears to have resulted from groundwater seepage.  相似文献   

11.
Based on high-resolution Chirp seismic, multibeam bathymetry and side scan sonar data collected in the ?zmir Gulf, Aegean Sea in 2008 and 2010, gas-related structures have been identified, which can be classified into three categories: (1) shallow gas accumulations and gas chimneys, (2) mud diapirs, and (3) active and inactive pockmarks. On the Chirp profiles, shallow gas accumulations were observed along the northern coastline of the outer ?zmir Gulf at 3-20 m below the seabed. They appear as acoustic turbidity zones and are interpreted as biogenic gas accumulations produced in organic-rich highstand fan sediments from the Gediz River. The diapiric structures are interpreted as shale or mud diapirs formed under lateral compression due to regional counter-clockwise rotation of Anatolian microplate. Furthermore, the sedimentary structure at the flanks suggests a continuous upward movement of the diapirs. Several pockmarks exist close to fault traces to the east of Hekim Island; most of them were associated with acoustic plumes indicating active degassing during the survey period in 2008. Another Chirp survey was carried out just over these plumes in 2010 to demonstrate if the gas seeps were still active. The surveys indicate that the gas seep is an ongoing process in the gulf. Based on the Chirp data, we proposed that the pockmark formation in the area can be explained by protracted seep model, whereby sediment erosion and re-distribution along pockmark walls result from ongoing (or long lasting) seepage of fluids over long periods of time. The existence of inactive pockmarks in the vicinity, however, implies that gas seepage may eventually cease or that it is periodic. Most of the active pockmarks are located over the fault planes, likely indicating that the gas seepage is controlled by active faulting.  相似文献   

12.
This study describes a new type of pockmark association from the Lower Congo Basin offshore West Africa, consisting of up to 8 stacked paleopockmarks separated by intervals of drape and onlap fill. The stacked paleopockmarks occur within the depocentres of polygonally-faulted Plio-Pleistocene sediments and are distributed evenly in the downslope parts of two salt mini-basins. The majority of the stacked pockmarks initiated synchronously in the late Pliocene (~ 3 Ma) with a subordinate initiation phase in the mid Pliocene (~ 4 Ma). The primary agents in pockmark formation are interpreted to be pore water expelled during early-stage compaction together with biogenic methane. Bottom simulating reflections (BSRs) associated with free gas overlain by gas hydrates are currently found in the area. It is speculated that biogenic methane accumulated within and below a clathrate cap, which was repeatedly breached, forming pockmarks at discrete horizons separated by intervals of draping sedimentation. The mid and late Pliocene pockmark initiations appear to coincide with sea-level falls following periods of relatively stable highstand conditions. Several subsequent pockmark horizons may similarly correlate with subsequent sea-level falls during the late Pliocene and early Pleistocene. The stacked paleopockmarks are completely surrounded by polygonal faults and consistently occur within polygonal fault cells that crosscut the succession containing the stacked pockmarks. Early-stage compaction and dewatering of the Pliocene sediments thus preceded polygonal faulting, providing a constraint on the conditions leading to polygonal faulting of the fine-grained host sediments. The relationship documented here is interpreted as due to the presence of a hydrate cap in the Plio-Pleistocene mini-basins which may have retarded the normal compaction processes and facilitated pockmark formation by allowing the build up of gas hydrate and free gas in the basin centres. The relative timing and spatial relationships implies that fluids expelled due to polygonal faulting were not implicated in pockmark formation in this area.  相似文献   

13.
A new seafloor observatory, the gas monitoring module (GMM), has been developed for continuous and long-term measurements of methane and hydrogen sulphide concentrations in seawater, integrated with temperature (T), pressure (P) and conductivity data at the seafloor. GMM was deployed in April 2004 within an active gas-bearing pockmark in the Gulf of Patras (Greece), at a water depth of 42 m. Through a submarine cable linked to an onshore station, it was possible to remotely check, via direct phone connection, GMM functioning and to receive data in near-real time. Recordings were carried out in two consecutive campaigns over the periods April–July 2004, and September 2004–January 2005, amounting to a combined dataset of ca. 6.5 months. This represents the first long-term monitoring ever done on gas leakage from pockmarks by means of CH4+H2S+T+P sensors. The results show frequent T and P drops associated with gas peaks, more than 60 events in 6.5 months, likely due to intermittent, pulsation-like seepage. Decreases in temperature in the order of 0.1–1°C (up to 1.7°C) below an ambient T of ca. 17°C (annual average) were associated with short-lived pulses (10–60 min) of increased CH4+H2S concentrations. This seepage “pulsation” can either be an active process driven by pressure build-up in the pockmark sediments, or a passive fluid release due to hydrostatic pressure drops induced by bottom currents cascading into the pockmark depression. Redundancy and comparison of data from different sensors were fundamental to interpret subtle proxy signals of temperature and pressure which would not be understood using only one sensor.  相似文献   

14.
Several incidents involving damage to submarine pipelines indicate that there will be potential hazards for many submarine structures if the geotechnical properties of the soil in pockmarks remain unclear. Based on a geophysical survey, geological drilling, in-situ measurement, and shallow gas eruption experiment, two large pockmarks near the Zhongjieshan Archipelago in the East China Sea were analyzed comprehensively. The geophysical and in-situ data indicated that the soil below the two pockmarks contains free or dissolved shallow gas, which continues to migrate upward from the deep zones, but there is no high-pressure gas reservoir in the pockmark soil. In-situ piezocone data, geotechnical results, and shallow gas eruption experiments have demonstrated that the pockmark soil has strengthened mechanical properties and zoning characteristics. After analyzing the pockmark soil in the area where the study was conducted, it was concluded that the pockmark soil in this area is not suitable for the accumulation of high-pressure, shallow gas. It also was concluded that the pockmark soil had stronger mechanical properties than virgin sediment due to the compaction of the soil caused by the eruption of the shallow gas. The zoning characteristics of pockmark soil are due to the regional differences in the ability of shallow gas to carry soil particles, which is a new finding that is worthy of attention in off-shore engineering.  相似文献   

15.
Water tank experiments were performed in order to investigate the behaviour of currents in pockmarks. A particle-seeded flow was visualised and quantified with the aid of the particle tracking velocimetry technique. The employed analogue pockmark is a 1:100 idealised scale model of a natural pockmark, while the highest Reynolds number in the experiments was one order of magnitude smaller than in nature. Interaction of the flow with the pockmark geometry resulted in an upwelling current downstream of the pockmark centre, along with enhanced water turbulence in the depression. Scaling-up the experimental measurements, it is found that the upwelling would be capable of preventing the settling of particles as large as very fine sand. Furthermore, the increased turbulence would support the suspended fine material, which can thus be transported away before settling. The net effect for a variable-direction near-bed current over long periods of time would be to winnow the settling sediments and reduce the sedimentation rate in pockmarks. These mechanisms may be responsible for the observed lack of sediment infill and the typical presence of relatively coarser sediments inside pockmarks compared to the surrounding bed. In contrast, sediments transported as bedload are likely to be deposited in pockmarks because of the weakening of near-bed currents as well as lateral flow convergence associated with the upwelling. Bedload, however, may not be the dominant mode of sediment transport in areas covered by cohesive sediments, where pockmarks are found.  相似文献   

16.
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.  相似文献   

17.
Pockmarks in the inner Oslofjord,Norway   总被引：5，自引：3，他引：2  
Multibeam bathymetric surveys of the Inner Oslofjord, Norway have revealed a high density of pockmarks in the 179-km2 inner fjord area, which contains over 500 pockmarks of varying size, typically 20–50 m in diameter and 2–10 m deep. These pockmarks have been investigated with a variety of techniques, including acoustic subbottom profiling, sedimentological and geochemical analyses of cores, remotely operated vehicle observation, and morphometry. Both the distribution and shapes of the pockmarks suggest that they are related to structures in the bedrock underlying relatively thin (<50 m) unconsolidated glacial and postglacial sediments. The data provide no direct indication of a particular mode of pockmark formation, but release of large amounts of biogenic, shallow methane seems unlikely. Several lines of evidence point to a continuous process of pockmark formation followed by inactivity, with some pockmarks recently active whereas others have been inactive for a considerable time. Some pockmarks are characterised by coarse sediment in their centres. The density, variety and easy access make this pockmark field an ideal model area for pockmark research. John S. Gray is deceased.  相似文献   

18.
The Nyegga region, located at water depths of about 600–800 m on the NW European continental margin, contains more than 200 pockmarks. Recently collected TOPAS seismic profiles and EM1002 bathymetric records now provide high-resolution information on their seabed and shallow sub-seabed geological setting. The identified pockmarks are up to 15 m deep, between 30 m and 600 m across and reach a maximum area of ca. 315,000 m2. The pockmarks are sediment-empty features. They do not have any preferred direction of orientation and show large variations in their shape. The pockmarks are restricted to a <16.2 cal. ka old sediment unit. This unit comprises sandy mud and is characterised by sedimentation rates of ca. 1 mm/year. The pockmarks are localised over a thick late Plio-Pleistocene prograding sediment package and a polygonal faulted Miocene-Oligocene ooze-rich unit. The late Plio-Plistocene deposits host bottom simulating reflectors, indicative of gas hydrate-bearing sediments. Inspection of the newly collected high-resolution dataset, combined with previously analysed sediment cores and 2D multichannel seismic profiles, reveals that the Nyegga pockmark field does not show any strong relationship between seabed features, sub-seabed structures and the sedimentary setting. This suggests a more complex evolution history of the Nyegga pockmark field then previously thought.  相似文献   

19.
Methane fluxes in the southeastern Baltic Sea   总被引：2，自引：2，他引：0  
New data from surveys of gas-bearing mud areas in the Gdansk Deep (southeastern Baltic Sea) were collected during four research cruises in 2009–2011. These revealed the presence of seven large pockmarks apart from the three already known, and enabled significant improvement of the existing digital map of gassy mud distribution. Based on geochemical sediment analyses, calculated diffusive methane fluxes from the upper (0–5?cm) seabed layer into near-bottom waters were highest—3.3?mmol/(m2?day)—in pockmark mud, contrasting strongly with the minimum value of 0.004?mmol/(m2?day) observed in typical, background mud. However, fluxes of less than 0.1?mmol/(m2?day) were observed in all sediment types, including pockmarks. In a newer attempt to roughly estimate budgets at a more regional scale, diffusive methane venting amounts to 280?×?106?mmol/day for southeastern Baltic Sea muddy sediments. Elongated pockforms in the southern Gotland Deep, known since the end of the 1980s as pockmarks, had methane concentrations that were similar to those of gassy mud from the Gdansk Basin, and there was no geo-acoustic evidence of considerably increased gas levels.  相似文献   

20.
The study of the bottom sediments accumulated during the last 60–65 ka in the pockmark craters of the Chukchi Plateau in the Arctic Ocean showed that their composition and lithostratigraphy in general are similar to those of the background areas. A specific feature is the presence of sedimentary interlayers and horizons with signs of gravitational stirring, carbonate mineralization, and accumulation under H2S contamination conditions. The micropaleontological remains found in the pockmark sediments accumulated during the glacial periods are represented by redeposited species from Paleozoic, Mesozoic, and Cenozoic deposits, which are locally exposed in the pockmark walls. It was concluded that the pockmarks were formed under the influence of pulsed fluid flows. The present-day topography of the pockmarks was formed at the last stage of active defluidization (35?20 ka ago).  相似文献   

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