Acoustic monitoring of gas emissions from the seafloor. Part I: quantifying the volumetric flow of bubbles

Three decades of continuous ocean exploration have led us to identify subsurface fluid related processes as a key phenomenon in marine earth science research. The number of seep areas located on the seafloor has been constantly increasing with the use of multi-scale imagery techniques. Due to recent advances in transducer technology and computer processing, multibeam echosounders are now commonly used to detect submarine gas seeps escaping from the seafloor into the water column. A growing number of en-route surveys shows that sites of gas emissions escaping from the seafloor are much more numerous than previously thought. Estimating the temporal variability of the gas flow rate and volumes escaping from the seafloor has thus become a challenge of relevant interest which could be addressed by sea-floor continuous acoustic monitoring. Here, we investigate the feasibility of estimating the volumetric flow rates of gas emissions from horizontal backscattered acoustic signals. Different models based on the acoustic backscattering theory of bubbles are presented. The forward volume backscattering strength and the inversion volumetric flow rate solutions were validated with acoustic measurements from artificial gas flow rates generated in controlled sea-water tank experiments. A sensitivity analysis was carried out to investigate the behavior of the 120-kHz forward solution with respect to model input parameters (horizontal distance between transducer and bubble stream, bubble size distribution and ascent rate). The most sensitive parameter was found to be the distance of the bubble stream which can affect the volume backscattering strength by 20 dB within the horizontal range of 0–200 m. Results were used to derive the detection probability of a bubble stream for a given volume backscattering strength threshold according to different bubble flow rates and horizontal distance.     

Acoustic tomography for monitoring the Sea of Japan: a pilot experiment

A pilot experiment was conducted in the Sea of Japan (also called the East Sea) in September-October 1999, to assess the possibility of using acoustic tomographic techniques for monitoring water mass structure and dynamics. Acoustic m-sequence signals at various frequencies between 250 and 634 Hz were transmitted from bottom-mounted acoustic sources in shallow water off the coast of Vladivostok to vertical-array receiving systems deployed off the north coast of Ulleung-Do island (S. Korea), 558 km to the south. The data are analyzed for temporal correlation, time spread, and transmission loss and are interpreted in terms of a tomographic system for monitoring the East Sea.     

Wave dispersion study for tsunami propagation in the Sea of Marmara

In this paper the aim is to investigate whether there are differences between the dispersion and non-dispersion solutions on tsunami propagation. For this purpose, two numerical models of tsunami propagation are compared. One of these numerical models is a nondispersive model that uses Saint Venant equations and the other is a dispersive model that uses Boussinesq equations. The tsunamis resulting from a submarine mass failure (SMF) which is settled at the bottom of the north eastern Sea of Marmara are examined. An analytical solution considering wave dispersion is developed for obtaining near-field tsunami amplitudes above the submarine mass failure. Numerical modeling is used at the sea surface from the common boundary called as liquid boundary with incident waves up to the coastal regions to get the tsunami amplitudes. The output of the analytical model is taken as the disturbances for the numerical method. In the numerical solutions TELEMAC-2D software system is used for both dispersive and nondispersive modeling. The results of the dispersive and nondispersive models are compared to each other. Both temporal and spatial differences in the amplitudes and wave shapes are examined. The obtained results demonstrate that there are no noticeable differences between the dispersion and non-dispersion solutions except some special cases and some special landslide velocities.     

Acoustic response of gas hydrate formation in sediments from South China Sea

Gas hydrate has been recognized as a potential energy resource in South China Sea (SCS). Understanding the acoustic response of gas hydrate formation in the SCS sediments is essential for regional gas hydrate investigation and quantification. The sediments were obtained from gravity core sampling at E 115°12.52363′ N 19°48.40299′. Gas hydrate was formed within a “gas + water-saturated SCS sediments” system. Combination of a new bender element technique and coated time domain reflectometry (TDR) was carried out to study the acoustic response of hydrate occurrence in SCS sediments. The results show the acoustic signal becomes weak when hydrate saturation (S_h) is lower than 14%. The acoustic velocities (Vp, Vs) of the sediments increase with S_h during hydrate formation, and Vs increases relatively faster when S_h is higher than 14%. These results indicate that tiny hydrate particles may firstly float in the pore fluid, which causes a significant acoustic attenuation, but has little influence on shear modulus. As time lapses and S_h approaches 14%, numerous particles coalesce together and contact with sediment particles. As a result, Vs has a sharp increase when hydrate saturation exceeds 14%. Several velocity models were validated with the experimental data, which suggests a combination of the BGTL (Biot–Gassmann Theory modified by Lee) model and the Weighted Equation is suitable to estimate S_h in SCS.     

Tracing seafloor methane emissions with benthic foraminifera: Results from the Ana submarine landslide (Eivissa Channel,Western Mediterranean Sea)

The hypothesis that benthic foraminifera are useful proxies of local methane emissions from the seafloor has been verified on sediment core KS16 from the headwall of the Ana submarine landslide in the Eivissa Channel, Western Mediterranean Sea. The core MS312 from a nearby location with no known methane emissions is utilised as control. The core was analysed for biostratigraphy, benthic foraminiferal assemblages, Hyalinea balthica and Uvigerina peregrina carbon and oxygen stable isotope composition, and sedimentary structures. The upper part of the core records post-landslide deglacial and Holocene normal marine hemipelagic sediments with highly abundant benthic foraminifera species that are typical of outer neritic to upper bathyal environment. In this interval, the δ¹³C composition of benthic foraminifera indicates normal marine environment analogous to those found in the control core. Below the sedimentary hiatus caused by the emplacement of the slide, the foraminiferal assemblages are characterised by lower density and higher Shannon Index. Markedly negative δ¹³C shifts in benthic foraminifera are attributed to the release of methane through the seabed. The mean values of the ¹³C anomaly in U. peregrina are ? 0.951 ± 0.208 in the pre-landslide sediments, and ? 0.269 ± 0.152 in post-slide reworked sediments deposited immediately above the hiatus. The δ¹³C anomaly in Hyalinea balthica is ? 2.497 ± 0.080 and ? 2.153 ± 0.087, respectively. To discard the diagenetic effects on the δ¹³C anomaly, which could have been induced by Ca–Mg replacement and authigenic carbonate overgrowth on foraminifera tests, a benthic foraminifera subsample has been treated following an oxidative and reductive cleaning protocol. The cleaning has resulted, only in some cases, in a slight reduction of the anomaly by 0.95% for δ¹³C and < 0.80% for δ¹⁸O. Therefore, the first conclusion is that the diagenetic alteration is minor and it does not alter significantly the overall carbon isotopic anomaly in the core. Consequently, the pre-landslide sediments have been subject to pervasive methane emissions during a time interval of several thousand years. Methane emissions continued during and immediately after the occurrence of Ana Slide at about 61.5 ka. Subsequently, methane emissions decreased and definitely ceased during the last deglaciation and the Holocene.     

Toward remote seafloor classification using the angular response ofacoustic backscattering: a case study from multiple overlapping GLORIAdata

While the average seafloor backscatter strength within a narrow range of grazing angles can be used as a first-order classification tool, this technique often fails to distinguish seafloors of known differing geological character. In order to resolve such ambiguities, it is necessary to examine the variation in backscatter strength as a function of grazing angle. For this purpose, a series of multiply overlapping GLORIA sidescan sonar images (6.5 kHz) have been obtained in water depths ranging from 1000 to 2500 m. To constrain the placement of acoustic backscatter measurements and to measure the true impinging angle of the incident wave, the corresponding seafloor was simultaneously surveyed using the Seabeam multibeam system. As a result of the multiple overlap, the angular response of seafloor backscatter strength may be derived for regions much smaller than the swath width. By using the derived angular response of seafloor backscatter strength in regions for which sediment samples exist, an empirical seafloor classification scheme is proposed based on the shape, variance, and magnitude of the angular response. Because of the observed variability in the shape of the angular response with differing seafloor types, routine normalization of single-pass swath data to an equivalent single grazing angle image cannot be achieved. As a result, for the case of single-pass surveys, confident seafloor classification may only be possible for regions approaching the scale of the swath width     

基于同轴差距测量法的南海深水海底沉积物声衰减特性研究

通过分析沉积物声波测量过程能量损失的实质,阐述平行轴差距衰减测量法和垂直轴差距衰减测量法的研究基础——差动式衰减测量方法原理,并推导了其衰减系数公式。根据南海沉积物的柱状样品分装的特点,结合以上两种方法,提出了同轴差距衰减测量法,这种方法具有原理上的合理性并且其可操作性强;运用此方法测量了沉积物在常温和温度控制变化条件下的声波信号,计算了衰减系数,研究了温度对沉积物声波传播能量的影响,得出南海深水海底沉积物具有以下声衰减特性：沉积物含砂量高,声衰减系数大;随着温度的升高,沉积物的声衰减系数变化具有不均匀性,整体呈非线性减小趋势。以上研究将为声学遥测和反演海底沉积物的物理力学特性提供数据和方法支持。     

The mesoscale variability in the Caribbean Sea. Part II: Energy sources

The processes which drive the production and the growth of the strong mesoscale eddy field in the Caribbean Sea are examined using a general circulation model. Diagnostics of the simulations suggest that:(1) The mean currents in the Caribbean Sea are intrinsically unstable. The nature of the instability and its strength vary spatially due to strong differences of current structure among basins.(2) The greatest and most energetic eddies of the Caribbean Sea originate in the Venezuela Basin by mixed barotropic-baroclinic instability of an intense jet, formed with waters mostly from the surface return flow of the Meridional Overturning Circulation and the North Equatorial Current which converge and accelerate through the Grenada Passage. The vertical shear of this inflow is enhanced by an eastward undercurrent, which flows along the south American Coast between 100 and 250 m depth. The shallow eddies (less than 200 m depth) formed in the vicinity of the Grenada Passage get rapidly deeper (down to 1000 m depth) and stronger by their interaction with the deep interior flow of the Subtropical Gyre, which enters through passages north of St. Lucia. These main eastern Caribbean inflows merge and form the southern Caribbean Current, whose baroclinic instability is responsible for the westward growth and strengthening of these eddies from the Venezuela to the Colombia Basin.(3) Eddies of lesser strength are produced in other regions of the Caribbean Sea. Their generation and growth is also linked with instability of the local currents. First, cyclones are formed in the cyclonic shear of the northern Caribbean Current, but appear to be rapidly dissipated or absorbed by the large anticyclones coming from the southern Caribbean. Second, eddies in the Cayman Sea, which impact the Yucatan region, are locally produced and enhanced by barotropic instability of the deep Cayman Current.(4) The role of the North Brazil Current (NBC) rings is mostly to act as a finite perturbation for the instability of the mean flow. Their presence near the Lesser Antilles is ubiquitous and they appear to be linked with most of the Caribbean eddies. There are some evidences that the frequency at which they form near the Grenada Passage is influenced by the frequency at which the NBC rings impinge the Lesser Antilles. But large Caribbean eddies also form without a close influence of any ring, and comparison between simulations shows that mean eddy kinetic energy and eddy population in the Caribbean Sea are not substantially different in absence or presence of NBC rings: their presence is not a necessary condition for the generation and growth of the Caribbean eddies.     

Benthic foraminifera for heavy metal pollution monitoring: A case study from the central Adriatic Sea coast of Italy

Benthic foraminifera are increasingly used as environmental bio-indicators of pollution in coastal and marginal marine settings. Their community structure provides information on the general characteristics of the environment and some species are sensitive to specific environmental parameters. Among various criteria, the occurrence of test abnormalities may represent a useful bioindicator for monitoring environmental impacts in coastal regions. A study of living benthic foraminifera was carried out in 42 sediment samples collected from the central Adriatic coast of Italy. Benthic foraminiferal assemblages from this area are rich, well preserved, and dominated by Ammonia parkinsoniana, and subordinately by Ammonia tepida, Aubignyna perlucida, Eggerella scabra, and Nonionella turgida. Heavy metal concentrations have been analysed which indicate low polluted environmental conditions. Foraminiferal species and heavy metal concentrations were investigated both with bivariate (correlation matrix) and multivariate techniques of principal component analysis (PCA) and cluster analysis. Statistical analysis shows a possible control of these pollutants both on the taxonomic composition of the benthic foraminiferal assemblages and the development of test malformations. Increasing heavy metal contents lead to an increase in relative abundance of A. tepida A. perlucida, N. turgida and E. scabra, and a relative concurrent decrease in relative abundance of A. parkinsoniana and higher percentages of deformed specimens (FAI) and species (FMI). Our results confirm that A. parkinsoniana prefers clean to low polluted environments and show that it is a very sensitive and un-tolerant species to heavy metal pollution being deeply affected by heavy metal content even at low concentrations. Our findings also confirm the capacity of the A. tepida to tolerate increasing heavy metal concentrations, and highlights that A. perlucida, N. turgida and E. scabra can be considered as tolerant species at least in low polluted environments. Following this, A. parkinsoniana and A. tepida can be reciprocally considered good bioindicator of heavy metal pollution over the surveyed area. The development of test abnormalities with a variety of malformations is a noticeable feature over the study area where the living deformed assemblages are largely dominated by a few species. The low percentages of deformed specimens (Foraminiferal Abnormality Index up to 4.7, with 2 on average) match well with the low concentrations of heavy metals that lead to low polluted environmental conditions. This study confirms and supports the suitability of studying benthic foraminifera as a technique for the in situ continuous bio-monitoring of heavy metal pollution of coastal marine sediments.     

Evidence of NW extension of the North Anatolian Fault Zone in the Marmara Sea: a new interpretation of the Marmara Sea (İzmit) earthquake on 17 August 1999

Active faults aligning in NW–SE direction and forming flower structures of strike-slip faults were observed in shallow seismic data from the shelf offshore of Avc?lar in the northern Marmara Sea. By following the parallel drainage pattern and scarps, these faults were traced as NW–SE-directed lineaments in the morphology of the northern onshore sector of the Marmara Sea (eastern Thrace Peninsula). Right-lateral displacements in two watersheds of drainage and on the coast of the Marmara Sea and Black Sea are associated with these lineaments. This right-lateral displacement along the course of these faults suggests a new, active strike-slip fault zone located at the NW extension of the northern boundary fault of the Ç?narc?k Basin in the Marmara Sea. This new fault zone is interpreted as the NW extension of the northern branch of the North Anatolian Fault Zone (NAFZ), extending from the Ç?narc?k Basin of the Marmara Sea to the Black Sea coast of the Thrace Peninsula, and passing through B üy ük çekmece and K ü ç ük çekmece lagoons. These data suggest that the rupture of the 17 August 1999 earthquake in the NAFZ may have extended through Avc?lar. Indeed, Avc?lar and ?zmit, both located on the Marmara Sea coast along the rupture route, were strongly struck by the earthquake whereas the settlements between Avc?lar and ?zmit were much less affected. Therefore, this interpretation can explain the extraordinary damage in Avc?lar, based on the newly discovered rupture of the NAFZ in the Marmara Sea. However, this suggestion needs to be confirmed by further seismological studies.     

Evidence of NW extension of the North Anatolian Fault Zone in the Marmara Sea: a new interpretation of the Marmara Sea (Izmit) earthquake on 17 August 1999

Active faults aligning in NW–SE direction and forming flower structures of strike-slip faults were observed in shallow seismic data from the shelf offshore of Avcılar in the northern Marmara Sea. By following the parallel drainage pattern and scarps, these faults were traced as NW–SE-directed lineaments in the morphology of the northern onshore sector of the Marmara Sea (eastern Thrace Peninsula). Right-lateral displacements in two watersheds of drainage and on the coast of the Marmara Sea and Black Sea are associated with these lineaments. This right-lateral displacement along the course of these faults suggests a new, active strike-slip fault zone located at the NW extension of the northern boundary fault of the ?ınarcık Basin in the Marmara Sea. This new fault zone is interpreted as the NW extension of the northern branch of the North Anatolian Fault Zone (NAFZ), extending from the ?ınarcık Basin of the Marmara Sea to the Black Sea coast of the Thrace Peninsula, and passing through B üy ük ?ekmece and K ü ? ük ?ekmece lagoons. These data suggest that the rupture of the 17 August 1999 earthquake in the NAFZ may have extended through Avcılar. Indeed, Avcılar and İzmit, both located on the Marmara Sea coast along the rupture route, were strongly struck by the earthquake whereas the settlements between Avcılar and İzmit were much less affected. Therefore, this interpretation can explain the extraordinary damage in Avcılar, based on the newly discovered rupture of the NAFZ in the Marmara Sea. However, this suggestion needs to be confirmed by further seismological studies.     

Morpho-tectonic evolution of the Marmara Sea inferred from multi-beam bathymetric and seismic data

In an initial stage, the Sea of Marmara developed as a graben and, in due course, considerable volumes of sediments were deposited in this basin. Before 200 ka, a new fault (New Marmara Fault) cutting through the whole basin developed, which postdated large sub-marine land sliding in the western part of the basin. This mass movement created the Western Ridge. The initiation of this strike-slip fault indicates that the extensional stress regime was replaced by a new, shearing stress field. In the eastern part of the Marmara Basin, the New Marmara Fault consists of two branches. The northern one replaces the normal faulting at the bottom of the northeastern slope of the basin. As a result, this slope has been rejuvenated. The southern branch is located along the central axis of the basin, forming the major extension of the North Anatolian Fault Zone within the region. Two restraining bends were formed because of the counterclockwise rotation of that part of the Anatolian Block. This resulted the uplifting of the Eastern Ridge and the formation of the positive flower structure within the Tekirdag Basin. The establishment of the compressional regime around the Sea of Marmara also resulted in the northwest–southeast shortening of the initial Marmara Basin.     

Acoustic monitoring using bamboo set net in the Southern Sea of Korea

High-temporal resolution profiles of acoustic backscatter were collected from a traditional bamboo set net along the coast of the Southern Sea, Korea, using sideward-looking multi-beam imaging sonar. These data were used to examine the impact of variations in tidal cycles and current speeds on the bamboo set net. The relatively high influx of fish during the nighttime compared to the low influx and high outflux of fish during the daytime suggests visual avoidance of the net by the fish during the daytime. The observed diel variation in the captured fish flux was significantly correlated with the current speed (day: r = 0.35, p = 0.002, night: r = 0.60, p < 0.001). The ratio of influx and outflux of fish, and current speed were correlated in a linear relationship (day: r = 0.45, p < 0.001; night: r = 0.56, p < 0.001). The fish activity of those inhabiting the bamboo set nets appears to be greatly influenced by day-night differences and current speed. The present study enhances understanding of fish behavior via utilization of a bamboo set net in the coastal zone.     

Postglacial floodings of the Marmara Sea: molluscs and sediments tell the story

The early Holocene marine flooding of the Black Sea has been the subject of intense scientific debate since the “Noah’s Flood” hypothesis was proposed in the late 1990s. The chronology of the flooding is not straightforward because the connection between the Black Sea and the Mediterranean Sea involves the intermediate Marmara Sea Basin via two sills (Dardanelles and Bosphorus). This study explores the chronology of late Pleistocene–Holocene flooding by examining sedimentary facies and molluscs from 24 gravity cores spanning shelf to slope settings in the southern Marmara Sea Basin. A late Pleistocene Ponto-Caspian (Neoeuxinian) mollusc association is found in 12 of the cores, comprising 14 mollusc species and dominated by brackish (oligohaline–lower mesohaline) endemic taxa (dreissenids, hydrobiids). The Neoeuxinian association is replaced by a Turritella–Corbula association at the onset of the Holocene. The latter is dominated by marine species, several of which are known to thrive under dysoxic conditions in muddy bottoms. This association is common in early Holocene intervals as well as sapropel intervals in younger Holocene strata. It is an indicator of low-salinity outflows from the Black Sea into the Marmara Sea that drive stratification. A marine Mediterranean association (87 species) represents both soft bottom and hard substrate faunas that lived in well-ventilated conditions and upper mesohaline–polyhaline salinities (ca. 25 psu). Shallower areas were occupied by hard substrate taxa and phytopdetritic communities, whereas deeper areas had soft bottom faunas. The middle shelf part of the northern Gemlik Gulf has intervals with irregular and discontinuous sedimentary structures admixed with worn Neoeuxinian and euryhaline Mediterranean faunas. These intervals represent reworking events (slumping) likely related to seismic activity rooted in the North Anatolian Fault system. The core data and faunas indicate an oscillating postglacial sea-level rise and phases of increased/decreased ventilation in the Marmara Sea during the Holocene, as well as palaeobiogeographic reorganisations of Ponto-Caspian and Mediterranean water bodies since the latest Pleistocene (<30 ka). The findings contribute to arguments against a single catastrophic flooding of the Black Sea at about 7.5 ka (Noah’s Flood).     

Estimation of seismic velocities and gas hydrate concentrations: A case study from the Shenhu area,northern South China Sea

In the Shenhu area of the northern South China Sea (SCS), canyon systems and focused fluid flow systems increase the complexity of the gas hydrate distribution in the region. It also induces difficulties in predicting the hydrate reservoir characteristics and quantitatively evaluating reservoir parameters. In this study, several inversion methods have been executed to estimate the velocities of strata and gas hydrate concentrations along a profile in the Shenhu area. The seismic data were inverted to obtain the reflection coefficient of each stratum via a spectral inversion method. Stratigraphic horizons were then delineated by tracking the inverted reflectivities. Based on the results of spectral inversion, a low-frequency velocity field of the strata was constructed for acoustic impedance inversion. Using a new iterative algorithm for acoustic impedance inversion, reflection coefficients were converted into velocities, and the velocity variations of the strata along a 2D seismic line were then obtained. Subsequently, gas hydrate saturations at well SH2 were estimated via the shale-corrected resistivity method, the chloride ion concentration method and three different rock physics models. The results were then compared to determine the optimal rock physics model, and the modified Wood equation (MWE) was found to be appropriate for this area. Finally, the inverted velocities and MWE were used to predict the distribution and concentrations of gas hydrates along the seismic line. The estimated spatial distribution of gas hydrates is consistent with that from sonic logging and resistivity data at well SH2, and with the drilling results. Therefore, this method is applicable in areas with no well data, or with few wells, and provides an effective tool for predicting and evaluating gas hydrates using seismic data.     

Tectonics related to the North Anatolian Fault in the Sea of Marmara: evidence from seismic reflection data

The North Anatolian Fault crosses the Sea of Marmara from east to west. Tectonic features of the Sea of Marmara were studied using multi-channel deep seismic reflection data. The northern branch of the North Anatolian Fault is active as a right lateral strike-slip fault zone and indicates both negative and positive flower structures. The North Anatolian Fault splays into two faults at the Sea of Marmara as a northern branch and north segment of the southern branch. The northern branch named the Main Marmara Fault extends in a complicated manner from the north of the Kapıdağı Peninsula to westward in the Sea of Marmara. The north segment of southern branch extends between the Gemlik and Bandırma gulfs in the south of the Sea of Marmara. In addition, uplift areas arose by compression and a push-up style in between the Kapıdağı Peninsula and the Main Marmara Fault. The North Anatolian Fault is characterized by a negative flower structure in basins and push-up style in uplift areas in the Sea of Marmara. An uplift area arose between the north segment of the southern branch and the northern branch of the North Anatolian Fault. The north segment of the southern branch of the North Anatolian Fault is a strike-slip fault and displays a pull-apart style in the seismic reflection data.     

Bohai Sea oil spill model: a numerical case study

An operational Bohai Sea oil spill serving module (BSOSSM) that can provide users with trajectory and movement information of the released oil is developed for the purpose of informing mitigation of oil spill incidents in the Bohai Sea, China. BSOSSM is one of the serving modules that had been integrated in China digital ocean prototype system, a marine information platform for managing, displaying and disseminating all the data investigated by China 908 Program. The oil spill trajectory is calculated by an oil spill model (OSM), which serves as a component in BSOSSM. The impacts of wind, current, as well as Stokes drift on oil spill trajectory are studied by sensitivity experiments conducted using OSM. Simulation results indicate that wind forcing is the most important factor in controlling the oil trajectory at the sea surface in Bohai Sea, whereas current and Stokes drift play relatively less important roles. However, because the direction of waves generally follows that of the wind, Stokes drift does lead to an increase in oil drift and spreading velocity. Case studies of the Penglai 19-3 oil spill incident (June 2011) and Xingang oil spill (April 2005) demonstrate that OSM can generally reproduce the oil spreading, and is therefore capable of supporting the emergency response of future oil spills in the Bohai Sea through BSOSSM.     

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.     

A multi-national,multi-parameter marine study of the Western Solomon Sea and region—Part II

In 1983–84, a cooperative geological/geophysical program was carried out in the western Solomon Sea and northeastern Bismarck Sea on the Japanese vesselNatsushima. Scientists representing Japan, Australia, Papua New Guinea, and the regional marine geoscience organization CCOP/SOPAC participated in the study. The first papers were published inGeo-Marine Letters, Volume 6, No. 4. This issue, containing six papers, is the second on the results of that work and the final of the special issues on the Western Solomon Sea and Region.     

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.