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Natural seafloor depressions, known as pockmarks, are common subaqueous geomorphological features found from the deep ocean trenches to shallow lakes. Pockmarks can form rapidly or over millions of years and have a large variety of shapes created and maintained by a large variety of mechanisms. In the sandy sediments of the southeastern North Sea, abundant shallow pockmarks are ubiquitous and occur at shallow water depths (<50 m). Their formation has previously been linked to methane seepage from the seafloor. Here, we characterize over 50,000 pockmarks based on their morphology, geochemical signature, and the subsurface pre‐conditions using a new integrated geoscientific data set, combining geophysical and sedimentological data with geochemical porewater and oceanographic analysis. We test whether the methane seepage is indeed responsible for pockmark formation. However, our data suggest that neither the seepage of light hydrocarbons nor groundwater is driving pockmark formation. Because of this lack of evidence for fluid seepage, we favor the previously suggested biotic formation but also discuss positive feedback mechanisms in ocean bottom currents as a formation process. Based on a comparison of pockmarks to the central and southeastern North Sea, we find that local lithology significantly affects pockmark morphology. Muddy lithologies favor the formation of larger, long‐lived structures, while sandy lithologies lead to short‐lived, small‐scale structures that are large in area but with shallow incision depth. We conclude that pockmarks in sandy environments might have been overlooked globally due to their shallow incision depth and recommend reevaluating the role of hydrocarbon ebullition in pockmark formation. Plain Language Summary: Pockmarks are natural depressions on the seafloor, found everywhere from deep oceans to shallow lakes. These underwater features can form quickly or over millions of years, sometimes reaching impressive sizes—tens of meters deep and kilometers wide. Pockmarks are created by different processes depending on where they are found, but they are often linked to the release of gases such as methane from the seafloor. In the southeastern North Sea, thousands of shallow pockmarks have been discovered in sandy areas less than 50 m deep. Initially thought to be caused by methane gas leaks, our research shows this is unlikely. We studied over 50,000 pockmarks using geological, chemical, and oceanographic data and found little evidence that methane or groundwater seepage is responsible. Instead, these pockmarks are more likely created by either marine animals such as harbor porpoises or turbulent ocean currents. We also found that the grainsize of the sediment greatly affects pockmark morphology and lifespan. In muddy areas, pockmarks are larger and longer‐lasting, whereas in sandy areas, they are shallower and shorter‐lived. Our findings suggest that many pockmarks in sandy environments may have been overlooked due to their shallow depth. We recommend reconsidering the role of methane in pockmark formation. Key Points: Pockmarks in the German Bight form in sandy environments and are likely not related to fluid ventingPockmarks in sandy sediments are shallow in depth, large in area and short‐lived structures compared to muddy host sedimentsDespite the widespread occurrence of pockmarks in the southeastern North Sea, the processes leading to their formation remain unclear [ABSTRACT FROM AUTHOR]

Abstract Natural seafloor depressions, known as pockmarks, are common subaqueous geomorphological features found from the deep ocean trenches to shallow lakes. Pockmarks can form rapidly or over millions of years and have a large variety of shapes created and maintained by a large variety of mechanisms. In the sandy sediments of the southeastern North Sea, abundant shallow pockmarks are ubiquitous and occur at shallow water depths (

Recent surface ship multibeam surveys of the Sur Pockmark Field, offshore Central California, reveal >5,000 pockmarks in an area that is slated to host a wind farm, between 500‐ and 1,500‐m water depth. Extensive fieldwork was conducted to characterize the seafloor environment and its recent geologic history, including visual observations with remotely operated vehicles, sediment core sampling, and high‐resolution, near‐bottom Chirp and multibeam surveys collected with autonomous underwater vehicles to capture the morphology and stratigraphy of the pockmarks. No evidence of high methane concentrations in sediments, chemosynthetic biological communities, or methane‐derived diagenetic byproducts was found. Chirp data and sediment cores showed alternating layers of slowly accumulating hemipelagic drapes interrupted by more reflective turbidite horizons that extend throughout the pockmark field and beyond. Chirp data showed multiple episodes of lateral migration over time in some of the pockmarks in association with erosion and infilling events. Laterally continuous turbidite horizons that overlay erosional surfaces indicated that pockmark migration occurred synchronously in multiple pockmarks separated by tens of kilometers. These shifts are presumed to be the result of asymmetrical erosion of the pockmark flanks caused by passing sediment gravity flows. While some pockmarks occur in chains, most are not clustered or randomly spaced but are regularly dispersed within the pockmark field. We hypothesize that intermittent, unconfined sediment gravity flows occurring over at least the last 280,000 years are the source of the regionally continuous turbidite deposits and the mechanism that maintained the regularly dispersed pockmarks. Plain Language Summary: Over 5,000 pockmarks were mapped in water depths between 500 and 1,500 m offshore Central California in an area slated to host a wind farm. Pockmarks are large circular seafloor depressions commonly believed to be formed and maintained by methane flux. Video footage from the pockmark field, detailed maps of the seafloor, sediment cores, and images of the subsurface were collected. Importantly, no significant evidence of methane gas was found in any of the collected data sets. Instead, the subsurface profiles and sediment samples indicated that the pockmarks contain layers of fine sediment deposited slowly over time alternating with sandy layers deposited by large sediment flows. These flows appear to erode the pockmark centers, and leave behind regionally continuous sandy deposits (turbidites). Some sediment flows have caused sufficient erosion to the pockmarks flanks to simultaneously shift the position of multiple pockmarks, 10s of kilometers apart. Most of the pockmarks are not randomly scattered or clustered on the seafloor but are distributed in a regular, equally spaced pattern. We hypothesize that intermittent, unconfined sediment gravity flows occurring over at least the last 280,000 years are the source of the regionally continuous turbidites and the mechanism that maintained the regularly spaced pockmarks over time. Key Points: No evidence of methane venting was found in the Sur PockmarksThe persistence and lateral migration of the Sur Pockmarks are attributed to erosion and deposition during unconfined sediment gravity flowsRecurrent, regional, and unconfined sediment gravity flows have occurred in the Sur Pockmark Field for at least ∼280,000 years [ABSTRACT FROM AUTHOR]

Pockmarks are morphological depressions commonly observed in ocean and lake floors. Pockmarks form by fluid (typically gas) seepage thorough a sealing sedimentary layer, deforming and breaching the layer. The seepage‐induced sediment deformation mechanisms, and their links to the resulting pockmarks morphology, are not well understood. To bridge this gap, we conduct laboratory experiments in which gas seeps through a granular (sand) reservoir, overlaid by a (clay) seal, both submerged under water. We find that gas rises through the reservoir and accumulates at the seal base. Once sufficient gas over‐pressure is achieved, gas deforms the seal, and finally escapes via either: (a) doming of the seal followed by dome breaching via fracturing; (b) brittle faulting, delineating a plug, which is lifted by the gas seeping through the bounding faults; or (c) plastic deformation by bubbles ascending through the seal. The preferred mechanism is found to depend on the seal thickness and stiffness: in stiff seals, a transition from doming and fracturing to brittle faulting occurs as the thickness increases, whereas bubble rise is preferred in the most compliant, thickest seals. Seepage can also occur by mixed modes, such as bubbles rising in faults. Repeated seepage events suspend the sediment at the surface and create pockmarks. We present a quantitative analysis that explains the tendency for the various modes of deformation observed experimentally. Finally, we connect simple theoretical arguments with field observations, highlighting similarities and differences that bound the applicability of laboratory experiments to natural pockmarks. Plain Language Summary: Pockmarks are pit‐like depressions common in ocean and lake floors, formed by gas seepage through underlying sediments. Despite relevance to both fossil fuel exploration and global warming, the mechanisms by which pockmarks evolve remain elusive. We conduct simple laboratory experiments in which we inject air into a layer of glass beads ("reservoir") overlain by a layer of clay ("seal"), all submerged underwater in a transparent box. We find that gas rises through the sand and accumulates at the base of the clay. Then, gas pressure rises until it suffices to deform the clay and escape, forming a pockmark. This occurs by one of three mechanisms, depending on clay thickness and stiffness: (a) heaving of a dome which then fractures in thin clay layers; (b) faulting in thick, stiff clays; and (c) bubbles ascend in thick, soft clays. Pipe‐like focused gas conduits connecting the clay bottom to the pockmark are created by the rise of a "trains" of bubbles that weaken their path. These pathways can also initiate in faults. Repeated seepage events push the clay particles, suspending them in water to create a pockmark. Our findings agree with field observations, improving our understanding of natural pockmark formation. Key Points: Sandbox experiments link pockmark morphology (irregular vs. conical) to gas seepage‐induced deformation of the host (seal) layerExperiments and theory show seal thickness and consolidation control deformation mechanism: doming, brittle (faults), or plastic (bubbles)Theoretical calculations predict that under field conditions, the preferred mechanism for gas escape will be bubbles rising in faults [ABSTRACT FROM AUTHOR]

The article presents the results of seismoacoustic studies carried out in Petrozavodsk Bay of Lake Onega. The goal was to study a near-surface geological section. Seismoacoustic studies were done by the reflection method in a modified ultra-high-resolution seismic survey. The data were office processed and interpreted: five seismic complexes were identified and correlated with information on the lake's structure, culminating in maps of all five identified reflectors covering all of Petrozavodsk Bay. It was possible to estimate the abundance and thickness of sediments corresponding to each complex. [ABSTRACT FROM AUTHOR]

The newly obtained results of the frequency-resonance (FR) technologies application for the study of the sources and processes of seeps and pockmarks formation in the Arctic Region (Norwegian, Barents Sea) and the North Sea are considered. The experimental study of the seepage processes' source formation was carried out using modified methods of the FR-processing, and decoding of satellite images and photographs with the vertical scanning of the cross-sections. The study results of the methane seeps and pockmarks fields show that the intensity and dynamics of their formation partly depend on the inflow of gas fluids from deep sources because of active degassing processes. The use of FR-sounding technologies allows us to remotely determine the spatial regularities of zones of intense gas emission formation and the geological sources of migration probable depth. The study of seep and pockmark field results confirms the significant, but insufficiently taken into account, the influence of crust-mantle gas fluids on the nature and degassing processes features in the structures of the continental margins. New results confirm the crust-mantle gas fluids influence on the nature and degassing processes features in the scan points of Polar marginal structures. These data are important arguments in favor of the "volcanic model" of various structural elements formation in this Region. The FR- technologies data also showed a possibility of seeps and pockmarks use as shallow and deep hydrocarbon field indicators in gas emission areas. These independent data can be used in modeling the deep lithosphere structure and possible mechanisms of abiogenic hydrocarbon formation in Arctic margin structures. We suppose that hydrocarbons through deep channels migrate (from 57 km deep) to the upper crustal horizons where can form hydrocarbon fields. During this migration, gas seeps and pockmarks are formed on the sea bottom and part of the gas can migrate into the atmosphere. Modern degassing centers' studies have confirmed the presence of zones of active migration of gases into the atmosphere, which is an important influencing factor in the dynamics of global climate change processes on Earth. The experience of FR- technologies used in the fields of methane seeps and pockmarks study found in the structures of the Arctic Polar seas showed versatility and the possibility of their use in the study of large gas emissions in structures of different regions.

The results of the study of the effect of low concentration of anti-corrosion additives in the coolant on corrosion, scaling and pitting of cooling jackets of the cylinder block of the internal combustion engines of the gas-diesel mining dump trucks are presented. The negative influence of the presence of coolant in the engine oil system has been studied. It is argued that even an insignificant concentration of coolant in the engine oil leads to cavitation with the concomitant occurrence of caverns or significant and deep pits—pockmarks—on especially stressed parts of the engine (crank-connecting rod liners, camshaft bushings). The conducted studies allow concluding that the coolant, in the presence of an insufficient concentration of anti-corrosion additives in it, as well as when it penetrates the oil system, has a sufficiently significant effect on the wear and destruction of the surface layer of the internal combustion engine parts. [ABSTRACT FROM AUTHOR]

Focused fluid flow through sub-seafloor pipes and chimneys, and their seafloor manifestations as pockmarks, are ubiquitous. However, the dynamics of flow localization and evolution of fluid escape structures remain poorly understood. Models based on geomechanical mechanisms like hydro-fracturing and porosity wave propagation offer some useful insights into fluid flow and escape dynamics, but face limitations in capturing features like mobilized granular matter, especially in the upper sediment layers where the link between fracture and pockmark is not always clear. Here, we propose a mathematical model based on the multiphase theory of porous media, where changes in subsurface and seafloor morphology are resolved through seepage-induced erosion, fluidization, transport, and re-deposition of granular material. Through simulation of an idealized scenario of gas escape from overpressured shallow gas reservoir, we demonstrate that our model can capture flow localization and formation of pipes, chimneys, and pockmarks. Our simulations show (1) formation of conical focused-flow conduits with a brecciated core and annular gas channels; (2) pockmarks of W and ring shapes; and (3) pulsed release of gas. Sediment erodibility and flow anisotropy control the morphology of focused fluid flow and escape structures, while permeability shows negligible impact. While the geological setting for this study is theoretical, we show that our results have real-world analogs. [ABSTRACT FROM AUTHOR]

The Patras Gulf Pockmark field is located in shallow waters offshore Patras City (Greece) and is considered one of the most spectacular and best-documented fluid seepage activities in the Ionian Sea. The field has been under investigation since 1996, though surveying was partially sparse and fragmentary. This paper provides a complete mapping of the field and generates new knowledge regarding the fluid escape structures, the fluid pathways, their origin and the link with seismic activity. For this, data sets were acquired utilising high-resolution marine remote sensing techniques, including multibeam echosounders, side-scan sonars, sub-bottom profilers and remotely operated vehicles, and laboratory techniques focusing on the chemical composition of the escaping fluids. The examined morphometric parameters and spatial distribution patterns of the pockmarks are directly linked to tectonic structures. Acoustic anomalies related to the presence of gas in sediments and in the water column document the activity of the field at present and in the past. Methane is the main component of the fluids and is of microbial origin. Regional and local tectonism, together with the Holocene sedimentary deposits, appear to be the main contributors to the growth of the field. The field preserves evidence that earthquake activity prompts the activation of the field. [ABSTRACT FROM AUTHOR]

The results of detailed geological and geomorphological studies in the waters of the Pechora Sea using a complex of hydroacoustic methods during the 52nd cruise of R/V Akademik Nikolaj Strakhov revealed numerous gas funnels (pockmarks) at the bottom of the Southern Novaya Zemlya Trench. These forms are widespread over a considerable part of the bottom of the Barents Sea, but for the Pechora shelf, there is little information about their distribution. The analysis of the seismoacoustic sections at the sites of development of gas funnels showed the presence of vertical chimneys in unconsolidated sediments of the Southern Novaya Zemlya Trench, which allow gas to rise to the bottom surface and to escape into the water column with the formation of negative landforms, which is accompanied by the detachment of bottom sediments. [ABSTRACT FROM AUTHOR]

We investigated fluid seepage within the Nyegga pockmark field (600–900 m water depths) off mid-Norway from Remotely Operated Vehicle dives at the so-called CNE sites (CNE01 to CNE17). The seafloor morphology of some of these features corresponds to pockmarks and adjacent ridges, with the latter being the focus of present seepage activity. These structures are underlain by chimneys above a gas-charged zone with, in some cases, a substantial body of hydrate-invaded sediment (down to 1.3 s in two-way travel time at CNE03). Present-day methane-rich fluid seepage through the seabed is indicated by chemosynthetic fauna, in particular Siboglinidae polychaetes (Oligobrachia haakonmobiensis webbi and Sclerolinum contortum), microbial mats and associated Rissoidae gastropod (Alvania sp.) grazers, and confirmed by measured in situ bottom-water methane anomalies, up to 2,130 nL/L. No free-gas bubble emissions were observed or acoustically identified. The presence of authigenic carbonates reveals past seepage with very low δ13C values (down to −58‰) indicating that the major source of carbon was methane carried by the venting fluids. The ages of major periods of methane venting are provided by vesicomyid bivalve shells (Isorropodon nyeggaensis) present in two sedimentary layers, 14,930 and 15,500 14C yr BP (ca. 17,238 and 17,952 cal yr BP), respectively, corresponding to the time of Melt Water Pulse IA. The seafloor morphology and pattern of seepage -chemosynthetic fauna and microbial mat distribution and dissolved methane concentration-are remarkably heterogeneous. Pore-water chemistry profiles in a gravity core taken only 40 m from major seepage sites indicate no seepage and anaerobic methane oxidation at a sub-bottom depth of about 2 m. Present-day seepage from the studied pockmark-chimney fluid-flow system charged with gas hydrate is dominated by the advection of methane solution in pore water. Some of this methane could result from the dissolution of hydrate in the chimney, most of which would have formed during an earlier period (post-LGM times) of history of the chimney, when it was venting free gas. However, the presence of free gas beneath this chimney is probably why the water entering the chimney is already saturated with methane and the process of hydrate formation in the chimney continues today.

The Baltic Basin is known for its numerous Paleozoic hydrocarbon reservoirs. There is published evidence that hydrocarbons are leaking from the seafloor, however, little is known about the hydrocarbon migration pathways from Paleozoic source and reservoir rocks toward the seafloor and their escape structures. To investigate these processes, we utilize a new set of multibeam, parametric sediment sub‐bottom profiler and 2D seismic reflection data. The integrated analysis of seismic profiles, diffraction imaging and bathymetric maps allow to identify a hydrocarbon migration system within Silurian and Devonian strata that consists of layer parallel and updip migration beneath sealing layers, migration across seals along faults, and seafloor escape structures in form of elongated depressions. The general migration trend is directed updip, from the Paleozoic reservoirs below the southeastern Baltic Sea toward the Gotland Depression in the northwest. The locations of the hydrocarbon escape structures at the seafloor and their elongated shape are mainly controlled by the regional geological setting of outcropping Paleozoic layers. In addition, iceberg scouring may have facilitated hydrocarbon migration through the Quaternary deposits. The description of this hydrocarbon migration system fills the gap between the known reservoirs and the observed hydrocarbon accumulations and seepages. With regard to potential Carbon Capture and Storage projects, the identification of this hydrocarbon migration system is of great importance, as potential storage sites may be leaking. Plain Language Summary: The Baltic Basin including the Baltic Sea is well known for its hydrocarbon reservoirs with ongoing oil production since the 1940s. While there is some published evidence that hydrocarbons are leaking from the seafloor, little is known about the pathways from the reservoirs toward theses leakages. In this study, we use three imaging techniques for the seafloor, the uppermost sediments and the first few kilometers of the subsurface to image the hydrocarbon migration pathways and their escape structures. We find that hydrocarbons are migrating along dipped geological layers from the reservoirs in the southeast toward the Gotland Deep in the northwest. Additionally, we also observe that hydrocarbons are penetrating through these geological layers at locations of pre‐existing small‐scale fractures. The locations, at which the hydrocarbons escape from the seafloor, are mainly controlled by the regional tectonic setting. In addition, iceberg scouring may have had an influence on the exact escape locations. With our findings in this study, we fill the gap between the known reservoirs and the observed seepages and can contribute to questions regarding the potential storage of CO2 in the Baltic Basin. Key Points: Numerous elongated fluid escape depressions are observed at the eastern margin of the Gotland Deep, central Baltic SeaFirst evidence for fluid migration pathways from Paleozoic toward Quaternary strata in the regionLocations of fluid escape is controlled by the regional tectonic setting [ABSTRACT FROM AUTHOR]

Many features indicative of natural gas and oil leakage are delineated in the deep-water Orange Basin offshore South Africa using 3D reflection seismic data. These features are influenced by the translational and compressional domains of an underlying Upper Cretaceous deep-water fold-and-thrust belt (DWFTB) system detaching Turonian shales. The origin of hydrocarbons is postulated to be from both: (a) thermogenic sources stemming from the speculative Turonian and proven Aptian source rocks at depth; and (b) biogenic sources from organic-rich sediments in the Cenozoic attributed to the Benguela Current upwelling system. The late Campanian surface has a dense population of > 950 pockmarks classified into three groups based on their variable shapes and diameter: giant (> 1500 m), crater (~ 700–900 m) and simple (< 500 m) pockmarks. A total of 85 simple pockmarks are observed on the present-day seafloor in the same area as those imaged on the late Campanian surface found together with mass wasting. A major slump scar in the north surrounds a ~ 4200 m long, tectonically controlled mud volcano. The vent of the elongated mud volcano is near-vertical and situated along the axis of a large anticline marking the intersection of the translational and compressional domains. Along the same fold further south, the greatest accumulation of hydrocarbons is indicated by a positive high amplitude anomaly (PHAA) within a late Campanian anticline. Vast economical hydrocarbon reservoirs have yet to be exploited from the deep-water Orange Basin, as evidenced by the widespread occurrence of natural gas/fluid escape features imaged in this study. [ABSTRACT FROM AUTHOR]

This is the first high-resolution seismic study showing how the Chicxulub impact shaped the eastern slope of the Campeche Bank in the south-eastern Gulf of Mexico. The induced shock wave fractured Cretaceous strata causing the collapse of the upper slope and shelf over a length of ca. 200 km. Failed material was either transported downslope or remained in parts on the accommodation space created by the collapsed. In the Cenozoic, the East Campeche Plastered Drift developed within the created accommodation space, controlled by the inflowing surface current from the Caribbean, which forms the Loop Current. The internal reflection configuration of the drift shows that the closure of the Suwannee Strait in the Late Oligocene and the closure of the CAS in the Mid to Late Miocene controlled the variability of the southern Loop Current in time. Since the Loop Current transports heat and moisture from the western Atlantic warm water pool into the North Atlantic and further to NW Europe by the Gulf Stream, the drift represents an archive for controlling factors that influenced climate of the northern hemisphere. This first high-resolution seismic reflection study from the eastern Campeche Bank expands the understanding of destructive processes that a meteorite impact induces into the earth system. Furthermore, these data document that the East Campeche Plastered Drift bears the potential to understand the link between the climate variability of the northern hemisphere and oceanic processes in the equatorial western Atlantic. [ABSTRACT FROM AUTHOR]

Seafloor pockmarks are abundant around Aotearoa New Zealand, occurring across a diverse range of tectonic, sedimentological and geomorphological settings. Globally, the formation and source of pockmarks is widely researched because they: 1) have potential links to subsurface hydrocarbon systems, 2) can provide important habitats for benthic organisms and 3) may be indications of fluid escape pathways or areas of sediment disturbance, which influence seafloor stability and could pose a risk to infrastructure. Pockmarks are widely associated with fluid release (such as gas or water) from subsurface reservoirs. However, the formation of pockmarks, the processes that shape and modify their morphology over time, and the relative timing of these events, remains enigmatic. Here, we compile the first national database of over 30,000 pockmarks around Aotearoa New Zealand, allowing us to begin to comprehend the dynamic processes that shape and affect pockmarks by exploring regional and inter-regional patterns in pockmark geometry and seabed characteristics. This compilation reveals several significant trends, including a distinct lack of correlation between active seafloor seeps and pockmarks, and a strong association of pockmarks with mud-rich seafloor substrate. Furthermore, we highlight key knowledge gaps that require further investigation moving forward, including a lack of constraint on the timing of pockmark formation, and limited modelling of the processes involved in their formation.

Fresh submarine groundwater discharge (FSGD) influences the biogeochemistry of coastal areas and can be a proxy for potential untapped resources of offshore freshened groundwater (OFG). In most areas however, the onshore-offshore connection and the recharge characteristics of offshore aquifers are poorly constrained, making a potential exploitation of this resource challenging. Offshore Wellington (New Zealand), a well-defined onshore aquifer system extends beneath the harbour, where substantial amounts of freshwater seep out from the ocean floor. The aquifer system has been studied in detail and recently the first attempts worldwide have been made here to use the offshore groundwater as a future source of drinking water. However, the locations and extent of FSGD as well as its influence on seafloor morphology are still poorly understood. Exact localisation of FSGD sites is essential to sample and quantify discharging waters but remains challenging due to a lack of robust and appropriate measurement procedures. Novel sensing strategies, such as the influence of seeping groundwater on hydroacoustic water column reflectivity could greatly improve the identification of groundwater discharge locations worldwide. Therefore, we use a multidisciplinary dataset and evaluate different methodologies to map the spatial extent of FSGD sites and determine their geomorphologic expressions on the seafloor of Wellington Harbour. In this study, single and multibeam hydroacoustics and towfish (temperature, salinity and turbidity) transects were combined with remotely operated vehicle (ROV) dives and sediment cores to better characterise FSGD sites. We observed several hundred seafloor depressions (pockmarks) that we attribute to continuous seepage of gas and groundwater from the seafloor. Different pockmark morphologies indicate different fluid flow regimes and the persistent flow allows even small pockmarks to remain unchanged over time, while the geomorphologic expressions of anchor scours on the seafloor diminish in the same region. Enhanced hydroacoustic reflections in the water column within and above the pockmarks indicate suspended sediment particles, which are likely kept in suspension by discharging groundwater and density boundaries.

Hydroacoustic observations of shallow marine environments reveal a variety of seafloor structures–both of natural and anthropogenic origin. Natural processes can result in features with circular geometries on the seafloor, such as kettles, sinkholes or iceberg pits, but human activities such as dredging, dumping, or detonating explosives can also cause similar shapes. Explaining the origin of these features is difficult if there are only few observations or if competing natural and anthropogenic processes have acted in the same area. Even though the location of dredging and dumping operations and munition blasting may be well documented in many parts of the global coastal ocean today, little information might be available about human practices in the past. In this study, more than 3,000 circular features were identified in side-scan sonar (SSS) datasets covering 1,549 km2 of shallow waters in the southwestern Baltic Sea. Additional data obtained by multibeam echosounder (MBES), sub-bottom profiler (SBP), and different SSS was considered in the analysis of 205 circular features that were characterized based on their sedimentology, morphology, SSS and SBP acoustic signatures. Characteristic differences between the structures allow their classification into six classes, which provide insight into their formation mechanisms. The obtained parameters (morphology, MBES and SSS acoustic backscatter, SBP characteristics and spatial distribution) allow the classification to be applied to the entire SSS dataset, resulting in the classification of 2,903 features. The mapped circular features have diameters between 6 and 77 m and correspond to pockmarks, dumping spots and explosion craters in water depths ranging from 8 m up to 25 m. Despite this rather multi-methodological approach, the origin of some observed features still cannot be explained with certainty, leaving room for further investigations of natural processes and human impacts on the seafloor.

Marine Geological Institute acquired high-resolution seismic and acoustic data during a survey in the Waipoga Trough-West Papua in 2018. The profiles show acoustically blanking, turbid, and cloudy occur along the trough floor over 3,000 km2. Many accoustic chimneys are breaking through the seabed of recent and subrecent sediment layers. It is suggested due to the local expulsion of methane gas. The pockmarks occur as transparent and turbid acoustic characteristics. They locally have relief down to 2-10 m below the seafloor with horizontal dimensions up to 25 -100 m in width. Methane formation due to the rapid deposition of organic-rich sediment down the uplifted area around the trough, associated with thrust fold tectonics is suggested as a triggering agent for the acoustic anomaly formation. [ABSTRACT FROM AUTHOR]

The new results of the frequency-resonance technologies application for the study of the sources and processes of seeps and pockmarks formation in the Arctic and the North Sea are considered. The study of the methane seeps and pockmarks fields shows that the intensity and dynamics of their formation depend on the inflow of gas fluids from deep sources because of a result of active degassing processes. The use of FR-sounding technologies allows us to remotely determine the spatial regularities of intense gas emission zone formation and the probable depth of geological sources of migration. The seep and pockmark field study results confirm the crust-mantle gas fluids' influence on the nature and degassing processes features in the structures of the continental margins. Modern degassing centers' studies have confirmed the presence of zones of active migration of gases into the atmosphere, which is an important factor in the dynamics of global climate change processes. The experience of FR- technologies used in the fields of methane seeps and pockmarks study in the structures of the Arctic polar seas showed their versatility and the possibility of their use in the study of gas emissions in structures of different regions. [ABSTRACT FROM AUTHOR]

Depleted chalk oilfields and chalk structures in the Danish Central Graben, North Sea, are potential CO2 storage sites. In most of these fields, the main reservoir is the Upper Cretaceous – Danian Chalk Group and the Eocene – Miocene mudstones of the Horda and Lark Formations constitute the primary seal. In a few fields, the reservoir is composed of the Lower Cretaceous Tuxen and Sola Formations. Here the main seal is assumed to be the Chalk Group which however has poor gas sealing characteristics; the Horda and Lark Formations constitute an efficient secondary seal although they are quite high in the section. This study documents a workflow that may help to evaluate the seal integrity of the structures from an integration of mud gas data from wells with seismic data. Mud gas data provide detailed information about the distribution and types of gas (biogenic or thermogenic) throughout the seal section and overburden. The presence of higher carbon number gases (C3–C5, propane to pentane) in the seal indicates migration of thermogenic gas into the thermally immature sealing mudstones; whereas the dominance of C1 (methane) and partly C2 (ethane) likely reflects the presence of in situ generated biogenic gas in the mudstones, thus indicating that there are no seal integrity issues. The vertical thermogenic gas migration front has been determined, and a "traffic light" indicator system has been used for seal integrity evaluation. Where no or minor migration of thermogenic gas into the primary seal has occurred and a primary seal >30 m thick is present, the seal is considered to have good matrix seal integrity (green). If some significant thermogenic gas migration has occurred into the primary seal but more than 30 m of primary seal is present above the thermogenic gas migration front, the seal integrity is reduced (yellow). In structures where thermogenic gas migration is recorded through the primary seal and into the overburden, seal integrity is considered to be poor (red). In areas where significant leakage of thermogenic gas has occurred into the seal, high density, low porosity carbonate beds frequently occur encapsulated within the sealing mudstones and are interpreted to be composed of methane‐derived authigenic carbonates (MDACs). Seismic data show that there is a convincing correlation between leakage as indicated from mud gas data and the presence of vertical wipe‐out zones (gas chimneys), bright zones (gas‐charged sediments or MDACs), and depressions (pockmarks). In general, potential CO2 storage sites in the study area in tectonically inverted structures show good seal integrity, but this may locally be reduced and require additional analyses. Storage sites associated with salt diapirs generally show poor seal integrity and are likely to be poor candidates for CO2 storage. In combination, mud gas and seismic data are therefore powerful tools to investigate (palaeo‐) leakage phenomena and provide support for seal integrity evaluation at local to regional scales. [ABSTRACT FROM AUTHOR]

We present the results of very high-resolution seismic profiling (VHR) and underwater electrical resistivity tomography (ERT) measurements in the Petrozavodsk Bay of Lake Onega, which were carried out to research the structure of Quaternary sediments. According to the VHR, five seismic sequences were identified during the seismic stratigraphy analysis, and a three-layer geoelectric section was obtained using ERT. The next step was an accomplishment of 2D ERT inversion based on reflectors from the VHR data. The joint interpretation of geophysical data was completed on the basis of comparative and cluster analyses with reference to the well and known geological data. During the research new information was obtained about the physical properties of the bottom sediments. The results indicate the need to apply such an approach to the interpretation of geophysical data and the prospects for the joint application of VHR and ERT acquisitions. [ABSTRACT FROM AUTHOR]

The Patras Gulf Pockmark field is located in shallow waters offshore Patras City (Greece) and is considered one of the most spectacular and best-documented fluid seepage activities in the Ionian Sea. The field has been under investigation since 1996, though surveying was partially sparse and fragmentary. This paper provides a complete mapping of the field and generates new knowledge regarding the fluid escape structures, the fluid pathways, their origin and the link with seismic activity. For this, data sets were acquired utilising high-resolution marine remote sensing techniques, including multibeam echosounders, side-scan sonars, sub-bottom profilers and remotely operated vehicles, and laboratory techniques focusing on the chemical composition of the escaping fluids. The examined morphometric parameters and spatial distribution patterns of the pockmarks are directly linked to tectonic structures. Acoustic anomalies related to the presence of gas in sediments and in the water column document the activity of the field at present and in the past. Methane is the main component of the fluids and is of microbial origin. Regional and local tectonism, together with the Holocene sedimentary deposits, appear to be the main contributors to the growth of the field. The field preserves evidence that earthquake activity prompts the activation of the field.

Abrupt fluid emissions from shallow marine sediments pose a threat to seafloor installations like wind farms and offshore cables. Quantifying such fluid emissions and linking pockmarks, the seafloor manifestations of fluid escape, to flow in the sub‐seafloor remains notoriously difficult due to an incomplete understanding of the underlying physical processes. Here, using a compositional multi‐phase flow model, we test plausible gas sources for pockmarks in the south‐eastern North Sea, which recent observations suggest have formed in response to major storms. We find that the mobilization of pre‐existing gas pockets is unlikely because free gas, due to its high compressibility, damps the propagation of storm‐induced pressure changes deeper into the subsurface. Rather, our results point to spontaneous appearance of a free gas phase via storm‐induced gas exsolution from pore fluids. This mechanism is primarily driven by the pressure‐sensitivity of gas solubility, and the appearance of free gas is largely confined to sediments in the vicinity of the seafloor. We show that in highly permeable sediments containing gas‐rich pore fluids, wave‐induced pressure changes result in the appearance of a persistent gas phase. This suggests that seafloor fluid escape structures are not always proxies for overpressured shallow gas and that periodic seafloor pressure changes can induce persistent free gas phase to spontaneously appear. Plain Language Summary: Thousands of pockmarks, circular depressions in the seafloor, were reported in North Sea, presumably formed in response to wave motions during major storms. It has been hypothesized that these pockmarks formed as pre‐existing shallow free‐gas pockets were mobilized by pressure changes of the waves. However, mechanisms that could have mobilized free‐gas are not yet constrained. Moreover, large scale free‐gas accumulations have not been reported in this region, and therefore, commonly invoked mechanisms like tensile failure and breaching of capillary seals are hard to justify as they rely on the presence of pre‐existing gas pockets. Here, through modeling studies, we tackle the question of the source of the observed free‐gas. Our study consists of two parts: First, assuming that some hitherto unknown shallow free‐gas pocket is indeed present, we test whether storm‐induced pressure changes could breach capillary seals. We find that free‐gas damps pressure changes due to its high compressibility, making the mobilization of pre‐existing gas unlikely. In the second part, we propose an alternative mechanism where free‐gas spontaneously appears due to exsolution from pore‐fluids. We test the feasibility of this mechanism and show how periodic pressure changes can lead to a persistent gas phase, that could explain the elusive gas source linked to these pockmarks. Key Points: Storm‐induced pressure changes can lead to spontaneous appearance of free gas phase near the seafloorThis process is driven by pressure‐sensitive phase instabilitiesThis mechanism could help explain elusive gas sources in recently observed pockmarks in the North Sea [ABSTRACT FROM AUTHOR]

High‐resolution bathymetry and three‐dimensional seismic data along the Cocos Ridge reveal a 245 km2 field of ∼1–4 km in diameter seafloor depressions. The seafloor depressions are part of a two‐tiered honeycomb pattern. The lower‐tier depressions have steep faults that truncate strata with chaotic internal reflections consistent with sediment collapse into the depression. These extend into a lens shaped interval just above igneous basement. Overlying these depressions is a second broader set with rough seafloor morphology with gently dipping boundaries defined by pinch‐out stratigraphic patterns. Drilling results indicate that the lens‐shaped zones that host the deeper depressions represent anomalous regions of high porosity, low velocity, and low density within calcareous rich sediment. Analysis of nannofossils from IODP Site U1414 suggests the collapse structures formed during the late Miocene, whereas the younger shallower depressions likely formed between the early Pliocene and the Pliocene‐Pleistocene boundary. Geochemical and petrological analysis at Site U1414 suggests that hydrothermal circulation during the late Miocene led to carbonate dissolution and collapse. Following collapse, focused fluid‐flow and bottom current scouring resulted in formation of the overlying set of depressions and a honeycomb seafloor morphology. Similar sets of depressions along the Carnegie Ridge to the south support the hypothesis that two‐tiered depressions formed in response to processes that occurred broadly across the Panama Basin between the late Miocene and the Pliocene‐Pleistocene transition. Geochemical results at Site U1414, combined with geophysical data, suggest this two‐tiered system of depressions currently guides ongoing fluid outflow. Plain Language Summary: We characterize a vast field of mega seafloor depressions along the Cocos Ridge using three‐dimensional geophysical data tied to drilling results. The mega depressions display distinct two‐tiered structures that form a honeycomb‐like pattern on the seafloor. By imaging faulting and sediment layers within the depressions, we reveal that the older, steeper‐walled depressions formed through collapse processes, whereas the younger, broader depressions likely formed through a combination of fluid‐flow and current scouring. Drilling results indicate that the older collapses formed during the late Miocene, and geochemical results suggest that hydrothermal circulation, likely along ridge‐parallel faults, dissolved carbonate‐rich sediments and led to collapse. In contrast, drilling results suggest the younger depressions formed sometime between the Pliocene and Pliocene‐Pleistocene boundary. Similar depressions imaged within the Panama Basin and along the Carnegie Ridge to the south suggest widespread Galapagos Hotspot volcanism and hydrothermal circulation led to dissolution and collapse of carbonate‐rich sediments across the eastern equatorial Pacific. Fluid‐flow through the existing collapses and water current activity following the closing of the Central American Seaway likely led to the formation of the younger depressions. Drilling results, seismic reflection indicators of fluid pathways, and indicators of seafloor seepage suggests these depressions are actively guiding fluid outflow. Key Points: The study provides three‐dimensional seismic tied to drilling results for understanding the development of mega‐depressions along the Cocos RidgeThe depressions formed initially through dissolution and collapse, driven by volcanism and hydrothermal circulationThe younger depressions likely formed through bottom current activity and fluid‐flow, the latter of which appears to be active today [ABSTRACT FROM AUTHOR]

Abstract High‐resolution bathymetry and three‐dimensional seismic data along the Cocos Ridge reveal a 245 km2 field of ∼1–4 km in diameter seafloor depressions. The seafloor depressions are part of a two‐tiered honeycomb pattern. The lower‐tier depressions have steep faults that truncate strata with chaotic internal reflections consistent with sediment collapse into the depression. These extend into a lens shaped interval just above igneous basement. Overlying these depressions is a second broader set with rough seafloor morphology with gently dipping boundaries defined by pinch‐out stratigraphic patterns. Drilling results indicate that the lens‐shaped zones that host the deeper depressions represent anomalous regions of high porosity, low velocity, and low density within calcareous rich sediment. Analysis of nannofossils from IODP Site U1414 suggests the collapse structures formed during the late Miocene, whereas the younger shallower depressions likely formed between the early Pliocene and the Pliocene‐Pleistocene boundary. Geochemical and petrological analysis at Site U1414 suggests that hydrothermal circulation during the late Miocene led to carbonate dissolution and collapse. Following collapse, focused fluid‐flow and bottom current scouring resulted in formation of the overlying set of depressions and a honeycomb seafloor morphology. Similar sets of depressions along the Carnegie Ridge to the south support the hypothesis that two‐tiered depressions formed in response to processes that occurred broadly across the Panama Basin between the late Miocene and the Pliocene‐Pleistocene transition. Geochemical results at Site U1414, combined with geophysical data, suggest this two‐tiered system of depressions currently guides ongoing fluid outflow.

Abstract Abrupt fluid emissions from shallow marine sediments pose a threat to seafloor installations like wind farms and offshore cables. Quantifying such fluid emissions and linking pockmarks, the seafloor manifestations of fluid escape, to flow in the sub‐seafloor remains notoriously difficult due to an incomplete understanding of the underlying physical processes. Here, using a compositional multi‐phase flow model, we test plausible gas sources for pockmarks in the south‐eastern North Sea, which recent observations suggest have formed in response to major storms. We find that the mobilization of pre‐existing gas pockets is unlikely because free gas, due to its high compressibility, damps the propagation of storm‐induced pressure changes deeper into the subsurface. Rather, our results point to spontaneous appearance of a free gas phase via storm‐induced gas exsolution from pore fluids. This mechanism is primarily driven by the pressure‐sensitivity of gas solubility, and the appearance of free gas is largely confined to sediments in the vicinity of the seafloor. We show that in highly permeable sediments containing gas‐rich pore fluids, wave‐induced pressure changes result in the appearance of a persistent gas phase. This suggests that seafloor fluid escape structures are not always proxies for overpressured shallow gas and that periodic seafloor pressure changes can induce persistent free gas phase to spontaneously appear.

The amplitude and coherence attributes of three-dimensional (3D) seismic data are used to confirm gas hydrate occurrence and to delineate its distribution in the Zhongjiannan basin, South China Sea. High amplitude anomalies (HAAs) are distributed above or below the regional base of gas hydrate stability zone (BGHSZ), which intersect with the bottom simulating reflectors (BSRs) or are interrupted by different types of pockmarks. The maximum amplitude attribute extracted along T1 (5.5 Ma) horizon is controlled by the widely distributed faults. The layer-bound polygonal faults (PFs) show networks of small normal faults, and the dominant orientations of PFs are similar or orthogonal to the regional tectonic faults, which provide the fluid migration pathways for gas and fluids to form HAAs. BSR shows the strong amplitude and continuous reflection where the faults or PFs can reach the BGHSZ without the influence of the pockmarks. Most of the pockmarks are related to the reactivation of faults and magmation, and some pockmarks are caused by the dissociation of gas hydrate. Around the matured pockmark, the BSR is discontinuous, and HAAs locally appear within the pockmarks. The inverted acoustic impedance profile shows obviously high values of HAAs except in pockmark zones. Partial HAAs occur above BGHSZ, and the continuity is interrupted by the pockmarks with only high values around the pockmarks. We propose that BSR, HAAs, pockmarks, and different types of faults are closely related to the occurrence and distribution of gas hydrates in the study area. This work allows us to understand the relationship between gas hydrate occurrence and accumulation with pockmarks, faults, and magmatic activities.

The data presented in this paper are related to the research article “Sub-bottom and bathymetry sonar inspection of postglacial lacustrine infill of the alpine lakes (Tatra Mts., Slovakia)” (Dhavamani et al., 2022). An implementation of acoustic sonar protocols provided data for the interpretation of glacigene, glaciolacustrine, postglacial, mass-movement deposits, and geodynamic factors influencing the sedimentation in seven alpine and sub-alpine Tatra Mountains lakes. The field data document the survey track lines of the sonars and allow to identify the location of the geomorphologic phenomena described in (Dhavamani et al., 2022). The laboratory data obtained by micro-CT document the lithology of glaciolacustrine and postglacial lake infill and support the interpretation of sub-bottom sonar record.

Paeonia lactiflora Pall. is a world-famous ornamental plant, whose roots have been used as an important traditional Chinese medicine, Shaoyao, to treat diseases for more than 1,000 years. Because of the excellent curative effect of Shaoyao, its quality has attracted wide attention, however, there is a lack of comprehensive research on the different influencing factors of quality of Shaoyao. In this study, ultra-performance liquid chromatography-quadrupole time-of-flight mass spectrometry (UPLC-Q/TOF-MS) and high-performance liquid chromatography with diode-array detection (HPLC-DAD) were utilized to systematically analyze the Shaoyao of different ages, diameters and roots with “pockmarks.” 60 metabolites were detected and identified from Shaoyao using the UPLC-Q/TOF-MS, of which 20 potential quality markers of dissected roots with and without “pockmarks” were selected for the first time using the orthogonal partial least squares discriminant analysis (OPLS-DA) and the variable importance for projection (VIP) plot. Then, a selective and accurate HPLC-DAD quantitative assay has been developed for the simultaneous determination of 11 bioactive components in Shaoyao. The results showed that the total content of five monoterpene glycosides including oxypaeoniflorin, albiflorin, paeoniflorin, lactiflorin, and benzoylpaeoniflorin and six phenols including gallic acid, catechin, methyl gallate, ethyl gallate, apiopaeonoside and benzoic acid in the 3-year-old Shaoyao was higher than that of 4-year-old and 5-year-old Shaoyao. In Shaoyao of the same age, the total content of five monoterpene glycosides and six phenols decreased with an increase in diameter. In addition, regardless of whether it is a whole or a divided root, the contents of five monoterpene glycosides and six phenols in Shaoyao with “pockmarks” were higher than those of Shaoyao without “pockmarks.” In summary, this work has explored several factors that might affect the quality of Shaoyao, and provide a guide for more comprehensive quality evaluation in its further production, processing, and rational utilization.

Paeonia lactiflora Pall. is a world-famous ornamental plant, whose roots have been used as an important traditional Chinese medicine, Shaoyao , to treat diseases for more than 1,000 years. Because of the excellent curative effect of Shaoyao , its quality has attracted wide attention, however, there is a lack of comprehensive research on the different influencing factors of quality of Shaoyao. In this study, ultra-performance liquid chromatography-quadrupole time-of-flight mass spectrometry (UPLC-Q/TOF-MS) and high-performance liquid chromatography with diode-array detection (HPLC-DAD) were utilized to systematically analyze the Shaoyao of different ages, diameters and roots with "pockmarks." 60 metabolites were detected and identified from Shaoyao using the UPLC-Q/TOF-MS, of which 20 potential quality markers of dissected roots with and without "pockmarks" were selected for the first time using the orthogonal partial least squares discriminant analysis (OPLS-DA) and the variable importance for projection (VIP) plot. Then, a selective and accurate HPLC-DAD quantitative assay has been developed for the simultaneous determination of 11 bioactive components in Shaoyao. The results showed that the total content of five monoterpene glycosides including oxypaeoniflorin, albiflorin, paeoniflorin, lactiflorin, and benzoylpaeoniflorin and six phenols including gallic acid, catechin, methyl gallate, ethyl gallate, apiopaeonoside and benzoic acid in the 3-year-old Shaoyao was higher than that of 4-year-old and 5-year-old Shaoyao. In Shaoyao of the same age, the total content of five monoterpene glycosides and six phenols decreased with an increase in diameter. In addition, regardless of whether it is a whole or a divided root, the contents of five monoterpene glycosides and six phenols in Shaoyao with "pockmarks" were higher than those of Shaoyao without "pockmarks." In summary, this work has explored several factors that might affect the quality of Shaoyao , and provide a guide for more comprehensive quality evaluation in its further production, processing, and rational utilization. [ABSTRACT FROM AUTHOR]

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

Ubiquitous pockmarks in continental margins are important to understand the tectonic and sedimentary history and flow activity of their host margins. The study of an extensive field of pockmarks in the South China Sea (SCS) is important both scientifically and economically, for the exploration of oil, gas, and gas hydrate resources and the prevention and control of marine geological disasters. We report our new findings on the characteristics, distribution, and genesis of pockmarks in the SCS based on interpretation of high-resolution multibeam bathymetric data and multichannel seismic profiles collected in the past decade. Three pockmark fields (total area 572,824 km2), are recognized in the northern, western, and southern continental margins of the SCS. The pockmarks are highly variable in shape: circular, elliptical, crescent-shaped, elongated or forming a chain in plan view and U-, V- or W-shaped in cross-section. Pockmarks are normally 10–200 m in diameter, with maximum diameters/axial lengths of 7.3 km for circular, 5.5 km for elliptical, 8.6 km for crescent-shaped, 9.5 km for elongated, and 30 km for chain-type pockmarks. Pockmarks are normally less than 35 m deep, with a maximum depth of 341 m at the southwestern SCS margin. These pockmarks might have originated from methane seepages or from pore fluids from deeper sources, which migrated vertically along gas chimneys, mud diapirs, mud volcanoes, faults or buried channels. In addition to possible structural controls, the pockmark morphologies are affected by bottom currents. [ABSTRACT FROM AUTHOR]

The northeastern region of Nansha Waters, located in the southern margin of the South China Sea, is abundant in gas resources. However, it remains unclear if the central and northern areas of Nansha Waters possess gas resources and if there are any associated leaks. The Jiuzhang Basin is located in the northern Nansha Waters. Pre-stack depth migration, analysis of seismic attributes and multibeam data processing were conducted to image the seabed topography and backscatter intensity, as well as the seismic structure of the water and sedimentary layers in the Jiuzhang Basin. The result reveals a presence of large plumes, approximately 1500 m in width and up to 1300 m in height, from a pockmark. The plume is characterized by a strong seismic amplitude featuring chaotic reflections, a high instantaneous frequency (100–150 Hz), and a strong instantaneous amplitude. These seismic features commonly appear in hydrocarbon-bearing seawater, indicating that the plume is composed of gas. The relationship between the volume fraction of bubbles in the gas plumes and the amplitude of multi-channel seismic waves suggests that the volume fraction of bubbles within the gas plume can reach up to 5.0 × 10−3. Meanwhile, a new pockmark with a length of 248 m, a width of 90 m, and a depth of 20.4 m, and an abrupt 22 m drop in water depth of an existing pockmark are observed in the gas plume area. These observations, combined with the gas plume revealed by the multi-channel seismic collected in 2013 and multibeam collected in 2020, suggest a very strong and recently active gas leaks in the pockmark area of the northern Jiuzhang Basin. Furthermore, in the pockmark area, there are gas-bearing formations with weak reflections, and mud diapirs charactered by low seismic velocity, low amplitude chaotic reflections, and pulled up reflections on both sides, and faults extend to the seabed. These observations suggest that gas plume originate from deep strata of the northern Jiuzhang Basin. Large gas plumes and mud diapirs are often indicative of the existence of deep gas reservoirs. Therefore, the presence of such active gas plumes and mud diapirs in the northern region of the Jiuzhang Basin signifies the presence of abundant gas resources. • Largest gas plumes in the north Nansha Waters, South China Sea to date. • Multi-channel seismic imaging of the gas plume. • Pockmark, mud diapirs, and faults are observed below the gas plume. • A deepening pockmark and a formation of a new pockmark were observed. [ABSTRACT FROM AUTHOR]

Based on an analytical review of factual materials, the article discusses the peculiarities of the distribution of gas hydrates in the development zones of mud volcanism of the Mediterranean, Black and Caspian seas, which are direct indications of sedimentary gas complex. A comparative analysis of the composition of hydrate-forming fluids, sources and depths of gas hydrates distribution was performed. The article provides information on gas seeps and pokmarks. The presence of gas seepsand pokmarks in the water column and at the bottom of the Southern Caspian is assumed.

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A high-resolution 3D reflection seismic dataset acquired over a deepwater exploration block in the Orange Basin, offshore of South Africa was used to achieve a detailed description of the contractional domain within a gravity-driven system. Conventional seismic analysis combined with novel horizon-based and volumetric seismic attributes highlight 5 main stratigraphic intervals, ranging from Cenomanian-Turonian to present-day in age. Coniacian to Santonian stratigraphic sequences were disrupted by numerous folds and a thrust fault system. The thrust fault system soles into two detachment layers, which coincide with the shale hydrocarbon source rocks at the Cenomanian−Turonian boundary and the top of the Turonian marine shale unit. The thrust fault system also propagates upwards into the Maastrichtian and Palaeogene sedimentary units and exploits S to WSW dipping normal faults along which hydrocarbons appear to have been transported to the present-day seafloor, forming pockmarks. Seismic attributes show 6 large pockmarks with diameters between 700 m and 1100 m and depths between 75 and 103 m. Thus, we suggest that hydrocarbons from the Cenomanian−Turonian source rocks were channelled up-section by the Cenomanian-Santonian thrust faults, followed by smaller-scale S-WSW dipping normal faults and in turn escaping into the ocean, resulting in pockmark formation. [ABSTRACT FROM AUTHOR]

Abstract Submarine groundwater discharge into coastal areas is a common global phenomenon and is rapidly gaining scientific interest due to its influence on marine ecology, the coastal sedimentary environment, and its potential as a future freshwater resource. We conducted an integrated study of hydroacoustic surveys combined with geochemical pore water and water column investigations at a well‐known groundwater seep site in Eckernförde Bay (Germany). We aim to better constrain the effects of shallow gas and submarine groundwater discharge on high‐frequency multibeam backscatter data and to present acoustic indications for submarine groundwater discharge. Our high‐quality hydroacoustic data reveal hitherto unknown internal structures within the pockmarks in Eckernförde Bay. Using precisely positioned sediment core samples, our hydroacoustic‐geochemical approach can differentiate intrapockmark regimes that were formerly assigned to pockmarks of a different nature. We demonstrate that high‐frequency multibeam data, in particular the backscatter signals, can be used to detect shallow free gas in areas of enhanced groundwater advection in muddy sediments. Intriguingly, our data reveal relatively small (typically

Submarine groundwater discharge into coastal areas is a common global phenomenon and is rapidly gaining scientific interest due to its influence on marine ecology, the coastal sedimentary environment, and its potential as a future freshwater resource. We conducted an integrated study of hydroacoustic surveys combined with geochemical pore water and water column investigations at a well‐known groundwater seep site in Eckernförde Bay (Germany). We aim to better constrain the effects of shallow gas and submarine groundwater discharge on high‐frequency multibeam backscatter data and to present acoustic indications for submarine groundwater discharge. Our high‐quality hydroacoustic data reveal hitherto unknown internal structures within the pockmarks in Eckernförde Bay. Using precisely positioned sediment core samples, our hydroacoustic‐geochemical approach can differentiate intrapockmark regimes that were formerly assigned to pockmarks of a different nature. We demonstrate that high‐frequency multibeam data, in particular the backscatter signals, can be used to detect shallow free gas in areas of enhanced groundwater advection in muddy sediments. Intriguingly, our data reveal relatively small (typically <15 m across) pockmarks within the much larger, previously mapped pockmarks. The small pockmarks, which we refer to as "intrapockmarks," have formed due to the localized ascent of gas and groundwater; they manifest themselves as a new type of "eyed" pockmarks, revealed by their acoustic backscatter pattern. Our data suggest that, in organic‐rich muddy sediments, morphological lows combined with a strong multibeam backscatter signal can be indicative of free shallow gas and subsequent advective groundwater flow. Plain Language Summary: Groundwater that seeps out of the ocean floor is a common global phenomenon. Marine ecosystems are highly dependent on nutrient supply from land, and recent studies have suggested that groundwater seeping out of the seafloor supplies even more nutrients to the world's oceans than rivers do. Also, nearshore freshwater springs have been used as a source of drinking water for decades and large offshore groundwater reserves in continental shelves can potentially prevent future freshwater shortages. We use echo sounding methods to search for indications for submarine groundwater. The methods enable us to carry out detailed investigations of intriguing seafloor depressions (i.e., "pockmarks"). Our accurate measurements give new insights into the morphology and characterization of the Eckernförde Bay pockmarks and reveal a new type of pockmark that is related to seeping groundwater. In the muddy sediment of Eckernförde Bay, methane gas forms in the sediments due to microbial decomposition of biomass. In areas of enhanced groundwater advection, this methane gas is brought closer to the seafloor, where pockmarks form and where we can detect the gas with our sonar system. Given the abundant global distribution of muddy gaseous sediments, our findings have important implications for the future detection of offshore groundwater systems. Key Points: We present a new form of eyed pockmarks caused by shallow gasWe establish acoustic indicators for submarine groundwater discharge associated with gas [ABSTRACT FROM AUTHOR]

Pockmarks consists of depressions on the seafloor and are often considered to originate from fluid escape activities. In this study, we report 125 pockmarks near the Andu Seamount in the southern margin of the South China Sea (SCS) and preliminarily investigate their genesis based on multi-beam bathymetry and multichannel seismic reflection data. Most of them are giant and mega pockmarks and display circular, elliptical, elongated, crescent, comet and irregular shapes on a plan-view. The mapped pockmarks cover a total surface of 291.354 km2. Most of them occur in strings or clusters, while some are isolated. The filled volume of pockmark is calculated for the first time in this region, which may have significant implications for sedimentary process. Beneath most of the pockmarks, high-angle faults, high-amplitude reflections, filled-up structures and other fluid escape features are imaged on seismic sections. The structural settings, especially the fault systems, may control the genesis and development of pockmarks in this area. We also speculate that seabed gradients may contribute to generation of crescent pockmarks. According to seismic sections, sediment types and morphological analysis, we finally proposed that the genesis of these pockmarks may mainly be triggered by gas escape activities in the study area. This work may have important implications for the gas exploration in the Nansha Region, southern South China Sea. [ABSTRACT FROM AUTHOR]

Soft-sediment deformation structures (SSDS) have been the focus of attention for over 150 years. Existing unconstrained definitions allow one to classify a wide range of features under the umbrella phrase “SSDS”. As a consequence, a plethora of at least 120 different types of SSDS (e.g., convolute bedding, slump folds, load casts, dish-and-pillar structures, pockmarks, raindrop imprints, explosive sand–gravel craters, clastic injections, crushed and deformed stromatolites, etc.) have been recognized in strata ranging in age from Paleoproterozoic to the present time. The two factors that control the origin of SSDS are prelithification deformation and liquidization. A sedimentological compendium of 140 case studies of SSDS worldwide, which include 30 case studies of scientific drilling at sea (DSDP/ODP/IODP), published during a period between 1863 and 2017, has yielded at least 31 different origins. Earthquakes have remained the single most dominant cause of SSDS because of the prevailing “seismite” mindset. Selected advances on SSDS research are: (1) an experimental study that revealed a quantitative similarity between raindrop-impact cratering and asteroid-impact cratering; (2) IODP Expedition 308 in the Gulf of Mexico that documented extensive lateral extent (>12 km) of mass-transport deposits (MTD) with SSDS that are unrelated to earthquakes; (3) contributions on documentation of pockmarks, on recognition of new structures, and on large-scale sediment deformation on Mars. Problems that hinder our understanding of SSDS still remain. They are: (1) vague definitions of the phrase “soft-sediment deformation”; (2) complex factors that govern the origin of SSDS; (3) omission of vital empirical data in documenting vertical changes in facies using measured sedimentological logs; (4) difficulties in distinguishing depositional processes from tectonic events; (5) a model-driven interpretation of SSDS (i.e., earthquake being the singular cause); (6) routine application of the genetic term “seismites” to the “SSDS”, thus undermining the basic tenet of process sedimentology (i.e., separation of interpretation from observation); (7) the absence of objective criteria to differentiate 21 triggering mechanisms of liquefaction and related SSDS; (8) application of the process concept “high-density turbidity currents”, a process that has never been documented in modern oceans; (9) application of the process concept “sediment creep” with a velocity connotation that cannot be inferred from the ancient record; (10) classification of pockmarks, which are hollow spaces (i.e., without sediments) as SSDS, with their problematic origins by fluid expulsion, sediment degassing, fish activity, etc.; (11) application of the Earth's climate-change model; and most importantly, (12) an arbitrary distinction between depositional process and sediment deformation. Despite a profusion of literature on SSDS, our understanding of their origin remains muddled. A solution to the chronic SSDS problem is to utilize the robust core dataset from scientific drilling at sea (DSDP/ODP/IODP) with a constrained definition of SSDS.

Sub-seabed gas is commonly associated with seabed depressions known as pockmarks—the main venting sites for hydrocarbon gases to enter the water column. Sub-seabed gas accumulations are characterized by acoustically turbid or opaque zones in seismic reflection profiles, taking the form of gas blankets, curtains or plumes. How the migration of sub-seabed gas relates to the origin and distribution of pockmarks in nearshore and fjordic settings is not well understood. Using marine geophysical data from Loch Linnhe, a Scottish fjord, we show that shallow sub-seabed gas occurs predominantly within glaciomarine facies either as widespread blankets in basins or as isolated pockets. We use geospatial ‘hot-spot’ analysis conducted in ArcGIS to identify clusters of pockmarks and acoustic (sub-seabed) profile interpretation to identify the depth to gas front across the fjord. By combining these analyses, we find that the gas below most pockmarks in Loch Linnhe is between 1.4 m and 20 m deep. We anticipate that this work will help to understand the fate and mobility of sedimentary carbon in fjordic (marine) settings and advise offshore industry on the potential hazards posed by pockmarked seafloor regions even in nearshore settings.

Multibeam echo‐sounding and seismic data were obtained during three geophysical surveys by Guangzhou Marine Geological Survey (GMGS) on the north‐eastern slope of the South China Sea (SCS). Nineteen seafloor domes and numerous elongated pockmarks were distinguished in the study area. The seafloor domes vary on number of summits, shape, and spatial interrelation, based on which they were classified into four types which are single‐summit, double‐summits, elongated, and connected. Bottom simulating reflectors and blanking zone underneath were observed in the seismic profiles, indicating the potential existence of gas hydrate and free gas. Multiple faults and diapirs exist beneath the seafloor domes and pockmarks, which were inferred to provide efficient pathways for fluid migrating from deep subsurface. The seafloor domes exhibits high backscatter feature, which is potentially associated with the seep‐related gas hydrate and authigenic carbonate in the shallow sediment. It is speculated that fault and diapir activity, fault sliding caused by region extension and the interaction between neighbour domes control and affect the activity and formation of the domes. However, further investigations, including geological sampling and seafloor observation, are still needed to confirm whether they are mud volcanoes. [ABSTRACT FROM AUTHOR]

We present the results of an interpretation of 2D and 3D seismic from offshore Lebanon in which we identify a suite of 5 linearly distributed trails of fluid escape pipes with pockmarks at their upper terminus. These features transect the thick Messinian evaporites and root within prominent NE-SW oriented pre-salt folds. The pipe trails are oriented orthogonal to the strike of the pre-salt folds, with a synchronous initial expulsion episode in each trail dated at 1.7 Ma (±0.3 Ma), approximately coeval with the onset of salt-detached growth faulting along the basin margin. Each expulsion episode has been systematically offset to the NW away from the pre-salt fold by the flow of the salt, resulting in deformation of the fluid escape pipes in the salt. The orientation of the pipe trails thus provides a direct kinematic indicator for the flow direction of the salt layer during early stages of gravity-driven deformation of the salt and overburden, concomitant with basin margin uplift and tilting. The unidirectional NW oriented flow is recorded over a region of some 50 km width within the translational domain of the salt tectonic deformation. Synchronicity in the onset of fluid expulsion from overpressured reservoirs within the pre-salt succession evidenced by these pipe trails and growth fault development at the basin margins implies that the pipe trails record the kinematics of the deforming salt layer throughout its post-Messinian phase of deformation. The deformed pipe trails demonstrate a Couette flow regime for the salt layer and document subtle changes in cumulative strain and velocity (2–4 mm/yr; ±0.3 mm/yr) over distances of a few km. It is proposed that this novel method of using fluid flow features as natural markers for the kinematics of deforming salt layers could be utilized in other parts of Eastern Mediterranean, as well as other salt basins on Earth. • Calibration of the 3D kinematics of a salt sheet deforming under gravity. • Multi episode fluid expulsion through 'sealing' salt. • Fluid escape pipes as natural strain markers through a salt sheet. • Pipe trails reveal unidirectional flow, Couette flow regime and salt flow velocity. • Lateral variation in salt flow velocity due to converging streamlines in salt. [ABSTRACT FROM AUTHOR]

Seismic imaging techniques are widely used to determine fluid migration pathways in the subsurface. Understanding the timing and distribution of focused fluid migration pathways is important for assessing reservoir seal quality and hydrocarbon exploration risk, as well as seafloor biogeochemical processes in the case of fluid venting from the seafloor. The frontier Canterbury Basin southeast of New Zealand is well studied, but questions remain regarding the dynamic movement of fluids through the basin. Using 3D seismic data combined with high-resolution boomer seismic data, we characterise focused fluid migration and sediment remobilisation events in the Canterbury Basin. We show how polygonal fault formation and nucleation is affected in the vicinity of vertically ascending fluids and sediments. A cylindrical region devoid of polygonal faulting around the feeder system of a buried sediment volcano points to the possible existence of an axisymmetrical stress field induced by upward migrating fluids and sediments. Sediment remobilisation and intrusions of permeable stingers (i.e. sand injectites) could enable dewatering of their host strata and thereby prevent polygonal faults from nucleating. Amplitude variations with angle (AVA) and a detailed analysis of two tiers of polygonal faults reveal three phases of fluid migration and sediment remobilisation and intrusion through, and into, low permeability sediments. Our integrated conceptual model provides insights into vertical fluid migration mechanisms and the implications for polygonal fault formation. • Cylindrical faults surround a fluid conduit in a 2 km radius. • Polygonal faults don't nucleate within a cylindrical region around a fluid conduit. • Buried pockmarks formed on the upper tips of polygonal faults. • Shallow bright spots form beneath present day seafloor pockmarks. • Radial faults within a polygonal fault system form around buried Mounds. [ABSTRACT FROM AUTHOR]

Seafloor pockmarks of varying size occur over an area of 50,000 km2 on the Chatham Rise, Canterbury Shelf and Inner Bounty Trough, New Zealand. The pockmarks are concentrated above the flat‐subducted Hikurangi Plateau. Echosounder data identify recurrent episodes of pockmark formation at ~100,000‐year frequency coinciding with Pleistocene glacial terminations. Here we show that there are structural conduits beneath the larger pockmarks through which fluids flowed upward toward the seafloor. Large negative Δ14C excursions are documented in marine sediments deposited next to these subseafloor conduits and pockmarks at the last glacial termination. Modern pore waters contain no methane, and there is no negative δ13C excursion at the glacial termination that would be indicative of methane or mantle‐derived carbon at the time the Δ14C excursion and pockmarks were produced. An ocean general circulation model equipped with isotope tracers is unable to simulate these large Δ14C excursions on the Chatham Rise by transport of hydrothermal carbon released from the East Pacific Rise as previous studies suggested. Here we attribute the Δ14C anomalies and pockmarks to release of 14C‐dead CO2 and carbon‐rich fluids from subsurface reservoirs, the most likely being dissociated Mesozoic carbonates that subducted beneath the Rise during the Late Cretaceous. Because of the large number of pockmarks and duration of the Δ14C anomaly, the pockmarks may collectively represent an important source of 14C‐dead carbon to the ocean during glacial terminations. Key Points: Pockmarks of varying size occur across a vast portion of the Chatham Rise and Bounty Trough in the Southwest PacificPockmarks overlie subsurface deformation features indicative of overpressurized vertical fluid flowNegative Δ14C nomalies occur near pockmarks indicating these were where 14C‐dead carbon was released to the ocean at the last glacial termination [ABSTRACT FROM AUTHOR]