Pockmark Field Research Articles

Localized strata-bound domino faulting offshore Espírito Santo Basin (southeastern Brazil): The case for sudden release of fluid in salt-withdrawal basins

Hydraulic-driven faults and fractures are important subsurface fluid-flow pathways, yet their seismic expressions are poorly documented. This paper uses high-quality three-dimensional seismic data to investigate curved, strata-bound domino-style faults above which a pockmark field is observed and interpreted to be associated with their formation. The faulted interval covers an area of 24 km2 and is ∼100 m thick. Faults show a concentric pattern with an average spacing of 120 m, whereas their throws approach 10 ms two-way time. Strata-bound domino faults were formed because of overpressure buildup and release in a mud-dominated succession, with their geometry responding to the local dip of the salt-withdrawal basin in which they occur. Further overpressure buildup resulted primarily from fluid migration through underlying crestal faults and secondarily through capillary permeability within permeable strata. Apart from the strata-bound domino faults and associated pockmark fields, other randomly distributed pockmarks indicate episodic fluid flow in the study area. Random pockmarks were likely formed during rapid burial of continental slope strata, resulting in the elevation of fluid pressure in underlying soft sediment. This paper concludes that strata-bound domino faults are a typical feature of fluid flow on salt-rich continental margins, such as southeastern Brazil, the Gulf of Mexico, and the western African continental margin. During their formation, strata-bound domino faults form significant conduits for migrating fluid into soft, porous strata, away from regional and local structural traps.

Production of methane and gaseous compounds by surface microbial activity in a small pockmark field, Dunmanus Bay, Ireland

Marine pockmarks are globally widespread seabed depressions, conventionally thought to be formed by the accumulation and expulsion of microbial and thermogenic gas. However, other putative fluids and processes have been implicated in pockmark formation and gas escape to the atmosphere may be underestimated. Given the complex spectrum of aquatic settings, morphologies and sizes, there may also exist a spectrum of physical, chemical and biological processes that form pockmarks. Pockmarks in shallow coastal waters are now understood to be widespread, but the influence of physical dynamics (e.g. tides, storms, etc.), terrestrial processes and anthropogenic activities add considerable spatiotemporal complexity and uncertainty to our understanding of these features. Here, we revisit a field of small (ca. 2 m diameter), shallow (<1 m depth) pockmarks in Dunmanus Bay, Ireland. The presence of muddy surface sediments overlying sand in the pockmarked area indicates that gas accumulation within fine-grained surface sediments contributes to formation of the features. Previous work indicates that CH4 is an important seepage fluid in Dunmanus and neighbouring bays. However, based on evidence from multiple surveys, we observe considerable spatiotemporal complexity, and the transient nature of the gas within sediments points to the potential for fluids other than traditional microbial or thermogenic CH4, migrating from sources tens to hundreds of metres below the seafloor. We observed atypical porewater profiles where millimolar concentrations of H2S concentrations are observed in surface sediments in the absence of SO42− depletion, together with NH4+ build-up from ammonification of sedimentary organic matter. Archaeal methanogens, anaerobic methanotrophic archaea and SO42--reducing Deltaproteobacteria co-occur in surface sediments in the pockmark field and NMR revealed the presence of non-competitive substrates for methanogens. We hypothesize that in-situ methanogenesis and production of other volatile metabolites besides CH4 (e.g. CO2, dimethyl disulfide) from microbial degradation of organic matter are potential gaseous fluids and could contribute to the formation of small pockmarks.

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

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

Types, characteristics, distribution, and genesis of pockmarks in the South China Sea: insights from high-resolution multibeam bathymetric and multichannel seismic data

ABSTRACT 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.

Clay Mineralogy and Geochemistry of the Pockmarked Surface Sediments from the Southwestern Xisha Uplift, South China Sea: Implications for Weathering and Provenance

In the northern South China Sea, pockmarks are widely distributed on the seabed offshore on the southwestern Xisha Uplift. The mineralogy and geochemistry of the clay minerals and surface sediments from the pockmark field were identified using X-ray diffraction (XRD) analysis and X-ray fluorescence (XRF) analysis to trace the provenance, weathering, and sediment transportation system in the area. The clay minerals are primarily comprised of illite, smectite, kaolinite, and chlorite, showing a distribution of average weight percentages of 35%, 35%, 18%, and 13%, respectively. Based on the surrounding fluvial drainage basins and various transport mechanisms (current or monsoon), illite and chlorite primarily originate from rivers in Taiwan and the Mekong and Red Rivers. Kaolinite primarily originates from the Pearl River, and smectite derived from the Luzon arc system is primarily transported by surface currents with significant influence from the Kuroshio intrusion.

A threefold perspective on the role of a pockmark in benthic faunal communities and biodiversity patterns

Pockmarks are circular-shaped depressions that increase seabed heterogeneity and are characterized by discontinuous fluid emissions. To understand how environmental conditions of pockmarks affect the structure of macro- and meiofauna, we investigated two sites in a pockmark field in the northwestern Madagascar margin. In a comparative approach, we explored the community structure of the dominant taxa (Polychaeta, Nematoda and hyaline foraminifera) in each component (macro-, metazoan meiofauna and foraminifera, respectively). The investigated active pockmark showed approximately two times higher meiofauna abundance compared to in a site away from another pockmark field, but macrofauna showed the opposite trend, with almost half density at the pockmark site. However, at both sites, macro- and meiofauna showed higher richness and abundance values in the top well-oxygenated layers of the sediment than in the underlaying ones. Polychaeta and Nematoda showed lower richness in the pockmark, opposed to hyaline foraminiferans, but lower evenness in the pockmark was found for the three groups. The detection of gas flares in the water column attests of the recent activity within the pockmark. High amount of sulfur-bearing minerals (mainly pyrite) evidences a production of dissolved free sulfides (not detected at the time of sampling) by sulfate reduction process driven by organic matter degradation and anaerobic oxidation of methane. Furthermore, recent increase in sedimentation rates in the past 70 years and organic matter inputs could have led to higher organic matter degradation rates resulting in reduced conditions and a high oxygen consumption. All this together seem to act as key factors in the determination of variation in richness, abundance and community composition of macrofauna and meiofauna. Additionally, some taxa seem to be more tolerant to these extreme conditions, such as species belonging to the Nematoda genus Desmodora and the phylum Kinorhyncha, which are highly abundant in the pockmark, and hence, may be considered as potential bioindicators of pockmark activity in this area. Further studies are required for a better assessment.

Dragons of the Deep Sea: Kinorhyncha Communities in a Pockmark Field at Mozambique Channel, With the Description of Three New Species

Cold seep areas are extremely reduced habitats with spatial-temporally variation of hydrocarbon-rich fluids seepage, low oxygen levels and great habitat heterogeneity. Cold seeps can create circular to ellipsoid shallow depressions on the seafloor called pockmarks. We investigated two selected pockmarks, characterized by different gas emission, and two sites outside these geological structures at the Mozambique Channel to understand whether and how their environmental conditions affect the kinorhynch fauna in terms of density, richness and community composition. A total of eleven species have been found living in the studied area, of which three are new species: Fissuroderes cthulhu sp. nov., Fujuriphyes dagon sp. nov. and Fujuriphyes hydra sp. nov. Densities outside the pockmarks are low and regularly decrease from the upper sediment layers, whereas inside the pockmarks density reaches its highest value at layer 1-2 cm, strongly decreasing along the vertical profile from this depth. Areas under pockmark influence and locations outside pockmarks are similar in terms of species richness, but kinorhynchs showed a significant remarkable higher density at the pockmark sites. Additionally, species composition changes between habitats (inside and outside pockmarks) and between the two sampled pockmarks, with most of the species restricted to one of the studied habitats, except for Condyloderes sp. and Echinoderes unispinosus present both outside and inside the pockmarks. Echinoderes hviidarum, E. unispinosus and Fi. cthulhu sp. nov., present at sites with gas emission, do not only survive under the specific pockmark conditions (characterized by hydrogen sulphide toxicity, methane high concentration and low availability of dissolved oxygen) but even profit from a habitat with a likely lower competition for space and resources, flourishing and enhancing the density, most likely through the replacement with specialized species. Contrarily, species that only appear outside the pockmarks do not seem to cope with the presence of hydrogen sulphide and methane. Therefore, environmental factors linked to gas emissions have a major role driving the kinorhynch community composition.

The Cost for Biodiversity: Records of Ciliate–Nematode Epibiosis with the Description of Three New Suctorian Species

Epibiosis is a common phenomenon in marine systems. In marine environments, ciliates are among the most common organisms adopting an epibiotic habitus and nematodes have been frequently reported as their basibionts. In the present study, we report several new records of peritrich and suctorian ciliates-nematode association worldwide: from a deep-sea pockmark field in the NW Madagascar margin (Indian Ocean), from a shallow vent area in the Gulf of Naples (Mediterranean, Tyrrhenian Sea), in a MPA area in the Gulf of Trieste (Mediterranean, Adriatic Sea), from a mangrove system in French Guiana (South America, Atlantic Ocean), and from the Maldivian Archipelago. In addition, three new species of Suctorea from the Secca delle Fumose shallow vent area (Gulf of Naples) were described: Loricophrya susannae n. sp., Thecacineta fumosae n. sp. and Acinetopsis lynni n. sp. In the light of these new records and data from the existing literature, we discuss the suctorian–nematode epibiosis relationship as a lever to biodiversity.

Poly-phased fluid flow in the giant fossil pockmark of Beauvoisin, SE basin of France

The giant Jurassic-aged pockmark field of Beauvoisin developed in a 800 m wide depression for over 3.4 Ma during the Oxfordian; it formed below about 600 m water depth. It is composed of sub-sites organized in clusters and forming vertically stacked carbonate lenses encased in marls . This fine-scale study is focused on a detailed analysis of petrographical organization and geochemical signatures of crystals that grew up in early to late fractures of carbonate lenses, surrounding nodules, and tubes that fed them. The isotopic signature (C, O and Sr) shows that at least three different episodes of fluid migration participated to the mineralization processes. Most of the carbonates precipitated when biogenic seepage was active in the shallow subsurface during the Oxfordian. The second phase occurred relatively soon after burial during early Cretaceous and thermogenic fluids came probably from underlying Pliensbachian, Late Toarcian or Bajocian levels. The third phase is a bitumen-rich fluid probably related to these levels reaching the oil window during Mio-Pliocene. The fluids migrated through faults induced by the emplacement of Triassic-salt diapir of Propiac during the Late Jurassic and that remained polyphased drain structures over time.

Structural controls on shallow fluid flow and associated pockmark fields in the East Breaks area, northern Gulf of Mexico

Three-dimensional (3D) seismic data, combined with semi-automated mapping methods in ArcGIS, were used to analyse the morphology and distribution of 720 pockmarks and 62 mud volcanoes in the northern Gulf of Mexico. The relationship amongst salt bodies, faults and the distribution of pockmarks and mud volcanoes stresses the significance of these structures in focusing fluid flow on continental margins. The pockmarks were classified according to their structural setting and depth of source, and later correlated with seep data from the Bureau of Ocean Energy Management (BOEM). Key findings include: a) half of the pockmarks are located within faults rooted on the top of salt diapirs, whilst 96% of the pockmarks are associated with salt diapirs – emphasising the importance of salt and crestal faults in focusing fluid flow to the sea floor; b) diffusion flow through the salt minibasins is evident due to the presence of soft amplitude anomalies (indicating fluids) and pockmarks located far from salt or faults; c) oil and gas are actively leaking to the sea floor; d) a higher density of fluid flow features are found in areas with steeper minibasin dips and greater catchment areas. While no clear correlation is evident between the morphological attributes and depth of source, the shallow plumbing system is dominated by pockmarks whereas mud volcanoes are sourced from the deeper parts of the salt minibasins. In summary, this study uses a novel approach to analyse the plumbing system in a salt-rich basin based on the surface fluid flow features. The importance of characterising the fluid flow features and associated structures to reduce risk and uncertainty is stressed in terms of both shallow gas hazards and hydrocarbon leakage from deeper reservoirs.

Distribution and classification of pockmarks on the seabed around western Scotland

ABSTRACT Pockmarks are seabed depressions that represent primary evidence of rapid biogenic/thermogenic gas build up and fluid release from seabed sediments to the water column. We use a Geographical Information System (GIS) to analyse multibeam echo-sounder bathymetric data and use a range of semi-automated tools to map seabed pockmarks in fjords and adjacent coastal waters around western Scotland. We map 1019 individual pockmarks in 12 different hydrographic areas covering ca. 2019 km2. We use morphological metrics and statistical procedures to classify and analyse the variety of pockmark forms. A k-means clustering algorithm identifies three classes of pockmark morphology: deep, elongate and regular. The recognition of separate pockmark classes could aid understanding of their age, activity and origin. This work presents the first detailed mapping of pockmark fields in Scottish west coast waters and highlights the use of pockmarks as an indicator of the quantity, mobility and fate of stored carbon.

New evidence of perfect overlapping of Haploops and pockmarks field: Is it a coincidence?

In the shallow waters of the Bay of Concarneau (South Brittany, France), previous studies reported a dense settlement of the species Haploops (tube-dwelling bioengineer amphipod) that perfectly overlaps a field of pockmarks. A possible mechanistic link may therefore exist between the Haploops colonies and the shallow pockmarks. To test this hypothesis, interferometry sonar and sub-bottom profiler chirp data were acquired in two new areas located in the marine estuary of the Loire River, “La Lambarde” and “Le Croisic”. The new sites share the same sedimentary and geological characteristics as those found in the Bay of Concarneau. The three sites are similarly located in shallow and muddy environments above a faulted Middle Eocene formation that is incised by paleo-valleys. Despite different hydrodynamic conditions and estuarine influence, in the two new investigated areas, the Haploops also settle within the pockmarks field limits. To better constrain the association pockmarks/Haploops, we compared the sedimentary features, seismic signatures and total organic carbon (TOC) content of the sediment. Active pockmark chimneys displaying highly reworked profiles and slightly higher TOC contents suggest a local deeper source of organic matter, which might be an important source of nutrient for the Haploops. In Southern Brittany the pockmarks/Haploops spatial overlap is not a simple coincidence and may therefore present a general pattern for identification of settlement preferences of the specie.

Geophysical and geochemical analysis of shallow gas and an associated pockmark field in Bantry Bay, Co. Cork, Ireland

An integrated geophysical, geological, and geochemical investigation of seabed fluid venting was carried out in upper Bantry Bay, a large marine inlet on the southwest coast of Ireland. The results provide evidence of the seafloor venting of gas rich fluids, resulting in the formation of a pockmark field identified here for the first time. The pockmarks occur in an area where sub-bottom profiles provide evidence of chimney-like features interpreted to record upward gas migration through Quaternary sediments to the seafloor. Three vibrocores up to 6 m long were acquired in water depths of 24–34 m, two from the pockmark field and one from outside. Methane of predominantly biogenic origin was quantified in all three cores by headspace analysis of sediment sub-samples. Well-defined sulfate methane transition zones (SMTZs) were observed in two of the cores, the shallowest (1.25 m below sea floor (mbsf)) inside the pockmark field and the other (3.75 mbsf) outside. It is likely that an SMTZ occurs at the location of the third core, also within the pockmark field, although deeper than the samples obtained during this study. Gas migration towards the seafloor is suggested to involve both diffuse pore fluid migration across wide areas and focused flow through the pockmarks, together driven by methanogenesis of pre-glacial lacustrine sediments preserved in a bedrock basin, and possible gas release from the Owenberg River Fault. Analysis of phospholipid fatty acids (PLFAs) and archaeal isoprenoid hydrocarbons was used to investigate the microbial ecology of these sediments. Anaerobic oxidation of methane (AOM) may play a role in controlling release of CH4 to the water column and atmosphere in this shallow gas setting, potentially mediated by syntrophic sulfate reducing bacteria (SRB) and anaerobic methanotrophic archaea (ANME).

Pockmarks in the Witch Ground Basin, Central North Sea

AbstractMarine sediments host large amounts of methane (CH4), which is a potent greenhouse gas. Quantitative estimates for methane release from marine sediments are scarce, and a poorly constrained temporal variability leads to large uncertainties in methane emission scenarios. Here, we use 2‐D and 3‐D seismic reflection, multibeam bathymetric, geochemical, and sedimentological data to (I) map and describe pockmarks in the Witch Ground Basin (central North Sea), (II) characterize associated sedimentological and fluid migration structures, and (III) analyze the related methane release. More than 1,500 pockmarks of two distinct morphological classes spread over an area of 225 km2. The two classes form independently from another and are corresponding to at least two different sources of fluids. Class 1 pockmarks are large in size (&gt;6 m deep, &gt;250 m long, and &gt;75 m wide), show active venting, and are located above vertical fluid conduits that hydraulically connect the seafloor with deep methane sources. Class 2 pockmarks, which comprise 99.5% of all pockmarks, are smaller (0.9–3.1 m deep, 26–140 m long, and 14–57 m wide) and are limited to the soft, fine‐grained sediments of the Witch Ground Formation and possibly sourced by compaction‐related dewatering. Buried pockmarks within the Witch Ground Formation document distinct phases of pockmark formation, likely triggered by external forces related to environmental changes after deglaciation. Thus, greenhouse gas emissions from pockmark fields cannot be based on pockmark numbers and present‐day fluxes but require an analysis of the pockmark forming processes through geological time.

Effects of an active pockmark field on the distribution of heavy metals of surficial sediments in the Gulf of Patras, Greece

Factor analysis carried out in thirteen sediment cores, collected from an active pockmark field area in the eastern part of the Gulf of Patras, discriminated a domestic sewage pollution factor, the Mn/Fe oxides and the carbonate sedimentation. The increased content in sand, the low C content as well as the lack of C /heavy org org" J metals association follow a specific NW-SE transect in the centre of the surveyed pockmark field, suggesting pockmarks activity. This seems to control sedimentation and heavy metals distributions.

Metal/Ca ratios in pockmarks and adjacent sediments on the SW Atlantic slope: Implications for redox potential and modern seepage

Metal/Ca ratios and their corresponding enrichment factors (EFs) in bulk sediment samples collected at a mixed carbonate-siliciclastic continental slope setting with an extensive pockmark field were measured and analyzed to find evidence of lateral differences in the redox potential and recent seepage activity inside the pockmarks. The analyzed ratios point to metal enrichments that indicate variations in the redox potential inside the pockmarks compared to the adjacent areas, as shown by the Cu/Ca, Ni/Ca and Zn/Ca EFs. Slight but statistically significant differences in the Mg/Ca ratios indicated that the formation of Mg-enriched carbonates within the pockmarks is presently taking place, attesting the maintenance of seepage in the shallow sedimentary column. Recent seepage activity was also confirmed by the upward enrichment of Ba in the sedimentary column. Our study provides direct evidence for pockmark activity and reinforces the need for an extended analysis of the geochemistry of natural seepages on the SW Atlantic margin, an area that has still not been studied in relation to this subject.

Origin of High Density Seabed Pockmark Fields and Their Use in Inferring Bottom Currents

Some of the highest density pockmark fields in the world have been observed on the northwest Australian continental shelf (&gt;700/km2) where they occur in muddy, organic-rich sediment around carbonate banks and paleochannels. Here we developed a semi-automated method to map and quantify the form and density of these pockmark fields (~220,000 pockmarks) and characterise their geochemical, sedimentological and biological properties to provide insight into their formative processes. These data indicate that pockmarks formed due to the release of gas derived from the breakdown of near-surface organic material, with gas accumulation aided by the sealing properties of the sediments. Sources of organic matter include adjacent carbonate banks and buried paleochannels. Polychaetes biodiversity appears to be affected negatively by the conditions surrounding dense pockmark fields since higher biodiversity is associated with low density fields. While regional bi-directionality of pockmark scours corresponds to modelled tidal flow, localised scattering around banks suggests turbulence. This multi-scale information therefore suggests that pockmark scours can act as proxy for bottom currents, which could help to inform modelling of benthic biodiversity patterns.

Seafloor pockmarks and gas seepages, northwestern Bay of Fundy, New Brunswick, Canada

Bathymetric images of linear and circular pockmark depressions on the Bay of Fundy seafloor, offshore eastern Canada, are interpreted to have been caused by escaping gas from underlying sediment or bedrock. Shallow interstitial gas within marine sediments restricts seismic reflection imaging (acoustic masking) of sub-bottom units resulting in obscured reflections within seismic profiles, confirming that interstitial gas is present in the underlying units. Pockmark fields are frequent in several bays and shallow coastal areas along the northern coast of the Bay of Fundy. The largest field containing over 10 000 pockmarks occurs in Passamaquoddy Bay, an estuary underlain by igneous and metamorphic bedrock. These features are interpreted to be caused by generation of biogenic methane from the microbial breakdown of organic matter buried within Holocene-age sediments and along the underlying Pleistocene/Holocene unconformity. The unconformity is recorded as a distinctive horizon that represents a time when glacier recession resulted in exposure of parts of the bay to sub-aerial erosion and growth of terrestrial vegetation at locations subsequently submerged by post-glacial transgression. Three areas of potential thermogenic gas occurrence were identified in seismic profiles collected south and east of The Wolves islands, New Brunswick. The underlying bedrock has not yet been precisely mapped, although outliers of Carboniferous-age bedrock that is the major petroleum source in New Brunswick may extend into this area of the Bay of Fundy.

Sedimentary processes and cold-water coral mini-mounds at the Ferrol canyon head, NW Iberian margin

The western Iberian margin has a complex morphology controlled by both geological and oceanographic processes. Compared to the submarine canyons in the southern part of this margin (e.g. Nazaré and Setubal), the canyons in the northern part (e.g. Ferrol and A Coruña) have received little attention. This study maps the geomorphological features around the Ferrol canyon head and combines them with oceanographic observations to infer the sedimentary and oceanographic processes active in this area. Furthermore, the occurrence of a cold-water coral (CWC) mini-mound province near the canyon head and the Ortegal Spur pockmark field is investigated with regard to seepage processes, oceanographic conditions and anthropogenic impact.The Ferrol canyon head and outer Ortegal Spur are characterized by erosional (erosional and abraded surfaces, contourite channels and furrows), depositional (contourite drifts and sediment waves) and mixed (contourite terrace) features, indicating a dominant control of bottom currents on the sedimentary processes. Bottom currents are related to the interaction of the canyon head topography with both the Mediterranean Outflow Water (MOW) slope current and with M2 internal tides associated to the interface (pycnocline) between Eastern North Atlantic Central Water (ENACW) and MOW. Due to this interaction, the Ferrol canyon head is subject to active sediment resuspension and forms a potential source area for the intermediate nepheloid layers along the NW Iberian upper slope.The CWC mini-mounds (1.9–2.7 m high and 75–141 m in diameter) occur between 400 and 560 m depth. Seismic facies signatures indicative of fluid-flow and morphometric analysis implies that larger CWC mini-mounds adjacent to the pockmark field initiated by colonization of pre-existing pockmarks, while smaller more clustered mounds developed independently. The observed mini-mounds are relict features likely related to their position outside of the contemporary ENACW-MOW interface which favours CWC growth through (1) increased bottom currents, enhancing food supply and (2) the presence of a potential density envelope σθ = 27.35–27.65 kg/m3, proposed to control coral larvae transport along the NE Atlantic margin. Alternatively, the lack of contemporary coral growth might be related to habitat destruction by bottom trawling. However, a preliminary age constraint reveals CWC growth occurred during the early Holocene, coeval to relict CWC mini-mounds near canyon heads on the Armorican margin. Their existence suggests a regional shift in the NE Atlantic density profile where the ENACW-MOW interface occurred up to 200 m above its contemporary position.

Deep-sea geohazards in the South China Sea

Various geological processes and features that might inflict hazards identified in the South China Sea by using new technologies and methods. These features include submarine landslides, pockmark fields, shallow free gas, gas hydrates, mud diapirs and earthquake tsunami, which are widely distributed in the continental slope and reefal islands of the South China Sea. Although the study and assessment of geohazards in the South China Sea came into operation only recently, advances in various aspects are evolving at full speed to comply with National Marine Strategy and ‘the Belt and Road’ Policy. The characteristics of geohazards in deep-water seafloor of the South China Sea are summarized based on new scientific advances. This progress is aimed to aid ongoing deep-water drilling activities and decrease geological risks in ocean development.