Active Seepage Research Articles

FMAW-YOLOv5s: A deep learning method for detection of methane plumes using optical images

Natural gas hydrates stored in the subsurface seabed of continental margins are one of the most important carbon reservoirs on Earth. Research on natural gas hydrates is of great significance to global warming and ecological protection. Methane plumes caused by crustal dynamics are usually considered as a sign of existence of natural gas hydrates. Detection of methane plumes thus becomes the first step of cold seep research. This paper conducts comprehensive research on detection of methane plumes based on deep learning methods and optical images. First, we proposed a method of creating high quality and balanced datasets for methane plumes detection tasks using open-source videos. We then proposed a FMAW-YOLOv5s method for methane plumes detection. The FMAW-YOLOv5s method improves the traditional YOLOv5s in design of backbone network, neck network and loss function. The FMAW-YOLOv5s method can realize accurate and fast detection of methane plumes with a precision of 96.9% and FPS of 141.7. The lightweight feature of FMAW-YOLOv5s also enables the deployment in edge computing devices such as AUVs and ROVs. This research can not only promote the study of cold seep activities, but also provide meaningful insights for detection of other underwater events such as gas pipelines leakage.

Postglacial ocean methane release from the northern South China Sea

Methane release into seawater and the atmosphere as a result of cold seepage activity is a sensitive indicator of global environmental change during the last glacial period. However, the link between postglacial ocean methane escape and possible increases in atmospheric methane concentration remains unclear, which hinders the evaluation of methane impact on climatic variability. In this study, lipid biomarkers were analyzed on samples from the Shenhu area in the South China Sea to reconstruct the marine gas hydrate variability since the last glacial time. The reconstructed methane release during the cool Last Glacial Maximum and Younger Dryas agreed well with previous findings. Interestingly, our results identified a pronounced methane outgassing at the end of Heinrich 1, which was likely triggered by the abrupt increase in intermediate water temperature. The occurrence of this gas hydrate dissociation before the drastic rise in atmospheric methane during Bølling-Allerød implied that the change in the gas hydrate reservoir could likely indirectly contribute to the initiation of the warming event, though more global evidence needs to be provided in the future.

Tidal and seasonal influence on cold seep activity and methanotroph efficiency in the North Sea

The ocean’s methane emission to the atmosphere is dominated by continental shelves where cold seeps are globally common features. Seeps emit methane into the hydrosphere, but temporal variations and controls of seep activity and the efficiency of the microbial methane filter in the water column are scarce. Here we address these knowledge gaps by measuring whole water column methane inventories and methanotrophic activity at a temporal resolution of 2 hours at a North Sea cold seep (Doggerbank) in summer and autumn. We found that bottom water methane inventories were 68% (summer) and 11% (autumn) higher during low tide compared to high tide coinciding with increased methanotrophic activity. The activity of methanotrophs was reduced during autumn when the water column was fully mixed and matched by higher methane emissions to the atmosphere. Our results show that tides are underappreciated controls on seepage and methanotrophic activity and methane sea–atmosphere fluxes.

Enrichment pattern of tungsten in sediments under methane seepage environments: Applicability as a proxy for tracing and reconstructing (paleo-)methane seepage

The release of hydrogen sulfide by sulfate-driven anaerobic oxidation of methane (SD-AOM) during methane seepage activities has a strong impact on the evolution of marine redox conditions and the cycles of redox-sensitive elements (e.g. Mo and U). Consequently, the sedimentary enrichment or depletion of these elements is often used to trace and reconstruct (paleo-)methane seepage activity. In recent years, tungsten (W) has shown promise as an indicator of marine redox changes, but its applicability to trace methane seepage activity is unknown. This study identified significant differences in the W content of sediments in methane seepage environments compared to suboxic environments not influenced by methane seepage and oxic environments in the South China Sea. The sediments subjected to methane seepage were characterized by low W contents, similar to those of euxinic Black Sea sapropels. It is believed that the sulfidic environment resulting from SD-AOM is the primary cause of W depletion in sediments subjected to methane seepage. By examining W's interaction with its twin element, Mo, we present WMo enrichment patterns in various environments. In methane seepage environment (sulfidic environments), sediments had high MoEF (> 10) and low WEF (< 2) with a high Mo/W molar ratio (> 10); in suboxic environments not influenced by methane seepage, sediments exhibited a WMo enrichment pattern similar to the UCC (the MoEF and WEF values are predominantly below 1 and 2, respectively, with a low Mo/W molar ratio, <1.5); in oxic environments (enrichment of FeMn (oxy)hydroxides), sediments exhibited high MoEF (2 < MoEF < 10) and high WEF (> 2), with a medium Mo/W molar ratio (1.5–10). This further indicates that the Mo/W molar ratio has great potential in tracing and reconstructing (paleo-)methane seepage activities.

Carbon budget of methane seepages in the Haiyang 4 Area in the northern slope of the South China sea

Methane seepage records information of the local carbon cycle with respect to the generation, consumption and sequestration of carbon. Here presents the investigation of 7 gravity cores retrieved in 2004 during cruise SO-177 in the Haiyang 4 Area at the northern slope of the South China Sea. Porewater solutes, sulfate, methane, total alkalinity, sulfide and calcium demonstrate currently the weak seep activity. Local carbon cycling and sequestration is also revealed, that dominates by anaerobic oxidation of biogenic methane to dissolved bicarbonate inducing calcium carbonate and iron sulfide minerals (mainly pyrite) precipitation. A reactive transport model was employed to quantify the carbon cycle and budget. Model results show that current methane fluxes contribute to less than 2 wt % of authigenic carbonates and 2 wt % iron sulfide minerals being precipitated in 600–800 cm sediment depth. The sequestration of carbon could be > 50 mmol C cm−2 yr−1 under in situ condition. The observed increase of carbonate and iron sulfide minerals at ∼100 cm, however, require higher methane fluxes to shift the zone of anaerobic oxidation of methane upwards to around 1 m below the seafloor, which have occurred during sea level low stands in the geological past. The oscillation of seepage flux contributed to the formation of multiple layers of authigenic carbonates and pyrite, which indicates the high capability of carbon sink and is speculated to be induced by the dissociation of the underlying hydrates triggered by sea level drop and or temperature increase.

Impacts of temperature and fluid seepage on organic matter composition in sediments of an active hydrothermal basin

Marine sediments are one of the largest organic carbon (OC) sinks on Earth. Yet, major knowledge gaps remain in our understanding of sedimentary OC cycling, particularly regarding temperature-induced alteration processes of OC from different sources. Here, we investigate OC-rich sediments of Guaymas Basin (Gulf of California) across two hydrothermal areas, one with only conductive geothermal heating and the other additionally experiencing seepage of hydrocarbon-rich fluids from deeper layers. We use Pyrolysis-Gas Chromatography/Mass Spectrometry (Py-GC/MS) to investigate diagenetic OC changes and show that cold control sites in both hydrothermal areas are dominated by similar contributions of lipid-derived compounds, nitrogenous (likely protein-derived) compounds, carbohydrates, and polyaromatic hydrocarbons (PAHs). These OC compound groups are largely derived from phytoplankton detritus from overlying water. Conductively heated sediments, which reach in situ temperatures of ∼ 80 °C, have similar general OC compositions and contents to these cold sites, but show evidence of diagenetic modifications of individual carbohydrate groups in deeper layers. By contrast, strong decreases in carbohydrate and nitrogenous compound abundances at the seep sites indicate that these compound groups are not only modified but also selectively degraded at the higher temperatures (>80 °C) of these sites. Increases in pyrolysis products of PAHs, prist-1-ene, and alkanes with depth, moreover, show that import of OC by deep hydrothermal fluids contributes significantly to sedimentary OC pools mainly in deeper layers of these sites. Our study provides the first comprehensive analysis of major OC compound groups in Guaymas Basin sediment and indicates that the supply of OC by hydrothermal fluid flow only has minor impacts on particulate organic matter compositions at the seafloor, even at active seep sites. We furthermore show that temperatures up to ∼ 80 °C already result in thermochemical modifications of organic matter (OM) that are potentially linked to the onset of kerogen formation. The sequence and time scales of chemical modifications and activations in relation to temperature are an important subject for future investigations.

Contrasting Methane Seepage Dynamics in the Hola Trough Offshore Norway: Insights From Two Different Summers

AbstractThis study investigates the temporal variations in methane concentration and flare activity in the Hola trough (offshore Norway) during May 2018 and June 2022. Between these time periods, methane seep activity exhibits 3.5 times increase, as evidenced by hydroacoustic measurements. As the seep area in the Hola trough is constantly within the hydrate stability zone, the observed increase cannot be attributed to migration of its shallow boundary due to temperature increase. However, a combination of low tide conditions resulting in a lower sediment pore pressure and a bottom water temperature increase resulting in a lower methane solubility is likely to explain the increase in the number of seeps observed in June 2022. The hypothesis of tide influence is supported by data collected from a piezometer deployed and recovered during the cruise showing that the tidal effect was observed 3 m below the seafloor. Despite the numerous methane seeps detected, methane concentration and gas flow rates near the seafloor were low (&lt;19 nM and &lt;70 mL min−1, respectively) compared to other areas with methane seep activity. This is likely due to strong currents rapidly dispersing methane in the water column. Sub‐seafloor investigations identified pathways for gas migration in methane seep areas, influenced by topography. This study provides valuable insights into the temporal dynamics of methane concentrations, flare activity, and gas distribution in the Hola trough, contributing to our understanding of offshore methane dynamics in the region.

Formation of dolomites from the newly discovered ancient cold seep in the Middle Dongsha area of the South China Sea

Remotely operated vehicle (ROV) observations have recently revealed seep dolomite deposits covering the seafloor on Middle Dongsha of the South China Sea (SCS). Spurred by these findings, we studied the mineralogical and geochemical properties to examine the seep dolomite’s origin, precipitation mechanism, and formation environment. These carbonates mainly comprise dolomite (from 64 to 84 wt%), followed by small amounts of calcite (from 1 to 3 wt%), locally containing magnesite (∼4 wt%), and goethite (∼14 wt%). Extensively developed mineralized cells containing high Mg and Ca contents found in chimneys imply that microbially-mediated processes probably participated in the reactivation of dolomite precipitation. The moderate and concentrated δ13C values (from −36.64 to −39.16‰, V-PDB) indicated that the carbon in carbonates was mainly derived from thermogenic methane. The methane fluids most likely ascended beneath the volcaniclastic unit and inherited enriched positive Eu anomalies due to fluid–rock interactions during their seepage along tectonic faults to the seafloor. The δ18O values (4.2–4.8‰, V-PDB) of carbonate suggested that an 18O-depleted fluid was involved in carbonate precipitation. The high total Rare-Earth Element contents (ΣREE) of seep carbonates varied from 61 ppm to 92 ppm, reflecting the chemical composition of surrounding pore waters at the time of carbonate precipitation, exhibiting less mixing with seawater. It may also indicate higher alkalinity and pH of the pore water. Although the sea-like REE patterns (HREE-enriched, negative Ce anomalies) are not caused by seawater, they may be due to the complexation competition between humic acid (HA) and carbonate for REE at high pore-water alkalinity and high CO32− concentrations. This is consistent with the dolomite chimney pointing toward a formation environment in relatively deep sediments, in particular, close to the sulfate-methane interface in sediments, where the bicarbonate concentration reaches its maximum due to AOM. Moderate Mo and no U enrichments appeared in the carbonates, suggesting a relatively lower intensity of methane seepages with relatively reduced AOM rates. The wide range of (Mo/U)EF values and U–Mo covariation indicates the very dynamic AOM rate (slow to moderate) and redox conditions (mostly suboxic–anoxic, occasionally sulfidic conditions) during carbonate formation due to episodically changing seepage activity.

Trophic relationship between mussels and scale worms under various seepage intensities in the haima cold seep: Insights from stable isotopes (δ13C and δ15N) and C:N:P stoichiometry

The deep-sea mussels Gigantidas haimaensis (Mytilidae: Bathymodiolinae) usually contain one scale worm Branchipolynoe pettiboneae in their mantle cavity in the Haima cold seep, South China Sea. To explore their environmental adaptation and coexistence mechanisms, the stable isotopes (δ13C and δ15N) and C:N:P ecological stoichiometry of G. haimaensis and the associated B. pettiboneae were investigated under different methane seepage intensities. In the presence of seepage, most mussels harbored one scale worm in their mantle cavity. However, under seepage cease, the physiological status of mussels looked unhealthy, and no scale worm appeared in their bodies. The variation in δ13C values was great among different mussel tissues, ranging from −49.7‰ to −57.8‰. The δ13C values of mussel tissues followed the order of foot > gill > mantle under active seepage, while no regular trend was found under seepage cease. The δ13C and δ15N of scale worms were averagely enriched by 2.9 ‰ and 3.2 ‰ relative to their mussel hosts, and the trophic niche separation between scale worms and mussels was more significant under active seepage. The δ13C value of mussel foot was significantly higher under active seepage (average −50.8‰) than under seepage cease (average −55.1‰), indicating that mussels might ingest and assimilate more 13C-rich suspended particulate organic matter (POM) under active seepage. In mantle tissues, the high C:N ratio and low δ13C value should be attributed to the high content of energy storage substances. The C:N ratio of mantle dramatically declined with the exhaustion of energy storage materials under seepage cease. The variation of P content was most significant in the gill, which might be regulated by the abundance of symbiotic bacteria. It can be speculated that the dramatic decline of P content in the gill was attributed to the loss of high-P bacterial symbionts under seepage cease. Both the C:P and N:P ratios of gill tissues significantly increased under seepage cease, which suggested the decline of food quality for scale worms. Mussel-dwelling scale worms might detect the change of food quality and abandon their starving mussel host. Our results help to better understand the environmental adaptation of chemoautotrophic mussels and scale worms under unstable seepage intensity in cold seep ecosystems.

Influence of methane seep activities and sea-level changes on elemental and isotopic compositions and abundance of carbonates in sediments of the Okinawa Trough since the last glacial period

The elemental and isotopic compositions and abundance of carbonates in marine sediments are used to reconstruct ancient climate and ocean conditions. These carbonates usually include biogenic and authigenic carbonates in origin, which are controlled by sedimentary and diagenetic processes, respectively. However, the specific mechanisms involved in controlling the origin of biogenic and authigenic carbonates are still unclear. To address this knowledge gap, we analyzed core CSHC-4 sediment from the Okinawa Trough to examine the elemental and isotopic compositions of carbonates and their relationship to methane seepage and sea-level changes. Our results indicate that high-Mg calcite is the dominant authigenic carbonate, indicating moderate methane seep intensity. Negative carbon isotope values as low as −4.06‰ V-PDB of the bulk sediments deposited in the glacial period suggest a connection between the formation of authigenic carbonates and methane seepage. Negative oxygen isotope values as low as −2.6‰ V-PDB may be attributed to the input of 18O-depleted fluids influenced by gas hydrate formation. Biogenic carbonates contribute the majority (96.3%) of the total carbonate contents in sediments. Biogenic carbonate contents are lower during glacial periods and higher during interglacial periods. The reduced carbonate content during glacial periods may be due to lower export productivity influenced by the deepening of North Pacific Intermediate Water, as well as terrestrial inputs and anoxic conditions during low sea level periods. Our findings highlight the importance of considering both biogenic and authigenic carbonates when reconstructing paleoclimate and paleoceanography based on their geochemical characteristics.

Characterizing Håkon Mosby Mud Volcano (Barents Sea) cold seep systems by combining ROV-based acoustic data and underwater photogrammetry

Cold-seeps have a unique geo-ecological significance in the deep-sea environment. They impact the variability of present-day submarine sedimentary environments, affecting the evolution of the landscape over time and creating a variety of submarine landforms, one of which is Mud Volcanoes (MVs). MVs form due to the extrusion of mud, fluids, and gas, mainly methane, from deeper sedimentary layers. These natural gas seepage systems could significantly affect climate change and the global carbon cycle. We present a comprehensive method that combines ROV-based multibeam mapping and underwater photogrammetry to enhance the understanding of the physical relationships between geomorphic units characterizing the Håkon Mosby Mud Volcano (HMMV) and the distribution of associated habitats. HMMV is indeed characterized by high thermal and geochemical gradients from its center to the margins resulting in a clear zonation of chemosynthetic communities. Our approach integrates multi-resolutions and multi-sources data acquired using a work-class ROV. The ROV-based microbathymetry data helped to identify the different types of fine-scale submarine landforms in the central part of HMMV. This revealed three distinct geomorphic units, with the central hummocky region being the most complex. To further study this area, ROV images were analyzed using a defined Structure from Motion workflow producing millimetric resolution 2D and 3D models. Object-Based Image Analysis (OBIA), applied on orthomosaics, allowed us to obtain a fine classification of main benthic communities covering a total area of 940m2, including the active seepage area of the hummocky rim. Four major substrate types were identified in these regions: uncovered mud, bacterial mats high-density, bacterial mats low-density, sediments and tubeworms. Their relationship with terrain morphology and seepage activity were investigated at different spatial scales, contributing to a deeper understanding the ecological functioning of cold seep ecosystems in MVs. The applied workflow is proposed as an innovative processing technique for future studies on cold-seep systems. Geomorphic and ecological processes in extreme environments are inherently linked and marked by spatial patterns typifying associated habitats and sedimentary environments. This is poorly investigated in previous studies, leaving a substantial gap in the geomorphological drivers responsible for habitat distribution and extent in cold seep systems.

Microbial communities and mineral assemblages in sediments from various habitats at the Haima Cold Seep, South China Sea

Cold seeps create diverse habitats in the deep sea and play an important role in the global carbon cycling. Anaerobic oxidation of methane (AOM) and biogenic mineralization are essential carbon pathways of methane and carbon transformation in cold seeps, however, the effects of habitat heterogeneity on the processes are still poorly understood. In this study, we investigated the microbial communities and mineral assemblages at distinct habitats in the Haima cold seep and their relationships with environmental factors. These habitats were classified as methane seep site (MS), seep-free faunal habitat (FH), and control site (CS). Bacterial communities were significantly different among the three habitats. ANME-3 archaea, Sulfurovum bacteria, and mineralization-associated microbes (e.g., Campylobacterales) were detected in high relative abundances at ROV2. Mineralogical analysis revealed abundant calcite minerals at the seep site, indicating that authigenic carbonate minerals were formed at highly active seep. Multivariate statistical analysis demonstrated that the concentrations of SO42–, Ca2+, and Mg2+ were significantly correlated with the presence of calcite minerals and bacterial communities. These results suggested that AOM-accompanied authigenic carbonate formation is an important factor influencing the mineral assemblages in seep habitats. This finding improves our understanding of marine microbial carbon cycling.

Methane-flow system within the Nyegga pockmark field, offshore mid-Norway

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.

Exploring carbon content variation in microplastics sequestrated from seawater to sediment in the Haima cold seep area

In the decades since plastic has become widely used, deep-sea areas, specifically cold seeps, have developed into plastic sinks. Cold seeps contain clean energy natural gas hydrates and act as a barrier reducing methane migration to the upper water column. However, the impacts of microplastics (MPs) on the carbon content in the cold seep remain unclear. In this study, we explored spatial changes in the MPs’ carbon content (MPC) selecting the Haima cold seep (HCS) as the study area. The main conclusions are as follows: (1) For active seepage areas, the mass abundance of the MPs increases with the methane seepage strength in all water columns and sediment of strong seepage areas. It decreases with the seepage strength in the sediment cores in other areas. (2)The MPC is positively correlated with the depth of the water column in the non-seepage area, while it is negatively correlated in the sediment core. (3) The surface roughness of the MPs was greater in the middle of the water column and the sediment core at ROV1. In the high-pressure and oligotrophic cold seep, the amount and method of microbial utilization of carbon from the MPs deserve greater attention.

Stress Constraints From Shear‐Wave Analysis in Shallow Sediments at an Actively Seeping Pockmark on the W‐Svalbard Margin

AbstractMechanisms related to sub‐seabed fluid flow processes are complex and inadequately understood. Petrophysical properties, availability of gases, topography, stress directions, and various geological parameters determine the location and intensity of leakage which change over time. From tens of seafloor pockmarks mapped along Vestnesa Ridge on the west‐Svalbard margin, only six show persistent present‐day seepage activity in sonar data. To investigate the causes of such restricted gas seepage, we conducted a study of anisotropy within the conduit feeding one of these active pockmarks (i.e., Lunde Pockmark). Lunde is ∼400–500 m in diameter, and atop a ∼300–400 m wide seismic chimney structure. We study seismic anisotropy using converted S‐wave data from 22 ocean‐bottom seismometers (OBSs) located in and around the pockmark. We investigate differences in symmetry plane directions in anisotropic media using null energy symmetries in transverse components. Subsurface stress distribution affects fault/fracture orientations and seismic anisotropy, and we use S‐wave and high‐resolution 3D seismic data to infer stress regimes in and around the active seep site and study the effect of stresses on seepage. We observe the occurrence of changes in dominant fault/fracture and horizontal stress orientations in and around Lunde Pockmark and conclude minimum (NE‐SW) and maximum (SE‐NW) horizontal stress directions. Our analysis indicates a potential correlation between hydrofractures and horizontal stresses, with up to a ∼32% higher probability of alignment of hydrofractures and faults perpendicular to the inferred minimum horizontal stress direction beneath the Lunde Pockmark area.

Geophysical and Geochemical Exploration of the Pockmark Field in the Gulf of Patras: New Insights on Formation, Growth and Activity

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.

Occurrence and Genesis of Cold‐Seep Authigenic Carbonates from the South‐Eastern Mediterranean Sea

AbstractMethane‐derived authigenic seep carbonates occur globally along continental margins. These carbonates are important archives to identify seep dynamics, the source of the ascending methane‐enriched fluids together with their timing, and are an important carbon sequestration mechanism. Recently, seep carbonates were discovered in the Levant Basin in the south‐eastern Mediterranean Sea. To elucidate past seepage activity and dynamics across the basin, different seep carbonate morphologies (chimneys, crusts and pavements) retrieved from the Levant Basin were mapped based on remotely operating vehicle data and analysed using standard sediment petrographic techniques, X‐ray diffraction and stable carbon and oxygen isotope analyses. Carbonate chimneys consist of micrite (δ13CVPDB of −10‰ to +5‰) with dispersed baryte and dolomite crystals, fan‐shaped aragonite (δ13CVPDB of −52‰ to −30‰) and high‐magnesium calcite cements, with the latter often growing from low‐magnesium calcite spherules. Botryoidal low‐magnesium calcite cements are forming in small cavities. Carbonate crusts consist of micrite with low‐magnesium calcite breccias, high‐magnesium calcite nodules (δ13CVPDB of −35‰ to −20‰) and cements, and partially replaced fan‐shaped aragonite cements. Carbonate pavements consist of low‐magnesium calcite microsparite, micritic dolomite and high‐magnesium calcite. Fan‐shaped aragonite is locally present as pore‐lining cement. Iron oxides are often seen coating the low‐magnesium calcite, high‐magnesium calcite and dolomite cements. Chimneys and crusts, characterised by high amounts of high‐magnesium calcite and aragonite, are interpreted to have formed through advective methane fluxes. Pavements, with high quantities of dolomite, are explained as the product of diffusive methane flux. Sediment petrographic and geochemical analysis of the different carbonate morphologies and cement phases therein witness distinct modes of ascending fluid fluxes and their mixing with marine pore water and/or sea water during precipitation of the individual phases.

Diving deeper into seep distribution along the Cascadia convergent margin, United States

Previous margin-wide studies of methane seep distribution along the Cascadia Subduction Zone indicate peaks in seep density within the landward limit of the of gas hydrate stability zone (GHSZ; ≤500 m depth), suggesting a link between current ocean warming, acceleration of hydrate dissociated, and methane emissions. This inferred connection, however, may not account for regional geologic and/or structural complexities driving methane seepage. Expanding upon an existing seep database by adding new seep data, we conducted statistical and spatial analyses to determine margin-wide distribution trends and offer a tectonic framework for understanding the tendency toward non-normality and spatial clustering. We then highlight the role of local-scale drivers of seep formation in addition to the first-order tectonic framework, using systematic geologic/geomorphic characterization of seep emission sites in southern Cascadia and case studies using meta-attribute analysis of seismic reflection data. Seep distribution along the margin is non-random, but instead of clustering along the 500-m isobath, regions of high seep density occur in canyons and topographic highs. New findings from this study conclude that co-location of the outer arc high (OAH) and the landward limit of the GHSZ may explain high concentrations of seeps where deformation is the greatest and hydrates are unstable. Detailed analysis of the spatial relationships between seep sites and geologic-geomorphic features in southern Cascadia reveal a link between seeps and anticlines, with 52% of the seeps found in association with anticlines, 36% found at faults, 16% associated with canyons, and 11% at seafloor failure scarps. Given that a majority of anticlines are located along or seaward of the OAH in the actively deforming outer wedge, we suggest that the location of the OAH is a primary structural control on seep distribution. This scenario is supported by neural network analysis of multichannel seismic data revealing zones of probable fluid migration along vertical pipes, faults, and chimneys in the vicinity of active seep sites on anticlines. Determining linkages between seeps and submarine tectonic geomorphology is a crucial first step for understanding and forecasting the distribution of methane seepage, but also a necessity for evaluating causal relationships between ocean warming and gas hydrate stability.

Late Quaternary paleoceanography of Vestnesa Ridge, Fram Strait: Ostracode species as a potential indicator of cold seep activity

Abstract Past intensity of methane release from deep-ocean methane hydrates continues to be challenging to reconstruct reliably. Here, we used fossil ostracode fauna paired with foraminiferal δ13C values in a marine sediment core from Vestnesa Ridge, western Svalbard margin, to reconstruct methane seepage activity during the late Quaternary and to examine faunal response to deglacial climatic changes. Benthic foraminiferal δ13C values indicate methane seepage activity was relatively strong during marine isotope stage 2, corresponding to a high percentage of the ostracode Rosaliella svalbardensis in the assemblage. In contrast, this species was absent under conditions of no or very strong seepage of methane. Faunal changes in other taxa were more related to global climate changes regardless of the seepage activity. This result indicates that Rosaliella svalbardensis is a potential new useful proxy for past methane release.

Sedimentary deformation relating to episodic seepage in the last 1.2 million years: a multi-scale seismic study from the Vestnesa Ridge, eastern Fram Strait

Seafloor hydrocarbon seepage is a natural fluid release process that occurs worldwide on continental shelves, slopes, and in deep oceanic basins. The Vestnesa sedimentary ridge in the eastern Fram Strait hosts a deep-water gas hydrate system that became charged with hydrocarbons ∼2.7 Ma and has experienced episodic seepage along the entire ridge until a few thousand years ago, when seepage activity apparently ceased in the west but persisted in the east. Although it has been documented that faults and fractures play a key role in feeding the seeps with thermogenic gases, the mechanisms controlling seepage periodicity remain poorly understood. Here we integrate high-resolution P-cable 3D seismic and Chirp data to investigate the spatial and temporal evolution of high-resolution fractures and fluid flow features in the west of the Vestnesa Ridge. We characterize sediment deformation using a fracture density seismic attribute workflow revealing two highly deformed stratigraphic intervals and associated small-scale pockmarks (&amp;lt;20 m diameter). Chronostratigraphic constraints from the region show that these two highly deformed intervals are influenced by at least three major climatic and oceanic events during the last 1.2 million years: the Mid-Pleistocene Transition (∼1.25–0.7 Ma), the penultimate deglaciation (∼130 ka) and the last deglaciation (Heinrich Stadial 1: ∼16 ka). These periods of deformation appear associated with seismic anomalies potentially correlated with buried methane-derived authigenic carbonate and have been sensitive to shifts in the boundary of the free gas-gas hydrate interface. Our results show shifts (up to ∼30 m) in the depth of the base of the gas hydrate stability zone (GHSZ) associated with major changes in ocean bottom water temperatures. This ocean-driven effect on the base of the GHSZ since the Last Glacial Maximum coincides with the already highly deformed Mid-Pleistocene Transition sedimentary interval and likely enhanced deformation and gas leakage along the ridge. Our results have implications for understanding how glacial cycles impact fracture formation and associated seepage activity.