Shallow Gas Hydrate Research Articles

Authigenic carbonates from active methane seeps offshore southwest Africa

The southwest African continental margin is well known for occurrences of active methane-rich fluid seeps associated with seafloor pockmarks at water depths ranging broadly from the shelf to the deep basins, as well as with high gas flares in the water column, gas hydrate accumulations, diagenetic carbonate crusts and highly diverse benthic faunal communities. During the M76/3a expedition of R/V METEOR in 2008, gravity cores recovered abundant authigenic carbonate concretions from three known pockmark sites—Hydrate Hole, Worm Hole, the Regab pockmark—and two sites newly discovered during that cruise, the so-called Deep Hole and Baboon Cluster. The carbonate concretions were commonly associated with seep-benthic macrofauna and occurred within sediments bearing shallow gas hydrates. This study presents selected results from a comprehensive analysis of the mineralogy and isotope geochemistry of diagenetic carbonates sampled at these five pockmark sites. The oxygen isotope stratigraphy obtained from three cores of 2–5 m length indicates a maximum age of about 60,000–80,000 years for these sediments. The authigenic carbonates comprise mostly magnesian calcite and aragonite, associated occasionally with dolomite. Their very low carbon isotopic compositions (–61.0 < δ13C ‰ V-PDB < –40.1) suggest anaerobic oxidation of methane (AOM) as the main process controlling carbonate precipitation. The oxygen isotopic signatures (+2.4 < δ18O ‰ V-PDB < +6.2) lie within the range in equilibrium under present-day/interglacial to glacial conditions of bottom seawater; alternatively, the most positive δ18O values might reflect the contribution of 18O-rich water from gas hydrate decomposition. The frequent occurrence of diagenetic gypsum crystals suggests that reduced sulphur (hydrogen sulphide, pyrite) from sub-seafloor sediments has been oxidized by oxygenated bottom water. The acidity released during this process can potentially induce the dissolution of carbonate, thereby providing enough Ca2+ ions for pore solutions to reach gypsum saturation; this is thought to be promoted by the bio-irrigation and burrowing activity of benthic fauna. The δ18O–δ13C patterns identified in the authigenic carbonates are interpreted to reflect variations in the rate of AOM during the last glacial–interglacial cycle, in turn controlled by variably strong methane fluxes through the pockmarks. These results complement the conclusions of Kasten et al. in this special issue, based on authigenic barite trends at the Hydrate Hole and Worm Hole pockmarks which were interpreted to reflect spatiotemporal variations in AOM related to subsurface gas hydrate formation–decomposition.

Thermal anomalies associated with shallow gas hydrates in the K-2 mud volcano, Lake Baikal

Thermal measurements and hydrate mapping in the vicinity of the K-2 mud volcano in Lake Baikal have revealed a particular type of association of thermal anomalies (29–121 mW m–2) near hydrate-forming layers. Detailed coring within K-2 showed that hydrates are restricted to two distinct zones at sub-bottom depths exceeding 70–300 cm. Temperature data from stations with hydrate recovery and degassing features all display low thermal gradients. Otherwise, the thermal gradients within the mud volcano are generally increased. These findings imply a more complicated thermal regime than often assumed for mud volcanoes, with important roles for both fluids and hydrates. The coexistence of neighbouring low and high thermal anomalies is interpreted to result from discharging and recharging fluid activity, rather than hydrate thermodynamics. It is suggested that hydrates play a key role in controlling the fluid circulation pattern at an early stage. At a later stage, the inflow of undersaturated lake water would favour the dissolution of structure I hydrates and the formation of structure II hydrates, the latter having been observed on top of structure I hydrates in the K-2 mud volcano.

Gas hydrate pingoes: Deep seafloor evidence of focused fluid flow on continental margins

Gas hydrates in the shallow subsurface form one of the largest reservoirs of methane in the global organic carbon cycle. Seafloor seeps and associated features represent the venting points of methane released from the shallow lithosphere to the hydrosphere and atmosphere. Here we document the discovery of seep-related seafloor mounds in the Kwanza Basin, offshore Angola, and employ high-resolution three-dimensional seismic analysis to unravel the subsurface plumbing system and the origin of mounds. Mounds with distinct morphologies and geophysical signatures illustrate different development stages associated with the formation and dissociation of shallow gas hydrate, linked to thermogenic fluid migration along salt diapir flanks draining deeply buried salt minibasins. The mounds are more than an order of magnitude larger than previously described submarine hydrate pingoes, and comparable to hydraulic pingoes commonly found in terrestrial periglacial environments, suggesting hydrate volumes of individual mounds up to 1.1 × 106 m3 (equivalent to 2.0 × 108 m3 of methane gas). The interpretation of seismically well-defined seep-related seafloor mounds brings new insight to the occurrence and development of concentrated near-surface gas hydrate accumulations and their relationship with thermogenic fluid migration and host sediment properties along continental margins.

MD179調査より得られた日本海東縁域海底表層堆積土の物理・力学的性質

Gas hydrates are attracting attention as a next-generation energy source. On the other hands, Methane gas contained in the natural gas hydrates has approximately 20 times the greenhouse effect of CO2. There are concerns that dissociation of methane gas from the gas hydrates distributed in submarine surface layers (shallow type gas hydrates) by rising ocean temperatures or by vaporization at recovery of the hydrates for energy may contribute to global warming, which in turn may raise the sea level and causing climatic instability. In addition, many gas-hydrate-bearing areas are distributed near the boundaries of tectonic plates, as shown in the figure. Gas hydrates in the surface layer of seafloor may dissociate when seismic activities cause seafloor landslides that in turn cause gas hydrate-bearing layers to fail. There have been concerns over the environmental effects of shallow type gas hydrates, but surveys for shallow type hydrates have been few.The purpose of this study is to understand the soil properties of the sea-bottom sediment obtained from the Eastern Margin of Japan Sea. The samples were collected from 5 regions (Umitaka Spur, Joetsu Ridge, Toyama Trough, West of Tsugaru and west of Okushiri Ridge), and the physical (soil density and water content) and mechanical (cone penetration ratio and vane shear strength) tests were performed by on-board used for the sea-bottom sediments.From the test results, it is found that the physical properties of the sediment samples obtained in these areas have no marked difference regardless of the locations or depth of sampling. However, the soil strength of sea-bottom sediments obtained gas hydrates was lower than those of other sediments. It would seem that this is because the effect of methane hydrates dissociation and vaporization of dissolved gas in the pore water at sampling.

表層型ガスハイドレート賦存海底・湖底地盤から採取した堆積土の土質特性

表層型ガスハイドレートが存在しているオホーツク海サハリン島沖およびロシア・バイカル湖において，海底・湖底表層地盤から堆積土を採取した。採取した堆積土に対して，調査船上で小型のベーンせん断試験とコーン貫入試験を行い堆積土の力学特性を調べた。また，試料引き上げ時の間隙水圧減少により，溶存ガスが気泡化し試料に乱れが生じることから，ガス濃度と強度との関係についても調べた。さらに，試料採取から室内試験に至るまでの応力条件を再現した実験を行った。その結果，間隙水中の溶存ガスの濃度が高いほど，船上試験においても再現実験においても強度低下が大きくなる傾向にあった。また，溶存ガスの気泡化により乱れた試料からも，乱れの影響のない強度をある程度推定可能であることも示された。

High-intensity gas seepage causes rafting of shallow gas hydrates in the southeastern Black Sea

Submarine gas hydrates are a major global reservoir of the potent greenhouse gas methane. Since current assessments of worldwide hydrate-bound carbon vary by one order of magnitude, new technical efforts are required for improved and accurate hydrate quantifications. Here we present hydrate abundances determined for surface sediments at the high-flux Batumi seep area in the southeastern Black Sea at 840 m water depth using state-of-the art autoclave technology. Pressure sediment cores of up to 2.65 m in length were recovered with an autoclave piston corer backed by conventional gravity cores. Quantitative core degassing yielded volumetric gas/bulk sediment ratios of up to 20.3 proving hydrate presence. The cores represented late glacial to Holocene hemipelagic sediments with the shallowest hydrates found at 90 cmbsf. Calculated methane concentrations in the different cores surpassed methane equilibrium concentrations in the two lowermost lithological Black Sea units sampled. The results indicated hydrate fractions of 5.2% of pore volume in the sapropelic Unit 2 and mean values of 21% pore volume in the lacustrine Unit 3. We calculate that the studied area of ~ 0.5 km 2 currently contains about 11.3 kt of methane bound in shallow hydrates. Episodic detachment and rafting of such hydrates is suggested by a rugged seafloor topography along with variable thicknesses in lithologies. We propose that sealing by hydrate precipitation in coarse-grained deposits and gas accumulation beneath induces detachment of hydrate/sediment chunks. Floating hydrates will rapidly transport methane into shallower waters and potentially to the sea–atmosphere boundary. In contrast, persistent in situ dissociation of shallow hydrates appears unlikely in the near future as deep water warming by about 1.6 °C and/or decrease in hydrostatic pressure corresponding to a sea level drop of about 130 m would be required. Because hydrate detachment should be primarily controlled by internal factors in this area and in similar hydrated settings, it serves as source of methane in shallow waters and the atmosphere which is mainly decoupled from external forcing.

Authigenic carbonates from the eastern Black Sea as an archive for shallow gas hydrate dynamics – Results from the combination of CT imaging with mineralogical and stable isotope analyses

Authigenic carbonates associated with cold seeps provide valuable archives of changes in the long-term seepage activity. To investigate the role of shallow-buried hydrates on the seepage strength and fluid composition we analysed methane-derived carbonate precipitates from a high-flux hydrocarbon seepage area (“Batumi seep area”) located on the south-eastern Black Sea slope in ca. 850 m. In a novel approach, we combined computerized X-ray tomography (CT) with mineralogical and isotope geochemical methods to get additional insights into the three-dimensional internal structure of the carbonate build-ups. X-ray diffractometry revealed the presence of two different authigenic carbonate phases, i.e. pure aragonitic rims associated with vital microbial mats and high-Mg calcite cementing the hemipelagic sediment. As indicated by the CT images, the initial sediment has been strongly deformed, first plastic then brittle, leading to brecciation of the progressively cemented sediment. The aragonitic rims on the other hand, represent a presumably recent carbonate growth phase since they cover the already deformed sediment. The stable oxygen isotope signature indicates that the high-Mg calcite cement incorporated pore water mixed with substantial hydrate water amounts. This points at a dominant role of high gas/fluid flux from decomposing gas hydrates leading to the deformation and cementation of the overlying sediment. In contrast, the aragonitic rims do not show an influence of 18O-enriched hydrate water. The differences in δ 18O between the presumably recent aragonite precipitates and the older high-Mg cements suggest that periods of hydrate dissociation and vigorous fluid discharge alternated with times of hydrate stability and moderate fluid flow. These results indicate that shallow-buried gas hydrates are prone to episodic decomposition with associated vigorous fluid flow. This might have a profound impact on the seafloor morphology resulting e.g. in the formation of carbonate pavements and pockmark-like structures but might also affect the local carbon cycle.

Role of 3D seismic for quantitative shallow hazard assessment in deepwater sediments

Increased drilling activities in deep water since 1985, especially in the Gulf of Mexico (GOM), have revealed numerous hazards in the shallow sediments below the seabed, such as fault scarps, gas vents, unstable slopes, and reefs and shallow subsurface geologic hazards such as faults, gas-charged sediments, buried channels, abnormally pressured sands, and gas hydrates. Drilling risks associated with shallow aquifer-pressured sands or shallow water flow (SWF) sands and gas hydrates have received the most attention. These are global problems. In the GOM alone, more than US$250 million has been lost due to SWF. While the industry has matured in handling these problems, it is estimated that the losses associated with SWF sands continue to be nearly $1.7 million per well in the GOM alone.

High-resolution P-Cable 3D seismic imaging of gas chimney structures in gas hydrated sediments of an Arctic sediment drift

The newly developed P-Cable 3D seismic system allows for high-resolution seismic imaging to characterize upper geosphere geological features focusing on geofluid expressions (gas chimneys), shallow gas and gas hydrate reservoirs. Seismic imaging of a geofluid system of an Arctic sediment drift at the Vestnesa Ridge, offshore western Svalbard, provides significantly improved details of internal chimney structures from the seafloor to ∼500 m bsf (below seafloor). The chimneys connect to pockmarks at the seafloor and indicate focused fluid flow through gas hydrated sediments. The pockmarks are not buried and align at the ridge-crest pointing to recent, topography-controlled fluid discharge. Chimneys are fuelled by sources beneath the base of gas hydrate stability zone (GHSZ) that is evident at ∼160–170 m bsf as indicated by a bottom-simulating reflector (BSR). Conduit centres that are not vertically straight but shift laterally by up to 200 m as well as discontinuous internal chimney reflections indicate heterogeneous hydraulic fracturing of the sediments. Episodically active, pressure-driven focused fluid flow could explain the hydro-fracturing processes that control the plumbing system and lead to extensive pockmark formation at crest of the Vestnesa Ridge. High-amplitude anomalies in the upper 50 m of the chimney structures suggest formations of near-surface gas hydrates and/or authigenic carbonate precipitation. Acoustic anomalies, expressed as high amplitudes and amplitude blanking, are irregularly distributed throughout the deeper parts of the chimneys and provide evidence for the variability of hydrate and/or carbonate formation in space and time.

The transient electromagnetic response of a resistive sheet: an extension to three dimensions

SUMMARY Controlled-source electromagnetic prospecting systems which produce vertical current flow in the Earth are sensitive to horizontal, electrically resistive structures such as hydrocarbon deposits and fresh water lenses. The electromagnetic response of such 3-D targets can sometimes be approximated by the fields produced by an arrangement of thin resistive sheets buried in a conductive host medium. We have previously shown, through simplified 2-D modelling, that the resistive sheet problem is more subtle than the conductive case commonly used in mineral exploration. Not only does the resistive sheet require the continuity of the normal current density as opposed to the continuity of the tangential electric field, interaction terms within the coupling matrix must also be calculated in full rather than approximated by point dipole sources representing each element. The purpose of this paper is to extend these results to the more realistic and practical situation of a 3-D resistive sheet buried in a multilayered earth. The algorithm is validated against 1-D calculations for a very large sheet and 3-D finite difference modelling for a smaller sheet. A current deflection number characterizes the effect of a resistive sheet's transverse impedance and governs its response in the same way that the current channelling number does for a conductive target. The sheet can be represented to first order by a single point dipole located at its centre; at most receiver locations, arrival times are nearly identical to those found from the full solution although amplitudes show some deviation near the target. As an example of the use of the algorithm, the marine controlled-source electromagnetic response of a simple anticlinal hydrocarbon reservoir underlying near surface resistors such as shallow gas or gas hydrate deposits is modelled. Results show that such near surface anomalies, even those with low deflection numbers, significantly affect the response of the deeper target and must be included in modelling and interpretation.

Mapping shallow geology and gas hydrate with marine CSEM surveys

Karen Weitemeyer and Steven Constable of the Scripps Institution of Oceanography have developed new marine CSEM technology to map gas hydrates and other geological features in the shallow seafloor section, and show examples from recent surveys in the Gulf of Mexico.

The potential influence of shallow gas and gas hydrates on sea floor erosion of Rock Garden, an uplifted ridge offshore of New Zealand

Regional erosion of the Rock Garden ridge top, a bathymetric high within New Zealand’s Hikurangi Subduction Margin, is likely associated with its gas hydrate system. Seismic data reveal gas pockets that appear partially trapped beneath the shallow base of gas hydrate stability. Steady-state fluid flow simulations, conducted on detailed two-dimensional geological models, reveal that anomalous fluid pressure can develop close to the sea floor in response to lower-permeability hydrate-bearing sediments and underlying gas pockets. Transient simulations indicate that large-scale cycling of fluid overpressure may occur on time scales of a few to tens of years. We predict intense regions of hydro-fracturing to preferentially develop beneath the ridge top rather than beneath the flanks, due to more pronounced overpressure generation and gas migration through hydrate-bearing sediments. Results suggest that sediment weakening and erosion of the ridge top by hydro-fracturing could be owed to fluid dynamics of the shallow gas hydrate system.

Geochemical evidence for a large methane release during the last deglaciation from Marmara Sea sediments

Recent methane inventories have revealed the potential impact of gas hydrates on the global carbon cycle, and hence in climate change ( Milkov, 2004). However, only a few studies have traced methane release in the geologic record. Here, we show geochemical evidence for a large scale methane release at mid-latitudes during the last deglaciation. The Sea of Marmara, an enclosed sea between the Mediterranean and Black Seas, is located in a tectonically active basin with gas hydrate expulsion and the formation of shallow gas hydrates. Since depths in the basin are shallower than 1100 m, future global temperatures are expected to have a great influence in destabilizing methane clathrates. Among the suite of biomarkers, we have focused on diplopterol and diploptene profiles in core MD012430, retrieved from the central basin in the Marmara Sea. Our results indicate that during the last 15,000 years, hopanoids showed important concentration variations with a pronounced peak during the deglaciation. The lack of a relationship between diplopterol/diploptene and phytoplanktonic biomarker concentrations, as well as a depleted isotopic composition, have linked the hopanoid maxima to methanotrophic activity, suggesting that an intense methane release occurred at the onset of deglaciation in the Marmara Sea. The vulnerability of the hydrate stability zone to changes in temperature and pressure under this range of shallow water depths, as well as the relative timing of the hopanoid maxima and sea surface temperature rise, points to thermal destabilization of hydrates as a trigger for methane release in the water column.

Molecular and isotopic partitioning of low-molecular-weight hydrocarbons during migration and gas hydrate precipitation in deposits of a high-flux seepage site

Detailed knowledge of the extent of post-genetic modifications affecting shallow submarine hydrocarbons fueled from the deep subsurface is fundamental for evaluating source and reservoir properties. We investigated gases from a submarine high-flux seepage site in the anoxic Eastern Black Sea in order to elucidate molecular and isotopic alterations of low-molecular-weight hydrocarbons (LMWHC) associated with upward migration through the sediment and precipitation of shallow gas hydrates. For this, near-surface sediment pressure cores and free gas venting from the seafloor were collected using autoclave technology at the Batumi seep area at 845 m water depth within the gas hydrate stability zone. Vent gas, gas from pressure core degassing, and from hydrate dissociation were strongly dominated by methane (> 99.85 mol.% of ∑[C1–C4, CO2]). Molecular ratios of LMWHC (C1/[C2 + C3] > 1000) and stable isotopic compositions of methane (δ13C = − 53.5‰ V-PDB; D/H around − 175‰ SMOW) indicated predominant microbial methane formation. C1/C2+ ratios and stable isotopic compositions of LMWHC distinguished three gas types prevailing in the seepage area. Vent gas discharged into bottom waters was depleted in methane by > 0.03 mol.% (∑[C1–C4, CO2]) relative to the other gas types and the virtual lack of 14C–CH4 indicated a negligible input of methane from degradation of fresh organic matter. Of all gas types analyzed, vent gas was least affected by molecular fractionation, thus, its origin from the deep subsurface rather than from decomposing hydrates in near-surface sediments is likely. As a result of the anaerobic oxidation of methane, LMWHC in pressure cores in top sediments included smaller methane fractions [0.03 mol.% ∑(C1–C4, CO2)] than gas released from pressure cores of more deeply buried sediments, where the fraction of methane was maximal due to its preferential incorporation in hydrate lattices. No indications for stable carbon isotopic fractionations of methane during hydrate crystallization from vent gas were found. Enrichments of 14C–CH4 (1.4 pMC) in short cores relative to lower abundances (max. 0.6 pMC) in gas from long cores and gas hydrates substantiates recent methanogenesis utilizing modern organic matter deposited in top sediments of this high-flux hydrocarbon seep area.

The Soil Properties of Lake-Bottom Sediments in the Lake Baikal Gas Hydrate Province

The purpose of this study is to understand the soil properties of grounds which contain shallow type gas hydrates. For this purpose, the surveys were conducted in Lake Baikal, Russia from 2005 to 2007, where shallow gas hydrates existed. For the lake-bottom sediments, physical and mechanical properties were tested (on-board and laboratory tests). The tested samples were retrieved from the mud volcano ground which contains the shallow gas hydrates, and reference ground at the same area. From these results, it was found that the gas hydrates distribute in large amounts in the lake-bottom sediments of Lake Baikal, and that form of the gas hydrates are varied. In addition, the reference samples in Lake Baikal have no marked differences in the soil properties of the sediments obtained from other sea-bottom grounds. On the other hands, the strengths of the mud volcano samples were lower than those of the reference samples. It would seem that these results are due to the effect of the disturbance of sedimentary layers by upwelling of gas and water from underground and the pressure release during the sampling.

Evaluation of gas hydrate deposits in an active seep area using marine controlled source electromagnetics: Results from Opouawe Bank, Hikurangi Margin, New Zealand

Several known gas seep sites along the Hikurangi Margin off the east coast of New Zealand were surveyed by marine controlled source electromagnetic (CSEM) experiments. A bottom-towed electric dipole–dipole system was used to reveal the occurrence of gas hydrate and methane related to the seeps. The experiments were part of the international multidisciplinary research program “New Vents” carried out on German R/V Sonne in 2007 (cruise SO191) to study key parameters controlling the release and transformation of methane from marine cold vents and shallow gas hydrate deposits. Two CSEM lines have been surveyed over known seep sites on Opouawe Bank in the Wairarapa region off the SE corner of the North Island. The data have been inverted to sub-seafloor apparent resistivity profiles and one-dimensional layered models. Clearly anomalous resistivities are coincident with the location of two gas seep sites, North Tower and South Tower on Opouawe Bank. A layer of concentrated gas hydrate within the uppermost 100 m below the seafloor is likely to cause the anomalous resistivities, but free gas and thick carbonate crusts may also play a role. Seismic data show evidence of fault related venting which may also indicate the distribution of gas hydrates and/or authigenic carbonate. Geochemical profiles indicate an increase of methane flux and the formation of gas hydrate in the shallow sediment section around the seep sites. Takahe is another seep site in the area where active venting, higher heat flow, shallow gas hydrate recovered from cores, and seismic fault planes, but only moderately elevated resistivities have been observed. The reasons could be a) the gas hydrate concentration is too low, even though methane venting is evident, b) strong temporal or spatial variation of the seep activity, and c) the thermal anomaly indicates rather temperature driven fluid expulsion that hampers the formation of gas hydrate beneath the vent.

Seismic imaging of gas conduits beneath seafloor seep sites in a shallow marine gas hydrate province, Hikurangi Margin, New Zealand

We present recently-acquired high-resolution seismic data and older lower-resolution seismic data from Rock Garden, a shallow marine gas hydrate province on New Zealand's Hikurangi Margin. The seismic data reveal plumbing systems that supply gas to three general sites where seeps have been observed on the Rock Garden seafloor: the ‘LM3’ sites (including LM3 and LM3-A), the ‘Weka’ sites (including Weka-A, Weka-B, and Weka-C), and the ‘Faure’ sites (including Faure-A, Faure-B, and Rock Garden Knoll). At the LM3 sites, seismic data reveal gas migration from beneath the bottom simulating reflection (BSR), through the gas hydrate stability zone (GHSZ), to two separate seafloor seeps (LM3 and LM3-A). Gas migration through the deeper parts of GHSZ below the LM3 seeps appears to be influenced by faulting in the hanging wall of a major thrust fault. Closer to the seafloor, the dominant migration pathways appear to occupy vertical chimneys. At the Weka sites, on the central part of the ridge, seismic data reveal a very shallow BSR. A distinct convergence of the BSR with the seafloor is observed at the exit point of one of the Weka seep locations (Weka-A). Gas supply to this seep is predicted to be focused along the underside of a permeability contrast at the BGHS caused by overlying gas hydrates. The Faure sites are associated with a prominent arcuate slump feature. At Faure-A, high-amplitude reflections, extending from a shallow BSR towards the seafloor, are interpreted as preferred gas migration pathways that exploit relatively-high-permeability sedimentary layers. At Faure-B, we interpret gas migration to be channelled to the seep along the underside of the BGHS — the same scenario interpreted for the Weka-A site. At Rock Garden Knoll, gas occupies shallow sediments within the GHSZ, and is interpreted to migrate up-dip along relatively high-permeability layers to the area of seafloor seepage. We predict that faulting, in response to uplift and flexural extension of the ridge, may be an important mechanism in creating fluid flow conduits that link the reservoir of free gas beneath the BGHS with the shallow accumulations of gas imaged beneath Rock Garden Knoll. From a more regional perspective, much of the gas beneath Rock Garden is focused along a northwest-dipping fabric, probably associated with subduction-related deformation of the margin.

Anomalous Strong Reflections on High Resolution Seismic Data from the Turkish Shelf of the Eastern Black Sea: Possible Indicators of Shallow Hydrogen Sulphide-Rich Gas Hydrate Layers

Some strong reflections about 3 to 5 m thick were observed at depths of 25-60 metres below the sea floor using deep-towed, 5 kHz subbottom profiler data in the Turkish shelf and upper slope of the Eastern Black Sea at water depths of 250 to 700 m. Strong reflections of this kind are generally attributed to shallow and localized gas accumulations. We, however, observed that the reflection polarity of these strong reflections was positive, suggesting that they do not correspond to reflections from the upper boundary of a possible gas front. In this study, we evaluate these reflections to determine if they represent hydrogen sulphide-rich shallow gas hydrate layers, which would be an unusual gas hydrate occurrence in a shallow marine environment. The existence of gas hydrate formations in thermobaric conditions, as in our study area (shallower water depths and relatively higher temperatures), depends completely on the gas composition in the hydrate structure; it is possible for gas hydrates to be stable only if they are formed by a certain amount of hydrogen sulfide together with methane. We closely examined the hydrogen sulphide potential of the area and found that the maximum total hydrogen sulfide concentration in the surficial sediments in the area was, at 5550 ppm, enough to produce hydrogen sulphide-rich gas hydrates. Using geoacoustic and geochemical data, we propose a conceptual model for the formation of shallow hydrogen sulfide-rich gas hydrates. According to this model, conjectural gas hydrate layers in the area should be formed along the boundary between a sulphate-reducing zone and an underlying carbonate reducing zone, where methane comes into contact with hydrogen sulphide. We also propose that the strongly reflective appearance of these layers on the subbottom profiler data indicates that the hydrate zone consists of a number of gas hydrate sheets with a decreasing thickness towards the seabed, interbedded with non-hydrate-bearing sediments.

Gas hydrate drilling transect across northern Cascadia margin - IODP Expedition 311

Abstract A transect of four sites (U1325, U1326, U1327 and U1329) across the northern Cascadia margin was established during Integrated Ocean Drilling Program Expedition 311 to study the occurrence and formation of gas hydrate in accretionary complexes. In addition to the transect sites, a fifth site (U1328) was established at a cold vent with active fluid flow. The four transect sites represent different typical geological environments of gas hydrate occurrence across the northern Cascadia margin from the earliest occurrence on the westernmost first accreted ridge (Site U1326) to the eastward limit of the gas hydrate occurrence in shallower water (Site U1329). Expedition 311 complements previous gas hydrate studies along the Cascadia accretionary complex, especially ODP Leg 146 and Leg 204 by extending the aperture of the transect sampled and introducing new tools to systematically quantify the gas hydrate content of the sediments. Among the most significant findings of the expedition was the occurrence of up to 20 m thick sand-rich turbidite intervals with gas hydrate concentrations locally exceeding 50% of the pore space at Sites U1326 and U1327. Moreover, these anomalous gas hydrate intervals occur at unexpectedly shallow depths of 50–120 metres below seafloor, which is the opposite of what was expected from previous models of gas hydrate formation in accretionary complexes, where gas hydrate was predicted to be more concentrated near the base of the gas hydrate stability zone just above the bottom-simulating reflector. Gas hydrate appears to be mainly concentrated in turbidite sand layers. During Expedition 311, the visual correlation of gas hydrate with sand layers was clearly and repeatedly documented, strongly supporting the importance of grain size in controlling gas hydrate occurrence. The results from the transect sites provide evidence for a structurally complex, lithology-controlled gas hydrate environment on the northern Cascadia margin. Local shallow occurrences of high gas hydrate concentrations contradict the previous model of gas hydrate formation at an accretionary prism. However, long-lived fluid flow (part of the old model) is still required to explain the shallow high gas hydrate concentrations, although it is most likely not pervasive throughout the entire accretionary prism, but rather localized and focused by the tectonic processes. Differences in the fluid flow regime across all of the transect drill sites indicate site-specific and probably disconnected (compartmented) deeper fluid sources in the various parts of the accretionary prism. The data and future analyses will yield a better understanding of the geologic controls, evolution and ultimate fate of gas hydrate in an accretionary prism as an important contribution to the role of gas hydrate methane gas in slope stability and possibly in climate change.

Gas and gas hydrate distribution around seafloor seeps in Mississippi Canyon, Northern Gulf of Mexico, using multi-resolution seismic imagery

To determine the impact of seeps and focused flow on the occurrence of shallow gas hydrates, several seafloor mounds in the Atwater Valley lease area of the Gulf of Mexico were surveyed with a wide range of seismic frequencies. Seismic data were acquired with a deep-towed, Helmholz resonator source (220–820 Hz); a high-resolution, Generator-Injector air-gun (30–300 Hz); and an industrial air-gun array (10–130 Hz). Each showed a significantly different response in this weakly reflective, highly faulted area. Seismic modeling and observations of reversed-polarity reflections and small scale diffractions are consistent with a model of methane transport dominated regionally by diffusion but punctuated by intense upward advection responsible for the bathymetric mounds, as well as likely advection along pervasive filamentous fractures away from the mounds.