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Submarine methane seepage can potentially be promoted by the dissociation of the marine hydrates surrounding the continental margins due to oceanic warming since the Last Glacial Maximum. This seepage could be archived by authigenic carbonates at seeping sites, but the time lag caused by heat transmission through the sediment column leads to an inconsistency between the ages of the carbonate and the period of bottom water warming. Here we present the records of the authigenic carbonate crust from drilling site D1 in the Mid‐Okinawa Trough. Uranium–thorium dating results show that the carbonate crust mainly grew downwards during 14–6 ka. Gas hydrates hosted in the relatively thin stability zone dissociated in a rapid response to bottom water warming and intensified the methane seepage. Our study better supports the hypothesis that a considerable amount of methane can be released from marine hydrates due to global climatic changes. Plain Language Summary: Methane hydrates are ice‐like crystalline compounds and often found in deep‐ocean marine sediments. Ocean warming in the past could have destabilized methane hydrates and led to a rapid discharge of free methane gas. Geological information on this methane escape could be preserved by carbonate rocks. To date, the hypothesis of the control of ocean warming on hydrate melting since the Last Glacial Maximum (19,000–26,000 years ago) has not been fully tested. To examine this hypothesis, we used the rock samples of seep‐carbonate retrieved by seafloor drilling in the Mid‐Okinawa Trough. Uranium–thorium dating results show that the seep carbonates formed between 6,000 and 14,000 years ago. It took less time for heat to diffuse downwards from warming seawater to trigger hydrate melting at this location than in most of other oceans at similar water depths. The smaller time lag makes the carbonate rock age close to the period of ocean warming. Our results better support that global ocean warming could potentially release methane from gas hydrates during glacial–interglacial transitions. The consequent methane transport into the oceans and likely also the atmosphere might impact ocean acidification and climatic warming. Key Points: Presentation of the chronology of methane seepage by dating authigenic carbonates collected from drilling core D1 in the Okinawa TroughOcean warming–induced hydrate dissociation led to the downward growth of authigenic carbonate for 14–6 ka after the Last Glacial MaximumThis carbonate record better supports the link of hydrate dissociation with ocean warming due to a smaller thermal lag effect [ABSTRACT FROM AUTHOR]

Natural gas hydrates have been hailed as a new and promising unconventional alternative energy, especially as fossil fuels approach depletion, energy consumption soars, and fossil fuel prices rise, owing to their extensive distribution, abundance, and high fuel efficiency. Gas hydrate reservoirs are similar to a storage cupboard in the global carbon cycle, containing most of the world's methane and accounting for a third of Earth's mobile organic carbon. We investigated gas hydrate stability zone burial depths from the viewpoint of conditions associated with stable existence of gas hydrates, such as temperature, pressure, and heat flow, based on related data collected by the global drilling programs. Hydrate‐related areas are estimated using various biological, geochemical and geophysical tools. Based on a series of previous investigations, we cover the history and status of gas hydrate exploration in the USA, Japan, South Korea, India, Germany, the polar areas, and China. Then, we review the current techniques for hydrate exploration in a global scale. Additionally, we briefly review existing techniques for recovering methane from gas hydrates, including thermal stimulation, depressurization, chemical injection, and CH4–CO2 exchange, as well as corresponding global field trials in Russia, Japan, United States, Canada and China. In particular, unlike diagenetic gas hydrates in coarse sandy sediments in Japan and gravel sediments in the United States and Canada, most gas hydrates in the northern South China Sea are non‐diagenetic and exist in fine‐grained sediments with a vein‐like morphology. Therefore, especially in terms of the offshore production test in gas hydrate reservoirs in the Shenhu area in the north slope of the South China Sea, Chinese scientists have proposed two unprecedented techniques that have been verified during the field trials: solid fluidization and formation fluid extraction. Herein, we introduce the two production techniques, as well as the so‐called "four‐in‐one" environmental monitoring system employed during the Shenhu production test. Methane is not currently commercially produced from gas hydrates anywhere in the world; therefore, the objective of field trials is to prove whether existing techniques could be applied as feasible and economic production methods for gas hydrates in deep‐water sediments and permafrost zones. Before achieving commercial methane recovery from gas hydrates, it should be necessary to measure the geologic properties of gas hydrate reservoirs to optimize and improve existing production techniques. Herein, we propose horizontal wells, multilateral wells, and cluster wells improved by the vertical and individual wells applied during existing field trials. It is noteworthy that relatively pure gas hydrates occur in seafloor mounds, within near‐surface sediments, and in gas migration conduits. Their extensive distribution, high saturation, and easy access mean that these types of gas hydrate may attract considerable attention from academia and industry in the future. Herein, we also review the occurrence and development of concentrated shallow hydrate accumulations and briefly introduce exploration and production techniques. In the closing section, we discuss future research needs, key issues, and major challenges related to gas hydrate exploration and production. We believe this review article provides insight on past, present, and future gas hydrate exploration and production to provide guidelines and stimulate new work into the field of gas hydrates. [ABSTRACT FROM AUTHOR]

Abstract In response to the accelerating processes of marine natural gas hydrate exploration and gas production from hydrate-bearing sediments, their potential impacts on the environment have been attracting extensive attention from international academia as well as from industry, especially in recent years. Methane seeps related to gas hydrate degradation on the seafloor are ubiquitous on continental slopes in both active and passive continental margins. A previous series of articles suggest that hydrate dissociation (gas release or ebullition at large scale) mainly occurs as a result of faulting or decreased lithostatic pressure triggered by many external driving forces (mainly including tectonic activities, overpressure zone, earthquake and tides), and temperature change. These frequently affect methane inputs into the atmosphere, fueling seafloor oxygen consumption or ocean acidification, and even posing submarine geohazards. Gas hydrate-related areas are usually estimated from indirect biochemical indications (cold-seep communities, element and isotopic anomalies in pore water) and geophysical indications (surface morphology, gas plumes and pathways). Therefore, appropriate application of monitoring and detection methods is of crucial significance for assessing the temporal and spatial variability of related environmental indicators in gas hydrate reservoirs. Monitoring is also as an essential support for harvesting this potential alternative energy in ways that are safer, more economical, and more environmentally friendly. Following the three structural elements of a hydrocarbon seep pumping system (gas/fluid source, fluid migration pathway, and seeping structures at or near the seafloor), this paper sets forth the significance of indicators for dynamics resulting from the destabilization of reservoirs of natural gas hydrates, reviews the corresponding monitoring or detection methods and integrated monitoring systems; and especially, expatiates the in situ observation networks regarding gas hydrate reservoirs and gas hydrate production tests. In the closing section, we discuss and draw conclusions regarding the challenges for future development of hydrate environmental monitoring and significant environmental issues requiring special attention. We intend this paper to provide references and to set the scene for future research activities about gas hydrates, so as to ignite the interest of more research groups around the world. Graphical abstract Image 1 Highlights • Environmental impacts of marine natural gas hydrate are reviewed comprehensively. • Cutting-edge new pieces of monitoring technologies for gas hydrate are presented. • Perspectives on the future of research and technologies are outlined. [ABSTRACT FROM AUTHOR]

The East China Sea is an important marginal sea of the Western Pacific Ocean, from which natural gas hydrate sample has not been acquired so far. Recently, copious carbonate chimneys have been discovered in turbidite deposits in the olistostrome zone located on the west slope of the northern section of Okinawa Trough. Here, the petrology, geochemistry and chronology of an iron-rich carbonate chimney were characterized, confirming a close relationship between its formation and the dissociation of natural gas hydrate beneath the chimney in OT. A distinctive relationship has been observed between goethite and total carbonate contents along with a negative correlation between Fe and Ca contents. Conversely, abundant Fe accumulated on carbonate substrate by mineralized microorganisms. The δ 13 C values of the chimney wall were from −27.56 to −43.66‰ (average: −37.18‰, V-PDB), implying anaerobic oxidation of methane (AOM) as a predominant controlling factor on carbonate precipitation. As no pyrite and organic residues were identified in the iron-rich chimney, it was assumed that AOM was coupled to the iron reduction reaction at least to some extent during the chimney growth owing to the local deficiency of sulfate supply. The δ 56 Fe values of bulk chimney wall (ranging from −0.316‰ to −0.023‰, average −0.134‰) suggest mass and isotope exchanges between the chimney and ambient environment during its growth history, whereas the enrichment of δ 18 O of the carbonate implies these carbonate sourcing from hydrate dissociation underlying our sampling site. This assumption has been supported by a distinct bottom simulation reflector (BSR) and a well-developed fault system beneath the sampling site. This is the first report of cold seepage inside the OT and the identified iron-dependent AOM has shed a new light to the Carbon cycle related to the marine methane oxidation, particularly before the Great Oxidation Event ~2.45 Ga ago. [ABSTRACT FROM AUTHOR]

Cold seep carbonate represents a faithful record of the ancient methane seepage and provides a nonnegligible contribution to the global carbon reservoir. On the western slope of the Northern Okinawa Trough (NOT), a recent seafloor visualized survey has discovered widespread crust of cold seep carbonate. Here we study mineralogy and geochemistry of these authigenic carbonate to investigate source origin and reconstruct its growth history. Mineralogically, the carbonate crusts are mainly composed of micritic aragonite, with botryoidal aragonite, framboidal pyrite, and microcrystalline authigenic gypsum. Petrographic characteristic unambiguously indicates that this carbonate precipitates in relatively open systems due to a considerable rate of sulfate-dependent anaerobic oxidation of methane (AOM). Regarding geochemistry, strongly 13C-depleted carbon isotope values (as low as −56.1‰, V-PDB) demonstrate that the carbon in the carbonate crusts is mainly derived from biogenic methane coupled with AOM. In contrast, the δ18O enrichment (up to +2.7‰, V-PDB) suggests that the fluid flow from which carbonate precipitated is sourced from dissociation of underlying natural gas hydrates. The U–Th ages of authigenic carbonates fall in the timescale of 22.8–55.7 ka BP, consistent with the period of sea-level lowstand in the late Pleistocene. Overall, several lines of evidence of this study indicate that extensive methane was released by gas hydrate decomposition during sea level fall, consequently resulting in the precipitations of carbonate crust in the NOT. Furthermore, the obviously episodic methane seepages led to the constant accretion from the interior to the exterior within the preformed crust, ultimately inducing the carbonate blocks, slabs and crusts to be exposed on the seafloor. The existence of large-scale carbonate crusts represents a good trapper of the later released carbon especially the isotopically light methane from the deep. • Aragonite is the dominated mineral phase in carbonate crust of Okinawa Trough. • Carbonate is sourced from gas hydrate and carbon mainly from biogenic methane. • U–Th age of carbonate crust is within the periods of sea-level lowstand. [ABSTRACT FROM AUTHOR]

Submarine methane seepages constitute an important part of global carbon cycle and may be promoted by the dissociation of gas hydrate. The migration pattern of a seepage is commonly dominated by vertical transport of methane-rich pore fluids, and then lateral migration of these fluids probably occurred due to self-sealing effect of carbonate crust above vertical migration conduit. Some studies found the evidence of this lateral migration, but the one from the record of seep carbonate in nature is very rare. In this study, we present the results of mineralogical, isotopic, and dating analyses of a core containing seep carbonates obtained from seafloor drilling site D5 in the Middle Okinawa Trough. The core contains 0.98-m-long carbonate interval that predominantly consists of aragonite, with an average content of ∼86.6%. The carbonate exhibits moderate depletion in 13C (δ13C values: −37.2‰ to −16.3‰ Vienna Pee Dee Belemnite; VPDB) and enrichment in 18O (δ18O values: 4.3‰–5.8‰ VPDB), with relatively constant 87Sr/86Sr values (0.709167–0.709197). These results suggest that the precipitation of the seep carbonate was induced by the sulfate-driven anaerobic oxidation of methane close to the seabed and this methane could be from the dissociation of gas hydrates. U–Th age–depth profile shows that two carbonate intervals grew downwards coevally to coalesce into a thicker crust, and its upper segment also grew upwards. We interpret that lateral migration of the fluid flows at two levels are responsible for the formation of upper and lower segments of carbonate intervals because they have (a) the distinct clustering of δ13C and δ18O values and (b) a coeval period of fast growth. It is likely that upward migration of methane was redirected by self-sealing of overlying carbonate crust, leading to lateral migration at the flank of seabed mound Db4 during 6.2–4.6 ka. Our findings highlight the interaction between fluid flows and authigenic carbonates during Late Quaternary, which implies that more methane could be consumed by oxidation because it takes longer time for methane to bypass self-sealing of carbonate to escape into the ocean. • U–Th dating of aragonite-rich carbonate from a drilling core D5 in the Okinawa Trough yields ages of 6.2–4.6 ka. • Enrichment of 18O in carbonate crust indicates the contribution of gas hydrate dissociation to intensified methane seepage. • U–Th ages with depth imply the occurrences of lateral migration of methane at two depths at flank of seabed mound. • Prolonged migration due to bypassing carbonate sealing may cause more of methane consumed close to seabed at seeps. [ABSTRACT FROM AUTHOR]

• Cold seeps were linked to fault scarps and dome-structures in the Okinawa Trough. • Numerical models were used to quantify CH 4 fluxes and biogeochemical processes. • Cold seeps are C sources to the seawater, and Seep-wide budgets were constrained. The methane-enriched fluids in cold seeps are likely to crystallize as gas hydrates and serve as crucial sources of carbon to seawater. In this research, we analyzed the pore-water composition in terms of CH 4 , dissolved inorganic carbon (DIC), Cl−, Br−, SO 4 2−, Na+, Mg2+, Ca2+, Sr2+, and NH 4 +, and the δ13C DIC , δ13C CH4 , and δD CH4 values of two gravity cores and six remotely operated vehicle (ROV) video-guided push cores retrieved from fault scarps and dome-like structures (DSs) on the western slope of the mid-Okinawa Trough. In addition, a reaction–transport model was applied to quantify the methane fluxes and related biogeochemical processes. Active seepage of biogenic (δ13C CH4 ~ –70‰ V-PDB) and thermogenic (δ13C CH4 = −40‰ to −56‰ V-PDB) methane was identified on fault scarps and dome structures, respectively. Methane seepage was controlled by the transport and dissolution of the ascending gas rather than by clay dehydration or gas hydrate dissociation-induced fluid advection. The high methane concentrations and shallow sulfate–methane transition zones (SMTZs; between 0.1 and 0.4 mbsf) at sites R3-C2, R4-C4, and R6-C1 suggest strong methane seepage at three of the four studied DS (the highest gas dissolution rates are 6450, 1475, and 515 mmol m−2 yr−1). Site GC08, located along a fault scarp, exhibits a moderate methane seepage; the SMTZ is located at ~2.5 mbsf, and the rate of anaerobic oxidation of methane (AOM) is 130 mmol m−2 yr−1. The methane migrating from depth is mainly consumed by AOM. However, ~12%–66% of the methane released from the two most intensive seep sites escapes to the water column. The precipitation of high–Mg calcite (at all sites) and aragonite (only at site R3-C2) has fixed 27%–50% (average = 39%) of the DIC. Therefore, the carbon outputs to the water column have been reduced. In the study area, the area-weighted seafloor CH 4 and DIC fluxes are ~30 and 20 mmol m−2 yr−1, respectively. Together, they correspond to ~7%–14% of the organic carbon burial rate, indicating that sediments could not be simply regarded as a stable carbon sink because they provide methane and a certain amount of DIC to the water column. Our findings contribute to the results of the ongoing efforts in understanding carbon cycling in submarine cold seep systems. [ABSTRACT FROM AUTHOR]

The formation and dissociation of hydrate at the pore scale can be observed qualitatively using μ -CT X-ray imaging. However, a quantitative criterion for hydrate pattern is not currently available in the literature. To fill this research gap, we generated numerically-simulated sediments with specific hydrate formation patterns and accurate hydrate saturation levels, by integrating X-ray computed tomography techniques, morphological operation algorithm, and quartet structure generation set method. Using the topological information extracted from the generated hydrate-bearing sediments, we proposed a quantitative criterion for the microscopic classification of hydrates (namely pore-floating, cementing, and bridging) based on the complexity of pore topology (T p) and hydrate topology (T h). In this work, we visualized 3D distribution of hydrate utilizing μ -CT imaging technology, as well as analyzed the topology of hydrate and its dynamic evolution through extracted dual-network models and proposed classification method. Our findings demonstrate that when the hydrate saturation exceeds 30%, the dominating hydrate patterns are bridging and pore-floating, while sediment is more likely to exhibit cementing and pore-floating patterns when the hydrate saturation is below this threshold. Additionally, we applied the lattice Boltzmann method to numerically analyze the variation of normalized permeability and tortuosity under different hydrate saturation levels. The permeability monotonically increases as hydrate dissociates from the sediments, while the behaviour of tortuosity is non-monotonic. • A quantitative measure is proposed to characterize the micro-occurrence types of hydrate. • The topology of hydrate and its dynamic evolution are analyzed via proposed methods. • The hydrodynamic properties are investigated under different hydrate saturation levels. [ABSTRACT FROM AUTHOR]

It is not currently well understood how a changing climate controls methane releases in the Okinawa Trough. In this study, a piston core was collected from a seep-impacted area at a water depth of approximately 1000 m from the Okinawa Trough, East China Sea, and was studied using comprehensive analyses of carbon and oxygen stable isotopes, lipid biomarkers, and major and trace element geochemistry. Extremely low δ13C values of total inorganic carbon and organic carbon, increased Methane Index, and molybdenum (Mo) and uranium (U) enrichments at depths of 225–255 cm below the sea-floor (bsf) and 75–142.5 cm bsf were found. Combined with published pore water data, fossil and current sulfate-methane transitions (SMTs) were identified. Mass balance equations were applied to estimate the fraction and content of authigenic carbonates at the two SMTs and their corresponding δ18O values. The calculations revealed that the estimated δ18O values of authigenic carbonates for the fossil SMT (from 4.9‰ to 5.8‰) and the current SMT (from −2.1‰ to 4.4‰) were higher and lower than the theoretical equilibrium δ18O value (4.8‰), respectively. These results suggest that fossil seepage is possibly induced by the dissociation of gas hydrates. The fluids of the current seepage may be derived from the equilibrium between the ambient seawater and the fluids of gas hydrate dissociation. Based on the accumulation of authigenic carbonate in sediments, previously published pore-water calcium and magnesium fluxes, and foraminifera 14C dating, the fossil methane seepage was confirmed to have occurred during the period 8.2 to 4.5 ka B.P., with its SMT depth less than 105 cm bsf, while the present-day methane seepage with a low fluid intensity was estimated to have started before 1.0 ka and formed its SMT at approximately 250 cm bsf. The timing of this fossil methane seepage suggests that the rising temperature of the North Pacific Intermediate Water during the early and middle Holocene mainly controlled the methane release in the Okinawa Trough, whereas the ongoing methane emission was likely induced by decreased pressure driven by back-arc extension. The geochemical data demonstrate past methane release as a response to environmental changes and suggest that the Okinawa Trough gas hydrates were sensitive to temperature fluctuations during the middle Holocene. [Display omitted] [ABSTRACT FROM AUTHOR]