The National Gas Hydrate Program Expedition 01 cored ten sites in the Krishna-Godavari basin, located on the southeastern margin of India. A comprehensive suite of pore water solute concentrations and isotope ratios were analyzed to investigate the distribution and concentration of gas hydrate along the margin, in situ diagenetic and metabolic reactions, and fluid migration and flow pathways. Gas hydrate was present at all of the sites cored, and in situ microbial methanogenesis leads to estimates of depth-integrated average gas hydrate saturations that are typically <5%. Deep-sourced fluid and gas migration produces gas hydrate saturations up to 68% along an isolated coarser-grained stratigraphic horizon at Site 15 and up to 41% within a fractured clay-dominated system at Site 10. Our results show that the CO2 produced through net microbial methanogenesis is effectively neutralized by silicate weathering throughout the sediment column drilled at each site (similar to 100-300 m), buffering the pore water pH and generating excess alkalinity via the same reaction sequence as continental silicate weathering. Most of the excess alkalinity produced by silicate weathering in the Krishna-Godavari basin is sequestered in Caand Fe-carbonates as a result of ubiquitous calcium release from weathering detrital silicates and dissolved Fe production within the methanogenic sediments. Formation of secondary hydrous silicates (e.g. smectite) related to incongruent primary silicate dissolution acts as a significant sink for pore water Mg, K, Li, Rb, and B. The consumption of methane through anaerobic oxidation of methane, sequestration of methane in gas hydrate, and sequestration of dissolved inorganic carbon in authigenic carbonates keeps methanogenesis as a thermodynamically feasible catabolic pathway. Our results combined with previous indications of silicate weathering in anoxic sediments in the Sea of Okhotsk, suggest that silicate weathering coupled to microbial methanogenesis should be occurring in continental margins worldwide, providing a net sink of atmospheric CO2. (C) 2014 Elsevier Ltd. All rights reserved.

Two sealed borehole hydrologic observatories (CORKs) were installed in two active hydrogeochemical systems at the Costa Rica subduction zone to investigate the relationship between tectonics, fluid flow, and fluid composition. The observatories were deployed during Ocean Drilling Program (ODP) Leg 205 at Site 1253, similar to 0.2 km seaward of the trench, in the upper igneous basement, and at Site 1255, similar to 0.5 km landward of the trench, in the decollement. Downhole instrumentation was designed to monitor formation fluid flow rates, composition, pressure, and temperature. The two-year records collected by this interdisciplinary effort constitute the first co-registered hydrological, chemical, and physical dataset from a subduction zone, providing critical information on the average and transient state of the subduction thrust and upper igneous basement. The continuous records at ODP Site 1253 show that the uppermost igneous basement is highly permeable hosting an average fluid flow rate of 0.3 m/yr, and indicate that the fluid sampled in the basement is a mixture between seawater (similar to 50%) and a subduction zone fluid originating within the forearc (similar to 50%). These results suggest that the uppermost basement serves as an efficient pathway for fluid expelled from the forearc that should be considered in models of subduction zone hydrogeology and deformation. Three transients in fluid flow rates were observed along the decollement at ODP Site 1255, two of which coincided with stepwise increases in formation pressure. These two transients are the result of aseismic slip dislocations that propagated up-dip from the seismogenic zone over the course of similar to 2 weeks terminating before reaching ODP Site 1255 and the trench. The nature and temporal behavior of strain and the associated hydrological response during these slow slip events may be an analog for the response of the seaward part of the subduction prism during or soon after large subduction zone earthquakes. (C) 2009 Elsevier B.V. All rights reserved.

Gas hydrates outcrop on the seafloor at the Bush Hill hydrocarbon seep site in the northern Gulf of Mexico. Four newly designed fluid flux meters/chemical samplers, called the MOSQUITO, were deployed for 430 days at Bush Hill to determine how dynamic subsurface fluid flow influences gas hydrate stability and to quantify the associated methane fluxes into the ocean. Three of the flux meters were deployed adjacent to an outcropping gas hydrate mound, while the fourth monitored background conditions. The flux meter measurements reveal that the subsurface hydrology in the vicinity of the mound is complex and variable with frequent changes from downward to upward flow ranging from -161 to 273 cm/yr, and with temporal variations in the horizontal component of flow. The continuous record of fluid chemistry indicates that gas hydrate actively formed in the sediments. We propose that long periods of downward flow of seawater adjacent to gas vents (up to 4 months) are driven by local sub-pressure resulting from gas ebullition through faults and fractures due to overpressure at depth. High frequency variations in flow rates (days to weeks) are likely due to temporal changes in sediment permeability and the 3-D fluid flow field as a result of active gas hydrate and authigenic carbonate precipitation, as well as the presence of free gas. Gas hydrate formation occurred as a result of long-term emanation of CH4 at focused gas vents followed by a more diffuse intergranular methane flux. The estimated CH4 flux to the water column from focused gas vents across the Bush Hill seep is similar to 5.10(6) mol/yr. This significant flux suggests that Bush Hill and similar hydrocarbon seeps in the northwestern Gulf of Mexico may be important natural sources of methane to the ocean and possibly the atmosphere. (C) 2008 Elsevier B.V. All rights reserved.

Natural gas hydrates occur on most continental margins in organic-rich sediments at water depths >450 m (in polar regions >150 m). Gas hydrate distribution and abundance, however, varies significantly from margin to margin and with tectonic environment. The National Gas Hydrate Program (NGHP) Expedition 01 cored 10 sites in the Krishna-Godawari (K-G) basin, located on the southeastern passive margin of India. The drilling at the K-G basin was comprehensive, providing an ideal location to address questions regarding processes that lead to variations in gas hydrate concentration and distribution in marine sediments. Pore fluids recovered from both pressurized and non-pressurized cores were analyzed for salinity, Cl , SO4 2, alkalinity, Ca 2+ , Mg 2+ , Sr 2+ , Ba 2+ , Na + , and Li + concentrations, as well as 13 C-DIC, 18 O, and 87/86 Sr isotope ratios. This comprehensive suite of pore fluid concentration and isotopic profiles places important constraints on the fluid/gas sources, transport pathways, and CH4 fluxes, and their impact on gas hydrate concentration and distribution. Based on the Cl and 18 depth profiles, catwalk infrared images, pressure core CH4 concentrations, and direct gas hydrate sampling, we show that the occurrence and concentration of gas hydrate varies considerably between sites. Gas hydrate was detected at all 10 sites, and occurs between 50 mbsf and the base of the gas hydrate stability zone (BGHSZ). In all but three sites cored, gas hydrate is mainly disseminated within the pore space with typical pore space occupancies being 2%. Massive occurrences of gas hydrate are controlled by high-angle fractures in clay/silt sediments at three sites, and locally by lithology (sand/silt) at the more “diffuse” sites with a maximum pore space occupancy of ~67%. Though a majority of the sites cored contained sand/silt horizons, little gas hydrate was observed in most of these intervals. At two sites in the K-G basin, we observe higher than seawater Cl concentrations between the sulfate-methane transition (SMT) and ~80 mbsf, suggesting active gas hydrate formation at rates faster than Cl diffusion and pore fluid advection. The fluids sampled within this depth range are chemically distinct from the fluids sampled below, and likely have been advected from a different source depth. These geochemical results provide the framework for a regional gas hydrate reservoir model that links the geology, geochemistry, and subsurface hydrology of the basin, with implications for the lateral heterogeneity of gas hydrate occurrence in continental margins.

To establish the structural and lithological controls on gas hydrate distribution and to assess the potential energy resource and environmental hazards in the Indian Ocean, non-pressurized and pressurized cores were recovered from the Krishna-Godavari (K-G) and Mahanadi Basins offshore east India, and from an Andaman Sea site. The pore fluids were analyzed for: salinity, Cl, sulfate, sulfide, carbonate alkalinity, Ca, Mg, Sr, K, Na, Ba, and Li concentrations, δC-DIC, δO, D/H, and Sr/Sr ratios; together with infra-red imaging they provided important constraints on the presence and distribution of gas hydrates, thus on the subsurface hydrology. Evidence for methane hydrate was obtained at each of the sites. Only in the K-G Basin, between the sulfate-methane transition zone (SMT) depth and ~80 mbsf, higher than seawater chloride concentrations are observed; below this zone to the depth of the base of the gas hydrate zone (BGHSZ), chloride concentrations and salinity are lower than seawater value. In the Andaman Sea and Mahanadi Basin, only lower than seawater chloride concentrations are observed, and the shallowest gas hydrates occur at 100-200 m below the sulfate-methane transition zone (SMT) and extend to the depth of the BGHSZ. In the K-G Basin, the highest methane hydrate concentrations are associated with fracture zones in clay-rich sediments and/or in some coarser grained horizons. In the Andaman Sea, however, they are primarily associated with volcanic ash horizons. Assuming dilution by water released from dissociated methane hydrate, chloride and salinity anomalies suggest pore volume occupancies on the order of <1% to a maximum of ~61% at two sites (10, 21) in the K-G Basin and <1% to a maximum of ~76% at the Andaman Sea site. Overall, the percent pore volume occupancies based on pressure core methane concentrations and the chloride concentrations in conventional cores are similar. Variations in sulfate gradients were observed with the steepest gradient having the SMT at 8 mbsf in the K-G Basin and the deepest SMT at ~25 mbsf at the Andaman Sea site. The extreme negative δC values of the dissolved inorganic carbon (DIC), ranging from -38‰ to -47‰ at the SMT at some of the sites, indicate that anaerobic oxidation of methane (AOM) is an important reaction responsible for sulfate reduction at these sites. At several sites in the K-G Basin, however, the δC-DIC values indicate that organic matter oxidation is the dominant reaction.

Pore fluids from Atwater Valley (AT 13/14) and Keathley Canyon (KC 151) in the northern Gulf of Mexico are surprisingly similar with respect to ionic concentrations and oxygen and strontium isotope values, as well as hydrocarbon geochemistry, suggesting that these widely separated localities share common deep subsurface fluid origins. Seafloor mounds with focused fluid migration pathways and inferred near-seafloor gas hydrates characterize the AT 13/14 region, whereas the KC 151 region has a bottom simulating reflector (BSR) at ∼310 mbsf, which is rather uncommon in the Gulf of Mexico (GOM). At these sites seafloor gas hydrates were not observed but the sediment surface in the vicinity and particularly at the mounds is populated with chemosynthetic communities that are commonly associated with seafloor gas emission. The geochemical results, together with the pressure core data, suggest that at the AT region methane hydrate mostly occurs in near-surface sediments at mounds, consistent with focused migration pathways. In the KC region methane hydrate mostly occurs deeper in the section, in highly fractured silty-clayey sediments from ∼220 to 300 mbsf. The pore fluids at the AT mounds and KC 151 are characterized by higher than seawater salinity. The more saline pore fluids at the AT mound and at KC151 sites, located ∼350 km apart, are almost chemically indistinct. Ionic ratios indicate that this distinct high salinity fluid is not from in situ salt dome halite dissolution. Rather, this fluid is a subsurface brine derived from Jurassic or Cenozoic evaporite formation, modified by fluid-sediment reactions, and migrated to the two sites analyzed. Despite porewater salinities elevated above that of seawater, the sediment temperatures are within the range of methane hydrate stability for each of the sites. Based on Cl− dilutions the maximum gas hydrate pore volume occupancy at the AT mound sites would be ∼9%. At KC, Cl− concentrations in pressure cores imply that in situ hydrate is unevenly distributed, with pore volume occupancy of 1–12%. Significant variations in sulfate gradients were observed, with the sulfate-to-methane transition zone (SMTZ) at or near the seafloor at the AT mound sites. At AT 13#2 the well-defined SMTZ is at ∼8 mbsf, and at KC 151#3 it is at ∼9 mbsf. There is no coincidence between the steepness of the sulfate gradients and the presence or depth of a BSR, suggesting that the SMTZ interfaces are measuring different aspects of the subsurface methane hydrology. At both AT and KC the δ13C-DIC values clearly indicate that anaerobic oxidation of methane (AOM) is the dominant reaction responsible for sulfate reduction and the increased alkalinities observed. The most negative δ13C-DIC values obtained are −46.3‰ and −49.6‰ at the SMTZs at AT 13#2 and KC 151#3, respectively.

Detailed near-bottom investigation of a series of giant, kilometer scale, elongate pockmarks along the edge of the mid-Atlantic continental shelf confirms that methane is actively venting at the site. Dissolved methane concentrations, which were measured with a commercially available methane sensor (METS) designed by Franatech GmbH mounted on an Autonomous Underwater Vehicle (AUV), are as high as 100 nM. These values are well above expected background levels (1-4 nM) for the open ocean. Sediment pore water geochemistry gives further evidence of methane advection through the seafloor. Isotopically light carbon in the dissolved methane samples indicates a primarily biogenic source. The spatial distribution of the near-bottom methane anomalies (concentrations above open ocean background), combined with water column salinity and temperature vertical profiles, indicate that methane-rich water is not present across the entire width of the pockmarks, but is laterally restricted to their edges. We suggest that venting is primarily along the top of the pockmark walls with some advection and dispersion due to local currents. The highest methane concentrations observed with the METS sensor occur at a small, circular pockmark at the southern end of the study area. This observation is compatible with a scenario where the larger, elongate pockmarks evolve through coalescing smaller pockmarks. (C) 2007 Elsevier B.V. All rights reserved.

Four newly designed flux meters called the MOSQUITO (Multiple Orifice Sampler and Quantitative Injection Tracer Observer), capable of measuring fluid flow rates and sampling pore fluid chemistry simultaneously, and two temperature loggers were deployed for 430 days adjacent to the Bush Hill hydrate mound in the northern Gulf of Mexico (GC 185). The main objective of the deployment was to understand how chemistry, temperature, and subsurface hydrology dynamically influence the growth and dissociation of the gas hydrate mound. The flux meters were deployed in a mussel field, in bacterial mats, in a tubeworm field, and at a background site approximately 100 m southwest of the hydrate mound. Results from the longterm chemical monitoring suggest that this system is not in dynamic equilibrium. Gas hydrate actively formed within the mussel field adjacent to the most active gas vent, in the tubeworm field, and at the background site. The hydrology is variable with upward flow rates ranging from 1-90 cm/yr and downward flow rates from 3-130 cm/yr. Two distinct hydrologic pulsing events were sampled across the three mound sites, which advect a fluid from depth that further stabilizes the gas hydrate deposit. The hydrogeochemistry at Bush Hill seems to be influenced by multiple mechanisms such as active formation of gas hydrate, fluid influx and outflux due to active venting of CH4 at transient methane seeps at and near the mound, local salt tectonics, and density driven convection. The fluxes of fluid, solutes, and methane may have a significant impact on the seafloor biochemical environment and the water column chemistry at Bush Hill. (Author)