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ABSTRACT: 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.
Earth and Planetary Sciences Letters 03/2008; 267(1-2):341-352.
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ABSTRACT: 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, ~ 0.2 km seaward of the trench, in the upper igneous basement, and at Site 1255, ~ 0.5 km landward of the trench, in the décollement. 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 (~ 50%) and a subduction zone fluid originating within the forearc (~ 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 décollement 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 ~ 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.
Earth and Planetary Science Letters.
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ABSTRACT: 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.
Marine and Petroleum Geology.
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ABSTRACT: 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 ~ 5•106 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.
Earth and Planetary Science Letters.
