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2014
Solomon, EA, Spivack AJ, Kastner M, Torres ME, Robertson G.  2014.  Gas hydrate distribution and carbon sequestration through coupled microbial methanogenesis and silicate weathering in the Krishna-Godavari Basin, offshore India. Marine and Petroleum Geology. 58:233-253.   10.1016/j.marpetgeo.2014.08.020   AbstractWebsite

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.

2008
Torres, ME, Trehu AM, Cespedes N, Kastner M, Wortmann UG, Kim JH, Long P, Malinverno A, Pohlman JW, Riedel M, Collett T.  2008.  Methane hydrate formation in turbidite sediments of northern Cascadia, IODP Expedition 311. Earth and Planetary Science Letters. 271:170-180.   10.1016/j.epsl.2008.03.061   AbstractWebsite

Expedition 311 of the Integrated Ocean Drilling Program (IODP) to northern Cascadia recovered gas-hydrate bearing sediments along a SW-NE transect from the first ridge of the accretionary margin to the eastward limit of gas-hydrate stability. In this study we contrast the gas gas-hydrate distribution from two sites drilled similar to 8 km apart in different tectonic settings. At Site U1325, drilled on a depositional basin with nearly horizontal sedimentary sequences, the gas-hydrate distribution shows a trend of increasing saturation toward the base of gas-hydrate stability, consistent with several model simulations in the literature. Site U1326 was drilled on an uplifted ridge characterized by faulting, which has likely experienced some mass wasting events. Here the gas hydrate does not show a clear depth-distribution trend, the highest gas-hydrate saturation occurs well within the gas-hydrate stability zone at the shallow depth of similar to 49 mbsf. Sediments at both sites are characterized by abundant coarse-grained (sand) layers up to 23 cm in thickness, and are interspaced within fine-grained (clay and silty clay) detrital sediments. The gas-hydrate distribution is punctuated by localized depth intervals of high gas-hydrate saturation, which preferentially occur in the coarse-grained horizons and occupy up to 60% of the pore space at Site U1325 and >80% at Site U1326. Detailed analyses of contiguous samples of different lithologies show that when enough methane is present, about 90% of the variance in gas-hydrate saturation can be explained by the sand (>63 mu m) content of the sediments. The variability in gas-hydrate occupancy of sandy horizons at Site U1326 reflects an insufficient methane supply to the sediment section between 190 and 245 mbsf. (C) 2008 Elsevier B.V. All rights reserved.

Malinverno, A, Kastner M, Torres ME, Wortmann UG.  2008.  Gas hydrate occurrence from pore water chlorinity and downhole logs in a transect across the northern Cascadia margin (Integrated Ocean Drilling Program Expedition 311). Journal of Geophysical Research-Solid Earth. 113   10.1029/2008jb005702   AbstractWebsite

A transect of four sites drilled by Integrated Ocean Drilling Program Expedition 311 provides an ideal data set to investigate the distribution of gas hydrates across the northern Cascadia convergent margin. We quantify gas hydrate saturation ( fraction of pore space occupied by gas hydrate) in a joint interpretation of pore water chlorinity data and downhole logs of porosity and electrical resistivity. The estimated saturation profiles define a gas hydrate occurrence zone (GHOZ), the depth interval where gas hydrates are actually found. In three of the Expedition 311 transect sites (U1326, U1325, and U1327), the top of the GHOZ systematically deepens moving landward of the deformation front of the Cascadia accretionary wedge. The farthest site from the deformation front ( U1329) shows no clear evidence of gas hydrates. We apply a simple diagenetic model to explain the observed landward deepening of the GHOZ. The model computes the methane concentration in the pore fluid for a given in situ bacterial methane production, sedimentation rate, and fluid advection velocity. Model results show that lower rates of sedimentation or fluid advection result in slower increases in methane concentration with depth and deeper tops of the GHOZ. Sedimentation rates in the Expedition 311 sites decrease landward, and fluid advection rates due to the dewatering of the accretionary wedge are expected to decrease moving landward of the deformation front as well. A combination of these two mechanisms can explain the deepening of the top of the GHOZ observed in the Expedition 311 transect sites.

Solomon, EA, Kastner M, Jannasch H, Robertson G, Weinstein Y.  2008.  Dynamic fluid flow and chemical fluxes associated with a seafloor gas hydrate deposit on the northern Gulf of Mexico slope. Earth and Planetary Science Letters. 270:95-105.   10.1016/j.epsl.2008.03.024   AbstractWebsite

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.