Velocity structure of a gas hydrate reflector.
09 Apr 1993-Science (American Association for the Advancement of Science)-Vol. 260, Iss: 5105, pp 204-207
TL;DR: Waveform inversion of seismic reflection data can be used to estimate from seismic data worldwide the velocity structure of a BSR and its thickness, and predicts that sediment pores beneath the BSR contain free methane for approximately 30 meters.
Abstract: Seismic reflection profiles across many continental margins have imaged bottom-simulating reflectors (BSRs) parallel to the seabed; these are often interpreted as the base of a zone in which methane hydrate "ice" is stable. Waveform inversion of seismic reflection data can be used to estimate from seismic data worldwide the velocity structure of a BSR and its thickness. A test of this method at a drill site of the Ocean Drilling Program predicts that sediment pores beneath the BSR contain free methane for approximately 30 meters. The hydrate and underlying gas represent a large global reservoir of methane, which may have economic importance and may influence global climate.
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TL;DR: In this paper, the authors assume that gas hydrate behaves in a way analogous to ice in a freezing soil, and they predict that gas hydrates in a sequence of fine-grained sediments are inhibited by reduced pore water activity in the vicinity of hydrophilic mineral surfaces, and the excess internal energy of small crystals confined in pores.
Abstract: The stability of submarine gas hydrates is largely dictated by pressure and temperature, gas composition, and pore water salinity. However, the physical properties and surface chemistry of deep marine sediments may also affect the thermodynamic state, growth kinetics, spatial distributions, and growth forms of clathrates. Our conceptual model presumes that gas hydrate behaves in a way analogous to ice in a freezing soil. Hydrate growth is inhibited within fine-grained sediments by a combination of reduced pore water activity in the vicinity of hydrophilic mineral surfaces, and the excess internal energy of small crystals confined in pores. The excess energy can be thought of as a “capillary pressure” in the hydrate crystal, related to the pore size distribution and the state of stress in the sediment framework. The base of gas hydrate stability in a sequence of fine sediments is predicted by our model to occur at a lower temperature (nearer to the seabed) than would be calculated from bulk thermodynamic equilibrium. Capillary effects or a build up of salt in the system can expand the phase boundary between hydrate and free gas into a divariant field extending over a finite depth range dictated by total methane content and pore-size distribution. Hysteresis between the temperatures of crystallization and dissociation of the clathrate is also predicted. Growth forms commonly observed in hydrate samples recovered from marine sediments (nodules, and lenses in muds; cements in sands) can largely be explained by capillary effects, but kinetics of nucleation and growth are also important. The formation of concentrated gas hydrates in a partially closed system with respect to material transport, or where gas can flush through the system, may lead to water depletion in the host sediment. This “freeze-drying” may be detectable through physical changes to the sediment (low water content and overconsolidation) and/or chemical anomalies in the pore waters and metastable presence of free gas within the normal zone of hydrate stability.
611 citations
TL;DR: In this paper, the authors measured velocities measured in three drill holes through a gas hydrate deposit on the Blake Ridge, offshore South Carolina, indicate that substantial free gas exists to at least 250 meters beneath the bottom-simulating reflection (BSR).
Abstract: Seismic velocities measured in three drill holes through a gas hydrate deposit on the Blake Ridge, offshore South Carolina, indicate that substantial free gas exists to at least 250 meters beneath the bottom-simulating reflection (BSR). Both methane hydrate and free gas exist even where a clear BSR is absent. The low reflectance, or blanking, above the BSR is caused by lithologic homogeneity of the sediments rather than by hydrate cementation. The average methane hydrate saturation above the BSR is relatively low (5 to 7 percent of porosity), which suggests that earlier global estimates of methane in hydrates may be too high by as much as a factor of 3.
492 citations
TL;DR: In a cell‐free system based on Xenopus egg extracts, Bcl‐2 blocks apoptotic activity by preventing cytochrome c release from mitochondria, which induces apoptosis by activating CPP32‐like caspases, via unknown cytosolic factors.
Abstract: In a cell-free system based on Xenopus egg extracts, Bcl-2 blocks apoptotic activity by preventing cytochrome c release from mitochondria. We now describe in detail the crucial role of cytochrome c in this system. The mitochondrial fraction, when incubated with cytosol, releases cytochrome c. Cytochrome c in turn induces the activation of protease(s) resembling caspase-3 (CPP32), leading to downstream apoptotic events, including the cleavage of fodrin and lamin B1. CPP32-like protease activity plays an essential role in this system, as the caspase inhibitor, Ac-DEVD-CHO, strongly inhibited fodrin and lamin B1 cleavage, as well as nuclear morphology changes. Cytochrome c preparations from various vertebrate species, but not from Saccharomyces cerevisiae, were able to initiate all signs of apoptosis. Cytochrome c by itself was unable to process the precursor form of CPP32; the presence of cytosol was required. The electron transport activity of cytochrome c is not required for its pro-apoptotic function, as Cu- and Zn-substituted cytochrome c had strong pro-apoptotic activity, despite being redox-inactive. However, certain structural features of the molecule were required for this activity. Thus, in the Xenopus cell-free system, cytosol-dependent mitochondrial release of cytochrome c induces apoptosis by activating CPP32-like caspases, via unknown cytosolic factors.
395 citations
TL;DR: In this article, the authors test a hypothesis relating large pore water sulfate gradients to upward methane flux and the presence of underlying methane gas hydrate on continental rises by examining: (1) Pore water geochemical data available from the global data set of Deep Sea Drilling Project-Ocean Drilling Program (DSDP-ODP) sites; (2) sulfate data from 51 coring sites located at the Carolina Rise and Blake Ridge (offshore southeastern United States); and (3) the relationship between the distribution of bottom-simulating reflectors (BS
355 citations
TL;DR: In this article, the authors report the direct measurement of in situ methane abundances stored as gas hydrate and free gas in a sediment sequence from the Blake ridge, western Atlantic Ocean.
Abstract: Certain gases can combine with water to form solids—gas hydrates—that are stable at high pressures and low temperatures1,2. Conditions appropriate for gas-hydrate formation exist in many marine sediments where there is a supply of methane. Seismic reflection profiles across continental margins indicate the frequent occurrence of gas hydrate within the upper few hundred metres of sea-floor sediments, overlying deeper zones containing bubbles of free gas3–9. If large volumes of methane are stored in these reservoirs, outgassing may play an important role during climate change10–12. Gas hydrates in oceanic sediments may in fact comprise the Earth's largest fossil-fuel reservoir2,13. But the amount of methane stored in gas-hydrate and free-gas zones is poorly constrained2–9,13–18. Here we report the direct measurement of in situ methane abundances stored as gas hydrate and free gas in a sediment sequence from the Blake ridge, western Atlantic Ocean. Our results indicate the presence of substantial quantities of methane (˜15 GT of carbon) stored as solid gas hydrate, with an equivalent or greater amount occurring as bubbles of free gas in the sediments below the hydrate zone.
350 citations
References
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TL;DR: The estimated amount of organic carbon in the methane-hydrate reservoir greatly exceeds that in many other reservoirs of the global carbon cycle as discussed by the authors, such as the atmosphere (3.6 Gt), terrestrial biota (830 Gt); terrestrial soil, detritus and peat (1960 Gt).
1,074 citations
TL;DR: In this paper, the response of a stratified elastic half space to a general source may be represented in terms of the reflection and transmission properties of the regions above and below the source.
Abstract: Summary. The response of a stratified elastic half space to a general source may be represented in terms of the reflection and transmission properties of the regions above and below the source. For P-SV and SH waves and both buried sources and receivers, convenient forms of the response may be found in which no loss of precision problems arise from growing exponential terms in the evanescent regime. These expressions have a ready physical interpretation and enable useful approximations to the response to be developed. The reflection representation leads to efficient computational procedures for models composed of uniform layers, which may be extended in an asymptotic development to piecewise smooth models.
647 citations
TL;DR: In this paper, the authors present a model in which bottom-simulating reflectors (BSR) hydrate layers are formed through the removal of methane from upward moving pore fluids as they pass into the hydrate stability field.
Abstract: Bottom-simulating reflectors (BSR) are observed commonly at a depth of several hundred meters below the seafloor in continental margin sedimentary sections that have undergone recent tectonic consolidation or rapid accumulation. They are believed to correspond to the deepest level at which methane hydrate (clathrate) is stable. We present a model in which BSR hydrate layers are formed through the removal of methane from upward moving pore fluids as they pass into the hydrate stability field. In this model, most of the methane is generated below the level of hydrate stability, but not at depths sufficient for significant thermogenic production; the methane is primarily biogenic in origin. The model requires either a mechanism to remove dissolved methane from the pore fluids or disseminated free gas carried upward with the pore fluid. The model accounts for the evidence that the hydrate is concentrated in a layer at the base of the stability field, for the source of the large amount of methane contained in the hydrate, and for BSRs being common only in special environments. Strong upward fluid expulsion into the hydrate stability field does not occur in normal sediment depositional regimes, so BSRs are uncommon. Upward fluid expulsion does occur as a result of tectonic thickening and loading in subduction zone accretionary wedges and in areas where rapid deposition results in initial undercconsolidation. In these areas hydrate BSRs are common. The most poorly quantified aspect of the model is the efficiency with which methane is removed and hydrate is formed as pore fluids pass into the hydrate stability field. The critical boundary in the phase diagram between the fluid-plus-hydrate and fluid-only fields is not well constrained. However, the amount of methane required to form the hydrate and limited data on methane concentrations in pore fluids from deep-sea boreholes suggest very efficient removal of methane from rising fluid that may contain less than the amount required for free gas production. In most fluid expulsion regimes, the quantity of fluid moved upward to the seafloor is great enough to continually remove the excess chloride and the residue of isotope fractionation resulting from hydrate formation. Thus, as observed in borehole data, there are no large chloride or isotope anomalies remaining in the local pore fluids. The differences in the concentration of methane and probably of CO2 in the pore fluid above and below the base of the stability field may have a significant influence on early sediment diagenetic reactions.
413 citations
TL;DR: In this article, multichannel seismic reflection data have been analyzed from an area of clear bottom simulating reflectors (BSRs) on the northern Cascadia subduction zone margin off Vancouver Island.
Abstract: Multichannel seismic reflection data have been analyzed from an area of clear bottom simulating reflectors (BSRs) on the northern Cascadia subduction zone margin off Vancouver Island. The reflector at a depth of about 300 m subbottom is interpreted to represent the base of a layer of methane hydrate or clathrate. The shallow water depth of 1300 m and the 3600-m-long hydrophone array have allowed BSR amplitude-versus-offset and high-resolution velocity analysis, as well as modelling of vertical incidence data. The results of all three types of analysis can be best explained by a 10 to 30-m-thick high-velocity layer located immediately above the BSR about 300 m below the seafloor, having a sharp base and transitional top. In the layer, about one third of the sediment pore spaces must be filled with hydrate “ice”. There is no seismically detectable free gas beneath the BSRs. These results put important constraints on models for the distribution and formation of BSR hydrate.
398 citations
TL;DR: In this article, the authors present empirical sound velocity-density relations (in the form of regression curves and equations) in terrigenous silt clays, turbidites, and shale, in calcareous materials (sediments, chalk, and limestone).
Abstract: In studies in underwater acoustics,geophysics, and geology, the relations between soundvelocity and density allow assignment of approximate values of density to sediment and rock layers of the earth’s crust and mantle, given a seismicmeasurement of velocity. In the past, single curves of velocity versus density represented all sediment and rock types. A large amount of recent data from the Deep Sea Drilling Project (DSDP), and reflection and refraction measurements of soundvelocity, allow construction of separate velocity–density curves for the principal marine sediment and rock types. The paper uses carefully selected data from laboratory and i n s i t umeasurements to present empirical sound velocity–density relations (in the form of regression curves and equations) in terrigenous silt clays, turbidites, and shale, in calcareous materials (sediments, chalk, and limestone), and in siliceous materials (sediments, porcelanite, and chert); a published curve for DSDP basalts is included. Speculative curves are presented for composite sections of basalt and sediments. These velocity–density relations, with seismicmeasurements of velocity, should be useful in assigning approximate densities to sea‐floor sediment and rock layers for studies in marine geophysics, and in forming geoacoustic models of the sea floor for underwater acoustic studies.
311 citations