Carolyn D. Ruppel

Predicting the occurrence, distribution, and evolution of methane gas hydrate in porous marine sediments

Using a new analytical formulation, we solve the coupled momentum, mass, and energy equations that govern the evolution and accumulation of methane gas hydrate in marine sediments and derive expressions for the locations of the top and bottom of the hydrate stability zone, the top and bottom of the zone of actual hydrate occurrence, the timescale for hydrate accumulation in sediments, and the rate of accumulation as a function of depth in diffusive and advective end-member systems. The major results emerging from the analysis are as follows: (1) The base of the zone in which gas hydrate actually occurs in marine sediments will not usually coincide with the base of methane hydrate stability but rather will lie at a more shallow depth than the base of the stability zone. Similarly, there are clear physical explanations for the disparity between the top of the gas hydrate stability zone (usually at the seafloor) and the top of the actual zone of gas hydrate occurrence. (2) If the bottom simulating reflector (BSR) marks the top of the free gas zone, then the BSR should occur substantially deeper than the base of the stability zone in some settings. (3) The presence of methane within the pressure-temperature stability field for methane gas hydrate is not sufficient to ensure the occurrence of gas hydrate, which can only form if the mass fraction of methane dissolved in liquid exceeds methane solubility in seawater and if the methane flux exceeds a critical value corresponding to the rate of diffusive methane transport. These critical flux rates can be combined with geophysical or geochemical observations to constrain the minimum rate of methane production by biogenic or thermogenic processes. (4) For most values of the diffusion-dispersion coefficient the diffusive end-member gas hydrate system is characterized by a thin layer of gas hydrate located near the base of the stability zone. Advective end-member systems have thicker layers of gas hydrate and, for high fluid flux rates, greater concentrations near the base of the layer than shallower in the sediment column. On the basis of these results and the very high methane flux rates required to create even minimal gas hydrate zones in some diffusive end-member systems, we infer that all natural gas hydrate systems, even those in relatively low flux environments like passive margins, are probably advection dominated.

Mechanical properties of sand, silt, and clay containing tetrahydrofuran hydrate

[1] The mechanical behavior of hydrate-bearing sediments subjected to large strains has relevance for the stability of the seafloor and submarine slopes, drilling and coring operations, and the analysis of certain small-strain properties of these sediments (for example, seismic velocities). This study reports on the results of comprehensive axial compression triaxial tests conducted at up to 1 MPa confining pressure on sand, crushed silt, precipitated silt, and clay specimens with closely controlled concentrations of synthetic hydrate. The results show that the stress-strain behavior of hydrate-bearing sediments is a complex function of particle size, confining pressure, and hydrate concentration. The mechanical properties of hydrate-bearing sediments at low hydrate concentration (probably 50% of pore space), the behaviorbecomesmoreindependentofstressbecausethehydratescontrolbothstiffnessand strength and possibly the dilative tendency of sediments by effectively increasing interparticle coordination, cementing particles together, and filling the pore space. The cementation contribution to the shear strength of hydrate-bearing sediments decreases with increasing specific surface of soil minerals. The lower the effective confining stress, the greater the impact of hydrate formation on normalized strength.

Extensional processes in continental lithosphere

Since Vening-Meineszs realization that the East African Rift represented an extensional, not compressional, feature and since the widespread acceptance of plate tectonics two decades later, research on the nature and causes of extensional tectonism within continental lithosphere has intensified. Among the manifestations of extensional processes affecting continental lithosphere are passive margins (Atlantic margin), discrete intracontinental rift zones (East African Rift), diffuse rifts (Basin and Range Province), strike-slip dominated rifts (Dead Sea Rift), and rifts in zones of regional compression (Tibetan grabens). Although no two rift zones are alike, continental rifts can generally be characterized by normal faulting with subsidiary strike-slip faulting, lithospheric thinning which outpaces crustal stretching, varying amounts of alkaline magmatism, heat flow that is locally elevated near faults and magmatic centers, and crust that has experienced magmatic underplating and some amount of magmatic intrusion. Most aspects of rift related deformation can be explained in terms of three parameters: (1) lithospheric and (sometimes) asthenospheric thermal structure, (2) lithospheric (particularly crustal) rheology, and (3) temporal factors such as the absolute age, timing, and rate of extension. The interaction of these physical parameters determines the eventual outcome of rifting (failure or progression to complete continental breakup), the patterns of subsidence and uplift, and the mode of extensional deformation. Modes of rifting (the lithospheres mechanical response to extensional stress) can be broadly divided into pure shear, simple shear, and lower crustal flow mechanisms. In a general sense, these categories of rift mechanisms can account for observations at rifted margins, in the Basin and Range Province, and at metamorphic core complexes respectively. The mechanisms of continental lithosphere rifting (the effects) are here distinguished from the processes which actually drive extension (the causes). Following the terminology of previous authors, the causal processes are categorized as either passive or active: Passive processes originate at plate boundaries or in response to convective drag on the base of the lithosphere, while active processes are seated in the sublithospheric mantle and typically involve the interaction of mantle plumes and the lithospheric plates. A peculiar set of factors, including the local stress state, the retreat of a subducting plate, and convective instability of thickened crustal roots, may drive extension in compressional settings like Tibet. In the future, rifting studies are likely to focus on further clarification of the active and passive rifting terminology, better constraints on deformation rates and lithospheric rheology and thermal structure, along-axis segmentation of rifts, and the analogies and differences between rifting on Earth and nearby planets.

New evidence for geologically instantaneous emplacement of earliest Jurassic Central Atlantic magmatic province basalts on the North American margin

Dikes in the southeastern United States represent a major component of the Central Atlantic magmatic province and record kinematics of Pangean breakup near the critical, predrift junction of three major continental masses. Until now, the age of these dikes had not been determined with the same precision as those of Central Atlantic magmatic province basalts on other parts of the circum-Atlantic margin. Our new results for three dike samples from the South Carolina Piedmont yield plateau ages of 198.8 ± 2.2, 199.5 ± 1.8, and 199.7 ± 1.5 Ma. For comparison, we present new age determinations of the benchmark Watchung flows I and III of the Newark basin: 201.0 ± 2.1 and 198.8 ± 2.0 Ma, respectively. Collectively, these data suggest that basaltic volcanism responsible for the dikes, flows, and sills of eastern North America occurred within ∼1 m.y. of 200 Ma. The timing, brief duration, and extent of the Central Atlantic magmatism imply that it may have been causally related to Triassic-Jurassic mass extinctions. The distribution and timing of this magmatism and the absence of regional uplift or an identifiable hotspot track lead us to favor strong lithospheric control on the origin of the Central Atlantic magmatic province, consistent with the modern generation of plume incubation or edge-driven convection models.

Blake Ridge methane seeps: characterization of a soft-sediment, chemosynthetically based ecosystem

Observations from the first submersible reconnaissance of the Blake Ridge Diapir provide the geological and ecological contexts for chemosynthetic communities established in close association with methane seeps. The seeps mark the loci of focused venting of methane from the gas hydrate reservoir, and, in one location (Hole 996D of the Ocean Drilling Program), methane emitted at the seafloor was observed forming gas hydrate on the underside of a carbonate overhang. Megafaunal elements of a chemosynthetically based community mapped onto dive tracks provide a preliminary overview of faunal distributions and habitat heterogeneity. Dense mussel beds were prominent and covered 20 � 20 m areas. The nearly non-overlapping distributions of mussels and clams indicate that there may be local (meter-scale) variations in fluid flux and chemistry within the seep site. Preliminary evidence suggests that the mussels are host to two symbiont types (sulfide-oxidizing thiotrophs and methanotrophs), while the clams derive their nutrition only from thiotrophic bacteria. Invertebrate biomass is dominated by mussels (Bathymodiolus heckerae) that reach lengths of up to 364 mm and, to a lesser extent, by small (22 mm length) vesicomyid clams (Vesicomya cf. venusta). Taking into account biomass distributions among taxa, symbiont characteristics of the bivalves, and stable-isotope analyses, the relative importance of methanotrophic vs thiotrophic bacteria in the overall nutrition of the invertebrate

ANOMALOUSLY COLD TEMPERATURES OBSERVED AT THE BASE OF THE GAS HYDRATE STABILITY ZONE ON THE U.S. ATLANTIC PASSIVE MARGIN

In situ measurements to depths of ∼415 m below sea floor in methane hydrate–bearing sediments on the U.S. Atlantic passive margin indicate that temperatures at the bottom simulating reflector (BSR) are anomalously low by 0.5–2.9 °C if the BSR marks the base of gas hydrate stability (BGHS). Several hypotheses may explain the occurrence of the BSR at inappropriate pressure-temperature ( P-T ) conditions. (1) If the BSR does not mark the BGHS, then P-T conditions need not be sufficient to dissociate gas hydrate at this depth. (2) The BSR may lie at nonequilibrium P-T conditions due to incomplete readjustment in response to upper Pleistocene–Holocene climate change. However, the occurrence of the Blake Ridge BSR at an overly shallow depth cannot be easily explained by realistic combinations of pressure-driven deepening (sea-level rise) and temperature-driven shoaling (bottom water temperature changes). (3) The properties of sediments or pore fluids may inhibit the stability of gas hydrate. In particular, capillary forces arising in the fine-grained, montmorillonite-rich sediments of the Blake Ridge may lead to shoaling of the BSR in this setting.

Pressure‐temperature‐time paths from two‐dimensional thermal models: Prograde, retrograde, and inverted metamorphism

Two-dimensional thermal models of time-transitive crustal thickening and subsequent unroofing in large-scale overthrust terrains generate pressure-temperature-time (PTt) paths that generally resemble those produced by earlier one-dimensional instantaneous models, but that differ in detail. These differences in PTt path morphology are most pronounced proximal to major fault zones, where postthrusting geotherms are characterized by large temperature inversions in one-dimensional models but generally lack inversions in two-dimensional models. Tests using the two-dimensional fault model developed here indicate that (1) burial rate (proportional to dP/dt), not thrust fault geometry (dip angle), controls the topology of synthrusting PT paths, but plays only a minor role in determining the maximum temperature that rocks attain during the later, unroofing stage in their thermal histories; (2) normal ranges of thrusting and erosion rates and fault parameters result in PT paths with the usual sense (clockwise on conventional PT diagrams; counterclockwise in our diagrams); (3) the amount of heating and duration of heating following the end of thrusting are a function of the rate of unroofing (−dP/dt) during this period; (4) fast unroofing rates lead to the attainment of lower maximum temperatures after greater amounts of unroofing; (5) the initial thermal state of the lithosphere prior to thrusting has a profound effect on PT path morphologies and on the peak metamorphic conditions attained by samples; (6) for excess heat distributed across the entire lithosphere (e.g., due to increased background thermal gradients), a plot of peak temperatures experienced by metamorphic rocks versus structural depth (TMAX plot) closely represents the initial geotherm; (7) excess heat in the crust only (e.g., increased radioactive heating in a layer) yields a different result, with the TMAX plot corresponding to the initial geotherm only near the top of the hanging wall (TMAX plots for both (6) and (7) show no temperature inversion which exceeds the nominal uncertainties (±50 K) for geothermometric data); (8) shear heating can lead to significant temperature inversions at the fault zone if the frictional coefficient μ is 0.6 or greater; and (9) simultaneous thrusting and erosion produce PT loops significantly narrower than those resulting from sequential thrusting and erosion, suggesting that any formulation which fails to account for some degree of simultaneity between thrusting and erosion represents a far endmember model. Forward models like those presented here provide important guidelines for understanding the sensitivity of metamorphic PTt paths to various thermal, mechanical, and geometric factors related to tectonism, but they are generally inappropriate for reconstructing metamorphic thermal histories from actual petrologic and geochronologic data. Analytical inversion techniques that use a postthrusting thermal regime, consistent with two-dimensional forward models, and that integrate values of dP/dt, dT/dt, and radioactive heating rates extracted from suites of metamorphic rocks provide the best hope for furthering our understanding of the thermal evolution of metamorphic terrains.

Observations related to tetrahydrofuran and methane hydrates for laboratory studies of hydrate‐bearing sediments

The interaction among water molecules, guest gas molecules, salts, and mineral particles determines the nucleation and growth behavior of gas hydrates in natural sediments. Hydrate of tetrahydrofuran (THF) has long been used for laboratory studies of gas hydrate-bearing sediments to provide close control on hydrate concentrations and to overcome the long formation history of methane hydrate from aqueous phase methane in sediments. Yet differences in the polarizability of THF (polar molecule) compared to methane (nonpolar molecule) raise questions about the suitability of THF as a proxy for methane in the study of hydrate-bearing sediments. From existing data and simple macroscale experiments, we show that despite its polar nature, THFs large molecular size results in low permittivity, prevents it from dissolving precipitated salts, and hinders the solvation of ions on dry mineral surfaces. In addition, the interfacial tension between water and THF hydrate is similar to that between water and methane hydrate. The processes that researchers choose for forming hydrate in sediments in laboratory settings (e.g., from gas, liquid, or ice) and the pore-scale distribution of the hydrate that is produced by each of these processes likely have a more pronounced effect on the measured macroscale properties of hydrate-bearing sediments than do differences between THF and methane hydrates themselves.

Thermal conductivity of hydrate‐bearing sediments

[1] A thorough understanding of the thermal conductivity of hydrate-bearing sediments is necessary for evaluating phase transformation processes that would accompany energy production from gas hydrate deposits and for estimating regional heat flow based on the observed depth to the base of the gas hydrate stability zone. The coexistence of multiple phases (gas hydrate, liquid and gas pore fill, and solid sediment grains) and their complex spatial arrangement hinder the a priori prediction of the thermal conductivity of hydrate-bearing sediments. Previous studies have been unable to capture the full parameter space covered by variations in grain size, specific surface, degree of saturation, nature of pore filling material, and effective stress for hydrate-bearing samples. Here we report on systematic measurements of the thermal conductivity of air dry, water- and tetrohydrofuran (THF)-saturated, and THF hydrate-saturated sand and clay samples at vertical effective stress of 0.05 to 1 MPa (corresponding to depths as great as 100 m below seafloor). Results reveal that the bulk thermal conductivity of the samples in every case reflects a complex interplay among particle size, effective stress, porosity, and fluid-versus-hydrate filled pore spaces. The thermal conductivity of THF hydrate-bearing soils increases upon hydrate formation although the thermal conductivities of THF solution and THF hydrate are almost the same. Several mechanisms can contribute to this effect including cryogenic suction during hydrate crystal growth and the ensuing porosity reduction in the surrounding sediment, increased mean effective stress due to hydrate formation under zero lateral strain conditions, and decreased interface thermal impedance as grain-liquid interfaces are transformed into grain-hydrate interfaces.

Fluid, methane, and energy flux in an active margin gas hydrate province, offshore Costa Rica

Joint analysis of seafloor temperature data, borehole temperature measurements, and observations that constrain the position of the top and base of the gas hydrate zone (GHZ) at sites on the non-accretionary Costa Rican margin provide quantitative constraints on the flux of energy, fluid, and methane through the sediments on the overriding plate. A comparison of borehole temperature data obtained in the trench, at the toe of the deformed sedimentary wedge, and at a midslope location indicates that the thermal regimes are perturbed by appreciable advective flux at ∼1 km oceanward (19 mm yr−1) and ∼1.4 km landward (5–7 mm yr−1) of the deformation front. This observation is loosely consistent with models that predict the most rapid dewatering and highest rates of fluid expulsion within a few kilometers of the deformation front. Using data that constrain the thickness of the GHZ at three sites on the forearc, we estimate methane and fluid flux rates through application of a one-dimensional analytical model that constrains the nature of the plumbing system for the gas hydrate reservoir. Our results indicate a two-fold decrease in methane flux between a site located ∼400 m from the toe of the wedge and a site 12 km from the toe. An intermediate site located 1.4 km from the toe of the wedge has a thick GHZ (extends to at least 340–365 m below seafloor) that requires a very high methane supply rate (>345 mol m−2 kyr−1 for a mass flux of 5 mm yr−1). We conclude that this site has the most two-dimensional fluid flux regime, with fluids being fed into the zone laterally by conduits and fractures and vertically by expulsion during compaction of underthrusting sediments.