Kevin M. Brown

Fluid and chemical fluxes in and out of sediments hosting methane hydrate deposits on Hydrate Ridge, OR, I. Hydrological provinces

Extensive deposits of methane hydrate characterize Hydrate Ridge in the Cascadia margin accretionary complex. The ridge has a northern peak at a depth of about 600 m, which is covered by extensive carbonate deposits, and an 800 m deep southern peak that is predominantly sediment covered. Samples collected with benthic instrumentation and from Alvin push cores reveal a complex hydrogeologic system where fluid and methane fluxes from the seafloor vary by several orders of magnitude at sites separated by distances of only a few meters. We identified three distinct active fluid regimes at Hydrate Ridge. The first province is represented by discrete sites of methane gas ebullition, where the bulk of the flow occurs through channels in which gas velocities reach 1 m s−1. At the northern summit of the ridge the gas discharge appears to be driven by pressure changes on a deep gas reservoir, and it is released episodically at a rate of ∼6×104 mol day−1 following tidal periodicity. Qualitative observations at the southern peak suggest that the gas discharge there is driven by more localized phenomena, possibly associated with destabilization of massive gas hydrate deposits at the seafloor. The second province is characterized by the presence of extensive bacterial mats that overlay sediments capped with methane hydrate crusts, both at the northern and southern summits. Here fluid typically flows out of the sediments at rates ranging from 30 to 100 cm yr−1. The third province is represented by sites colonized by vesicomyid clams, where bottom seawater flows into the sediments for at least some fraction of the time. Away from the active gas release sites, fluid flows calculated from pore water models are in agreement with estimates using published flowmeter data and numerical model calculations. Methane fluxes out of mat-covered sites range from 30 to 90 mmol m−2 day−1, whereas at clam sites the methane flux is less than 1 mmol m−2 day−1.

Compositional and fluid pressure controls on the state of stress on the Nankai subduction thrust: A weak plate boundary

Abstract We show that both fault mineralogy and regional excess fluid pressure contribute to low resolved shear stresses on the Nankai subduction plate boundary off southwest Japan. Ring and direct shear tests indicate that saturated clay minerals in the fault possess intrinsically low residual friction coefficients ( μ r ) at stress levels between 1.0 and 40 MPa. The direct shear μ r values for purified smectite are ∼0.14±0.02, for illite ∼0.25±0.01, and for chlorite 0.26±0.02 (for point load velocities of 0.0001 mm/s). These clay minerals dominate the Nankai subduction decollement zone. Illite (plus quartz) is mechanically important in the altered incoming Muroto section and the predicted decollement μ r should lie between 0.2 and 0.32. This low residual strength, together with elevated fluid pressure, limits shear stresses to below ∼4 MPa within the frontal ∼50 km of the subduction system, consistent with the low wedge taper in this region. A higher wedge taper off the Ashizuri peninsula indicates basal shear stresses rise slightly along strike towards this region. Our analysis indicates lower fluid pressures must predominantly be responsible because only small second order along strike variations in μ r are predicted to occur as a result of variations in smectite and total clay content. These variations should be further reduced at depth under the wedge as smectite is diagenetically altered to illite. However, our data suggest the low μ r values of the clay-rich decollement still limit shear stresses to between ∼17 and 29 MPa within the frontal ∼50 km of the wedge, consistent with other estimates of plate boundary weakness. Indeed, we propose that it should be expected that subduction plate boundaries like Nankai will be weak because of the intrinsic presence of clay-rich faults and moderate fluid overpressures. Our data do not support the hypothesis that the smectite-to-illite reaction directly controls the onset of seismogenic behavior deep in the Nankai system because there is already a mechanical dominance of illite (rather than smectite) in the shallow decollement zone, and we find all the clay phases tend to velocity strengthen. However, temperature-activated clay diagenesis and dehydration may cause secondary changes in the fault properties and state of stress across the up-dip limit of the seismogenic zone.

Tectonics and hydrogeology of the northern Barbados Ridge: Results from Ocean Drilling Program Leg 110

Drilling near the deformation front of the northern Barbados Ridge cored an accretionary prism consisting of imbricately thrusted Neogene hemipelagic sediments detached from little-deformed Oligocene to Campanian underthrust deposits by a decollement zone composed of lower Miocene to upper Oligocene, scaly radiolarian claystone. Biostrati-graphically defined age inversions define thrust faults in the accretionary prism that correlate between sites and are apparent on the seismic reflection sections. Two sites located 12 and 17 km west of the deformation front document continuing deformation of the accreted sediments during their uplift. Deformational features include both large- and small-scale folding and continued thrust faulting with the development of stratal disruption, cataclastic shear zones, and the proliferation of scaly fabrics. These features, resembling structures of accretionary complexes exposed on land, have developed in sediments never buried more than 400 m and retaining 40% to 50% porosity. A single oceanic reference site, located 6 km east of the deformation front, shows incipient deformation at the stratigraphic level of the decollement and pore-water chemistry anomalies both at the decollement level and in a subjacent permeable sand interval. Pore-water chemistry data from all sites define two fluid realms: one characterized by methane and chloride anomalies and located within and below the decollement zone and a second marked solely by chloride anomalies and occurring within the accretionary prism. The thermogenic methane in the decollement zone requires fluid transport many tens of kilometers arcward of the deformation front along the shallowly inclined decollement surface, with minimal leakage into the overlying accretionary prism. Chloride anomalies along faults and a permeable sand layer in the underthrust sequence may be caused by membrane filtration or smectite dewatering at depth. Low matrix permeability requires that fluid flow along faults occurs through fracture permeability. Temperature and geochemical data suggest that episodic fluid flow occurs along faults, probably as a result of deformational pumping.

Measurements of transience and downward fluid flow near episodic methane gas vents, Hydrate Ridge, Cascadia

Aqueous flux measurements within cold seep regions on northern Hydrate Ridge, Cascadia, indicate a high degree of variability, with extended periods of downflow and reversals of flow direction over periods of weeks to months. Local episodic venting of free methane gas was also observed. The instruments recorded similar changes in hydrologic flow patterns both on and off clam fields, the magnitude of the flow rates decreasing away from the clam field. The coincidence of episodic gas venting with nearby highly variable aqueous fluid flow suggests that they may be coupled. We propose that these observations are consistent with the action of a gas-driven pump that operates somewhat like a geyser. The hypothesis of gas-driven pumping of seawater through northern Hydrate Ridge has important ramifications for the mass fluxes through this region.

Spatial heterogeneity of macrofauna at northern California methane seeps: influence of sulfide concentration and fluid flow

Relationships among fluid flow, sulfide concentration, sulfur bacteria and macrofaunal assemblages were examined at methane seeps on the northern California margin, near the mouth of the Eel River (512 to 525 m). Over a 6 mo period, sediments covered with microbial mats exhibited significant but variable outflow of altered fluids, with no flow reversals. This fluid flow was associated with high porewater sulfide concentrations (up to 20 mM) and almost no oxygen penetration of sedi- ments ( 300 µm), but biomass and diversity (no. species per core, E(S100), H ) were lower and composition varied in the sulfidic microbial mat sediments relative to clam-bed and non-seep sediments. The community in microbial mat-covered sediments consisted largely (82%) of 6 species in the polychaete family Dorvilleidae, whereas the clam-bed and non-seep microhabitats supported a mixture of annelids, peracarid crustaceans, nemerteans, and mollusks. Vertical microprofiling of sulfide in animal cores indicated that most taxa avoid H2S concentrations >1 mM. However, sulfide-oxidizing filamentous bacteria, dorvilleid polychaetes and bivalves (mainly V. pacifica) exhibited highest densities at sul- fide concentrations of 1 to 5 mM sulfide. Horizontal and vertical patterns of sulfide availability have a strong influence on the fine-scale distribution, structure and composition of macrofaunal assem- blages inhabiting methane seeps and must be accounted for when characterizing the microbiology and ecology of seep habitats.

Heterogeneous hydrofracture development and accretionary fault dynamics

Ocean Drilling Program observations at many accretionary wedges indicate that fluid flow in low-permeability sediments is focused along faults rich in clay minerals. Recent ring-shear experiments show that shear failure generally reduces permeability in muddy faults to values well below the surrounding sediments and that very little increase occurs in the fault permeability even when continued failure proceeds under steadily reduced effective loads. It is, thus, unlikely that observed focused flow occurs through the matrix of muddy faults, and we suggest that focused flow can only be explained by the development of open fracture systems. The lithostatic fluid pressures necessary for hydrofracturing in weak sediments cannot, however, exist throughout the imbricate thrust system and basal decollement zone because a zero basal shear stress condition would result in regional instability and accretionary wedge collapse. We propose that the regional hydrofracture networks must propagate heterogeneously, leaving relatively strong asperity regions at sublithostatic fluid pressures to balance forces in the wedge in three dimensions.

Smectite diagenesis, pore-water freshening, and fluid flow at the toe of the Nankai wedge

The presence of low-chloride fluids in the lowermost sediments drilled at Ocean Drilling Program Site 808, at the Nankai accretionary wedge, has been considered as prime evidence for long-distance, lateral fluid flow from depth. Here, we re-evaluate the potential role of in situ reaction of smectite (S) to illite (I) in the genesis of this low chloride anomaly. This reaction is known to be occurring at Site 808, with both the S content and S to I ratio in the mixed layer clays decreasing substantially with depth. We show that the bulk of the chloride anomaly can generate by in situ clay dehydration, particularly if pre-reaction smectite abundances (Ai) approach ∼10–15% of the bulk sediment. The Ai values, however, are not well constrained. At Ai values <10–15%, an additional source of low-Cl fluid centered close to the decollement could be required. Thus, there remains the important possibility that the observed low-Cl anomaly is a compound effect of both lateral flow and in situ smectite dehydration.

Friction experiments on saturated sediments and their implications for the stress state of the Nankai and Barbados subduction thrusts

The Nankai and Barbados forearcs have low-stress subduction thrusts. The sediments entering the subduction zone, and namely the material in the decollement zones, have been well characterized by numerous deep-sea drilling legs and studies of the recovered cores. Nankai has high heat flow and significant amounts of illite, while Barbados is a smectite-dominated system. Based on results from ring shear (<2 MPa normal stress) and direct shear (<30 MPa) tests on marine sediments and mineral standards, this translates into a residual frictional resistance of μr=∼0.25 and μr=∼0.11 in clay horizons, respectively. Such values agree with theoretical estimates from critical wedge theory (Nankai: μb=∼0.16–0.26 and Barbados: μb=∼0.06–0.09) and fault spacing geometries from seismic profiles (Nankai μb=∼0.12–0.23 and Barbados: μb=∼0.11–0.19). Maximum pore pressure ratios of λ*=0.85 and 0.73 for Nankai and Barbados, respectively, allow us to estimate effective shear stresses as a function of friction coefficient and density of the sediment gouge to reach only ∼10 MPa or less in the frontal ∼50 km of the decollement zone, respectively. Our data support the contention that fluid pressure transients and sediment composition contribute equally to the weakness along plate boundary faults down to the seismogenic zone, with the first probably dominating the shallow decollement. Shear velocity stepping tests show that the clay-dominated gouges strengthen velocity irrespective of the clay mineralogy, and hence suggest that clay transformation does not affect the onset of seismogenesis.

Stress-induced smectite dehydration: ramifications for patterns of freshening and fluid expulsion in the N. Barbados accretionary wedge

Abstract Presented is new experimental evidence that smectites can partially dehydrate from 18.5 A to 15.4 A hydrates when they are subjected to effective mineral framework stresses above 1.3 ± 0.3xa0MPa. We show here that this process can substantially freshen saline pore fluids and impact our understanding of the hydrogeologic processes in convergent margins where smectites are abundant. The general anomalous freshening of pore fluids in the N. Barbados accretionary wedge was previously proposed to relate to the widespread lateral expulsion of fluids derived from mineral dehydration reactions at great depth. In contrast, our analysis suggests that a significant part of the widespread anomalous freshening relates to stress-induced smectite dehydration that occurred during the Ocean Drilling Program (ODP) pore water extraction process. Clear evidence for long-distance lateral flow appears to be limited to localized chloride and isotopic anomalies developed in a 20xa0m thick, locally mineralized region at the top of decollement horizon at Site 948. Our reinterpretation is consistent with the fluid flow in the decollement zone being (1) highly transient on time scales of several hundreds of years, and (2) channeled laterally into heterogeneous finger-like flow paths that are apparently not intersected, and/or sampled by drilling in most regions at the toe of the wedge. The incipient stress-induced dehydration of smectite may also be buffering in situ fluid pressures in the wedge, maintaining the effective stresses at ˜1.3xa0MPa.

Complex flow patterns through Hydrate Ridge and their impact on seep biota

Our recent measurements of long-term fluid flux rates and output fluid chemistry at seep sites on Hydrate Ridge, Cascadia, suggest that the mechanisms driving fluid flow in gas hydrate bearing forearc settings may be significantly more complex than previously thought. Temporal changes in the polarity and magnitude of flow and in seep fluid chemistry were observed. The nature of flow appears to generate a strong response in the chemosynthetic seep community ecology. We document here: 1) the frequency and range of temporal variability in flow rates, 2) the relatively rapid changes in outflow composition, 3) strong outflow of seawater-like fluids, 4) the association of Calyptogena (sp) clams with oscillatory and inflow settings, and 5) microbial mat communities associated with strong advection of altered fluids. These results suggest that a reappraisal may be required of the nature of the hydrologic system and its effect on related processes such as benthic seep community ecology and biogeochemical cycles.