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2017
Brown, KM, Poeppe D, Josh M, Sample J, Even E, Saffer D, Tobin H, Hirose T, Kulongoski JT, Toczko S, Maeda L, shipboard party IODPL348.  2017.  The Action of Water Films at Å-Scales in the Earth: Implications for the Nankai Subduction System. EPSL. 463:266-276.   10.1016/j.epsl.2016.12.042   Abstract

Water properties change with confinement within nanofilms trapped between natural charged clay particles. We investigated nanofilm characteristics through high-stress laboratory compression tests in combination with analyses of expelled pore fluids. We utilized sediments obtained from deep drilling of the Nankai subduction zone at Site C0002 of the Integrated Ocean Drilling Program (IODP). We show that below 1–2 km, there should be widespread ultrafiltration of migrating fluids. Experiments to> 100 MPa normal compression collapse pores below a few ion monofilm thicknesses. A reduction towards a single condensing/dehydrating ion monofilm occurs as stresses rise >100–200 MPa and clay separations are reduced to <10–20 Å. Thus, porosity in high mineral surface area systems only consists of double and single monofilms at depths below a few km leaving little room for either bulk water or the deep biosphere. The resulting semipermeable properties result in variable segregation of ions and charged isotopes and water during active flow. The ultrafiltration and ion dehydration processes are coupled in that both require the partial immobilization of ions between the charged clay surfaces. The general effect is to increase salinities in residual pore fluids at depth and freshen fluids expelled during consolidation. Cessation of nanofilm collapse to a near constant ∼17 Å below 2 km depth at Nankai supports the contention for the onset of substantial geopressuring on the deeper seismogenic fault. The properties of monofilm water, thus, have considerable implications for the deep water properties of subduction zones generating major tremor and Mw 8+ earthquakes. Indeed, the combined effects of advective flow, ultrafiltration, diffusion, and diagenesis could provide a unifying explanation for the origins of overpressuring and pore water geochemical signals observed in many natural systems.

Keywords

nanofilms;
ultrafiltration;
ion dehydration;
pore fluid evolution;
porosity evolution;
overpressuring

2015
Mitchell, EK, Fialko Y, Brown KM.  2015.  Frictional properties of gabbro at conditions corresponding to slow slip events in subduction zones. Geochemistry Geophysics Geosystems. 16:4006-4020.   10.1002/2015gc006093   AbstractWebsite

We conducted a series of experiments to explore the rate and state frictional properties of gabbro at conditions thought to be representative of slow slip events (SSEs) in subduction zones. The experiments were conducted using a heated direct shear apparatus. We tested both solid and simulated gouge samples at low effective normal stress (5-30 MPa) over a broad range of temperatures (20-600 degrees C) under dry and hydrated conditions. In tests performed on dry solid samples, we observed stable sliding at low temperatures (20-150 degrees C), stick slip at high temperatures (350-600 degrees C), and a transitional "episodic slow slip'' behavior at intermediate temperatures (200-300 degrees C). In tests performed on dry gouge samples, we observed stable sliding at all temperatures. Under hydrated conditions, the gouge samples exhibited episodic slow slip and stick-slip behavior at temperatures between 300 and 500 degrees C. Our results show a decrease in the rate parameter (a - b) with temperature for both solid and gouge samples; friction transitions from velocity strengthening to velocity weakening at temperature of about 150 degrees C for both solid and gouge samples. We do not observe transition to velocity-strengthening friction at the high end of the tested temperature range. Our results suggest that the occurrence of slow slip events and the downdip limit of the seismogenic zone on subduction megathrusts cannot be solely explained by the temperature dependence of frictional properties of gabbro. Further experimental studies are needed to evaluate the effects of water fugacity and compositional heterogeneity (e. g., the presence of phyllosilicates) on frictional stability of subduction megathrusts.

2013
Mitchell, EK, Fialko Y, Brown KM.  2013.  Temperature dependence of frictional healing of Westerly granite: Experimental observations and numerical simulations. Geochemistry Geophysics Geosystems. 14:567-582.   10.1029/2012gc004241   AbstractWebsite

Temperature is believed to have an important control on frictional properties of rocks, yet the amount of experimental observations of time-dependent rock friction at high temperatures is rather limited. In this study, we investigated frictional healing of Westerly granite in a series of slide-hold-slide experiments using a direct shear apparatus at ambient temperatures between 20 degrees C and 550 degrees C. We observed that at room temperature coefficient of friction increases in proportion to the logarithm of hold time at a rate consistent with findings of previous studies. For a given hold time, the coefficient of friction linearly increases with temperature, but temperature has little effect on the rate of change in static friction with hold time. We used a numerical model to investigate whether time-dependent increases in real contact area between rough surfaces could account for the observed frictional healing. The model incorporates fractal geometry and temperature-dependent viscoelasoplastic rheology. We explored several candidate rheologies that have been proposed for steady state creep of rocks at high stresses and temperatures. None of the tested laws could provide an agreement between the observed and modeled healing behavior given material properties reported in the bulk creep experiments. An acceptable fit to the experimental data could be achieved with modified parameters. In particular, for the power-law rheology to provide a reasonable fit to the data, the stress exponent needs to be greater than 40. Alternative mechanisms include time-dependent gouge compaction and increases in bond strength between contacting asperities.

Niemann, H, Linke P, Knittel K, Macpherson E, Boetius A, Bruckmann W, Larvik G, Wallmann K, Schacht U, Omoregie E, Hilton D, Brown K, Rehder G.  2013.  Methane-carbon flow into the benthic food web at cold seeps - a case study from the Costa Rica Subduction Zone. Plos One. 8   10.1371/journal.pone.0074894   AbstractWebsite

Cold seep ecosystems can support enormous biomasses of free-living and symbiotic chemoautotrophic organisms that get their energy from the oxidation of methane or sulfide. Most of this biomass derives from animals that are associated with bacterial symbionts, which are able to metabolize the chemical resources provided by the seeping fluids. Often these systems also harbor dense accumulations of non-symbiotic megafauna, which can be relevant in exporting chemosynthetically fixed carbon from seeps to the surrounding deep sea. Here we investigated the carbon sources of lithodid crabs (Paralomis sp.) feeding on thiotrophic bacterial mats at an active mud volcano at the Costa Rica subduction zone. To evaluate the dietary carbon source of the crabs, we compared the microbial community in stomach contents with surface sediments covered by microbial mats. The stomach content analyses revealed a dominance of epsilonproteobacterial 16S rRNA gene sequences related to the free-living and epibiotic sulfur oxidiser Sulfurovum sp. We also found Sulfurovum sp. as well as members of the genera Arcobacter and Sulfurimonas in mat-covered surface sediments where Epsilonproteobacteria were highly abundant constituting 10% of total cells. Furthermore, we detected substantial amounts of bacterial fatty acids such as i-C15:0 and C17:1 omega 6c with stable carbon isotope compositions as low as -53 parts per thousand in the stomach and muscle tissue. These results indicate that the white microbial mats at Mound 12 are comprised of Epsilonproteobacteria and that microbial mat-derived carbon provides an important contribution to the crab's nutrition. In addition, our lipid analyses also suggest that the crabs feed on other C-13-depleted organic matter sources, possibly symbiotic megafauna as well as on photosynthetic carbon sources such as sedimentary detritus.

2012
Brown, KM, Fialko Y.  2012.  'Melt welt' mechanism of extreme weakening of gabbro at seismic slip rates. Nature. 488:638-+.   10.1038/nature11370   AbstractWebsite

Laboratory studies of frictional properties of rocks at slip velocities approaching the seismic range (similar to 0.1-1 m s(-1)), and at moderate normal stresses (1-10 MPa), have revealed a complex evolution of the dynamic shear strength, with at least two phases of weakening separated by strengthening at the onset of wholesale melting(1-4). The second post-melting weakening phase is governed by viscous properties of the melt layer and is reasonably well understood(5,6). The initial phase of extreme weakening, however, remains a subject of much debate. Here we show that the initial weakening of gabbro is associated with the formation of hotspots and macroscopic streaks of melt ('melt welts'), which partially unload the rest of the slip interface. Melt welts begin to form when the average rate of frictional heating exceeds 0.1-0.4 MW m(-2), while the average temperature of the shear zone is well below the solidus (250-450 degrees C). Similar heterogeneities in stress and temperature are likely to occur on natural fault surfaces during rapid slip, and to be important for earthquake rupture dynamics.

2011
Bekins, BA, Matmon D, Screaton EJ, Brown KM.  2011.  Reanalysis of in situ permeability measurements in the Barbados decollement. Geofluids. 11:57-70.   10.1111/j.1468-8123.2010.00310.x   AbstractWebsite

A cased and sealed borehole in the Northern Barbados accretionary complex was the site of the first attempts to measure permeability in situ along a plate boundary decollement. Three separate efforts at Hole 949C yielded permeability estimates for the decollement spanning four orders of magnitude. An analysis of problems encountered during installation of the casing and seals provides insights into how the borehole conditions may have led to the wide range of results. During the installation, sediments from the surrounding formation repeatedly intruded into the borehole and casing. Stress analysis shows that the weak sediments were deforming plastically and the radial and tangential stresses around the borehole were significantly lower than lithostatic. This perturbed stress state may explain why the test pressure records showed indications of hydrofracture at pressures below lithostatic, and permeabilities rose rapidly as the estimated effective stress dropped below 0.8 MPa. Even after the borehole was sealed, the plastic deformation of the formation and relatively large gap of the wire wrapped screen allowed sediment to flow into the casing. Force equilibrium calculations predict sediment would have filled the borehole to 10 cm above the top of the screen by the time slug tests were conducted 1.5 years after the borehole was sealed. Reanalysis of the slug test results with these conditions yields several orders of magnitude higher permeability estimates than the original analysis which assumed an open casing. Overall the results based on only the tests with no sign of hydrofracture yield a permeability range of 10-14-10-15 m2 and a rate of increase in permeability with decreasing effective stress consistent with laboratory tests on samples from the decollement zone.

2010
Furi, E, Hilton DR, Tryon MD, Brown KM, McMurtry GM, Bruckmann W, Wheat CG.  2010.  Carbon release from submarine seeps at the Costa Rica fore arc: Implications for the volatile cycle at the Central America convergent margin. Geochemistry Geophysics Geosystems. 11   10.1029/2009gc002810   AbstractWebsite

[1] We report total dissolved inorganic carbon (DIC) abundances and isotope ratios, as well as helium isotope ratios ((3)He/(4)He), of cold seep fluids sampled at the Costa Rica fore arc in order to evaluate the extent of carbon loss from the submarine segment of the Central America convergent margin. Seep fluids were collected over a 12 month period at Mound 11, Mound 12, and Jaco Scar using copper tubing attached to submarine flux meters operating in continuous pumping mode. The fluids show minimum (3)He/(4)He ratios of 1.3 R(A) (where R(A) is air (3)He/(4)He), consistent with a small but discernable contribution of mantle-derived helium. At Mound 11, delta(13)C(Sigma CO2) values between -23.9% and -11.6% indicate that DIC is predominantly derived from deep methanogenesis and is carried to the surface by fluids derived from sediments of the subducting slab. In contrast, at Mound 12, most of the ascending dissolved methane is oxidized due to lower flow rates, giving extremely low delta(13)C(Sigma CO2) values ranging from -68.2% to -60.3%. We estimate that the carbon flux (CO(2) plus methane) through submarine fluid venting at the outer fore arc is 8.0 x 10(5) g C km(-1) yr(-1), which is virtually negligible compared to the total sedimentary carbon input to the margin and the output at the volcanic front. Unless there is a significant but hitherto unidentified carbon flux at the inner fore arc, the implication is that most of the carbon being subducted in Costa Rica must be transferred to the (deeper) mantle, i.e., beyond the depth of arc magma generation.

2009
Barry, PH, Hilton DR, Tryon MD, Brown KM, Kulongoski JT.  2009.  A new syringe pump apparatus for the retrieval and temporal analysis of helium in groundwaters and geothermal fluids. Geochemistry Geophysics Geosystems. 10   10.1029/2009gc002422   AbstractWebsite

We present details of a newly designed syringe pump apparatus for the retrieval and temporal analysis of helium (SPARTAH). The device is composed of a commercially available syringe pump connected to coils of Cu tubing, which interface the syringe and the groundwater or geothermal wellhead. Through test deployments at geothermal wells in Iceland and California, we show that well fluids are drawn smoothly, accurately, and continuously into the Cu tubing and can be time-stamped through user-determined operating parameters. In the laboratory, the tubing is sectioned to reveal helium (He) characteristics of the fluids at times and for durations of interest. The device is capable of prolonged deployments, up to 6 months or more, with minimal maintenance. It can be used to produce detailed time series records of He, or any other geochemical parameter, in groundwaters and geothermal fluids. SPARTAH has application in monitoring projects assessing the relationship between external transient events (e. g., earthquakes) and geochemical signals in aqueous fluids.

Furi, E, Hilton DR, Brown KM, Tryon MD.  2009.  Helium systematics of cold seep fluids at Monterey Bay, California, USA: Temporal variations and mantle contributions. Geochemistry Geophysics Geosystems. 10   10.1029/2009gc002557   AbstractWebsite

We report helium isotope ratios ((3)He/(4)He) as well as helium and neon abundance results for submarine cold seep fluids from Extrovert Cliff in Monterey Bay, California. Samples were collected in copper tubing attached to submarine flux meters operating in continuous pumping mode. Following instrumentation recovery, the tubing was sectioned to produce for the first time a high-resolution time series of dissolved He and Ne variations over a time span of several days. Noble gas concentrations are variable and appear affected by interaction with a hydrocarbon phase within the aquifer. However, it is still possible to resolve the He signal into components associated with air equilibration, excess air entrainment, and terrigenic fluxes (both crustal and mantle-derived). The mantle He contribution reaches similar to 25-30% in some samples (up to 2.3 R(A), where R(A) = air (3)He/(4)He). Our quasi-continuous He-Ne record shows remarkable fluctuations over time scales of only a few hours and reflects the combined effects of gas stripping by hydrocarbons and an episodic input of mantle- derived fluids.

LaBonte, AL, Brown KM, Fialko Y.  2009.  Hydrologic detection and finite element modeling of a slow slip event in the Costa Rica prism toe. Journal of Geophysical Research-Solid Earth. 114   10.1029/2008jb005806   AbstractWebsite

We investigate transient fluid flux through the seafloor recorded near the Costa Rica trench during the 2000 Costa Rica Seismogenic Zone Experiment using a 2-D fully coupled poroelastic finite element model. We demonstrate that the observed hydrologic anomalies are consistent with a model of propagating slow slip at the subduction interface between the frontal prism and downgoing plate. There are two sources of volumetric strain that drive fluid flux at the seafloor in response to fault slip at depth: (1) compression and dilation in the vicinity of the tips of a slipping patch and (2) extension and compression due to flexure of the seafloor. The superposition of these two effects results in distinctive spatial and temporal patterns of fluid flow through the seafloor. In a forward modeling approach, time series from shear ruptures with a range of fault length-to-depth ratios in a heterogeneous crust are generated and compared with flow rate observations. Assuming a constant propagation rate and an elliptical profile for the distribution of slip along the decollement, the set of model predictions enables us to infer the probable rupture location, extent, propagation velocity, and duration from a single flow rate time series. The best fit model suggests that the slow slip event initiated within the toe at a depth of less than 4 km and propagated bilaterally at an average rate of 0.5 km d(-1). This interpretation implies that stress in the shallow subduction zone is relieved episodically. Furthermore, the Costa Rica data suggest that episodic slow slip events may initiate in the prism toe without being triggered by a seismic event further downdip.

2007
LaBonte, AL, Brown KM, Tryon MD.  2007.  Monitoring periodic and episodic flow events at Monterey Bay seeps using a new optical flow meter. Journal of Geophysical Research-Solid Earth. 112   10.1029/2006jb004410   AbstractWebsite

[1] To enable testing of the hypothesis that fluid flow from cold seeps responds to episodic tectonic events, we have developed a flow meter with a temporal resolution on the order of minutes. The Optical Tracer Injection System (OTIS) measures flow rates through the sediment-water interface (SWI) of 0.1 to > 500 m/yr +/- 10%, samples fluid for postrecovery chemical analysis, and is adaptable for use as a long-term real-time monitoring station. Laboratory and in situ testing demonstrate that the instrument meets temporal accuracy and resolution requirements necessary to detect transient flow events. In a deployment at an active seep site in Monterey Bay, the OTIS measured flow rates with tidal period variability. Time series analysis indicates flow rate and tide height records are in phase. This flow rate response to tides, together with seep fluid temperatures measured at the SWI of greater than 9 degrees C, suggests the seep is connected via a highly permeable fracture or other conduit to an overpressured aquifer at a depth of at least 110 m. We infer the hydrogeology of this well-like system is further complicated by additional fluid sources since (3)He/(4)He ratios are elevated, and there is a lack of correlation between flow rate and temperature records at frequencies lower than tidal frequencies. This short-duration deployment demonstrates the OTIS's potential to capture flow and chemistry transients associated with earthquakes and creep in the offshore environment.

2006
Weinberger, JL, Brown KM.  2006.  Fracture networks and hydrate distribution at Hydrate Ridge, Oregon. Earth and Planetary Science Letters. 245:123-136.   10.1016/j.epsl.2006.03.012   AbstractWebsite

Analysis of resistivity-at-the-bit (RAB) images acquired during Ocean Drilling Program (ODP) Leg 204 to southern Hydrate Ridge, Oregon reveals changes in the distribution and orientation of resistive fractures between the different hydrologic provinces sampled. Fractures on the ridge crest at Sites 1247, 1248, and 1250, are typically high-angle (> 60 degrees) with isotropically distributed strikes, while those on the ridge flanks at Sites 1244-1246 have lower dips (20-60 degrees) and strike parallel to the trend of the ridge. The eastern slope basin, Site 1251, lacks significant fracturing. These differences indicate that the minimum principal stress (sigma(3)) direction rotates to horizontal at the ridge crest as a result of topographically-driven extension and gravitational collapse of the ridge. Chaotic fracturing on the ridge crest results from the interaction of the stress state and free gas migration, which is confined to high permeability sediments and fracture zones by capillary pressure constraints. In contrast, on the ridge flanks well ordered fractures that agree with stress orientations determined from borehole breakouts result from an obliquely dipping 93 and the predominance of aqueous, rather than gaseous, flow. (c) 2006 Elsevier B.V. All rights reserved.

2005
Brown, KM, Tryon MD, DeShon HR, Dorman LM, Schwartz SY.  2005.  Correlated transient fluid pulsing and seismic tremor in the Costa Rica subduction zone. Earth and Planetary Science Letters. 238:189-203.   10.1016/j.epsl.2005.06.055   AbstractWebsite

Continuous measurements of fluid flow were made over a six month period across the Nicoya Peninsula, Costa Rica (Pacific), convergent margin utilizing osmotically-driven fluid flow meters designed to quantify both inflow and outflow rates on the order of similar to 10(-5) to 3 cm/d. Significant transience in flow was observed through the surface of the forearc. Three periods of correlated flow signals were seen on the subduction forearc among three instruments located in the out-of-sequence thrust (OOST) zone over along-margin strike distances of similar to 30 km. Amplitudes of ground velocity recorded on collocated ocean bottom seismometers (OBS) increase during the three correlated flow events. The seismic signal has frequency characteristics that resemble volcanic and non-volcanic tremor. We hypothesize that repeated plate boundary slow slip events, potentially originating at the up dip limit of the seismogenic zone, generate the observed signals within the toe of the forearc. We propose a model in which the poro-elastic stress/strain field around a series of creep dislocations simultaneously forces flow through fracture networks in the forearc and oceanic basement rocks and induces diffuse flow through the shallow sediments. The former generates the seismic tremor-like noise recorded by the OBSs and the latter generates the flow transients recorded by the fluid flow meters. We suggest that high sensitivity fluid flow meters can be utilized to detect transient tectonic strain events in offshore environments where traditional geodetic techniques lack resolution or are not possible. (c) 2005 Elsevier B.V. All rights reserved.

Weinberger, JL, Brown KM, Long PE.  2005.  Painting a picture of gas hydrate distribution with thermal images. Geophysical Research Letters. 32:n/a-n/a.   10.1029/2004GL021437   AbstractWebsite

A study of cold thermal anomalies identified in continuous down-core records of thermal structure acquired by an infrared (IR) camera during Ocean Drilling Program (ODP) Leg 204 to southern Hydrate Ridge, is combined with core-based observations of structure and lithology, to present new insights into the influence of lithology on hydrate distribution in an accretionary margin. Hydrate distribution is spatially associated with <0.5 cm–thick, laterally discontinuous sand horizons that compose <1% of the sedimentary section at all sites. Proximity to the fractured ridge crest determines the extent to which individual sand horizons host hydrate, with 70% of the horizons hosting hydrate at the crest and only 29% in the basin. This trend is consistent with focused up-dip migration of fluids along permeable lithologic conduits, and shows that lateral flow may be as important as vertical diffusion and fracture flow in determining gas hydrate distribution at Hydrate Ridge.

2004
Tryon, MD, Brown KM.  2004.  Fluid and chemical cycling at Bush Hill: Implications for gas- and hydrate-rich environments. Geochemistry Geophysics Geosystems. 5   10.1029/2004gc000778   AbstractWebsite

The results of a deployment of aqueous flux meters at the Bush Hill hydrate mound show that persistent hydrologic instability is a primary feature of the globally abundant gas- and hydrate-rich cold seep environment. Seven flux meters were deployed for 14 weeks in the area of the Bush Hill hydrate mound. Instruments were deployed on microbial mats, bivalves, adjacent to the surface hydrate mound, and a site without visible fauna. Flow rates were observed to range from downflow of 0.01 mm/day to upflow of >15 mm/day. Temporal variability and a major hydrological event were observed to occur on all instruments. There is evidence that this event was related to a gas expulsion episode. The two instruments which exhibited downflow during the event were near surface hydrates and bubbling vents. Nearby instruments recorded a rapid increase in flow rates at the time of the event with a subsequent decrease in rates to the previous background values. Two instruments showed significant output of seawater-like fluids, while the others output typical methane seep-type pore fluids. These results at a passive margin site, along with our previously published convergent margin results at gas- rich northeast Pacific cold seeps (Hydrate Ridge, Eel River margin), illustrate the hydrologic complexity of these environments. We now propose that these and our earlier results are characteristic of seafloor environments which have abundant free gas and hydrates. These mechanisms have major consequences for the near-surface geochemical and microbial environment and for the way we interpret measurements made in these areas.

2003
Brown, KM, Kopf A, Underwood MB, Weinberger JL.  2003.  Compositional and fluid pressure controls on the state of stress on the Nankai subduction thrust: A weak plate boundary. Earth and Planetary Science Letters. 214:589-603.   10.1016/s0012-821x(03)00388-1   AbstractWebsite

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 (mu(r)) at stress levels between 1.0 and 40 MPa. The direct shear mu(r) values for purified smectite are similar to0.14+/-0.02, for illite similar to0.25+/-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 mu(r) should lie between 0.2 and 0.32. This low residual strength, together with elevated fluid pressure, limits shear stresses to below similar to4 MPa within the frontal similar to50 kin 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 mu(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 mu(r) values of the clay-rich decollement still limit shear stresses to between similar to17 and 29 MPa within the frontal similar to50 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. (C) 2003 Elsevier B.V. All rights reserved.

Kopf, A, Brown KM.  2003.  Friction experiments on saturated sediments and their implications for the stress state of the Nankai and Barbados subduction thrusts. Marine Geology. 202:193-210.   10.1016/s0025-3227(03)00286-x   AbstractWebsite

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 mu(r) = similar to0.25 and mu(r) = similar to0.11 in clay horizons, respectively. Such values agree with theoretical estimates from critical wedge theory (Nankai: mu(b) = similar to0.16-0.26 and Barbados: mu(b) = similar to0.06-0.09) and fault spacing geometries from seismic profiles (Nankai mu(b) = similar to0.12-0.23 and Barbados: mu(b) = similar to0.11-0.19). Maximum pore pressure ratios of lambda* = 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 similar to10 MPa or less in the frontal similar to50 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. (C) 2003 Elsevier B.V. All rights reserved.

Levin, LA, Ziebis W, Mendoza GF, Growney VA, Tryon MD, Brown KM, Mahn C, Gieskes JM, Rathburn AE.  2003.  Spatial heterogeneity of macrofauna at northern California methane seeps: influence of sulfide concentration and fluid flow. Marine Ecology-Progress Series. 265:123-139.   10.3354/meps265123   AbstractWebsite

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 sediments (<0.1 mm). Vesicomya pacifica (clam) bed and non-seep sediments exhibited little net fluid outflow and similar oxygen penetration (3 and 4 mm, respectively); however, sulfide concentrations were higher in subsurface clam-bed sediments (up to 2 mM) than in non-seep sediments (<200 muM). Macrofaunal densities did not differ among the 3 habitats (13 800 to 16 800 ind. m(-2); >300 mum), but biomass and diversity (no. species per core, E(S-100), 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 sulfide 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 assemblages inhabiting methane seeps and must be accounted for when characterizing the microbiology and ecology of seep habitats.

2002
Spinelli, GA, Fisher AT, Wheat CG, Tryon MD, Brown KM, Flegal AR.  2002.  Groundwater seepage into northern San Francisco Bay: Implications for dissolved metals budgets. Water Resources Research. 38   10.1029/2001wr000827   AbstractWebsite

[1] Nonconservative excesses of dissolved metals in northern San Francisco Bay indicate that there are internal sources of metals within the bay. We quantified groundwater seepage and bioirrigation rates in this area to determine their roles in transporting dissolved metals from benthic sediments to surface waters. We deployed seepage meters and collected sediment, pore water, and bottom water samples at three sites. We determined seepage rates from seepage meters and modeled the transport of water through the sediment using pore water data to constrain rates of diffusion, advection, and bioirrigation. A groundwater flow model incorporating sediment physical properties and local topography constrains more regional seepage estimates. The seepage meters indicate upflow rates from 7 to 56 cm yr(-1) in March and April 1999 with some large (less than or equal to50 cm yr(-1)) daily fluctuations that greatly exceed predictions based on sediment physical properties and tidally induced pore pressure variations. During this period, results from modeling pore water chemical data are consistent with a small bioirrigation rate (<1.5 10(-7) s(-1)) relative to values determined for southern San Francisco Bay, and an average groundwater upwelling speed of 15 cm yr(-1). The speed and direction of flow changed throughout the year, with best fits to the data ranging from 20 cm yr(-1) upflow to 34 cm yr(-1) downflow and averaging 4 cm yr(-1) upflow. Confidence intervals (95%) are about +/-10 cm yr(-1) for this method, yet the range of acceptable seepage rates for temporally successive periods only overlap in one of four cases, suggesting that temporal variability can be discerned from potential artifacts. Groundwater flow modeling suggests that the seepage rates determined at our sites represent &SIM;45% of the average seepage rate for the area, applying one consolidation and permeability relationship to all sediments. If we apply these approximations to all of northern San Francisco Bay, benthic fluxes of dissolved metals to the surface waters could account for a relatively large amount (&LE;60%) of the unknown sources of dissolved cobalt and a relatively small amount (&LE;4%) of the unknown sources of dissolved silver, cadmium, copper, nickel, and zinc. More focused groundwater discharge or elevated metals concentrations are required to have a larger impact on trace element budgets in this setting.

Torres, ME, McManus J, Hammond DE, de Angelis MA, Heeschen KU, Colbert SL, Tryon MD, Brown KM, Suess E.  2002.  Fluid and chemical fluxes in and out of sediments hosting methane hydrate deposits on Hydrate Ridge, OR, I: Hydrological provinces. Earth and Planetary Science Letters. 201:525-540.   10.1016/s0012-821x(02)00733-1   AbstractWebsite

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 in, which is covered by extensive carbonate deposits, and an 800 in 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 similar to 6 X 10(4) 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 I mmol m(-2) day(-1). (C) 2002 Elsevier Science B.V. All rights reserved.

Tryon, MD, Brown KM, Torres ME.  2002.  Fluid and chemical flux in and out of sediments hosting methane hydrate deposits on Hydrate Ridge, OR, II: Hydrological processes. Earth and Planetary Science Letters. 201:541-557.   http://dx.doi.org/10.1016/S0012-821X(02)00732-X   AbstractWebsite

Multiple, long-term measurements of aqueous and chemical flux through regions of active fluid seeps and gas vents on Hydrate Ridge, Cascadia accretionary prism, were accomplished during 1998 and 1999 as part of the international TECFLUX project. These measurements indicate that flow is highly heterogeneous in both time and space with areas of inflow, outflow, and outflow of fluids of both altered and seawater-like composition. While tectonics is the dominant underlying driving force in fluid expulsion, we suggest that much of the shallow expression of this flow is modulated by more complex hydrological processes. These dynamic processes may include gas expulsion-driven pumping and aqueous entrainment in migrating gas, buoyancy-driven fracturing of overlying sediments, rapid changes in the permeability distribution due to injection of gas and the formation of gas hydrates, migrating flow conduits, and tidally driven flow oscillations. By placing the flow rate observations obtained during the two field seasons in the context of the distribution and nature of the seepage sites, we develop a conceptual model of the spatial and temporal interactions between various processes that appear be active at Hydrate Ridge. We further discuss: (1) evidence for the existence of a subsurface gas transport system and discuss the mechanisms which maintain it, (2) estimates of the in situ permeability of seep settings and evidence for, and the mechanisms controlling temporal changes in permeability, and (3) changes in seep output fluid chemistry which may relate to the above hydrological processes.

2001
Tryon, M, Brown K, Dorman LR, Sauter A.  2001.  A new benthic aqueous flux meter for very low to moderate discharge rates. Deep-Sea Research Part I-Oceanographic Research Papers. 48:2121-2146.   10.1016/s0967-0637(01)00002-4   AbstractWebsite

Significant quantities of fluids and dissolved geochemical components are expelled through the sediment surface in ocean margin and sedimented ridge environments. Recently, significant interest has been generated in constraining hydrological processes in these environments, but direct measurement of fluid flow in the marine environment has proven to be difficult and many aspects of marine hydrogeology remain poorly understood. To address the need for a means to make a significant number of direct measurements in a wide range of low to moderate flow environments, we have developed a new type of benthic aqueous flux meter that is capable of measuring diffuse fluid flow through the sediment surface on the order of 0.1 mm yr(-1)-15 m yr(-1) when the flow is through sediments with permeabilities of less than 10(-8) cm(2) (typical seafloor sediments). The instrument measures fluid flow by determining the degree of dilution of a chemical tracer that is injected by an osmotic pump at a known rate into the fluids venting into or out of a collection chamber situated on the sea bed. The pump also withdraws a subsample of this tracer/fluid mix into sample coils allowing a serial record of the flow rates to be determined. Both upward and downward flow can be measured and, when flux rates are high enough to effectively flush the collecting chamber, the instruments also act as geochemical samplers. Three years of laboratory testing and field use have constrained the effects of (1) temperature, pressure, and deployment duration on osmotic pump performance, (2) dispersion/diffusion in the sample coils, and (3) deflection of flow under a range of sediment permeabilities. Recent deployments on the Kodiak and Cascadia accretionary prisms document the range and capabilities of the instrument in the field. (C) 2001 Elsevier Science Ltd. All rights reserved.

Tryon, MD, Brown KM.  2001.  Complex flow patterns through Hydrate Ridge and their impact on seep biota. Geophysical Research Letters. 28:2863-2866.   10.1029/2000gl012566   AbstractWebsite

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.

Brown, KM, Saffer DM, Bekins BA.  2001.  Smectite diagenesis, pore-water freshening, and fluid flow at the toe of the Nankai wedge. Earth and Planetary Science Letters. 194:97-109.   10.1016/s0012-821x(01)00546-5   AbstractWebsite

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 (A(i)) approach similar to 10-15% of the bulk sediment. The A(i) values, however, are not well constrained. At A(i) 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. (C) 2001 Elsevier Science B.V. All rights reserved.

1999
Fitts, TG, Brown KM.  1999.  Stress-induced smectite dehydration: ramifications for patterns of freshening and fluid expulsion in the N-Barbados accretionary wedge. Earth and Planetary Science Letters. 172:179-197.   10.1016/s0012-821x(99)00168-5   AbstractWebsite

Presented is new experimental evidence that smectites can partially dehydrate from 18.5 Angstrom to 15.4 Angstrom hydrates when they are subjected to effective mineral framework stresses above 1.3 +/- 0.3 MPa. 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 20 m 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 similar to 1.3 MPa. (C) 1999 Elsevier Science B.V. AU rights reserved.