A new benthic aqueous flux meter for very low to moderate discharge rates

Deep-Sea Research I 48 (2001) 2121}2146

Instruments and methods

A new benthic aqueous #ux meter for very low to moderate discharge rates Michael Tryon*, Kevin Brown, LeRoy Dorman, Allan Sauter Scripps Institution of Oceanography, University of California, San Diego, 9500 Gilman Drive, La Jolla, CA, 92093-0244, USA Received 14 August 2000; accepted 1 November 2000

Abstract Signi"cant quantities of #uids and dissolved geochemical components are expelled through the sediment surface in ocean margin and sedimented ridge environments. Recently, signi"cant interest has been generated in constraining hydrological processes in these environments, but direct measurement of #uid #ow in the marine environment has proven to be di$cult and many aspects of marine hydrogeology remain poorly understood. To address the need for a means to make a signi"cant number of direct measurements in a wide range of low to moderate #ow environments, we have developed a new type of benthic aqueous #ux meter that is capable of measuring di!use #uid #ow through the sediment surface on the order of 0.1 mm yr\}15 m yr\ when the #ow is through sediments with permeabilities of less than 10\ cm (typical sea#oor sediments). The instrument measures #uid #ow by determining the degree of dilution of a chemical tracer that is injected by an osmotic pump at a known rate into the #uids venting into or out of a collection chamber situated on the sea bed. The pump also withdraws a subsample of this tracer/#uid mix into sample coils allowing a serial record of the #ow rates to be determined. Both upward and downward #ow can be measured and, when #ux rates are high enough to e!ectively #ush the collecting chamber, the instruments also act as geochemical samplers. Three years of laboratory testing and "eld use have constrained the e!ects of (1) temperature, pressure, and deployment duration on osmotic pump performance, (2) dispersion/di!usion in the sample coils, and (3) de#ection of #ow 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 "eld.  2001 Elsevier Science Ltd. All rights reserved. Keywords: Flow measurement; Fluid #ow; Hydrothermal #ow; Instruments; Seep meter

* Corresponding author. Tel.: #1-858-822-0591; fax: #1-858-822-3310. E-mail address: [email protected] (M. Tryon). 0967-0637/01/$ - see front matter  2001 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 7 - 0 6 3 7 ( 0 1 ) 0 0 0 0 2 - 4

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1. Introduction Subsurface #uid #ow is a key area of earth science research, because #uids a!ect almost every physical, chemical, mechanical, and thermal property of the upper crust. There are currently developing research "elds in the deep biosphere, gas hydrates, subduction zone #uxes, and seismogenic zone processes and continuing interest in hydrothermal processes. All these new research avenues are directly impacted by the transport of mass, heat, nutrients, and other chemical species in the hydrogeologic system. The coupling action of #uid #ow is particularly important in subduction and spreading ridge environments. Considerable recent interest has, for example, been generated in constraining the mass balance and cycling rates of volatiles (H O, CO , CH ) and    other geochemical components through subduction systems and the stability of large resevoirs of global warming gas contained in shallow methane hydrate bodies (Shipboard Scienti"c Party, 1997; Suess et al., 1999; Whiticar et al., 1995). Hydrogeologic processes at plate boundaries also exert a fundamental control on the stress state, fault dynamics, and thermal processes in these environments (Hyndman et al., 1993; Moore and Vrolijk, 1992) and may impact the nature and properties of the seismogenic zone in convergent margins where &90% of the global earthquake energy is released (Yeats et al., 1997). Surface manifestations of #uid #ow are widely evident in spreading ridge and subduction environments. For example, large amounts of seawater are cycled through the young oceanic crust in the extensive hydrothermal systems centered on the mid-ocean ridge axes resulting in black smokers and unusual biological communities (Campbell et al., 1988; Edmond et al., 1982; von Damm et al., 1985a). A more di!use component of #ow also exists in these environments, particularly at sedimented ridges such as Guaymas Basin, where it is evidenced by extensive areas of bacterial mats (Gieskes et al., 1988; von Damm et al., 1985b). At convergent margins there is signi"cant expulsion of #uids through the shallow oceanic crust and overlying sediments resulting in extensive carbonate chimneys and communities of chemoautotrophic clams and tube worms at cold seep sites (Kulm et al., 1986; Suess et al., 1985). Focused discharge rates can certainly be very large. Determinations made at seeps marked by obvious biologic communities o! Oregon suggests that H O and CH #ux rates can be as high as 640 m yr\ (average Darcy #ow) and 120 mmol m\   day\, respectively (Linke et al., 1994). Unfortunately, the relative ease of identifying sites of signi"cant out#ow has tended to bias our investigations toward these speci"c out#ow regions. While seeps certainly represent volumetrically signi"cant #uid expulsion, they typically represent an extremely small fraction of the surface area of most active systems. Even though it occurs at generally much lower rates, the di!use component of #ow is probably as important as focused #ow in terms of the total mass balance of #uids because of the greater areas involved. For example, extensive development of carbonate pavement and methane hydrates along continental margins indirectly suggests signi"cant di!use #ow is occuring through the surface of these regions (Carson et al., 1994; Hyndman and Davis, 1992; Paull et al., 1994; Xu and Ruppel, 1999). In#ow, whether focused or di!use, is not normally associated with any obvious diagenetic or biological activity and has generally been more di$cult to identify; however, volumetrically signi"cant di!use in#ow and out#ow is believed to exist out on spreading ridge #anks (Becker et al., 1998; Fisher and Becker, 2000; Mottl et al., 1998), and focused in#ow has been recorded in the forearc environment (Tryon et al., 1999). The major impediment to evaluate the magnitude of the #ux in these environments is that the #ux is believed to be extremely heterogeneous, both spatially and temporally, with regions of

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focused and di!use #ow existing in close proximity (Carson et al., 1990; Foucher et al., 1992; Linke et al., 1994; Moore and Vrolijk, 1992). Structure and lithologic and diagenetic patterns contribute to heterogeneity in #ow and expulsion patterns through their e!ect on the permeability distribution (Brown et al., 1994; Moore and Vrolijk, 1992). For a variety of reasons, direct quanti"cation and comparison of the relative signi"cance of the di!use and focused components have been di$cult. These factors have fundamentally limited our past ability to constrain the complete hydrologic system and associated geochemical #uxes in many environments. There have been several recent attempts to build quantitative predictive hydrogeologic models that simulate the hydrologic system of accretionary wedges (Bekins and Dreiss, 1992; Sa!er et al., 2000; Screaton et al., 1990; Wang and Davis, 1996). The building of improved models requires reasonably accurate determinations of a series of parameters that have proven di$cult to obtain including: (1) magnitude, distribution, and nature of internal #uid sources (i.e., consolidation and mineral dehydration reactions), (2) the permeability distribution, and (3) the surface boundary conditions including the di!usive and focused input and output #uxes. It is with quanti"cation of the latter parameter that we are concerned here. To address this need, we have developed a new type of benthic aqueous #ux meter that is capable of measuring di!use #uid #ow through the sediment surface on the order of 0.1 mm yr\}15 m yr\, which covers the range of most cold seep and non-seep environments (Tryon and Brown, 1999a; Tryon et al., 1997, 1999). In this paper, we introduce our #ux measurement technique with (1) a description of the meter, (2) lab testing of the meter, and (3) "eld testing and initial results.

2. Alternate methods of 6ux measurement The most substantial problem in measuring rates of #ow through the sediment}water interface is that its range may be over eight orders of magnitude ((0.1 mm to '1000 m yr\). This has necessitated a variety of techniques, each suitable for some subset of this range and having unique strengths and limitations. Benthic #ux meters have traditionally measured the upper end of this range, thermal pro"le modeling the mid-range, with chemical pro"le modeling used in the lowest #ow areas. 2.1. Chemical/thermal proxle modeling Commonly used methods for determining #ux rates through the sediment/water interface are through modeling of chemical and thermal pro"les. Concentration or temperature vs. depth pro"les are collected from either shallow temperature probe measurements or from pore waters extracted from push, gravity, or piston cores. Concave down and concave up pro"les are produced at any downstream interface by up#ow and down#ow, respectively; however, only the top (up #ow) is typically sampled because of the lack of a lower chemical or thermal boundary. These gradients are then matched to pro"les predicted by solutions to the steady state, one-dimensional di!usion/ conduction}advection equation (e.g., Wheat and McDu!, 1995). Shallow temperature measurements have been used successfully in a variety of settings to determine #ux rates of approximately 0.3}1000 m yr\. Rate limitations depend on the depth of the penetration of the temperature probe, #uctuations in bottom water temperature, and temperature

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probe accuracy. Probes carried by submersible are generally limited to depths of (50 cm and typically cannot de"ne #ow rates below &10 m yr\ (Henry et al., 1992) while the deeper penetration of surface deployed probes can attain rates as low as 30 cm yr\ (Fisher, pers. commun.). A signi"cant advantage to this method is that a large number of measurements may be made in a short period of time. Flux estimates based on porewater pro"les can be very useful in many seep and hydrothermal environments. Chemical di!usion through pore waters is about two to three orders of magnitude slower than thermal di!usion, which allows much lower #ux rates to be measured by chemical modeling than with thermal modeling; however, a distinct and relatively conservative or wellcharacterized tracer must be present in the pore #uid. The upper limit of this method is a few m yr\ as at this rate most of the curvature in the pro"le will be in the upper 0.5 cm of the core (Wheat and McDu!, 1995). The lower bound is limited primarily by the depth of the core and is believed to be in the order of a few mm yr\ (Wheat, pers. commun.). Some of the drawbacks to these methods are (1) down #ow may be detected but can rarely be measured, (2) time variance in #ow cannot be determined, (3) vertical advection must be assumed, and (4) a number of di$cult to obtain parameters such as the thermal or chemical di!usion coe$cient and porosity must be determined or estimated in order to model the pro"les. Additionally, in many environments shallow cementation in the sediment is often encountered, which results in poor core recovery or shallow temperature probe penetration. 2.2. Flux meters There are several di!erent types of #ux meter that have been used in the past to estimate relatively rapid seep discharge rates. A typical #ux meter consists of an inverted `barrela partially inserted into the sediments with some means of measuring the #ux into or through this barrel. The earliest examples of #ux meters were simply "tted with an in#atable bag on the output to measure total #ux (Brouwer and Rice, 1963; Cherkauer and McBride, 1988; Israelsen and Reeve, 1944). More recent methods measure a changing property which can be related to the #ux rate. The Seep Meter (Sayles and Dickinson, 1991) is a substantially more complex variant which uses the temperature or chemical concentration change within the barrel to derive a #ux rate. This technique requires relatively fast #ux rates and a clearly characteristic vent #uid temperature or composition. The OSU/GEOMAR Benthic Barrel (Carson et al., 1990; Linke et al., 1994) uses sequentially triggered Niskin bottles to ultimately record the #uid composition change within the barrel, but can also use a Bernoulli-type or a thermistor #owmeter mounted to the output port. The lower limit of this technique is typically on the order of 5 m yr\ (Carson et al., 1990) however, it has been used successfully to record rates below 1 m yr\ when the venting #uids are methane saturated so that a small net #ux produces a signi"cant concentration change within the barrel (Torres, pers. commun.). In environments with exceptionally high #ow rates, such as mid-ocean ridge hydrothermal "elds, the Medusa system (Schultz et al., 1996) has been used successfully. This instrument uses a rotary #ow sensor and requires #ow rates greater than &30,000 m yr\. Many of these types of #owmeters tend to create su$cient backpressure that #ow is substantially diverted around the meter. The problem of #ow restriction and de#ection of #ow around the meter has been noted for many types of benthic #ux meters (Brouwer and Rice, 1963; Carson et al., 1990; Cherkauer and McBride, 1988; Woessner and Sullivan, 1984) yet generally has not been well

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documented. This e!ect is intrinsic to all benthic #ux meters and is a fundamental constraint on all such measurements. Flux meters typically have a relatively restricted outlet port either to minimize mixing with outside #uid in the case of those measuring a changing property within the chamber, or to increase the #ow velocity to a measurable rate in the case of most other varieties. This restriction of #ow is entirely analogous to permeability. When the instrument's e!ective permeability is signi"cantly less than that of the sediment upon which it lies, then #uid will pass around the instrument along its path of least resistance. All of these are generally suitable for the more vigorous of seeps, requiring #ux rates of 10}100 m yr\, cannot measure in#ow, and can record for relatively short time periods.

3. The chemical and aqueous transport meter In order to address the need for a technique for making a signi"cant number of #uid #ux measurements over an extended range of rates, we have developed the chemical and aqueous transport (CAT) meter. Following is a general description of the instrument, its operating principles, and the methods used for #ux rate determination. Our #ux meter design is similar to others in that it has an open ended collecting chamber inserted into the sediment but di!ers principally in the way it measures the #uid #ux through the chamber. The initial design parameters were: (a) capacity to measure a broad range of #ux rates including very low #ux rates, (b) inexpensive, simple, robust construction so that a large number could be built and deployed for long periods of time, (c) low backpressure so that little or no corrections need be made to the #ux rates to adjust for #ow diverted around the meter, (d) modular design to allow the instrument package to be more adaptable and more easily modi"ed, and (e) capacity to measure #ow for periods of months to a year. The meter uses the dilution of a chemical tracer to measure #ow through the outlet tubing at the top of the chamber. Clear di!erences between seep and seawater compositions are not required. A tracer solution of similar density but di!erent composition than the seep #uid is injected at a constant rate by two osmotic pumps into the water stream as it moves through the outlet tubing (Fig. 1). These same pumps withdraw a sample of the seep #uid/tracer mixture from downstream of the tracer injection port giving a serial record of the tracer dilution. While our osmotic pumps work on the same principle as the Jannasch pumps (Jannasch et al., 1994), they di!er signi"cantly in design. Hydranautics of Oceanside, CA, has been supplying us with osmotic membrane material from which we fabricate membranes for speci"c pump rates. The osmotic pump contains an osmotic membrane that separates chambers containing pure water on one side and a saline side that is held at saturation levels by an excess of NaCl. Due to the constant gradient, distilled water is drawn from the fresh water chamber through the osmotic membrane into the saline chamber at a rate that is constant for a given temperature. The saline output side of the pump system is rigged to inject the tracer while the distilled input sides of the two pumps are connected to separate sample coils into which they draw #uid from either side of the tracer injection point (Fig. 1). Each sample coil is initially "lled with deionized water. Portions of the #uids moving out of the top of the chamber are collected in the coils, displacing the deionized water. A unique pattern of chemical tracer distribution is recorded in the two sample coils allowing a serial record of the #ow rates to be determined. Having two sample coils allows both positive and negative #ow to be measured.

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Fig. 1. Flux meter schematic. The meter uses the dilution of a chemical tracer to measure #ow through the outlet tubing at the top of the chamber. A tracer solution of equal density but di!erent composition than the #ux #uid is injected at a constant rate by an osmotic pump into the water stream as it moves through the outlet tube. The same pump withdraws a sample of the vent #uid/tracer mixture from downstream of the tracer injection port. Each sample coil is initially "lled with distilled water. Portions of the #uids moving into or out of the top of the chamber are collected in the coil, displacing the distilled water. A unique pattern of chemical tracer distribution in the two sample coils gives both polarity and aqueous #ow rates allowing a serial record of the #ow rates to be determined. Having two sample coils allows both positive and negative #ow to be measured.

The meters are suitable for deployment to depths of about 6 km. The simple design of these meters maintains costs low enough that a large number of meters may be deployed in an array to map out the surface hydrogeologic activity of an area. We have deployed arrays of up to 21 #ux meters. The meters can be rapidly redeployed by merely changing out the sample coils and re"lling the tracer bags if necessary. The collecting chambers cover an area of approximately 0.4 m and are &10 cm tall with thin edges so that they can penetrate the sediment easily. For use in soft sediments, an internal perforated ba%e is installed within the chamber to stop excessive settling. The outlet tube is 4.8 mm ID and is in four sections as shown in Fig. 1. The inner sections are 1.0 m in length and the outer ones 0.5}1.0 m, depending on the level of dampening of tidal e!ects we desire. 3.1. Method of yux determination Tracer injection and sampling are all done within the outlet tubing (Figs. 2a and b) so that there is no possibility of loss of tracer due to adsorption by sediment. The pattern of tracer concentration in the two sample coils gives the polarity and aqueous #ux rate. Described here and shown graphically in Fig. 2c is the method used to calculate the #ux rates. Di!erent relationships must be used depending upon whether the volumetric #ux rate through the meter is greater than or less than the upstream osmotic pump rate. Typically, one of the two coils will contain either no tracer or all tracer and the other will contain a seawater/tracer mixture. If the downstream coil contains all tracer then the volumetric #ux rate, q (#ux rate, Q* collection chamber area, A), is less than the upstream pump rate, P (Fig. 2a). If the upstream coil contains no tracer then q'P (Fig. 2b). V V These same relationships hold for both up#ow and down#ow; only the relative positions of P and V P are reversed. The #ux rate, Q, may then be determined by the following relationships, where f is W

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Fig. 2. Tracer concentration vs. #ux rate. (a) At low upward #ux rates (q(P , where q is the volumetric #ux rate and V P is the lower osmotic pump rate) only the lower coil contains a mixture of #ux #uid and tracer while the upper coil V contains pure tracer. (b) At #ux rates greater than the osmotic pump rate (q'P ) only the upper coil contains a mixture V while the lower contains pure #ux #uid. (c) Relative tracer concentrations (sample tracer concentration/injected tracer concentration) of 0.002$0.001 to 0.99$0.01 can be reliably determined by ICP-OES analysis resulting in the resolution of approximately "ve orders of magnitude of #ux rates. Resolution is greatest in the central portion of the range, with signi"cantly greater uncertainty at the extremes. Shown are the results of applying Eqs. (1b) and (2b) to an osmotic pump rate of 5 ml day\. For other pump rates the plot is essentially shifted to higher and lower #ux rates over a range of approximately an order of magnitude. Also shown are the results of applying the equations to the examples in Fig. 3.

the fraction of tracer in the sample (C /C , where C is the concentration of the chemical      used as a tracer in the sample and tracer, respectively): if q(P , f "1, V W

q f "1! V P V

P or Q" V (1!f ), V A

(1b)

P #P W if q'P , f "0, f " V V V W q#P W 1 P P V # W !P . or Q" W A f f W W




(1a)



(2a) (2b)

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Fig. 3. Typical results of a #ux meter deployment. There is an initial ramp up of salinity as seawater is drawn into the coils by the osmotic pump. The osmotic pump rate is determined by dividing the quantity of #uid drawn into the coils up to this salinity front by the deployment duration time. (A) Time required for tracer to reach the sample coils. (B) Transient period of settling of the meter which appears as a high #ux rate ramping toward (C) that associated with the #uid #ux. (a) upward #ux of &28 mm yr\, (b) downward #ux of 3.5 mm yr\ (see also Fig. 2c).

This is shown graphically in Fig. 2c. For a given tracer injection rate, a #ux rate range of approximately "ve orders of magnitude can be measured (Fig. 2c). The lowest measurable #ux rate is &0.1 mm yr\, and the upper limit is &15 m yr\ (these limits are addressed in a following section). The accompanying plots of the salinity/tracer concentration vs. time in the upper and lower sample coils are typical of the initial portions of our results and show several distinct features (Fig. 3). Initially, the coils are "lled with deionized water (DI) and so there is an initial ramp up toward seawater/tracer salinity values. The extent of this ramp is indicative of the amount of dispersion which has occurred in the oldest portion of the record due to its traveling through up to 100 m of tubing. The center of this ramp is considered to be the point at which the meter is turned on, i.e., dispersive mixing has spread the DI/salinity boundary in both directions. From this point, the volume of #uid collected may be determined and, along with the deployment time, is used to determine the in situ pump rate of each osmotic pump. This is necessary as the pump rate can vary with depth and temperature (this e!ect and that of dispersion are covered in the following testing section). Following this there is often a period of nearly pure seawater values (A#B, Fig. 3). This is caused by (1) the time lag necessary for tracer to reach the sampling ports, and (2) high apparent #ow rates as the meter settles slowly into the sea bottom. The length of this period is dependent upon the nature of the sediment and the means of deployment. When the meters are set in place by submersible or diver, they are pressed into the sediment and further settling generally does not occur. However, in the case of meters which have been allowed to settle to the bottom under their own weight, this period can last up to a week or two, particularly when deployed on soft mud.

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Following any period of settling the concentrations will ramp toward the level associated with the actual #ux rate (C, Fig. 3). The rate determinations for these examples are made by applying the appropriate equation (Eqs. (1b) and (2b)) to the resulting tracer concentrations and osmotic pump rates. These determinations are shown graphically in Fig. 2c. 3.2. Tracer analytical technique Almost any tracer may be used which is nonreactive and present in only trace amounts in the environment. The tracer we have been using is 0.300% RbCl in a NaCl solution adjusted to match the water density of the deployment site. Density matching negates any possible density-driven #ow caused by the meter itself. If a tracer of signi"cantly di!erent density than the ambient seawater is injected into the outlet tube, it would rise or sink and indicate #ow where none exists. This e!ect is also mediated by using a horizontal outlet tube, making any gravitational potential negligible. Upon recovery, the sample coils are cut and #uids dispensed into individual samples of 1.0}4.0 cm each. Subsamples are diluted 100 : 1 to remove salt e!ects and analyzed for Rb> concentration on a Perkin}Elmer Optima 3000XL optical emission spectrometer (ICP-OES) which has been upgraded to both axial and radial detectors. The tracer analytical range is 0.2}100% tracer with an accuracy at low tracer concentrations of 30 ppb or $0.1% tracer and at high concentrations of 300 ppb or $1% tracer. Resolution is typically 0.02 and 0.2%, respectively. The choice of Rb> as the tracer species was made in order to allow the measurement of other dissolved chemical components and nutrients in the seep discharge #uids to be undertaken where appropriate. As ICP-OES analysis allows simultaneous analysis of many elements we routinely analyze for the major element (Na, Mg, Ca, K, S) composition of the #uids.

4. Instrument performance evaluation In order to interpret the results of our "eld studies a number of factors need to be evaluated: (1) constancy of the osmotic pump rate and the associated accuracy of the #ux rate determination, (2) the impact of dispersion in the sample coils on the temporal resolution of the #ux rate, (3) conditions under which diversion of #ow around the instrument are not signi"cant, and (4) the maximum and minimum measurable #ux rates. 4.1. Pump rates The method of #ux rate determination relies upon a constant, predictable, and veri"able osmotic pump rate. These rates can be a!ected by di!erences in membrane size and material, chemical gradient across the membrane, temperature, and pressure. The instruments have been tested in the laboratory to determine the change in pump rate with temperature and have been found to respond in a predictable manner to viscosity and volumetric e!ects. Rate tests conducted in a temperature controlled bath indicate that osmotic pump rates change linearly by 3}5%3C\ over the tested range of 2}203C (Fig. 4). We currently use Onset Computer Tidbit威 temperature loggers (resolution 0.153C) on the CAT meters during deployments to monitor temperature variations. The typical temperature variations which we have observed in the deep oceans are generally ($0.253C and

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Fig. 4. Results of a laboratory test of pump rates vs. temperature. The osmotic pump tracer injection rate increases with temperature in a predictable manner. Temperature is logged to account for changes.

so are unlikely to cause signi"cant variations in pump rates in these settings; however, in shallower shelf environments, temperature monitoring is very important. Pump rates also increase signi"cantly as pressure rises from sea level to &500 kPa but remain constant beyond that. This is caused by an irreversible change in the physical properties of the membrane material. All membranes are pressurized in the lab to 7 MPa prior to being placed in service, e!ectively removing e!ects of pressure variations from rate calculations. Fig. 5 shows the in situ pump rates for a number of deployments at di!erent water depths and indicates that the pumps are not impacted by pressure e!ects. The average pump rate of any instrument during any deployment can also be veri"ed because it equals the volume of saline #uids collected in the sample coil up to the fresh water/seawater front (see Fig. 3, region A) divided by the total time of the deployment (in situ pump rate calibration). The tracer injection rate is always equal to the sum of the rates at which sample is drawn into the two sample coils (pump rate). We have now made deployments of the CAT meters in a variety of marine settings at di!erent water depths. In situ pump rate calibrations have been made for each instrument and, after adjusting to a common temperature, are plotted against membrane area in Fig. 5. The deployment duration and depths are also noted. The mean rates de"ne a linear trend with standard deviation of 12% occurring within groups of instruments having certain size membranes. It is important to note that the lack of signi"cant di!erences in average rates between long and short duration deployments indicates that long-term changes in membrane properties are not a factor in these instruments. Slight di!erences in individual membrane properties probably account for the majority of the scatter about the mean rate. Of course, in situ calibration of the pump rates is always determined for each pump individually to account for individual membrane properties and the conditions at the deployment site, so the variability between instruments does not translate into an error in the #ux rate determination made by an individual instrument. The one setting where we have encountered signi"cant pump rate variability is shallow water environments in which large ('13C) and rapid temperature changes occur, generally at tidal frequencies. The thermal expansion coe$cients of the pump housing and contents are similar causing a net di!erence in volume between pump and #uid of only 0.2 ml3C\ during long-period temperature changes. However, the rapid cycling of temperature causes a more complex

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Fig. 5. Summary of in situ pump rates for a wide range of deployment times and conditions. Error bars are one standard deviation. The results indicate that the pump rates are independent of depth and duration. The scatter in the data is due to small di!erences in membrane properties.

interaction between the expansion rates of the pump housing and its contents due to more rapid expansion/contraction of the housing. This basic problem has been noted before (Jannasch et al., 1994) and would be encountered in almost any osmotic pump design. Since tidal period temperature oscillations are the most rapid changes expected in most environments, laboratory simulations of these temperature oscillations were done to constrain their e!ect on pump rates. A typical con"guration of a 10 ml d\ tracer injection rate and temperature range of $0.33C on a tidal cycle yielded a net output rate that varied by $5%. A con"guration of 18 ml d\ and 1.03C yielded a variability in #ow rate about the long-term mean of $12%. It should be noted that these pump rate oscillations will have no e!ect on the determination of average #ux rates over periods of more than a day. By monitoring the temperature both inside and outside the pump we have determined that we can correct the pump rates for these temperature e!ects. Onset Computer Tidbit威 temperature loggers are now being used for this purpose on "eld deployments. 4.2. Dispersion and diwusion As #uids are drawn into and through the long sample coils (Fig. 1), dispersion tends to average out short-term #uctuations in concentration. These #uctuations are the record of changes in #ux rate during the deployment due to tidal variations or other factors. Dispersion is a property of #ow through tubes where viscous drag on the tube walls leads to #ow being more rapid in the tube's center than near the walls. This tends to smear out sharp concentration gradients. Di!usion, however, tends to counter this e!ect in small diameter tubes at slow #ow rates. Di!usion is relatively rapid over the short radial distances and tends to maintain plug #ow in the tube. The e!ect of di!usion in the axial direction is negligible as it is orders of magnitude less e!ective than dispersion. Ideally capillary tubing would lead to the greatest temporal resolution; however, for practical reasons (cost, availability, ease of handling, etc.) we use the largest tubing that will still give acceptable results (2.4 mm ID). The temporal resolution depends primarily on deployment time with the oldest portion of the record being more dispersed than the most recent due to its

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Fig. 6. Example of a tracer record with a large degree of variability. The samples from the oldest portion of this 70 day record have passed through &75 m of tubing and yet maintained large concentration gradients, indicating that di!usion and dispersion of the tracer are not signi"cant.

travel through a greater length of tubing. This dispersion a!ects only the temporal resolution of #ux rates, however. The average #ux rate will still be recorded accurately and the net #ux may still be determined. Our experience with deployments of several months indicates that dispersion is not a signi"cant issue. Fig. 6 is an example from a 10 week deployment on the Costa Rican Paci"c margin and shows similar levels of variability in both the early and later portions of the record. We have found that we typically can achieve resolutions of 0.25}0.50% of the deployment time in the latest portions of the record and &2% in the oldest portion for deployments of a month or more. At di!use, low #ux rate sites the pumps are set at low rates and normally obtain a resolution on the order of 1}2 days for deployments of up to 6 months. At seep sites, the pumps are set at higher rates and deploy for shorter periods (5}10 days to 2 months). The temporal resolution is on the order of 1}12 h. This allows us to look, for example, at how tidally induced #uctuations can a!ect discharge rates (see below). 4.3. Flow deyection As noted earlier, the problem of de#ection of #ow around the meter has been noted for many types of benthic #ux meters (Brouwer and Rice, 1963; Carson et al., 1990; Cherkauer and McBride, 1988; Woessner and Sullivan, 1984). This e!ect is inherent to all benthic #ux meters and is a primary limitation of this technique. In our design, we have minimized this de#ection by using the largest possible outlet tube that would still allow dispersive mixing of the tracer. Quantifying #ow de#ection over a range of sediment properties by actual physical testing, however, proved problematic. A testing facility was constructed using a silty sand bed of uniform thickness overlying a chamber with a 4 m screened top. Although extensive e!orts were made to produce a uniform bed, we were unable to achieve su$ciently homogeneous #ow through the test bed at appropriate #ow rates to properly evaluate the instrument's accuracy. For this reason we turned to numerical modeling of the CAT meter/sea#oor system, which also allowed us to evaluate the instrument's

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Fig. 7. (a) Results of the backpressure tests. Water was pumped through the #ux meter and the backpressure produced was recorded at a number of pump rates. These results were then used to quantify de#ection of #ow around the meter over a range of sediment properties and #ow rates. (b) Results of modeling the diversion of #ow around the meter. Numerical modeling of the meter/sea#oor system has shown that greater than 90% of the #ow should pass through the meter when deployed on sediments less permeable than 10\ cm (typical clay and silt rich sea#oor sediments).

performance under a wider range of sediment permeability than would be possible with the test bed. The instrument's resistance to #ow had to "rst be empirically determined. The di!erential pressure vs. #ow rate through the meter was determined by enclosing the chamber bottom and pumping water through the submerged meter at known rates with a Harvard syringe pump. A Validyne 0.2 psi pressure transducer was used to measure the pressure di!erential between the chamber and the testing tank. The resulting linear relationship between #ow rate and di!erential pressure (Fig. 7a) may then be used in the numerical model to de"ne the back pressure that would be produced beneath the instrument at any particular #ow rate. The modeling was performed with Waterloo Hydrologic's Visual MODFLOW, an extension of the standard USGS hydrogeological modeling package with the addition of a graphical interface. The three-dimensional (3D) simulation is done in a 2 m cube with a 5 cm cubic grid. Larger volumes and smaller grids were tried and were found only to increase computation time with no signi"cant change in "nal results. The meter is simulated by a block of cells atop a uniformly permeable bed

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1.8 m thick and overlain by a constant head boundary simulating the surrounding seawater. The cells simulating the meter are assigned a "xed pressure di!erential relative to the overlying seawater cells. The Zone Budget subprogram monitors the #ow rate through the meter. The remaining cells are assigned a permeability for which we wish to evaluate the instrument's performance. By assigning a head to the base cells of the model, a steady-state vertical #ow is setup. Through an iterative process the base cells' head is varied until a steady-state #ow is generated which produces a #ow through the meter which corresponds to the chosen backpressure. The model is then repeated with no di!erential pressure assigned to the meter cells and the change in #ow through the meter determined. This process is repeated for a large number of #ow rates and sediment permeabilities. The results of the modeling are shown in Fig. 7b. It was found that the meter e$ciency (100%;measured #ow rate/unperturbed #ow rate) is purely a function of the sediment permeability and is independent of #ow rate. A common misconception is that greater diversion of #ow around an instrument occurs at higher #ow rates; however, this can easily be shown to be untrue of any instrument that has a linear pressure response to #ow. For any given increase in #ow rate there is a corresponding proportional increase in the pressure gradient through the instrument, but there is also a proportionally equal increase in pressure gradient in the sediment beneath and surrounding the instrument. Therefore, for any #ow path, whether through the instrument or around the instrument, there will always be the same relative resistance to #ow, regardless of #ow rate. As long as the instrument has a `permeabilitya that is greater than or equal to the sediment permeability, there will be little diversion of #ow. Our results show that the meter should give reliable results ('90% of true #ux) when used to measure #ow di!using through sediments with permeabilities less than &10\ cm (silty sand). Typical clay and silt-rich marine sediments fall well within this range, which includes all but the most permeable sea#oor sediments (well sorted, clean sand). At higher permeabilities a correction to the indicated #ux rate could be made based on these model results if the sediment permeability could be measured or estimated from an associated core sample. 4.4. Maximum and minimum rates The maximum measurable #ux rate, with the current con"guration, is limited by the nature of the sediment through which the #uid is passing. The maximum head gradient cannot exceed lithostatic, which for typical near-surface sediment is &1.5 times hydrostatic or a head gradient of &0.5. The greatest #ow rate will be produced through the highest permeability sediment which will not cause de#ection of #ow, or 10\ cm from the previous section. Using Darcy's Law and these values results in a maximum #ux rate of &150 m yr\. Darcy's Law is k
g dh
" ,  dl

(4)

where
is the #ow rate, k is the permeability,
the #uid density, g the gravity,  the #uid viscosity, and dh/dl the head gradient. We suggest, however, that the practical maximum #ow rate the meters can measure accurately is probably closer to 15 m yr\. This is because at higher #ow there may be insu$cient mixing of the tracer between the injection and sampling port and we are approaching the permeability where

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de#ection of #ow becomes an issue. Theoretically, the tracer technique should be capable of measuring #ux rates as low as 0.01 mm yr\ at the minimum tracer injection rate; however, our experience with di!usion and other factors has shown that the practical lower limit is closer to &0.1 mm yr\. We have commonly been able to measure rates in the 0.1}10 mm yr\ range (see Section 7).

5. Tidal pumping Ocean bottom water pressure cycles with the tides with a typical amplitude of 10}20 kPa. These pressure changes penetrate into the sediment column a short distance. As the pressure builds or wanes, a pressure di!erential exists across the sediment leading to a small amount of oscillating #ow. In low permeability, clay-rich sediments this #ow is negligible; however, in silt or sediments dominated by fracture permeability signi"cant #ow can occur. Sediment permeability and tidal magnitude are the most sigi"cant of a number of factors in#uencing the magnitude of tidal #ux including porosity, matrix grain bulk modulus, and Poisson's ratio (Wang and Davis, 1996). This oscillatory #ow imparts `noisea to our net #ow signal which, under certain conditions, can dominate the observed signal. This process will in#uence measurements by all types of #ux meters and may fundamentally limit our ability to measure #ux rates in the oceanic environment on certain high-permeability substrates through which only a small net #ow is occurring. Examples of this phenomenon are shown in Fig. 8. Fig. 8a is a record of tracer concentraton and shows a low

Fig. 8. Examples of the e!ect of tidal #uctuations on the tracer record. (a) A large amount of tidal oscillation with little or no resolveable net #ow. The rapid reversals of #ow with relatively large tidal #ux result in rapidly varying tracer concentration in both the upper and lower coil. (b) Tidal oscillations overlaid on a net upward #ux that is larger than the tidal #ux results in no reversals of #ow and a varying amount of tracer in the upper coil only. The resolution of this record is insu$cient to resolve individual tidal oscillations but readily shows the net #ux and approximates the level of variability.

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signal-to-noise-ratio problem encountered on high-permeability sediments. While individual tidal cycles cannot be resolved in this record because of the sampling frequency, clearly #ow is oscillating up and down with #ow rates high enough in both directions to cause signi"cant dilution of the tracer (i.e. equivalent to the dilution produced by advective rates of 10 cm yr\). No net #ow can be resolved in this record. Fig. 8b is also a record of highly variable #ow in high-permeability sediment, but in this case the advective #ow is high enough that reversals of #ow do not occur and the net #ow is readily determined.

6. Adaptations for di4erent deployment strategies and environments The meters are currently being produced in three basic con"gurations; the basic meter suitable for submersible or diver deployment (Fig. 9a), the autonomous meter with acoustic release system

Fig. 9. Flux meter variations. (a) basic meter suitable for diver or submersible deployment, (b) autonomous #ux meter with #oatation and acoustic release system, and (c) OBS-mounted #ux meter with collection chamber incorporated into the anchor below the OBS.

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and #oatation (Fig. 9b), and the OBS meter designed to `piggybacka on LeRoy Dorman's Scripps/ONR Ocean Bottom Seismometers (OBS) (Fig. 9c). Currently there are 34 #ux meters (14 OBS type and 10 of each other con"guration). A number of changes can be made to the con"guration of the #ux meters to adapt them for di!erent uses. The most common change is in the osmotic pump rates. Although the meters are capable of measuring #ux rates covering "ve orders of magnitude, their resolution is greatest when the volumetric #ux rate is closest to the tracer injection rate (central portion of Fig. 2c). The osmotic pump rate is therefore adjusted higher when high #ux rates are expected. Another consideration in setting the rate is the length of the deployment. We need to collect enough #uids so that we have the desired temporal resolution at the minimum sample volume required for analytical purposes. Ideally, the combination of expected #ux rate, desired temporal resolution, and minimum sample size determines the length of the deployment and thus the osmotic pump rate, although in the real world, ship scheduling and weather are the true controlling factors. The pumps hold enough salt and tracer for approximately 35 days of deployment time at the fastest tracer injection rates (&40 ml d\) and can be rapidly re"lled and redeployed when appropriate. At the lowest rates (&4 ml d\) deployments can last for up to a year. When used on appropriate out#ow sites, the meters may also be used as geochemical samplers. At di!use #ow sites, where Darcy #ow rates may be on the order of 1 mm yr\, very little water is actually displaced through the meter and only the original bottom water labeled with tracer tends to be collected in the coils. In contrast, when the #ux rate and deployment time are su$cient to allow signi"cant #ushing of vented #uids to occur through the meters, a serial record of the composition of the #uid within the collection chamber will be recorded in the sampling coils (Fig. 13e). We are currently evaluating the ability to measure dissolved gasses in the sample coils. This is accomplished by the "tting of copper sample coils. These are held at ambient seabed pressures on recovery by valves "tted to the end of the coils which close automatically as the instrument leaves the sea bottom. After recovery these coils are separated into a serial record and each coil segment analyzed for the concentration or isotopic compositions of dissolved gasses.

7. Field testing and examples 7.1. Prototype deployments Prototypes of the present instruments were deployed, mounted within OBS frames, at the East Paci"c Rise during the MELT Project, o! northern California near the Eel River during the ONR-STRATAFORM Project, and at Monterey Bay, CA (Brown et al., 1995; Tryon et al., 1996). These early deployments proved invaluable in re"ning the current design and evaluating the instrument under in situ conditions. During these deployments both up#ow and down#ow were detected and rates of 0.1 to &25 mm yr\ were observed. These were isolated measurements, but they were in agreement with expectations and indicative of the capabilities of the present instruments. Following these deployments a year of extensive lab testing, construction, and "eld tests occurred followed by two years of deployments including Cascadia (Tryon and Brown, 1999a,b; Tryon et al., 1999), Brans"eld Straits, San Francisco Bay, Kodiak accretionary prism, and the

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Fig. 10. Kodiak deployment regional map showing the location of the deployment site and the three Sonne survey sites (A-Shumagin, B-Albatross, and C-Edge). Map based on Suess et al. (1998).

Costa Rica margin accretionary prism. A total of 80 instrument deployments have been made since completing "eld testing with a success rate of 32 of 35 on the most recent set of deployments o! Costa Rica. We present here partial results from two deployments as examples of the types and range of studies that can be done with this instrument. 7.2. Gulf of Alaska, Kodiak accretionary prism The Gulf of Alaska study is an example of a high rate, short duration, and high-resolution deployment in a deep water, cold seep environment. In the eastern Aleutian trench the Paci"c plate is being subducted beneath Alaska at a rate of 55 mm yr\ (DeMets et al., 1990), carrying with it 1}5 km of sediment, which has resulted in an extensive accretionary prism. Detailed dewatering estimates have been derived for the toe of the wedge in the EDGE region (Fig. 10) which suggest that 50% of the total net #uid loss occurs within the "rst 14 km of the toe of the wedge (Suess et al., 1998). During R/V Sonne cruises 96, 97, and 110 active #uid seeps with extensive clam and tubeworm colonies were found in all three regions studied (Fig. 10) (Suess et al., 1998) con"rming that this is a highly hydrologically active region. Southeast of Kodiak Island, Kodiak Seamount encroaches you the toe of the accretionary prism, possibly leading to the retreat of the toe of the prism by about 8 km. Oversteepening and slumping of the leading edge of the prism in this area has led to the development of slump scarps of up to 600 m (Eakins et al., 1999). Our deployment site was near the top of a transverse scarp bounding

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Fig. 11. Kodiak results showing (a) the upper coil tracer concentrations (lower coil contained no tracer) and the #ux rate determined from these values using Eq. (2b), and (b) the #ux rate compared to the tidal amplitude showing a dramatic increase in #ux rates at approximately the time of the lowest tides. This pattern of transience is presumed to be due either directly or indirectly to tidal forcing.

the southwestern end of this oversteepened area. The site was a fairly #at area of "ne-grained terrigenous sediment at a depth of 4430 m which exhibited dense clam and tubeworm clusters indicative of currently active seepage. A series of Alvin dives were made over approximately 1 week to investigate the seep biology, chemisty, and hydrology. As this was to be a short deployment on a presumably highly active site, the meters were setup for the highest #ow rates and sampled at a resolution of &1.4 h. The instruments were recovered after 4 days. The results from one of these instruments are shown in Fig. 11. Meter 5, located on the most dense portion of a small clam colony, exhibited variable up#ow of &10}400 cm yr\. The high sampling rate allowed us to show that variability is clearly diurnal, presumably due either directly or indirectly to tidal forcing. Diurnal variability is a common feature of our measurements in most environments although this is by far the largest amplitude signal we have seen. This diurnal pattern is interesting in that the peak #ow rate period is narrowly con"ned in time, occurring during and shortly after the principal tidal drop. Another meter, located on a less dense portion of the same clam colony, exhibited a similar pattern of accelerated #ow during the same periods. There is, however, little evidence of semidiurnal variability as would be expected with tidally driven #ow, with accelerated #ow also occurring during the smaller tidal drop. We can only speculate as to a mechanism to produce the observed diurnal transience. The pattern has the appearance of transience produced by a valving mechanism, i.e., relatively low #ow that periodically accelerates dramatically for a short period of time when some restriction or barrier is removed. This sort of pattern might be related to two-phase #ow dynamics if the #uid pressure gradient produced during the principle tidal drop exceeded the capillary pressure required

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to eject gas bubbles that were partially obstructing #ow (Ingebritsen and Rojstaczer, 1996; Tryon et al., 1999). There were, however, no observations of gas bubbles during any of the dives. Focused #ow seep sites such as this one are often the surface expression of underlying leaky fracture systems. An alternative valving mechanism might relate to the rapid partially undrained pressure release on the sediment column as the tide drops. In this case, rather than the #uid pressure gradient building up in the fracture, the fracture normal stress is reduced su$ciently during the nearly 3.5 m tidal drop to allow rough-walled fractures in the sediment to pop open slightly. Fracture #ow is approximately proportional to the third power of the fracture opening (simple Poiseuille Equation) (Lamb, 1945) so that at low normal stress a very small change in loading could lead to a signi"cant #ow response (e.g., Brown, 1995). 7.3. Hydrate Ridge, Cascadia The Hydrate Ridge study shows typical results of our work at moderate depth cold seep environments. Records of both high and low rates of #ow, downward #ow, and changes in seep #uid chemistry are illustrated in these examples. The Cascadia convergent margin has become a major focus area for the study of gas hydrates and the hydrogeology of accretionary prisms (i.e., ODP Leg 146). A thick incoming sedimentary sequence of continental derived turbidites and hemipelagic mud overlaid by Quaternary fans from the Columbia River and Straits of Juan de Fuca has led to seaward growth of the accretionary wedge of about 30}50 km over the last 2 Ma (Barnard, 1978). A series of short, subparallel ridges have formed along the lower slope due to folding along both seaward and landward verging thrust faults (Carson and Westbrook, 1995; Kulm et al., 1986; MacKay et al., 1992). Hydrate Ridge, located 90 km o! central Oregon, is cut by several out-of-sequence thrust faults, one of which was shown to be hydrologically active during ODP Leg 146 (Shipboard Scienti"c Party, 1994). Northern Hydrate Ridge is extensively covered by a massive carbonate `chemoherma (Bohrmann et al., 1998) and exhibits numerous methane gas vents and benthic organism assemblages diagnostic of methane-rich #uid seeps. Southern Hydrate Ridge sharply contrasts the northern site in being predominantly sediment covered. Seep biota is widely evident, particularly in large areas of bacterial mats. Gas venting is also present as well as surface outcrops of gas hydrates. During the TECFLUX'98 and '99 "eld seasons (Torres et al., 1998, 1999), 18 deployments of the autonomous, acoustic releaseable type #ux meter (Fig. 9b) were made for periods of 4}6 weeks on both north and south Hydrate Ridge. Using the ROPOS ROV in 1998 and submersible Alvin in 1999, the meters were placed on and around areas of presumed #uid seepage as indicated by biological marker species (Olu et al., 1997). Shown here are the results of four of these deployments as examples of the types and range of results that can be obtained with these instruments. A more complete analysis of the 1998 results can be found in Tryon et al. (1999). Flux meters E and F were deployed during TECFLUX'98 at the clam "eld designated as Clam 1 (Fig. 12), located &45 m northeast and 10 m downslope of an active gas vent. This clam "eld appeared to consist predominantly of dead clams with small pockets of live clams, typical of the many clam "elds observed in the study region. Meter F was deployed upon dense clam coverage and meter E upon moderately sparse clams near the edge of the "eld. Meter A was deployed about 4 m away from a small clam "eld (Clam 3) &150 m northeast and 12 m downslope of the gas vent

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Fig. 12. Cascadia, Hydrate Ridge map showing the locations of the CAT meters and a general description of the bottom type.

(Fig. 12). In contrast to Clam 1, this site is small ((2 m) and has predominantly live clams, suggesting that it may be younger. Meters E and F, located on Clam 1, both recorded down#ow, but at di!erent rates and with di!erent levels of variability. Meter F, located near the center of the clam "eld, indicates a moderate rate of downward #ux of '5 cm yr\ or more during the "rst week of deployment (Fig. 13a) which decreases rapidly. The rate decreased monotonically for the remaining 3 weeks to a few mm yr\ 1 month after deployment. Meter E, located to the side of the "eld, also indicates down#ow, but at an average rate of only &0.5 mm yr\, with a more subtle decrease in the downward #ux rate over the same time period (Fig. 13b). Meter A, located at Clam 3, recorded low #ux rates of (1 mm yr\ and long-period variability, with a reversal in #ow direction at day 11}12 (Fig. 13c). During TECFLUX'99, meter C was deployed in an area of dense bacterial mats located above a shallow escarpment (Fig. 12). It recorded variable #ow rates of &20}200 cm yr\, which show long-term changes in #ow on the scale of days to weeks (Fig. 13d). This is overprinted with a short-period transience that may be related to tidal forcing. This instrument also recorded a change in the composition of the #uids within the collection chamber as the initially contained bottom water was displaced with chemically unique pore #uids. The #uid chemistry asymptotically approaches a composition typical of the shallow subsurface pore #uids in this area (Torres, pers. commun.). In a manner similar to that used by the OSU/GEOMAR Benthic Barrel (Carson et al., 1990; Linke et al., 1994), the aqueous #ux rate may be determined from the change in #uid

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composition in the collection chamber. From the change in Ca concentration, a volume of 18.2 l (chamber inserted 5 cm into sediment) and an input concentration of 5.5 mM, an average #ux rate of 146 cm yr\, results. The average #ux rate from the tracer analysis is 124 cm yr\. Considering the uncertainly in the chamber volume this is in close agreement. The signi"cant and surprizing results of these studies are (1) downward directed #ow appears to be a common phenomena at clam colonies, and (2) there is a large amount of temporal variability in #ow rates that occurs on time scales of hours to weeks. This has signi"cant consequences for mass balance estimates in the gas hydrate bearing forearc environment. It also raises new questions regarding the mechanisms generating transience and downward #ow. Our current working hypothesis is that these phenomena are primarily a result of gas hydrate dynamics. Both down#ow and transience may be generated by the "lling and emptying of subsurface reservoirs of gas (Tryon et al., 1999). Down#ow occurs when subhydrostatic pressures are generated below a column of gas rising through conduits to the sea#oor. Fluid will #ow laterally and downward through highpermeability conduits, "lling the emptying gas reservoir. Down#ow may also result from convection driven by entrainment of #uids in the transient gaseous discharge observed in this area (O'Hara et al., 1995). Variability may result from migrating #uid conduits at depth as gas hydrate "lls and ultimately blocks permeable pathways. Clearly, the hydrogeology of this environment is much more complex than previously thought. Acknowledgements Development of the CAT meter was supported by NSF Grant OCE 9633378 to Brown and Dorman. The Kodiak "eld study was supported by NURP UAF 98-0036 to Brown. The Hydrate Ridge study was supported by NSF Grant OCE 9731157 to Brown. Thanks to Irv Shelby of Hydranautics for consultations and supply of osmotic membrane material and to Don Elliott for extensive help with construction and "eld testing of the instruments. References Barnard, W.D., 1978. The Washington continental slope: quaternary tectonics and sedimentation. Marine Geology 27, 79}114. 䉳 Fig. 13. Examples of results of the TECFLUX deployments. (A) Meter F, located at Clam 1, indicates a rapidly decreasing downward #ow during the "rst week which slows to a few mm yr\ after 1 month. Note that the volumetric #ow rate exceeds the pump rate until the last 2 days and therefore tracer appears in the upper coil only during the last 2 days. (B) Meter E also indicates down#ow on the periphery of Clam 1 but at only &0.5 mm yr\. The pump rates greatly exceed the volumetric #ow rate and we sample only tracer in the lower coil and only a small amount of seawater in the upper coil. (C) Meter A at Clam 3 shows #ow varying in magnitude and in direction. The direction changes cause a change in which coil is sampling seawater diluted tracer. (D) Meter C, located at the microbial mat site, shows very high up#ow rates with a signi"cant amount of variability. At these rates, only a small amount of tracer is found in the upper coil. In this example from the 1999 "eld season, the sampling rate is &4 times that of the earlier deployments. (E) High #ow rates of meter C caused #ushing of the collection chamber allowing the recording of the composition of the expelled pore #uid. Mg, Na, Cl, and K were indistinguishable from seawater, but Ca and S were signi"cantly modi"ed.

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Becker, K., Leg 174B Scienti"c Party, Davis, E.E., 1998. Leg 174B revisits Hole 395A: logging and long-term monitoring of o!-axis hydrothermal processes in young oceanic crust. JOIDES Journal 24 (1), 1}3,13. Bekins, B.A., Dreiss, S.J., 1992. A simpli"ed analysis of parameters controlling dewatering in accretionary prisms. Earth and Planetary Science Letters 109, 275}287. Bohrmann, G., Greinert, J., Suess, E., Torres, M., 1998. Authigenic carbonates from the Cascadia subduction zone and their relation to gas hydrate stability. Geology 26 (7), 647}650. Brouwer, H., Rice, R.C., 1963. Seepage meters in seepage and recharge studies. Journal of the Irrigation and Drainage Division, American Society of Civil Engineers 89, 17}42. Brown, K.M., 1995. Data report: hydraulic conductivity measurements on discrete samples collected from Leg 141, Site 863. In: Lewis, S.D., Behrman, J.H., Musgrave, R.J., Cande, S.C. (Eds.), Proceedings of the Ocean Drilling Program, Scienti"c Results, Vol. 141. Ocean Drilling Program, College Station, TX, pp. 401}405. Brown, K.M., Bekins, B., Clennell, B., Dewhurst, D., Westbrook, G., 1994. Heterogeneous hydrofracture development and accretionary fault dynamics. Geology 22 (3), 259}262. Brown, K.M., Sauter, A.W., Dorman, L.M., 1995. Di!use #ux measurement in convergent margin and ridge #ank environments: a new sea #oor #uid #ux meter system. Eos (Transactions, American Geophysical Union) 76 (46), 563. Campbell, A.C., Bowers, T.S., Measures, C.I., Falkner, K.K., Khadem, M., Edmond, J.M., 1988. A time series of vent #uid compositions from 213N, East Paci"c Rise (1979, 1981, 1985), and the Guaymas Basin, Gulf of California (1982, 1985). Journal of Geophysical Research 93 (B5), 4537}4549. Carson, B., Seke, E., Paskevich, V., Holmes, M.L., 1994. Fluid expulsion sites on the Cascadia accretionary prism: mapping diagenetic deposits with processed GLORIA imagery. Journal of Geophysical Research 99 (B6), 11959}11969. Carson, B., Suess, E., Strasser, J.C., 1990. Fluid #ow and mass #ux determinations at vent sites on the Cascadia margin accretionary prism. Journal of Geophysical Research 95 (B6), 8891}8897. Carson, B., Westbrook, G.K., 1995. Modern #uid #ow in the Cascadia accretionary wedge: a synthesis. In: Westbrook, G.K., Musgrave, R.J., Suess, E. (Eds.), Proceedings of the Ocean Drilling Program, Scienti"c Results. Ocean Drilling Program, College Station, TX, pp. 413}421. Cherkauer, D.A., McBride, J.M., 1988. A remotely operated seepage meter for use in large lakes and rivers. Ground Water 26 (2), 165}171. DeMets, C., Gordon, R.G., Argus, D.F., Stein, S., 1990. Current plate motions. Geophysical Journal International 101 (2), 425}478. Eakins, B.W., Lonsdale, P.F., Massell, C.G., Rathburn, A.E., 1999. Kodiak seamount in the Eastern Aleutian Trench: submersible study of an intact guyot entering the subduction zone. Eos (Transactions, American Geophysical Union) 80 (46), 1180. Edmond, J.M., von Damm, K.L., McDu!, R.E., Measures, C.I., 1982. Chemistry of hot springs on the East Paci"c Rise and their e%uent dispersal. Nature 297 (May 20), 187}191. Fisher, A.T., Becker, K., 2000. Channelized #uid #ow in oceanic crust reconciles heat-#ow and permeability data. Nature 403 (6 January), 71}74. Foucher, J.P., Henry, P., Lepichon, X., Kobayashi, K., 1992. Time-variations of #uid expulsion velocities at the toe of the Eastern Nankai accretionary complex. Earth and Planetary Science Letters 109, 373}382. Gieskes, J.M., Simoneit, B.R.T., Brown, T., Shaw, T., Wang, Y.-C., Magenheim, A., 1988. Hydrothermal #uids and petroleum in surface sediments of Guaymas Basin, Gulf of California: a case study. Canadian Mineralogist 26, 589}602. Henry, P., Foucher, J.-P., LePichon, X., Sibuet, M., Kobayashi, K., Tarits, P., Chamot-Rooke, N., Furuta, T., Schultheiss, P., 1992. Interpretation of temperature measurements from the Kaiko}Nankai cruise: modeling of #uid #ow in clam colonies. Earth and Planetary Science Letters 109, 355}371. Hyndman, R.D., Davis, E.E., 1992. A mechanism for the formation of methane hydrate and sea#oor bottom-simulating re#ectors by vertical #uid expulsion. Journal of Geophysical Research 97 (B6), 7025}7041. Hyndman, R.D., Wang, K., Yuan, T., Spence, G.D., 1993. Tectonic sediment thickening, #uid expulsion, and the thermal regime of subduction zone accretionary prisms: The Cascadia margin o! Vancouver Island. Journal of Geophysical Research 98 (B12), 21865}21876.

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