Gas seep induced interstitial water circulation: observations and environmental implications

ContinentalShelfResearch, Vol. 15, No. 8, pp. 931-948.1995

Pergamon

Copyright @)1995Elsevier Science Ltd Printed in Great Britain. All rights reserved 0278-4343/95s9.5o+o.oo 0278-4343(94)00050-6

Gas seep induced interstitial water circulation: observations and environmental implications S. C. M. O’HARA,* P. R. DANDO,? U. SCHUSTBR,$ J. D. BOYLE,8 F. T. W. CHUI,O T. V. J. HATHERELL,(j and L. J. TAYLOR?

A. BENNIS,$ S. J. NIVBN,t

(Received 4 November 1993; in revised form 14 March 1994; accepted 26 May 1994)

Ah&act-An interstitial water circulation, generated by gas flow through a permeable sediment, was observed at an intertidal site on the Kattegat coast of Denmark. Concentrations of methane dissolved in the interstitial water of the near-surface sediment decreased sharply only centimetres away from gas seeps venting almost pure methane (-99% methane). Water was driven out of the sediment by the rising bubbles of gas at the seep and was replaced by an equivalent draw-down of overlying, oxygenated water into the surrounding sediment. This process steepened the chemical gradients close to the gas flow channel, with the effects progressively diminishing with increasing distance from the seep. The position of the redox potential discontinuity (RPD) moved by as much as 7 cm deeper into the sediment close to the seep: this effect was less marked, but still detectable, 50 cm away. The degree of displacement from the “normal” sediment profiles depended on the magnitude of the interstitial flow rate. The distribution of pore water pH and sulphate: sodium ratios were also dependent on the flow rate of the circulating water. The concentrations of sulphide, thiosulphate and sulphite in the interstitial water from the top 10 cm of sediment, were high at a seep, decreased to a minimum at 20-30 cm distance, then increased again at 40-50 cm distance. Laboratory experiments confirmed that gas bubbling through a fluid filled permeable matrix generated a flow, out of the sediment at the gas exit and into the sediment over the peripheral surfaces surrounding the outlet. Experimentally determined rates of dispersion, for gas flow rates of 3-20 ml min-‘, for a 40 g 1-l sodium chloride solution, were 62.5 x 1r9 to 540 x W9 m* s-l, 4o-400 times the molecular diffusion coefficient. Linear interstitial fluid velocities of 3-12 mm mitt-‘, were recorded at 14-3 cm from the seep axis respectively, with a gas flow rate of 5 ml min-‘. Two-dimensional modelhng of the experimental system confirmed the flow pattern determined visually with dye. Implications of this process with regard to the recycling rates of elements generally, and of nutrient and waste materials, in particular, are discussed.

INTRODUCTION Circulation of water through sediments and its affects on the distribution and/or recycling of elements has been discussed for various situations; particularly movement of interstitial water generated by wave energy (Riedl and Machan, 1972; Riedl et al., 1972) and the enhancement of sediment-water exchange due to bioturbation, reviewed by Kersten

*Plymouth Marine Laboratory, Citadel Hill, Plymouth, PLl 2PB, U.K. tMarine Biological Association of the United Kingdom, Citadel Hill, Plymouth, PLI 2PB, U.K. $Department of Biology, University of Essex, Wivenhoe Park, Colchester, CO4 3SQ, U.K. @School of Engineering, University of Exeter, North Park Road, Exeter, EX4 4QF, U.K. 931

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S. C. M. O’Hara et al.

(1988). The stripping of dissolved gases from sediments by ebullition of biogenic methane has been referred to by Reeburgh (1969). This process was further studied by Martens and Klump (1980)) Klump and Martens (1981), and Kipphut and Martens (1982) who also observed increased fluxes of nutrient elements from the sediment, as a result of ebullition. This was generally held to be due to increased exchange with the sediment due to open tubes and fissures produced by the escaping gas bubbles. As part of an investigation into the mechanisms leading to phosphorus release from freshwater lake sediments, Woods et al. (1975) carried out some laboratory studies with a gas-lift apparatus, using nitrogen or glucose fermentation gas (predominantly C02). They concluded that gas bubbles, rising within vertical tubes and channels in bottom muds, pump fluids enriched in soluble phosphorus out of the organic-rich sediment. However, no conclusions were drawn as to the effect this water movement had on the main body of interstitial water in the sediment. Submarine methane seeps associated with carbonate-cemented sandstones are known from the Kattegat coast of northern Denmark, near Frederikshavn, (Jorgensen, 1992). Studies of sediment cores, taken by divers from close to the gas seeps, indicated that significant chemical differences occurred laterally over a few centimetres within the sediment (Jensen et al., 1992). The discovery of an intertidal site, with active methane seepage, within 5 km of Frederikshavn at Hedenstrand (Fig.1) described in Dando et al. (1994a,b), meant that it was possible to make more detailed studies of sediment chemistry and biological distribution around seeps. The present paper describes the effects of an active interstitial water flow on sediment chemical profiles, induced by gas seepage through the porous matrix. METHODS Mapping of the active seep area (see Fig. l), measurement of the range of seep flow rates and the variation in the flow at individual seeps with various parameters, including water and sand depth have been described elsewhere (Dando et al., 1994b). Samples of sediment were taken with cut-off polystyrene syringes, and these were sealed immediately with neoprene rubber stoppers; within 10-30 min sediment samples were transferred to 22 ml glass vials (containing 4 ml 2% sodium azide solution) under an atmosphere of zero-grade nitrogen and immediately closed with caps and PTFE lined butyl rubber seals. Head spaces were equilibrated with the sediment gases by mixing the bottle contents for a minimum of 1 h on roller mixers. Samples of the vial head-space gases were taken with Pressure-lok@ (Dynatech Precision Sampling Corporation, USA) syringes and analyzed by gas chromatography as described in Dando et al. (1994b). Seep outlet water was collected with the “switched Y-fitting, twin evacuated-bag” technique for analysis of phosphate and ammonia, as described in Dando et al. (1994a). Samples of surface sea water and sediment pore water were taken at different distances from seeps and at different depths beneath the surface, using plastic capillary tubing fitted with quartz wool plugs, attached to plastic 1 ml syringes. Aliquots (90 or 180 ~1) were transferred immediately after collection into argon filled 1.1 ml tapered glass screw-cap tubes for the derivatization (with monobromobimane) of reduced sulphur species, as described in Fahey and Newton (1987) and subsequently stored frozen at -20°C. The residual water samples were transferred into 1.5 ml polypropylene screw-cap vials, analyzed for pH, Na+ concentration, and then stored at 04°C for the short term, and deep frozen at -20°C for the long term. The bimane derivatives were analyzed by high pressure

933

Gas pumping in sediments

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liquid chromatography (HPLC) on a 250 x 4.6 mm Spherisorb XT18 column, as described in Dando et al. (1991). In situ redox potential measurements were made in August/September 1992 with a micro-combination platinum redox electrode inserted directly into the sediment surface. The sediments were typically covered with l-30 cm of calm water. Periods when there was rough water over the beach were avoided because excessive movement of the redox probe due to sand swirl and water movement made accurate positioning impossible. Horizontal (O-50 cm) and vertical (O-10 cm) sediment profiles around and over the seep outlets were taken, to map the influence that the induced water flow has on the position of the sediment redox potential discontinuity (RPD). For clarity the RPD points in this paper were taken as the y-axis intercepts (E/z = 0 mV) on the redox profiles. Measurements of the composition of the interstitial water around a seep, whether by electrodes or by syringe sampling disturbs and compresses the sediment structure; as does

934

S. C. M. O’Hara et al.

the presence of the sample-taker. To minimize these effects, samples were always taken as far as possible in front of the investigator and working towards the seep. The seep outlet was always sampled last in any transect. Different sample types (i.e. redox, pH, and pore water samples) from the same seep were taken along different undisturbed radii and were not sampled simultaneously. Heterogeneity within the sediment also meant that precise agreement between the profiles of different parameters cannot be expected. All laboratory flow experiments were carried out at the School of Engineering, University of Exeter, in glass tanks (39 x 29 X 21 cm for the initial verification of flow generation, and 28 x 28 X 3.0 cm for the flow pattern experiments), at ambient temperature. In these preliminary modelling exercises, to simply test the flow generation hypothesis, the tanks contained well washed coarse freshwater sand and distilled water. In an initial flow verification experiment, a glass tank was partially filled (to a depth of 21 cm) with water saturated sand (porosity : 0.44); the sediment bed was overlaid with a 7 cm depth of distilled water. A gas diffuser, with attached tubing, was installed beneath the sand, on the bottom of the tank. Regulated compressed-air (1 bar pressure) was supplied via a calibrated rotameter to deliver selected flow rates of gas within the 0.12-1.20 1 h-l range (2-20 ml mitt-r), typical of the flow rates found at the beach site. A 10 ml aliquot of a tracer solution (40 g 1-l sodium chloride) was injected 10 cm beneath the sediment surface. Detection of sodium chloride labelled water was via a digital conductivity meter and a platinum conductivity cell which was immersed in the overlying distilled water. The distribution pattern of a fluorescein dye solution was tracked visually through the side of the glass tank; flow tracks were traced with a marker pen. Compressed air, as described above, was injected at 10 cm depth into the sediment bed to generate a circulation. The flow structure within the laboratory system was also mathematically modelled in two dimensions in this preliminary study using a complex variable boundary element method (CVBEM) described by Chiu and Hatherell(l993). RESULTS Description of site

The beach at Hedenstrand, 5 km south of Frederikshavn, Denmark (see Fig. 1) described in detail in Dando et al. (1994a) is a site of active methane seepage (65 seeps in an area of about 2100 m*), associated with carbonate cemented sandstone, covered by a fine sandy sediment (0.3-0.1 mm grain size, and the porosity range = 0.30-0.54, mean = 0.41) to depths of up to 100 cm. One seep vented gas (-99% methane) at about 330 ml min-‘, a second vented at about 165 ml min-’ , the remaining seeps released gas in the range 2.5-80 ml min-‘. The sand contained horizontal bands of shell debris in the 0.3 to >2.0 mm size range at various depths below the surface, and a shell debris bank lay at the surface along the high water line. Water coverage of the site during visits was in the O-l.3 m range, dependent mainly on the weather conditions prevailing; the official tidal range for the area is only 0.4 m maximum. Water levels over the site were predominantly controlled by atmospheric pressure differentials between the North Sea and Baltic Sea areas. Typical wave heights over the beach site were 0.25 m with a frequency of 6-g min-‘. Site measurements

Interest of gas-driven pore-water circulation was first stimulated while surveying the beach site for sediment methane concentrations as part of a wider study of marine methane

Gas pumping in sediments

935

a

60

0 -30

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b

d

8060-

-30

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0 10 20 -10 Distance from seep (cm)

Fig. 2. Methane concentration profiles of west-east (a) and south-north (b) transects across a seep on the Hedenstrand beach sampled in May 1991. Sediment cores, 4 cm deep were taken, with cut-off polystyrene syringes, for gas analysis.

seepage and the sulphur chemistry and associated biological processes in the sediments involved. Concentrations of methane in 4 cm deep syringe cores of sediment taken from within a few centimetres of the methane seeps were unexpectedly low [Fig. 2(a) and (b)]. The sediment is clearly permeable; there appears to be a slight seaward drift of the peak value for dissolved methane [Fig. 2(a)] sampled on a receding tide, due possibly to downbeach drainage. The composition of the venting gas was greater than 99% methane (Dando et al., 1994a). In May 1992 a change in atmospheric conditions, resulted in lower salinity Baltic water (1.5% sodium chloride; -l&8%0 salinity) replacing the higher salinity (2.1% sodium chloride; ~25.0% salinity) water overlying the beach at Hedenstrand. The sodium chloride concentration of the interstitial water at 2 and 4 cm depth was measured at

936

S. C. M. O’Hara er al.

0.0

2.0 4.0 6.0 8.0 Distance from seep (cm)

10.0

Fig. 3. A transect of pore water sodium chloride concentrations, taken from a seep on the Hedenstrand beach in May 1992, when the surface water sodium chloride was 1.5% (Baltic water) and the sediment pore water sodium chloride concentration, away from the seep area, was 2.1%. Symbols; q, pore water from 2 cm below the surface; A, pore water from 4 cm below the surface.

distances of up to 10 cm from a seep outlet (gas flow rate = 26.7 ml min-l). Draw-down of the low salinity water into the sediment close to the seep is clearly indicated (Fig. 3). Tide and weather conditions on 31 August and 6 September 1992 allowed detailed beach sampling and measurement to be performed around individual seeps (marked A and B in Fig. 1). These seeps had a sufficient radius and depth of undisturbed sand surrounding them to allow adequate sampling to be carried out. Sediment profiles down to 10 cm depth were determined for redox potential (Fig. 4), pH (Fig. 5), reduced sulphur species (Fig. 6) and sulphate : sodium ratio (mmol 1-l SOi- : %NaCl; Fig. 7) at distances up to 50 cm from gas seeps. The gas flow rate for seep A was measured at 31.7 ml min-’ and data from this seep are shown in Figs 4 (a), 5(a), 6 and 7. The flow rate for seep B was much slower and intermittent and was estimated at 18.3 ml min-‘, data from this seep are shown in Figs 4(b), 5(b) and 7. The gradient of the RPD profile (thin broken line through the 0 mV points) across the 90 cm transect in Fig. 4(a) was steeper (lowered by 6-7 cm) from the high gas-flow seep than that for the lower gas-flow transect, Fig. 4(b) which was only lowered 2-3 cm. The depth to which the RPD is lowered will depend on the dynamic equilibrium between the oxygen consuming processes within the sediment and the flow rate of the surface water draw down. The interstitial flow rate of the water decreases progressively with distance away from the gas seep and the RPD horizon gradually rises in the sediment. The redox potential at the seep outlet channel was very anoxic, -92 to -354 mV over the 2-10 cm depth range of the redox electrode. The surface sea water pH for the beach site was in the range of 8.03 to 8.06. The pore water pH distribution patterns reflected the equilibria achieved within the sediment close to the seep; the observed pH profiles, in Fig. 5, for seeps less than 25 m apart on the site show a typical sediment range of between 7 .Oand 8.11. There were significant quantities of carbonate (shell debris) mixed into the sediment throughout the beach, so that a lowering of the pore water pH to 7.0, from the surface sea water value, represented a large hydrogen ion input. This occurred on sediment surrounding the seep, with a gas flow rate of 18.3 ml min-’ [Fig. 5(b)] and is typical of surface horizons of organic-rich sediments. The other

937

Gas pumping in sediments

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Fig. 4. Transects of sediment redox potential profiles of seeps on the Hedenstrand beach, taken in August&September 1992; (a), high gas flow rate (31.7 ml min-r) seep A, sampled 31/08/92; (b), low, intermittent gas flow rate (~8.3 ml min-‘) seep B, sampled 06/09/92; this seep was within 25 m of A. The thin broken line indicates the approximate profile of the 0 mV potential.

seep sediment [Fig. 5(a)], with a faster gas flow of 31.7 ml min-‘, was supplied with an induced influx of surface sea water sufficient to maintain the higher pH profile in the pore water. Soluble reduced sulphur species were almost completely absent from the upper sediment horizons, between 20 and 40 cm away from a seep (Fig. 6), in contrast to the water in the seep outlet channel, which contained l-17,12-68 and 130-950,~g atoms S 1-l for sulphite, thiosulphate and sulphide, respectively. These species were not detectable in the surface sea water. The interstitial water sulphate : sodium ratio is an indicator of the degree of sulphide

938

S. C. M. O’Hara et ul.

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Transects of pH profiles taken from seeps on the Hedenstrand beach in AugustSeptember 1992. Seep gas flows, in (a) and (b) as in Fig. 4.

oxidation or sulphate reduction within the sediment, which is influenced by both salinity variation of the overlying water (a phenomenon observed at Hedenstrand) and input of meteoric water. Given a constant sulphate : sodium ratio in the overlying water, an increase in the ratio in the interstitial water would indicate the oxidation of sulphides, whereas a decrease in the ratio would indicate an increase in sulphate reduction. The mean NaCl concentration was 2.1 f 0.1% (=25.0% salinity; IZ = 77) for all the Hedenstrand surface and interstitial water samples analyzed in August-September 1992. The sulphate : sodium ratio for the surface water at Hedenstrand ranged from 11.5 :l to 12S:l. The profiles (Fig. 7) for the seeps measured show deviations from these values. There was a clear increase in the ratio (up to 13.5:1) for pore water collected between 20 and 40 cm from the faster seep, at 4-8 cm below the surface. In contrast, this ratio for similar samples taken from around the slower seep showed evidence of substantial sulphate depletion (down to 8.7:1).

939

Gas pumping in sediments

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Fig. 6. A transect of profiles showing the concentrations of dissolved sulphide, thiosulphate, and sulphite in the pore water from the sediment between the seep outlet A and 50 cm away. Gas flow as in Fig. 4(a).

Experimental data The laboratory experiment to verify that flow was induced within the sediment by a gas vent, confirmed that a circulation would be generated. After injection of the tracer electrolyte, the times were recorded, in minutes, for the conductivity of the overlying water to rise in steps of O.S,~Siemens cm-’ until the rate of increase plateaued off; this was repeated for a variety of different gas flow rates in the 2-20 ml mine1 range. The mixing time (time taken for the tank contents to reach 90% of the final conductivity) was plotted against the gas flow rate (Fig. 8). Mixing times varied from 178 to 65 min for 3-20 ml min-’

940

S. C. M. O’Hara et al. 0 1Ocm

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Fig. 7. A transect of profiles for pore water sulphate :sodium ratio (mmol I-’ SG- : % NaCI). Symbols; W, high gas flow seep A, sampled 31/08/92; 0, low, intermittent gas flow seep B, sampled 06/09/92; q - - - -Cl, indicates the range of this ratio for surface seawater.

gas flow in an asymptotic relationship, the lower limit being the molecular diffusion rate and the upper rate being limited by the permeability characteristics of the sand-water mixture and possibly by gas saturation of the sediment which would lower the permeability to liquid flow. The time interval (Q between injection of the sodium chloride and the first recorded rise in conductivity was also noted; the depth of sediment traversed (10 cm) was divided by Tj and the resultant linear velocity plotted against gas flow rate (Fig. 9). A linear relationship is clearly demonstrated. Dispersion of the tracer solute through the sand was calculated using methods described in Brodkey (1966). It should be noted that the flow (advection) of interstitial water can be considered to have two related and interacting effects. The first, is the carriage of solute from one part of the sediment to another. The second is the enhancement of molecular diffusion by the spreading out and mixing of the flowing water as it moves along tortuous

941

Gas pumping in sediments

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Times taken for surface water in the experimental tank to reach 90% of the equilibrium conductivity v the model seep gas flow rate.

pathways, within the sediment matrix, which successively separate and rejoin in a random fashion. This dispersive process occurs not only in the direction of flow (axial dispersion), but also in the radial direction, so that what may start as a compact pulse of solute is simultaneously elongated and broadened. Experimentally determined values for dispersion (D) of the tracer through the sediment bed varied proportionally with the gas flow ranging from 62.5 x lo-‘to 540 x lo-’ m* s-* for 3-20 ml min-l flow rates. Values for the molecular diffusion coefficients (D,J of sodium chloride solutions were obtained from the literature (Weast, 1988). For these preliminary experiments a value of 1.4 X lo- ’ m * s -l, for 40 g 1-r sodium chloride solution at 20°C was taken. The ratio DID, for the tracer electrolyte under the experimental conditions ranged from 40 to 400 depending on the gas flow. For flow pattern studies the system was assembled and equilibrated by flowing gas for 4 h, to allow the gas “seep” to stabilize, before a fluorescein dye solution was introduced into the sediment bed below the gas injection point. The flow tracks of the dye were visually observed and manually timed; a typical circular flow pattern is shown diagrammatically in

s

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Gas flow ml min.’

Fig. 9. The linear flow rate of the pore water in the sediment, averaged over the cross-sectional area of the experimental tank, during conductivity experiments, versus the model seep gas flow rate.

942

S . C . M . O'Hara et al.

I C°ttP5rm~linI air

28 cm

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Fig. 10. Diagrammatic representation of the two-dimensional fluorescein flow pattern experiment in a 28 x 28 x 3 cm glass tank, showing the form of the flow tracks with typical visually determined linear flow rates marked.

Fig. 10. For a gas injection of 5 ml min -1 the linear flow rates, observed visually, ranged from 3 mm min -1 at 14 cm away from the seep, to 12 mm min -1 at <5 cm away. Two-dimensional modelling of the experimental system using the complex variable boundary element method (CVBEM) described in Chiu and Hatherell (1993) produced a series of solutions, depending on the assumptions made for the pressure distribution affecting the system's active boundaries. The simulation (Fig. 11), showing half the bed (the flow is symmetrical about the central axis, the injection point C), illustrates the twodimensional flow generated by assuming a constant pressure along side A (which is known) and a lower constant pressure along side B. Along the other boundaries a zeronormal-flow Neumann boundary condition is imposed. More experimental work will be required to verify the pressure distribution along side B. It is, however, important to make the following points: (a) within the experimental gas flow channel, it is highly unlikely that sharp variation of pressure could occur,

Gas pumping in sediments

a

943

b

Fig. 11. Computer simulations of a two-dimensional model of the flow pattern generated by a gas injection to a permeable sediment bed at point C. The pressure along boundary A is fixed and a lower constant pressure is set along boundary B. Plot (a) is a flow field simulation; (b) is a flow line simulation.

(b) the CVBEM software has been used to investigate other types of pressure distribution, such as different constant pressures, or gradual vertical pressure variation, along side B. A different constant pressure along side B would only influence the magnitude of the velocities, with no effect on the overall flow pattern. When a gradual vertical pressure variation is imposed, only the flow very close to side B will be modified, while the flow line pattern shown in Fig. 11(b) remains practically unchanged. As indicated by the results shown in Fig. 11 and other numerical tests with different pressure distributions along side B, the flow patterns presented by the model closely matches that obtained experimentally (Fig. 10). The model also predicted that the velocity of interstitial water flow was highest close to the gas injection axis and progressively decreased with distance away. DISCUSSION One explanation for the data on the surface distribution of dissolved methane (Fig. 2) was that the rising column of gas bubbles from the seep generated an outflow of interstitial water along the axis of the gas flow. This was replaced by an inflow of water from the surrounding sediment and overlying surface water. Surface sea water and its constituents would be drawn into and through the sediment close to the seep. The idea of a gas bubble lift-pump effect was supported by the occasional observation of black sulphidic sand

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(normally present at horizons 2-6 cm below the surface) coming out of the sediment with the released gas from some seep outlets. This hypothesis was confirmed by the results from the laboratory experiments. Generation of an interstitial water circulation was demonstrated (Fig. 8) and the pore water circulation rate was shown to be proportional to the gas flow rate driving the system (Fig. 9). The results from the preliminary two-dimensional modelling exercise indicated that the patterns and rates of flow can be predicted for systems where parameters such as porosity, gas flow rate, fluid density and boundary pressure differentials are defined. Early work on the effects of gas ebullition from sediments concentrated on the release of biogenic “bacterial” methane from the surface horizons of anoxic mud and the “stripping” effect this had on other dissolved gases in the sediment (Reeburgh, 1969). Rates of ebullition from the sediment in anoxic mud are variable and reported to be dependent on environmental temperatures; methane generation rates sufficient to cause bubbling of gas from the muds typically occurring during warmer periods, Martens and Klump (1980). Sediment-water exchange rates were enhanced up to 3-fold by this process. Another aspect of the gas “stripping” process was dealt with by Vroblesky and Lorah (1991), whilst describing the concentrations of volatile chlorinated organic contaminants released to the surface water and the atmosphere from a tidal creek. The sediment in the creek was a mixed gravel-sand-silt, with the potential to allow water to pass through it at linear flow rates in the range of 2.3 to 38 mm min-r. This study highlighted a problem that had developed, where toxic waste substances were being released back into the surface water and atmosphere (albeit slowly) by the natural out-gassing of biogenic methane. The present work indicates that soluble non-volatile substances and even fine particulates (as shown by the fine particles of black iron sulphide brought to the surface by seeps at Hedenstrand) may be carried to the surface, from buried material, at sites where gas generation can occur. The greater rates of sediment flushing generated by continuous gas seepage through permeable deposits containing waste materials, will result in faster release of the waste into the water column. Gas-lift pumping, caused by bubbles rising in vertical channels in gas-generating lake mud, was shown by Woods et al. (1975), to enhance phosphorus release into overlying waters. Possible circulation of interstitial water (as an alternative to diffusion) within the mud matrix, caused by the gas-lift, was not considered by these workers; the water flow in mud and fine silt, in any event, would be very slow and difficult to measure. Enhanced nutrient release from the Hedenstrand beach seeps has been reported. Phosphate and ammonia concentrations measured in water issuing from a seep were 0.88pmo11-1 and 20pmo11-1 , respectively, compared to levels of O.l3pmoll-’ and 1 pm01 1-l) respectively, detected in the overlying surface water (Dando et al. 1994a). The draw-down of water into the sediment carries with it dissolved oxygen and other oxidised chemical species as well as micro-particulates. The influx of oxygenated surface water creates a set of dynamic equilibria, which resulted in an increased surface area of the RPD (Fig. 4) influenced the pH (Fig. 5), soluble reduced sulphur species (Fig. 6) and the sulphate : sodium ratios (Fig. 7) in the sediment close to each seep. These data represent “snap-shots” of the equilibria around these particular seeps. Data from the seep with the higher gas flow rate, showed evidence of greater surface water influence [Figs 4(a), 5(a) and 71 than the data [Figs 4(b), 5(b) and 71 from the lower, intermittent gas flow seep, with a steeper RPD transect profile, higher pH and sulphate : sodium ratios for the interstitial water. The data from the high gas flow seep (Fig. 6), for the distribution of soluble reduced sulphur species in the interstitial fluid, showed that sufficient surface water was drawn into

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the zone between 20 and 40 cm radius of the outlet to flush or oxidize almost all the available reduced sulphur from the sediment, to a depth of 10 cm. The sediment in the field differs from that in an experimental tank, in that it is not well sorted and contains shells and small stones, as well as coarse fill channels. This will result in “non-ideal” profiles due to interruption of the interstitial water flow pattern; such obstructions or conduits will cause asymmetric flows to develop around the seep axis. Attempts to remove such objects would completely disrupt the natural system. Gas seep channels are rarely vertical (Dando etal., 1994a) and sampling along a transect at different depths from the seep may take some samples from closer to or further from the gas channel than expected. The presence of reduced sulphur species in interstitial water taken 10 cm from the seep in Fig. 6 may be due to such a sub-surface irregularity. Continuous gas seepage from sediment horizons below a muddy surface layer tends to form craters or small “pock-marks” in the under consolidated surface layer, by winnowing away the fine silt/clay particles in the water flow induced by the gas (Hovland and Judd, 1988). The coarser, more permeable fraction of the surface will remain. However, flow patterns within the relatively impermeable particle matrix below will probably be more restricted and considerably more difficult to observe or measure than has been experienced in this work. Fluid flow rates would be much slower and interstitial water sampling at intervals of a few centimetres would require more sophisticated instrumentation than used in this study. In situ linear rates of pore water circulation, driven by wave action through sediment with a mean particle diameter of 0.25 mm (a fine sand) have been measured in inter-tidal zones (mean flow = 18 mm min-‘; maximum flow in storms 1120 mm min-‘) by Riedl and Machan (1972), and in sub-tidal zones (range of flow = O-6 mm min-‘) by Riedl et al. (1972), for some high-energy beaches on the eastern coast of North America. The lower flow rates are in the same range as those obtained experimentally in this work (3-12 mm min-l), albeit with a coarser (2.2 mm particle diameter) sediment. These experimental flow rates were recorded for a gas flow rate of only 5 ml min-r, which represents the low end of the range of flow rates (2.5-80 ml min-l; mean = 20.3 ml min-‘) obtained from seeps on the Hedenstrand site (Dando et al., 1994b). The energy dissipated in the high energy beach environments (mean wave height of lm with a frequency of approximately 6 min-‘) is far greater than normally expended by wave action on the Hedenstrand beach. The contribution that the gas-flow generated fluid exchange makes to the overall watersediment exchange for this particular beach, from sediment close to seeps (within 1 m), is potentially of the same order as that expected from wave pumping. Kersten (1988) discussed enhancement of fluid transport mechanisms across the sediment-water interface due to bioturbation processes, and reported typical rate increases for the molecular diffusion of dissolved solutes of 6-20-fold, with a maximum of lOO-fold. The range of fluid transport enhancement reported here (40-400 times the molecular diffusion rate, up to 50 cm from seeps), is comparable to the ranges reported for the more significant influences identified to date. At the Hedenstrand site 65 significantly active seeps have been recorded (Dando et al., 1994b). Seeps at this site have been shown to have a radius of influence of at least 50 cm (equivalent to an area of 0.8 m2). The total reef area is approximately 2100 m2. A conservative estimate for the range of effective dispersion from the sediment of the overall reef area is 2-11 times the molecular diffusion rate. Since the sediment acts as a filter bed, this will produce “hot-spots” of chemical and

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SURFACE

SEAWATER

Fig. 12. Diagram showing the influence, confirmed by analyses to date, that methane seeps have on permeable marine sediments and the overlying water. RPD = redox potential discontinuity.

biological activity at the periphery of seep outlets. This is reflected in the biological distribution data reported for the Hedenstrand beach site (Dando et al., 1994a). The numbers (and biomass) of nematodes in the upper 20 cm of sediment around the seeps were greater (close to 2-fold for each parameter) than for the upper 20 cm of sediment from a non-seep area 30 m away. The diversity of nematode species, and their abundance, was also greater in the sediment zone around the seep compared to sediment at the seep outlet; this may be due, in part, to the physical disturbance of sediment at the outlet by the

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gas and water flow. The influx of oxygenated and organic-rich sea water into the “flow zone”, generated by the seep driven circulation system, creates conditions favourable to microbe feeding nematodes, which in turn support a population of larger predatory nematodes. A diagram of the main chemical and physical processes surrounding gas vents in permeable sediments is shown in Fig. 12. A continuously venting seep will develop a gas flow channel, as shown in the figure, which will be swept constantly with methane-rich, hypoxic water, rich in nutrients and possibly metal ions (although the high sulphide concentrations will favour precipitation of many metals). This circulation process will have the effect of confining the methane gradient (among others) within the sediment at seep sites, to within centimetres, or even millimetres of the gas flow channel. This may be one of the factors that, in the deeper sulphate reduction zone under suitable chemical and gas flow conditions, leads to the formation of tubular columns of carbonate concretion within the Kattegat sediments. The plate-like slabs of carbonate may possibly be formed at sediment horizons where non-venting methane is trapped within sediment in sufficient concentration over a wide area, in contact with a suitable oxidizing source (e.g. diffusing sulphate). Sediment structure and interstitial gradient characteristics combining to provide conditions suitable for precipitation and aggregation (Jensen et al., 1992; Jorgensen, 1992). CONCLUSIONS Wherever submarine seepage of gas occurs through permeable sediment, the circulation processes described above will develop, and water-sediment exchange processes will be enhanced to an extent at least as significant as that due to bioturbation. The process has been clearly demonstrated in sands of 0.1-0.3 mm grain size. Other types of sediment with different physical (and chemical) characteristics will need to be studied under laboratory and field situations, to determine the limits of this process. Submarine gas seeps are a widespread phenomenon that occur in waters throughout the world (Hovland and Judd, 1988) and methane is only one of the gases known to vent from the sediment (carbon dioxide, sulphur gases, hydrogen and steam, from hydro-thermal sites, are examples of other common venting gases). Some of these other gases would influence the sediment chemistry more directly and to a much greater degree than methane. The major effect described in this paper is the mass movement of fluid through the sediment and the concomitant chemical changes which are generated by any gas when flowing through a permeable, fluid-filled matrix. Acknowledgements-We

thank Preben Jensen and the University of Copenhagen for the use of the facilities at the Havbiologisk Feltlaboratorium, Frederikshavn. This study was funded in part by the Carlsberg Foundation and the CEC under MAST contract 0044C.

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