Introduction

Mud volcanoes occur in a wide variety of geological environments that permit escape of focused fluid from sedimentary basins. The geological field descriptions for feeder complexes are sparse (Davies and Stewart 2005; Roberts et al. 2010), as are the injection effects on local stratigraphy (Castilla and Audemard 2007). The geological definition of a mud volcano (Stewart and Davies 2006) requires the presence of a set of structures associated with an extrusive edifice and subaqueous plumbing system, which connects to a stratigraphic subsurface source. The geometries of these stratigraphic connectors have been described as diatremes (Robertson and Kopf 1998), vertical pipes (Graue 2000), mud pipes, dyke-sill complexes (Morley 2002; Stewart and Davies 2006) and conical mounds (Yamamoto et al. 2005).

Mud volcanoes develop mostly in convergent orogens but also in deltaic settings (e.g. the Nile delta; Loncke et al. 2004; Feseker et al. 2010; Dupré et al. 2014). In the present case, the first observation of sulphate-silica mud diapirs from an arid sabkha setting in the Persian Gulf is described. Detailed mapping of the shallow subsurface zones of sabkha-intrusive mud volcanoes is expected to elucidate the processes that govern fluid transport through the shallow sub-surface, thereby leading to improved modelling of seismic events in evaporitic stratigraphy records. In the Persian Gulf region, the Arabian plate is moving northeast against the Eurasian plate (Authemayou et al. 2006). The region is not recognized as a mud volcano environment, but compressional forces lead to pervasive gas venting on the seafloor (Uchupi et al. 1996). Onshore salt tectonics are known to occur in the Iranian sector of the Persian Gulf bordering the Sea of Oman, and favourable conditions for the development of sub-aerial mud volcanoes can therefore be expected (Negaresh and Khosravi 2008). However, there are few, if any, mud volcanoes reported from low-temperature salt-flat environments preserved together with siliceous sediments (cf. Grasby et al. 2009).

The main aim of the present study, therefore, was to describe the pseudo-stromatolithic features observed in the siliciclastic tidal flats of Kuwait Bay in northern Kuwait (Fig. 1) together with their petrography and isotope geochemistry, and to evaluate their combined influence on the geomorphology of this salt-flat environment. A secondary aim was to generate criteria for assessing the genesis of laminar structures and bioherms in sabkhas in general. Indeed, laminar bedding structures in tidal flats are commonly classified as algal and/or microbial in origin (Kendall and Skipwith 1968; Gerdes and Krumbein 1987; Gaillard et al. 1992) but, in fact, may be partly induced by liquefaction in areas of co-eval seismic activity.

Fig. 1
figure 1

Geological map of mud volcanoes, small craters and seismites in northern Kuwait. Note that the swarm of newly identified features is confined to three zones: zone 1 NW sector of exposed Quaternary beach, zone 2 centre of field, zone 3 SW sector

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Geological setting

A compressional tectonic regime in eastern Kuwait has been maintained since the Tertiary resulting in the formation of the Burgan Arch, a regional anticline bordering the study area. This anticlinal crest confines some of the largest hydrocarbon accumulations in the world (>60 bbs oil) and, like many other oilfields, has a component of thermogenic gas seepage. Mud volcanoes associated with evaporites are a new discovery, having not previously been reported from Arabian tidal-flat environments in spatial association with the eastern flank of the Burgan anticline. The majority of the onshore seismites and mud volcano outcrops are developed in sediments of the Miocene Ghar to lower Fars formations (Figs. 1 and 2), which are equivalents to the Ahwas sandstone member of the Asmari Formation of Iran/Iraq where marginal deposits were developed in sediments of Miocene age (Mukhopadhyay et al. 1996).

Fig. 2
figure 2

Stratigraphic column of the main units within the study area. Ghar Formation sediments are the main hosts of the mud volcanoes but the seismites are increasingly observed in younger formations

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The tidal-flat deposits along the coast of Kuwait Bay form an extensive arc of sabkhas deposited on a shallow ramp structure (Fig. 1). The sediments have been exposed to continuous flooding cycles since at least the Late Miocene. They consist of fine-grained evaporites and incorporate a considerable amount of aeolianite (carbonate-cemented wind-blown sands) and stromatolithic structures (Al-Shuaibi et al. 2012; Fig. 1). The present-day onshore mud volcanoes in Kuwait extrude through partially eroded sedimentary highs and pierce through Holocene strata in a NW–SE trend, oblique to the regional sedimentary arc.

Materials and methods

Three well-defined units were identified within the evaporites in the course of a field mapping exercise (zones 1, 2 and 3; Fig. 1). Only complete exposures were mapped, and extensions beneath Dibdibba gravel deposits to the north are expected to become exposed in the future as wind deflation of the sabkha continues. Grid mapping was carried out using a handheld global positioning system, and structural readings were measured with a compass clinometer and metre rule. Mud volcano samples were collected and processed for thin sections, and XRD mineral identification was carried out using a Bruker D8 X-ray instrument.

Carbonate isotopes were measured on powdered bulk calcite samples, handpicked from veins and disseminated dolomite. δ18O and δ13C of the carbonate material were determined by means of an automated preparation device (KIEL-III) coupled to a Finnigan Mat 252 gas ratio mass spectrometer. Powdered samples were reacted with phosphoric acid under vacuum at 70 °C. The measured isotope ratios are calibrated based on repeated measurements of NBS-19 and NBS-18, with precision of ±0.1‰ for δ180 and ±0.08‰ for δ13C (1σ). To test for the effect of any fine-grained calcite present in the bulk rock samples, three samples were reacted quickly with HCl prior to analysis in the Kiel device. The results obtained were identical with those for the corresponding splits that had not been reacted with HCl. The data presented here are therefore considered to represent dolomite, except in cases where there was evidence of calcite veining.

Sulphate from gypsum was extracted from bulk rock samples by leaching with dilute HCl and precipitated as BaSO4. δ34S was measured on SO2 gas in a Thermo Quest Finnigan Delta PlusXL continuous flow gas ratio mass spectrometer. Samples were combusted at 1,030 °C with O2 and V2O5 using an elemental analyser (Costech) coupled to the mass spectrometer. Standardization was based on international standards OGS-1 and NBS 123, and several other sulphide and sulphate materials that have been compared between laboratories. Calibration is linear in the range from –10 to +30‰. Precision is estimated to be ±0.15 or better (1σ), based on repeated runs on internal standards. δ18O of sulphate was measured on CO gas in a Thermo Electron Delta V continuous flow gas ratio mass spectrometer. Samples were combusted with excess C at 1,350 °C using a thermal combustion elemental analyser (Thermo Quest Finnigan) coupled to the mass spectrometer. Standardization was based on the international OGS-1 standard. Precision was estimated to be ±0.4‰ or better (1σ), based on repeated internal standards. Stable isotope ratios are reported relative to the VPDB (Vienna PDB) standard for carbonates, and VSMOW and VCDT for sulphates.

Results

Geological aspects of mounds

The plan view of the mud volcano system is broadly linear and can be subdivided into three distinct units (zones 1, 2 and 3; Fig. 1), separated by remnant tidal channels. A complete lack of vegetation, due to wind deflation, has fully exposed the structures. The boundary of the area is defined by the edge of the tidal flat that appears to coincide with a sharp and steep (faulted) contact zone. Fracturing is evident throughout the complex, the most common features being regular, grid-like fractures several metres in length and intersect sinuous fractures following several divergent trends (Fig. 3). Conjugate fractures occur closer to the centre of the swarm of mud volcanoes, such fractures being penetrated by small mud volcanoes 4–6 cm in diameter. Dip and strike of the bedded evaporites are consistent except for local areas of steep dip (zone 3) and highly variable strikes mirroring the chaotic bedding produced by syn-sedimentary deformation. Mud was evidently expelled from the feeder system in the form of watery pulses and precipitated in situ (Fig. 3, panel B1), the mud pulses attesting to significant reactivation within the recent past. Small brecciated sandstone clasts and marine bivalves occur within the mud matrix. Mapping revealed zones of similar structural elements but with distinct expulsion features. The central zone consists of small craters that range in size from 0.1 cm to 3 m in diameter, often with an ochreous colour contrasting with the surrounding salt-encrusted sabkha. Giant seismites are particularly well developed in zone C. The long axis of the structures (170°) suggests alignment with the ancient channel.

Fig. 3
figure 3

Collage of pertinent geomorphological features, sedimentary structures and micro-features recorded in the tidal flat of Kuwait Bay, including some mapping details. Panels A–C: mapped small-scale features on cemented, flat salt pan; A zone 1, B zone 2, C zone 3 (cf. Fig. 1 and main text)

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In plan view, isolated pillow structures (individual seismites; Fig. 3, panels C2, C3) occur with elliptical morphology and concentric ring structures, whereas only a few show circular morphology (panel C1). Sizes of pillows are approximately constant within a particular swarm and reach up to 3 m in diameter. The southern sector contains the largest structures (several are >5 m across ellipse axes) with a north–south trend. Mud volcano craters are characteristically perched on the apex of a pillow. Load structures are laterally discontinuous and merge with cemented sabkha material.

The mineralogy of a crater is variable but zoned (Fig. 3, panel B4). Spheroidal dolomite-megaquartz-calcite and occasional lithiophorite [(Al,Li)MnO2(OH)2] dominate the central core, followed by quartz-gypsum-calcite. The spheroidal dolomites range in size from 5–350 μm and have concentric zones, internal nuclei with or without a radial fabric.

Details of outcrop structures

In the panels of Fig. 3, A, B and C are representative of small-scale features on a cemented, flat salt pan: zone 1 structures = A, zone 2 = B and zone 3 = C. A: small prominent craters (diameters <1 cm to 1 m) confined by pseudo-hexagonal sinuous fractures, occasionally penetrated by small craters, are indicative of syn-sedimentary fluid expulsion. B: denser pattern of small craters with a tendency to merge and cluster in a chaotic fashion; fractures are discontinuous and appear to be covered by mud. C: small craters specifically associated with seismite pillow structures; craters are perched on the apex of the seismites and are associated often with concentric rings, indicating discharge of gas from the pillow.

B4: inverted field specimen showing the interior morphology of a small crater; the presence of vuggy mega-quartz, dolomite and gypsum breccia is characteristic of the apex mud volcano. B5: sinuous fracture with small post-development crater penetrating the fracture. A1, A2, A3: conjugate syn-sedimentary fractures in zone A; fractures are sealed with megaquartz, calcite, gypsum and flecks of lithiophorite, and were clearly tensional in the early stages of development.

B1: panoramic view of raised terrace showing recently exposed (wind-deflated) platform containing mud volcanoes and fossilized mud flow. B2: partly degraded mini cluster of mud volcanoes showing concentric laminae. B3: photomicrograph (diameter 25 mm) of dolomitized mud volcano fabric with matrix carbonate and gypsum, enclosing prominent spheroids of dolomite.

C1: giant seismites, weathered with small apex crater in southern zone 3. C2: perfectly preserved pillow with deformed aspect and small apex crater common in the southern study area. C3: largest observed seismite with an internal diameter of >3 m, displaying a deformed pillow structure.

Trigger mechanism of fluid expulsion

The pillow-like structures illustrated in Fig. 3 were clearly not formed by storm waves because artesian-derived volcanoes are morphologically different, having steeper to near-vertical walls tapering downwards (Mount 1993). Storm-generated waves can be discounted in any case because of the shallow nature and small dimensions of the Persian Gulf during the Miocene. The only probable alternative to seismic-induced liquefaction is overloading of the sediment pile, but this hypothesis can be discounted as well because (1) the units that host the mud volcanoes are shallow near-shore deposits, typically composed of thin laminated sediments; (2) the fact that these structures are laterally continuous in a cross-cutting aspect to the shoreline suggests a local catastrophic geological event; (3) soft-sediment deformation features (Leeder 1987) are found within the evaporites associated with the mud structures; (4) present-day earthquakes are known to occur in the area as a consequence of both oil extraction from the Burgan anticline, and proximity to the active Zagros fold-and-thrust belt; (5) observed sedimentary structures compare well with those generated by liquefaction, not biologically induced sedimentary structures. On the basis of the above considerations, episodic earthquakes are considered to be the most likely trigger mechanism for the fluid expulsion (cf. Fig. 4).

Fig. 4
figure 4

Compiled seismic and structural data from Kuwait (modified after Sadek 2004). MV Mud volcano field, eastern flank of Burgan anticline

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Stable isotope data

Paired δ34S, δ18O data for gypsum from the study area are plotted in Fig. 5a. Values of δ34S range from +16 to +23‰, and of δ18O from –10 to –15‰. Sulphate from zone 1 has lower δ34S values than that from zones 2 and 3. There is no correlation between δ18O and δ34S.

Fig. 5
figure 5

a Mud volcano sulphate isotope data: O vs. S for gypsum from edifice rocks: open diamond gypsum A from zone 1, filled diamond gypsum B from zone 2, circle gypsum C from zone 3. b Corresponding data from earlier studies in the Persian Gulf: seawater (Olsen 1975; Robinson and Gunatilaka 1991), continental groundwater from Al-Khiran (southern Kuwait), the Rus anhydrite regional aquiclude together with the regional gypsum getch (Wadi Al Batin surficial deposits), and sabkha minerals from Abu Dhabi (Olsen 1975). Polygon Field of data in a, dashed lines mixing between continental sulphate and evolved seawater (see main text), continuous line progressive gypsum recrystallization (Lu et al. 2001), open square Abu Dhabi gypsum and anhydrite, triangle Rus anhydrite, circle Al-Khiran groundwater in southern Kuwait, closed square Persian Gulf seawater

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Values of δ13C in dolomite form a triangular array (Fig. 6) and range from –6 to –2‰. δ18O signatures are –10 to –6‰. Calcite veins plot in the centre of the dolomite field in Fig. 6.

Fig. 6
figure 6

Plot of oxygen and carbon stable isotopes for dolomite and calcite from the study area, and carbonate stable isotope data from earlier studies of Persian Gulf sabkhas. AJA, AJC, AJD Aragonite, calcite and dolomite respectively from Al Jubayl, Saudi Arabia (Chafetz and Rush 1995), ADD dolomite, Abu Dhabi (McKenzie 1981), DD1, DD2, DC dolomites 1 and 2, and calcite from Dammam Formation, Qatar (Hanafy et al. 2005), DC water in equilibrium with Dammam Formation calcite at 60 °C (mineral isotope data from Hanafy et al. 2005). Isotope fractionation data are from Horita (2014) and Ohmoto and Rye (1979)

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Discussion

Mud volcanoes are usually an expression of mud and fluid eruption at the seafloor (Brown 1990; Kopf 2002; Judd and Hovland 2007) but eruption into a wet sabkha in the Miocene would have produced a unique set of sedimentary structures. The leakage mechanism that drives highly focused, cross-stratal fluid expulsion in sabkha sediments would have involved an enhancement of pressure-generated permeability by hydraulic fracturing (Hubbert and Willis 1957). Several lines of evidence support the pillow-like structures investigated in this study being interpreted as seismites (Seilacher 1969). Relationships between earthquakes and enhanced eruption of mud have been analysed elsewhere (e.g. Deville et al. 2003; Bonini 2009), and are here suggested to be the main trigger mechanism for fluid expulsion.

Hydraulic fracturing can be involved in the early emplacement of sedimentary structures—e.g. remobilization of mud and sand (Van Rensbergen et al. 1999). The studied soft-sediment deformation structures described herein occur at the margin of an elevated Quaternary tidal-flat shoreline. Numerous multilayered unstable density gradients were produced (seawater-mud-evaporites), which is to be expected in evaporative-carbonate systems. Ideal geotechnical conditions were developed during the Miocene to generate large load structures in these previously soft sediments. The origin of the concentric ring structures around the centre of the largest mud volcanoes is considered to be associated with periodic fluctuations in volume flux, where each ring would represent a near-circular mud flow event (Murton and Biggs 2003).

A key feature of the fracture density is its locus that increases from the east to the west, indicating the proximity of the Burgan anticlinal axis. The increasing intensity of the sinuous fracture patterns, on the other hand, points towards the original centre of the feeder complex in zone 2, indicating that the mud edifice itself is the principal control on fracture distribution, not the regional folding.

The non-tectonic fractures (those induced by syn-sedimentary deformation of evaporite sediment), on the other hand, may be explained by the mud intrusion process. Overpressured fluids below the edifice would have produced a pressure differential between the fluids in the fractures and the fluids in the sedimentary rocks (Fig. 7). This pressure vector would exceed the minimum principal stress field, causing dilation and fluid flow through the fractures (Morley and Crevello 1998; Jolly and Lonergan 2002; Campbell et al. 2002). Mud drapes in the fracture walls would inhibit exterior fluid leakage and aid in the sustenance of fluid pressure, facilitating fracturing deeper in the pile and the propagation of the fractures up to the surface (Morley 2003).

Fig. 7
figure 7

Schematic cross-section and interpretative model for the features discussed in this study. Fluid expulsion is considered to be coeval with seismic events

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Fluid sources and mixing trends

Several stable isotope studies of carbonate and sulphate minerals from carbonate formations, active sabkhas along the shoreline of the Persian Gulf and continental getch (gypsum soil enrichment) provide a useful background for the present study (Fig. 6). McKenzie (1981) established that secondary dolomitization of Ca-carbonate sediment in the Abu Dhabi sabkha occurs at temperatures of 34–49 °C. Chafetz et al. (1999) showed the presence of correlatable horizons with low values of δ18O and δ13C, which they interpreted as evidence of bacterial sulphate reduction (BSR), in Pleistocene dolomites at Al Jubayl (Saudi Arabia). An overlying low-magnesium calcitic facies at Al Jubayl has low values of δ18O as a result of diagenetic interaction with a lateral influx of meteoric water (Chafetz and Rush 1995). BSR is also considered as an explanation of low values of δ13C in dolomite of the Eocene Dammam Formation in Qatar, where late influx of meteoric water is the causative agent for overprinting diagenetic calcite (Hanafy et al. 2005). BSR and the presence of sulphate of both marine and continental origin (getch) are inferred from studies of sulphate isotopes in the south and north of Persian Gulf sabkhas (e.g. Olsen 1975; Robinson and Al-Ruwaih 1985; Robinson and Gunatilaka 1991). Furthermore, sulphate is present in groundwater in Kuwait and enters the sabkhas of the south via the Kuwait Group clastics (Robinson and Al-Ruwaih 1985). Those authors identified sulphate with δ34S values of 15–16‰ in the freshwater entering the sabkha.

Published S isotopic data on the northern Persian Gulf sabkhas indicate a seawater influence (δ34S of +21‰, δ18O of +2.4‰) but this applies only in the outer sabkha areas, decreasing towards the landward side. These values are lower than the Abu Dhabi sabkhas where seawater sulphate and BSR are more prevalent, indicating that at least in southern Kuwait the influence of water from continental reflux to the sabkha is greater. Evaluation of the Persian Gulf hydraulic model for sabkhas (Yechieli and Wood 2002; Wood et al. 2002) suggests that vertical flux of groundwater from the deeper formations could be a major source of solutes (but less so the water), while the seawater influence is confined to a narrow sliver of coastline. The present study supports this model.

Source of sulphate

Figure 5b shows available sulphur isotope data from earlier studies of Persian Gulf sabkhas, along with the δ34S, δ18O of +14, +13‰ reported by Olsen (1975) and Robinson and Gunatilaka (1991). Another vertex (+20, +10‰) corresponds to present-day Gulf seawater (Olsen 1975; Robinson and Gunatilaka 1991). The third vertex (+22, +15‰), or more positive values, may be related to seawater sulphate. The slope of the data trend between seawater and vertex 3 is too steep to be explained by dissolution and partial recrystallization of gypsum, which would generate a slope near 2 (Lu et al. 2001). The trend is unlikely to represent BSR because there is so little modification of the S isotope signature. A different mechanism for evolving seawater sulphate might involve exchanging O isotopes between evaporite sulphate and mildly oxidizing continental waters in the mixing sub-surface of the edifice mud mounds, but it would be difficult to achieve over a reasonable time scale at temperatures less than 100 °C (Chiba and Sakai 1985). Alternatively, cyclic recrystallization of gypsum in the edifice may be reasonably expected to mix with continental “getch” sulphate, a process ongoing in Kuwait since at least the Miocene. It is possible that such mixing has led to an apparent steepening of a recrystallization trend originating at the seawater vertex, as well as the generation of mixtures in the middle of the triangular array.

During the mid-Miocene, seawater sulphate briefly attained δ34S values of +22 to +23‰ (Claypool et al. 1980). With the possible exception of sample B6 (zone 2), such sulphate is not evident in the present database. The B6 data could also be explained by invoking BSR. It is not known whether the mud volcanoes were active during that critical part of the Miocene when δ34S values were high, or at another time when the δ34S values of marine sulphate were closer to those of modern seawater.

Carbonate isotopes

The mud-volcano dolomite does not resemble dolomite from the Persian Gulf sabkhas or from the Dammam Formation aquifer in carbon isotopes (Fig. 6). However, its isotopic composition overlaps with late diagenetic calcite from the Dammam aquifer.

The isotope compositions of water and dissolved bicarbonate in equilibrium with the mud-volcano dolomite and calcite were calculated using fractionation factors from Horita (2014) for oxygen, and data summarized by Ohmoto and Rye (1979) for carbon. The findings are shown in Fig. 6. The temperature range used for the calculations was 50 to 80 °C, consistent with fluid inclusion data for gypsum from other warm-water mud volcanoes. For temperatures of 70–90 °C, the mud-volcano water would have the same range of δ18O as water in equilibrium with the calcite cement of the Dammam Formation aquifer (calculated from the data by Hanafy et al. 2005), or with sabkha pore water from Kuwait (Robinson and Gunatilaka 1991). Gulf seawater can be in equilibrium with dolomite only at temperatures of 100 °C or greater.

The δ13C values of dissolved bicarbonate in equilibrium with dolomite (–4 to –8‰; Fig. 6) for the temperature range used here indicate the addition of low-δ13C bicarbonate to the near-0‰ bicarbonate of marine origin that is clearly present in most of the fields shown in Fig. 6. Low-δ13C bicarbonate can be generated by oxidation of buried organic matter, in this case most likely as a result of BSR or bacterial reduction of Fe3+ (BFR). The bleaching of Fe oxides around the edifice and the presence of spheroidal dolomites at the crest of the mud volcanoes point to localized BFR. The calcite veins are paragenetically younger than the dolomite but, if the calcite and the dolomite formed at similar temperatures, then it is likely that the calcite was in equilibrium with water having δ18O values 3–4‰ higher than that in equilibrium with the dolomite. Alternatively, they may have formed in equilibrium with Dammam Formation water at a temperature below 50 °C.

Conclusions

These exceptional outcrops comprising sulphate mud volcanoes and seismites preserved in a sabkha are a consequence of several unique conditions (Fig. 7): (1) the area is characterized by earthquakes of low to moderate magnitude; (2) the area is on the eastern flank of the most oil-endowed anticline on Earth; (3) empirical relationships enable linkages between large seismic events and liquefaction events (Ambraseys 1988). The dimensions of liquefaction structures result from a combination of earthquake parameters and sedimentary features. Different methods have been proposed to estimate the severity of earthquakes using liquefaction structures (dykes, fluid expulsion features, mud volcanoes; cf. Allen 1986; Leeder 1987; Castilla and Audemard 2007).

The primary nature of the dolomite spheroids in the matrix suggests an association between the migration of methane and the presence of microbial components in the mud fluids (Gunatilaka 1989). In this context, the present study identifies a hitherto unrecognized formation of mud injections in a shallow tidal-flat environment in the Persian Gulf. These cannot be demonstrated to be related to regional hydrothermal systems (Grasby et al. 2009), unless supporting evidence of high-temperature fluids can be obtained. Nevertheless, the presence of spheroidal dolomite, vein calcites and unusual stable isotope signatures in sulphate points to a sub-mud mound mixing of seawater-derived fluids with sulphate-bearing continental fluids of a deep nature. Stable isotope data in dolomite indicate a role for bacterial reduction of sulphate or Fe3+, and are consistent with a deep origin for water, possibly the Dammam aquifer. The ancient analogues of mud volcanoes in the geological record together with the Kuwait swarm (Gaillard et al. 1992; Campbell et al. 2002) are important scientific targets for evaluating the three-dimensional biological and physical nature of seeps in general.