Introduction

Modern continental shelves contain substantial amounts of methane, predominantly produced through bacterial degradation of organic matter (e.g. Martens and Berner 1974; Hovland and Judd 1992; Fleischer et al. 2001). Although the total mass of methane in sediment beneath shallow marine waters has not been quantified (Fleischer et al. 2001), methanogenesis probably occurs over at least 30% of the world’s continental shelves, potentially contributing 8–65 Tg of methane to the atmosphere annually (Hovland and Judd 1992). Attempts to quantify the gas budget have focused on individual sites (Hornafius et al. 1999), or restricted areas (e.g. UK continental shelf, Judd et al. 1997).

Methanogenesis in marine sediments typically proceeds after a specific sequence of microbially mediated reactions that degrade organic matter, each of which changes the pore-water chemistry (Claypool and Kaplan 1974; Berner 1975). Early diagenetic carbonates can provide important constraints on the nature and evolution of pore waters over long time scales (Malone et al. 2002).

Biogenic methane is produced by methanogenic archaea as an end product of the decomposition of organic matter. This process begins after the organic matter is deposited and occurs mainly at relatively shallow depths within sediments in which sulphate levels have been depleted by the activities of sulphate-reducing bacteria. Where methane concentrations can rise to exceed methane solubility, the formation of gas bubbles is permitted. As production rates may be seasonal (Martens et al. 1998), and the solubility of methane is temperature and pressure dependent, ebullition may be seasonal (controlled by bottom water temperatures; Wever et al. 1998), triggered by tidal falls and atmospheric pressure variations (McQuaid and Mercer 1990), or triggered by earthquake events (e.g. Hasiotis et al. 1996). The advection of groundwater may also promote methanogenesis and the flux of methane to the seabed in coastal sediments (Bussman and Suess 1998).

In the last three decades, a wealth of evidence has shown that various features demonstrate fluid migration at continental margins (Kaluzza and Doyle 1996; Baraza et al. 1999). Furthermore, it is evident that features, such as shallow gas accumulations, pockmarks, seeps, mud volcanoes and gas hydrates, often associated with cold seep communities and methane-derived authigenic carbonate, are present in a wide variety of geographical, oceanographic and geological environments: near-shore, continental shelf to deep ocean.

The sedimentary record in the rías and shelf gives an excellent opportunity to evaluate the sea-level changes during the Quaternary where the major gas accumulations appear. Also, this record can offer important information about the biogeochemical cycles, which involves the sediments and organic matter content, at different temporal scales (year, decades and geological scales). In this sense, the upwelling of Eastern North Atlantic Water (ENAW) within the sediments is recorded in the sediments by the occurrence of different organisms such as foraminifers, diatoms and other organic components.

The purpose of this paper is to show that the Rías Baixas are natural laboratories where interactions between different processes (fluid escapes, gas generation, sedimentation, biology, diagenesis, sea-level variations, etc.) can be observed. Coupling processes between fluid escapes and geosphere/biosphere/hydrosphere can be analysed on the scale of the small marine basins that constitute the rías environment.

Setting

The study area (Fig. 1) includes the shallow marine environments (rías) from Miño estuary (41°55′N) up to Ría de Muros (42°50′N). The shelf in this region is 30–50 km wide with a poorly defined shelf break that occurs at depths of 160–180 m. North of 42°30′ (Ría de Arousa), the shelf narrows and the shelf break is clearly defined. The coast of this sector is characterized by structurally controlled, incised valleys bounded by steep hills (horsts). These are the Rías Baixas (from south to north: Vigo, Pontevedra, Arousa and Muros).

Fig. 1
figure 1

Geographical location of the Rías Baixas on a satellite image. Main trending fault (white dashed lines) systems (NW–SE, N–S and NE–SW)

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The term ‘ría’ is an old Galician name for the marine inlets that are so well developed in this part of the NW coast of Spain. Von Richtofen introduced the term in 1886. He used the term for a funnel-shaped or wedge-shaped drowned valley on coasts that are transverse to the structural grain of the land. García-Gil et al. (1999a) provided new data supporting a tectonic–eustatic origin and evolution of the rías.

The average water depths within the rías range from 5 m in their inner parts to 55 m at the outer (south-west) entrance to the sea. They have a mean tidal range of 2–3 m.

The general pattern of grain-size distribution on the present seafloors of the rías consist of mixed siliciclastic and skeletal gravels in both the outer areas and the edges of the rías. The central and inner parts of the rías are dominated by clay and silt, which contain up to 10% organic matter.

The Rías Baixas are located on the passive Atlantic margin of south-western Galicia (NW Spain). They trend north-eastward at nearly right angles to the main-trending Palaeozoic basement structure.

The rías and their surroundings (Fig. 1) contain several major faults (N–S, NE–SW and NNW–SSE). To the east of the rías is a 170-km-long fault zone consisting of north–south-trending parallel highs and lows filled with Oligocene–Pliocene lignites and Quaternary sediments. This fault zone was probably formed during the Hercynian, was reactivated during the Miocene–Pliocene and may have experienced activity into the Quaternary (Pannekoek 1966; Pazos et al. 1995; García-Gil et al. 1999a). It is noticeable that this N–S-faulting zone controls the presence of San Simón Bay at the inner part of Ría de Vigo.

The basement is composed of Palaeozoic metamorphic and granitic rocks cut by NE–SW-, NW–SE- and N–S-trending fault systems. The islands located at the mouth of the Rías Baixas (except Ría de Muros) correspond to structural highs.

Materials and methods

From 1991 through to 2003, more than 3,000 km of acoustic and seismic records were recorded from the Rías Baixas and adjacent shelf (Fig. 2). Different survey patterns were acquired using the following equipment: sparker (4,500 J, EG&G multi-electrode Sparker fired at 3- to 5-s intervals), Uniboom (E.G. and G. Uniboom Catamaran Model 230 operating at 300 J), Bubble Pulser (Model SPR-1200, 400 Hz with a shoot frequency of 250 ms), ORE 3.5 kHz sub-bottom profiler, echo sounder (Atlas Deso 20 and KODEN CVS-106 models), sidescan sonar (Klein Model 595 operating at 100 kHz on three channels and C-MAX model CM-800 operating at 375 kHz on two channels, both with a lateral cover of 150 m). In 2001, a multibeam echo-sounder (Reson Seabat 8101) survey was performed. Navigation and position fixing were performed using a Trimble 4000 RL Differential GPS station and a FURUNO GP-35 Differential GPS combined with a transponder unit.

Fig. 2
figure 2

Position of high-resolution seismic profiles and location of well (PMB1) on the shelf as well as vibrocores. Thick line correspond to Sparker surveys and the thin line to Uniboom surveys

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A 50-point grid (1 km2) of sediment cores (>100) covering the whole surface of the Rías Baixas was sampled. Mineralogical analysis of sediments and the quantitative variations in mineral content related to the gas-escape areas were determined using XRD-Rietveld analysis in Ría de Vigo. In addition, vibracores and a well drilled by ENIEPSA (1984) were use for sedimentological analysis and chrono-stratigraphical correlations.

Analysis of gas bubbles collected in situ was carried out by gas chromatography GC-MS (HP 6890 Series GC and HP 5973 Mass Selective Detector). Total ion chromatograms were performed, whereas methane was better detected by scan chromatograms and compared with the standard methane sample of the mass selective detector.

Results

Previous work on the Quaternary sedimentary record of the area (García-García 1999; García-Gil et al. 1999a, 1999b, 2000) and new high-resolution seismic data tentatively correlated with a well (PMB1, depth 3,538 m, see Fig. 2 for location) from the close shelf allowed to establish a more complete seismic stratigraphic architecture for the Rías Baixas. This consists of several sedimentary sequences bounded by major unconformity surfaces (Figs. 3 and 4). From older to younger these are Palaeozoic Basement, granitic (GB) and metamorphic (MB); Upper Oligocene–Lower Miocene (T1); Lower Miocene–Middle Miocene? (T2); Pleistocene (Q1); and Upper Pleistocene–Holocene (Q2).

Fig. 3
figure 3

a High-resolution seismic section (Sparker source) from the innermost part of the Ría de Pontevedra (the whole seismic line goes from the edge of the shelf up to the inner areas of Ría de Pontevedra along its central axis). b Seismic interpretation showing the top of the gas blanket in the youngest Quaternary sequence (Q2). GB Granitic basement; T1 Upper Oligocene–Lower Miocene; Q1 Pleistocene; Q2 Upper Pleistocene–Holocene

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Fig. 4
figure 4

New stratigraphic model for the Rías Baixas based on well data, seismic interpretation and correlation with a curve of sea-level variations (Haq 1991). GB Granitic basement; MB metamorphic basement; T1 Upper Oligocene–Lower Miocene; T2 Lower Miocene–Middle Miocene; Q1 Pleistocene; Q2 Upper Pleistocene–Holocene; LGM last glacial maximum unconformity surface; YD Younger Dryas erosive surface; TST transgressive system tract; HST high system tract; ts transgressive surface; dls downlap surface; psb present seabed; U1, U2 Upper Pleistocene seismic units; U3,U4 Holocene seismic units

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These results enabled us to conclude that the oldest sediments overlying the Palaeozoic basement (granite and metamorphic rocks) within the rías are of upper Oligocene–Lower Miocene age (Fig. 4).

The youngest sequence (Q2) rests unconformably on the Q1 sequence. Within the Q2 sequence, four different seismic units can be distinguished (from U1 to U4).

Nine fluid migration and accumulation features have been distinguished in the Rías Baixas based on their acoustic signatures: (1) acoustic blankets, (2) domes, (3) diapirs, (4) dark patches, (5) pockmarks, (6) relict collapse depressions, (7) seafloor mounds, (8) acoustic plumes and (9) cloudy turbidity.

Shallow gas accumulations

Acoustic blankets, domes and diapirs correspond to ‘reservoirs’ of fluids that are trapped during their migration towards the surface (seabed).

Acoustic blankets are the most frequent type of gas accumulation in the Rías Baixas (Fig. 5). The 2-D seismic signature consists of an upper strong coherent or enhanced reflection over a complete acoustic masking of the underlying seismic record, without any morphological expression on the overlying seafloor. The acoustic blanking fronts mainly appear within unit U4 and less frequently in unit U3. The gas also accumulates nearer the surface in the internal areas of the rías than at their outer areas.

Fig. 5
figure 5

High-resolution seismic section (3.5 kHz source) from inner part of the Ría de Arousa. Shallow gas accumulation and a pockmark are identified

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In contrast, domes show a smooth relief over the surrounding seabed. The domes appear as rounded or ovoid features on side scan sonar with an average diameter of 50 m, but occasionally reaching up to 100 m. The average height of these domes is generally 0.7 m. The Uniboom and 3.5 kHz seismic records show a rise of the shallow gas front just underneath these domes. Grab samples taken in the dome area indicate mud is the dominant sediment type (García-Gil et al. 2001). The location of domes is restricted to the inner areas of Ría de Muros and Ría de the Arousa.

Diapirs are fluid migration/accumulation features, seismically characterized by the low impedance on the Uniboom seismic records, a fact that allows them to be differentiated from the plutonic (granite) intrusions that are also present on the same seismic records. It is noticeable that diapirs have been identified in the Uniboom seismic records only from Ría de Pontevedra where they intrude and deform (Fig. 6) the sediments that comprise the sequence (Q2) in the external areas of this ría (Fig. 1).

Fig. 6
figure 6

Uniboom seismic sections showing diapirs (D) in Ría de Pontevedra disrupting the oldest Quaternary sequence (Q2) and underlying the youngest Quaternary sequence (Q2). Both sequences are separated by the erosive surface (LGM) correlated with the regression during the last glacial maximum. a Strike line and b dip line

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Fluid/gas escapes features

Dark patches (Fig. 7) have been identified in the side scan sonar records as areas of high backscatter. They appear as circular or elliptical shallow depressions from several tens to hundred of metres across. Dark parches are frequently associated with the shallowest gas accumulations, domes, seafloor mounds and/or pockmarks in the Ría de Muros and Arousa. Analysis from grab samples indicates a muddy composition of the surface sediment.

Fig. 7
figure 7

Side scan sonar records from Ría de Muros where seafloor mounds, dark patches and gas depressions can be observed

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It is evident that pockmarks are formed by the expulsion of either gas or water (King and Maclean 1970; Hovland and Judd 1988) through seafloor sediments. Generally, they are formed in soft, fine-grained seabed sediments. In the rías, pockmarks are shallow seabed depressions, typically a few metres across (5 m average) and less than 1 m deep (0.5 m average). Nevertheless, in seismic profiles, as well as in side scan sonar from the shallowest inner part of Ría de Arousa, pockmarks of several tens of metres across and up to 1.5 metre deep have been identified (Fig. 8).

Fig. 8
figure 8

High-resolution seismic section (3.5 kHz source) from inner part of the Ría de Vigo next to San Simón Bay. Shallow gas accumulations, pockmarks and seeps are identified

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It is noticeable that gas collapse depressions have only been identified in Ría de Vigo where they disrupt the top of the seismic unit (U3) of the Quaternary sequence (Q2), i.e. these are relict structures (Fig. 4) fossilized by the youngest Holocene seismic unit (U4). They are, on average, 94 m across in the Uniboom records and 7.5 m deep. In the seismic records, pockmarks and collapse structures show different seismic characteristics. Pockmarks are typically v-shaped whereas collapse structures have steeper lateral boundaries and a flat bottom without a clear basal reflector. The lack of transverse seismic lines crossing individual collapse depressions makes it impossible to reconstruct their 3-D shape.

Seafloor mounds have been identified on sidescan sonar in Ría de Muros (Fig. 7). They are typically 5 m in diameter and 1 m high, located at water depths of 10–15 m. The mounds in the Ría de Arousa have diameters ranging from 15 to 45 m and heights of 1–2 m at water depths of 15–20 m. These mounds have seafloor expressions on the echo sounder as well as in the Uniboom records. Grab sampling of a mound shows muddy seabed sediments. Uniboom records show extensive acoustic blanking (3.7 km along the line) underlying the seafloor mounds due to the presence of shallow gas that is accumulating at almost the present seafloor (García-Gil et al. 2001).

Acoustic plumes and cloudy turbidity in the water column are better identified by the echo sounder and 3.5-kHz records and are frequently related to shallow underlying gas accumulations.

Mapping of gas accumulations has enabled several fields to be recognized in the four rías. Comparison of the locations of the gas accumulations with the grain-size distribution of sediments in the Rías Baixas (Vilas et al. 1995, 1996, 1999) reveals a spatial coincidence between gas fields and finer surface sediments, i.e. mainly muds (García-Gil et al. 1999c; Ferrín et al. 2003). These muds correspond to the youngest Holocene seismic unit, U4, which constitutes a seal for the gas accumulations.

X-ray photographs of gravity cores and vibrocores have been used as semi-direct evidence of the presence of gassy sediments. Gas-filled pores can be seen as white elongated spots with larger deformations at the edges of the cores and as voids in cut cores (Fig. 9).

Fig. 9
figure 9

Photograph of ZV1split vibrocore where voids due to gas in the muddy sediment can be recognized

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Mapping of fluid-escape areas shows that these are located either above the shallower gas accumulations or in the outermost zones of the gas accumulation fields, as well as at intersections of the main fault systems or along the contact surfaces between basement rocks and sediments (Fig. 6). In these areas, the permeability of the overlaying facies would not be enough to constitute an efficient seal, allowing the fluids to be released in different ways.

Shallow gas–geosphere–hydrosphere–biosphere processes

Samples of gas were collected in the shallow subtidal and intertidal areas from San Simón Bay (Ría de Vigo). Ebullition of gas bubbles can be observed during high tide over an ancient Holocene peat horizon that is partially exposed under coarse sands of the beach during low spring tides. Gas bubble escapes are also seen from the shallower subtidal areas close to San Simón Island during low tides (Fig. 10).

Fig 10.a
figure 10

Aerial Photograph from San Simón Bay (inner part of Ría de Vigo) showing the location of some gas bubble occurrences. b Photograph from Cesantes Beach during a low spring tide, where a peat horizon is partially exposed; the rectangle delimits the zooming area shown in c, small shovel for scale is 30 cm. c A detailed photograph where the coarse-sized sediment can be seen (rule divisions are 1 cm) with a black finer matrix covered by white mats normally described as Beggiatoa. d Streams of gas bubbles coming from the seabed (peat area) and rising to the surface during a high spring tide in Cesantes Beach

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Gas chromatography showed that methane is the dominant gas (Fig. 11).

Fig 11.a–d
figure 11

Chromatographic analysis. a–c TIC (total ion chromatography) mode of different samples showing that methane is the major gas component. d GC-MS (gas chromatography scan mode) analysis clearly detects the methane presence in other gas bubble samples

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Geochemical studies on 35 core samples from Ría de Vigo (García-Díez 2001) have shown authigenic aragonite, pyrite and gypsum occur within the shallower sediment record from Ría de Vigo. In order to evaluate the sequence and extension of the early diagenesis reactions, mineralogical percentage mapping was performed. To achieve this goal, samples from the first 25 cm of each core were analysed at 5-cm intervals using RXD, and their quantification was established by the Rietveld method. The result was the mapping of the 11 major minerals in samples at different stratigraphical horizons (from the present seabed at 5- to 25-cm depth). Carbonate (calcite and aragonite) was decreased by dissolution from the present seafloor up to 20 cm depth. At 25 cm depth, there is a change: the carbonates increase, but now the authigenic aragonite is a major component compared to calcite. Sulphur compounds (pyrite), sulphates and Fe-silicates were analysed and mapped in a similar way as the carbonates (García-Díez 2001). Positive anomalies of these latter minerals are at shallower depth in the inner areas of the Ría de Vigo and are deeper in the external areas.

A comparison between the distribution of gas accumulation and authigenic minerals reveals two noticeable coincidences: (1) there is a spatial coincidence between the gas accumulation and the increase in authigenic aragonite and sulphur; and (2) the shallower positive anomalies of the authigenic minerals corresponds with the shallower gas accumulations at the internal parts of the Ría de Vigo, whereas both authigenic mineral-positive anomalies and gas accumulations are deeper at the outer areas of the ría, i.e. there is a direct relationship between shallow gas and these authigenic minerals (García-Gil et al. 2003).

Preliminary radiotracers studies (Max Planck Institute for Marine Microbiology, in Germany) of the microorganisms that occur in the bubbling area sediments (i.e. San Simón Bay) show the presence of methane oxidation due to anaerobic microorganisms, although this activity is much lower than that found related to the gas hydrates. In San Simón Bay sediments, the rate is: 0.5 nmol/cm3 slurry/day whereas in gas hydrates, the values are ~2 µmol/cm3 per day.

Marine productivity and biological diversity are also very high in the Rías Baixas. Winds play an important role in circulation and cause upwelling and downwelling cycles with a strong seasonal pattern. During spring–summer (April to September), upwelling takes place when northerly winds are prevalent. The cooler and nutrient-rich Eastern North Atlantic Water (ENAW, Ríos et al. 1992) is driven into the Rías Baixas, and increases marine productivity.

Other researchers have focused on the biological communities of the rías (Fig. 12). Some of their findings are noteworthy to this study, for example, the appearance of the octocoral Leptogorgia sarmentosa (Fig. 13a). This species has a calcium carbonate skeleton and occurs in the rías as large branching colonies that can be up to 49 cm high (Mijón 1998). Normally, the branches are in a single plane perpendicular to the main current, but, in Ría de Vigo, 3-D-branching colonies are also found. These colonies are mainly located in the middle–inner part of Ría de Vigo, close to the areas where very shallow gas accumulations and/or escape features are present.

Fig. 12
figure 12

Spatial distribution of gas field accumulations, gas escape features (pockmarks and seeps) and Anthozoa fauna in the Ría de Vigo. It is noticeable that the location of Leptogorgia sarmentosa (octocoral) is probably related to methane escapes. Hydroid data from Ramil et al. (1998) and Anthozoa data from Mijón (1998)

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Fig. 13.a, b
figure 13

Photographs of some benthic fauna from Ría de Vigo. a Leptogorgia sarmentosa (octocoral); b Suberites ficus (sponge on muddy sediment)

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Thirty-one hydroid species have been reported from the soft bottom of Ría de Vigo; of these, five species were collected exclusively from soft bottoms (Ramil et al. 1998).

In addition, there are sponges several metres in length and orange in colour (Fig. 13b). Suberites ficus was found during one diving expedition. This sponge (a filter feeder) rests on muddy sediments in the inner areas of this ría where seeps occur.

Discussion

Sequence stratigraphic analyses based on both the seismic record of the Mesozoic–Cenozoic cycle chart (Haq et al. 1991) and correlation with well data show that the oldest sediments within the rías correspond to the Upper Oligocene–Lower Miocene sediments overlying the Palaeozoic basement (Figs. 3 and 4). This indicates that the origin of the rías is relatively recent, being not older than the Upper Oligocene–Lower Miocene. This is in agreement with Pannekoek (1970) who, based on the Nonn (1969) chronology for the three erosive surfaces in Galicia, suggested three erosional stages for the development of the rías: during Middle Miocene, after Pontian tectonics and at Early Pleistocene.

The glacio-eustatic sea-level curve for the late Quaternary describes an asymmetric fourth-order cycle periodicity of about 100,000 years, (Chappell and Shackleton 1986; Chappell et al. 1996) in which the last glacial maximum (LGM) occurred between 22 and 18 ka b.p. The Upper Pleistocene can be considered a cold stage beginning at 115–120 ka and ending at ~10–11 ka b.p. It is proposed that sequence Q1 in the Rías Baixas was generated during the Würm (Weichselian) glaciation (Pleistocene), and the origin of its top discontinuity (LGM, Fig. 4) was due to erosion during the regression (when sea level was at minus 120 m) of this last glacial episode at 18 ka b.p.

All of the fluid migration features can be observed in the acoustic/seismic records from the four rías within the youngest seismic Q2 sequence (units U3 and U4) with the exception of the diapiric structures, which, up until now, have only been recognized in Ría de Pontevedra (Figs. 4 and 6) within the oldest Quaternary sequence (Q1).

The spatial distribution of the gas escapes and gas accumulations, and their vertical variations, are interpreted as evidence of sedimentary facies control, i.e. depending upon the sediments permeability and the quantity of fluid (fluid pore pressure) within them.

Gas accumulates when the facies is sufficiently porous to accommodate it and where an overlying, low permeability facies is present to act as a seal.

In the Rías Baixas, the top of the gas accumulations are mainly located inside the uppermost seismic unit (U4) and, less frequently, in the underlying unit (U3). Facies analysis from cores and vibrocores (Fig. 9), as well as their correlation, shows that the finest-sized sediment constitutes a gas seal. These are mainly muds that are characterized by a low permeability. Nevertheless, it is possible to observe the gas bubbles as voids within the muds . The present seabed muds in the rías are very rich in organic matter content, which is up to 10% (Vilas et al. 1995, 1996, 1999). The microbial decomposition of this organic matter in the sub-seabed environment can generate biogenic methane shortly after deposition.

Whilst seeping fluids may include groundwater, the distribution of groundwater seeps is governed primarily by the availability of sufficient hydraulic head to drive their emission. On the continental shelf, groundwater emissions are restricted to coastal areas (Judd 2001). This could be the origin of some stream bubbles found in the intertidal area (Cesantes beach) from San Simón Bay where spring water is present. Nevertheless, the presence of peat, which also generates gas, could also be the source of the gas bubbles in this specific location (Fig. 10).

Pockmarks are formed by fluid escape and they occur only where the seabed is suitable (e.g. soft, silty clays). In the Rías Baixas, the fluid migrates from the underlying shallow gas reservoirs (Fig. 8) within seismic units U4 and/or U3, and the morphological expression is preserved by the presence of the overlying Holocene muds (silty clays, and silts). The fluid migration is normally vertical up towards to the seabed. However, when an impermeable barrier stops vertical fluid flow, horizontal flow may occur if the lateral facies changes allow.

In contrast, where the sediments are not suitable for pockmark formation (e.g. where there is porous sediment at the seabed) there is no surficial morphological evidence of fluid escape. However, acoustic plumes may indicate the presence of fluid release to the water column. If such seeps provoke sediment resuspension, it can then lead to acoustic turbidity in the water column (García-Gil et al. 2002).

Collapse depressions are formed by the collapse of the sediment as a result of dewatering or degassing, and not by the erosion of the sediment material (Hovland and Judd 1988). The gas escape is probably less violent at the collapse features than at the pockmarks.

At least two different explanations for the origins of the dark patches can be found in the scientific literature: (1) they are shallow depressions with only coarser materials at the surface as a result of the remobilization of the finer sediments during gas expulsion (Hovland 1984), or (2) they are areas where the gassy sediments rise up to the surface (Hagen and Vogt 1999). In the Rías Baixas, the local occurrence of very shallow accumulations, dark patches and mounds (Fig. 7) suggests that dark patches and mounds are both features related to gas escape. Gas accumulation almost at the seabed surface would be responsible for dispersing acoustic energy due to the presence of gas bubbles (dark pattern on side scan sonar).

There also are several coincidences between domes and large pockmarks in Ría de Arousa: (1) close spatial location in the inner part of this ría, and (2) similarity in the dimensions, i.e. 0.7 m height and average diameter of 50 m across. There is a direct relationship between domes and pockmarks: the domes could be interpreted as an early stage of the formation of the large pockmarks.

Special consideration must be given to the diapirs as they disrupt (Fig. 6) the oldest Quaternary sequence (Q2). Mapping of these features shows that they are located at intersections of recognizable faults at the seafloor. The diapirs are thought to have originated at depth due to fluid migration (most likely from thermal springs) up the Q2 sequence via a system of deep faults. The predominant fault trends in the rías (NW–SE, N–S and NE–SW) are late Hercynian, reactivated as strike-slip fault during the Tertiary–Quaternary (Figs. 1 and 4).

Shallow diagenesis studies in the Ría de Vigo have permitted the mapping of sulphate (gypsum) and sulphur compounds (pyrite), as well as authigenic aragonite from the seabed up to 25 cm depth (García-Díez 2001). The overlapping distribution of gas accumulations/seeps and authigenic minerals at different depths indicates the following: (1) gypsum- and pyrite-positive anomalies are in spatial coincidence with the Ría de Vigo gas fields; and (2) the peaks of both minerals appear shallower in the inner part and deeper at the external areas of this ría. A similar distribution pattern is found in the gas fields, i.e. shallower and deeper gas reservoirs in the inner and outer areas of the ría, respectively. This is taken as evidence of the coupled processes between methane generation, shallow diagenesis and microbially mediated activity (Fig. 10).

The interaction between upwelling events and the flooded tectonic valleys forming the Rías Baixas allow them to be highly productive systems supporting one of the highest production of edible mussels in the world. In addition, there are two other points: (1) the direct observation of mussel raft locations, some of which are placed in the most internal areas of the rías (e.g. Ría de Arousa, San Simón Bay in Ría de Vigo), and (2) the fishermens’ affirmations that “mussels grow more at the inner areas than at the external parts” of the Ría de Arousa. The inner sectors of the rías are not affected by the upwelling events. Therefore, the idea that active seeps may enhance this productivity is proposed here. The fluid escapes as seeps, and pockmark formation would provoke the resuspension of sediments and nutrients from the seabed. In addition, the presence of methane escapes may allow bacterial mat formation, which can have a positive effect on the local ecosystems (Fig. 10). The filter-feeding fauna (e.g. mussels) will be favoured by the presence of these seeps. Hovland and Thomsen (1997) suggested that there is a relationship between gas-associated seafloor features and local communities in the North Sea, and that carbon from the methane would enter the food chain.

Malone et al. (2002) indicated that methane inputs to the shelves have probably affected the global carbon cycle and climate over time.

The geological history of the Galician Shelf and the rías is highly complex, and is documented in the sedimentary sequences, faults, folding and erosive surfaces that result from rifting and spreading events, sea-level fluctuations, subsidence and the effects of climate change. It is proposed that the Q1 sequence (Fig. 4) originated during the Würm (Weichselian) glaciation (Pleistocene, i.e. isotopic stages 2 to 4). The discontinuity at its top was eroded during the regression (sea level at −120 m) at this last glacial maximum (LGM), ending at 18 ka b.p. The subsequent sea-level rise was followed by the deposition of the Q2 sequence (upper Pleistocene-Holocene). This last sea-level rise occurred in at least two steps, producing (U1 and U2) units corresponding to the transgressive system tract (TST), and both were separated by a hiatus surface. This cycle is interrupted by a smaller (~–45 m) sea-level fall that is correlated (García-Gil et al. 1999b) with the Younger Dryas cold event (11 ka). The next sea-level rise generated another transgressive system tract in the rías (U3). From that time, relative sea level has continued to rise and the next unit (U4) corresponds to the highstand system tract (García-Gil et al. 2000).

Concerning the generation of gas in the Rías Baixas, based on all the data discussed above, it is suggested that the deposition of organic-rich sediments that favour microbial methane generation was largely controlled by relative sea-level variations. Falling relative sea level (about 120 m) during the LGM resulted in a prominent unconformity due to sub-aerial erosion on the shallower continental shelf and within the Rías Baixas (Figs. 4 and 6). Relative sea-level rise from its lowest position (LGM erosive surface) allowed the infilling of the existing incised valleys by the transgressive system tract (seismic units U1 to U3) and early high stand system tract sediments (lower part of U4 unit). These sediments (TST) are characteristically organic-rich due to the high productivity of such areas. They were soon overlain by the finer sediments (muds) of the HST (U4), thus preserving the organic matter. Unit (U4) also acts as seal for the gas.

Worldwide concern for the warming of the planet through the greenhouse effect has highlighted the possibility that gas venting from the sediments beneath the oceans may be making a significant contribution to the atmospheric methane and CO2 concentrations (Dimitrov 2002; Judd et al. 2002). Calculations (Table 1) based on a similar method to that of García-Gil et al. (2002) in the Ría de Vigo, have allowed preliminary estimates in the other three rías (Preliminary results of PGDIT00PXI Project).

Table 1 Some quantitative data from the Rías Baixas and estimated fluxes from seabed to water column and to atmosphere
Full size table

Conclusions

The sedimentary record in the Rías Baixas gives an excellent opportunity to evaluate the sea-level changes during the Quaternary where the major gas accumulations appear. This record also offers important information about the biogeochemical cycles that involve the transformation of sediments and organic matter at different temporal scales (year, decades and geological scales).

Nine types of fluid migration features have been distinguished in the acoustic records from the Rías Baixas: (1) acoustic blankets, (2) domes, (3) diapirs, (4) dark patches, (5) pockmarks, (6) relict collapse depressions, (7) seafloor mounds, (8) acoustic plumes and (9) cloudy turbidity. The presence of voids in cores and vibrocores has been used as semi-indirect evidence of the presence of gas within the sediments.

Several factors controlling gas migration and seepage can be recognized: (1) sedimentary facies, which control the distribution and types of gas reservoirs (gas blankets, domes, diapirs) as well as the gas escapes (e.g. acoustic dark patches, pockmarks, mounds, etc.), and (2) faults that constitute efficient pathways of fluid migration from the lower stratigraphic levels (e.g. diapirs).

Direct gas sampling of bubbles coming from the seafloor in San Simón Bay (i.e. the inner sector of the Ría de Vigo) has been performed for the first time. Analysis of GC-MS confirms that methane is an important component of this gas.

The interrelationship of geological/chemical/biological processes results in complex feedback control for the presence of methane within the Rías Baixas. High productivity has been related to the upwelling events that periodically occur in the Rías Baixas. The observation that methane fluxes can play and important role contributing to both high productivity and diversity in the benthic–planktonic ecology of the Rías Baixas is also presented here.

Estimates of methane fluxes to the hydrosphere and to the atmosphere contribute to a better understanding of the role of gas seeps in the global carbon cycle, including their influence on atmospheric methane and climate change. In the Rías Baixas, the methane fluxes to the atmosphere have special significance in the sense that, due to the shallow water depths (less than 60 m) of these environments, higher percentages of methane reach the water–air interface.