Characteristics of Marine Methane Macroseeps

1Center for Geobiology, University of Bergen, Bergen, Norway, and Statoil ASA, Stavanger, Norway. E-mail: [email protected].

AbstractThe ebullition of methane through the seafloor (macro-

seepage) is a relatively rare occurrence. Such seepage has been proven to affect the seabed’s sediments, its topogra-phy, its life, and the seawater in various ways and at various scales. From the results of detailed surveys conducted on three distinct and continuous macroseeps in the North Sea (Tommeliten, Scanner, and Gullfaks) over several decades, using a range of tools and scientific disciplines and compar-ing the various results from these macroseeps with those of shallow methane macroseeps elsewhere, we conclude that macroseepage of methane at less than 160-m water depth most probably affects not only the local topography, geo-chemistry, biogeology, and water column but also the re-gion, including the downstream water column, the seafloor surface, and the seawater surface. In addition, some macro-seeps provide methane to the lower atmosphere. We also suspect that some macroseeps give birth to adjacent micro-seepage and therefore represent important geobiological systems that can only be understood properly by long-term studies performed at many scales and by cross-disciplinary scientific methods. Marine methane macroseeps are charac-terized by (1) visual ebullition through seafloor holes; (2) hydroacoustic flares (columnar midwater reflections); (3) ethane concentration anomalies in the water column and ad-jacent sediment porewater; (4) development of visual, bio-logical, and chemical aureoles surrounding the seep location; (5) anomalies (strong gradients) in chemical, temperature, and biological composition of the water column, especially downcurrent (e.g., pH, eH, carbon dioxide [CO2], oxygen [O2], methane [CH4], sulfate, sulfide); (6) topographical ef-fects (mounds, depressions, pockmarks); (7) carbonate ce-mentation of subsurface sediments surrounding the conduit

and adjacent sediments; (8) bacterial mats on the sediment surface adjacent to a seep; (9) upwelling of seawater; (10) downwelling (circulation) of seawater into conduit throats; (11) sea-surface effects, e.g., nutrients coming to the surface because of upwelling; (12) sea-surface slicks and seabirds feeding, downcurrent of the seep; (13) attraction of fish and other macrofauna to the seep; and (14) anomalies in meth-ane concentration in the lower atmosphere above the seep. These effects are listed in order of occurrence, with the most common first (1 and 2) and the less common at the bottom (13 and 14).

IntroductionOver the last decade, the study of fluid seepage through

the seafloor has disclosed the importance of macroseeps for the following reasons:

• Provide strong evidence of a working petroleum sys-tem in the subsurface (Thrasher et al., 1996; Dembicki and Samuels, 2007).

• Affect the local and regional food chains of the water column, sediment surface, and local subseafloor sedi-ments (Dando and Hovland, 1992; Niemann et al., 2005; Judd and Hovland, 2007; Wegener et al., 2008; Valentine et al., 2010).

• Transmit excess harmful gases (carbon dioxide [CO2], methane [CH4], ethane [C2H5], and perhaps hydrogen [H2]) to the lower atmosphere (Hovland et al., 1993; Etiope et al., 2008; Canet et al., 2010).

• Cause temperature and chemical anomalies in shallow sediments, the lower water column, and potentially the lower atmosphere (Dando and Hovland, 1992; Judd and Hovland, 2007).

Chapter 4

Characteristics of Marine Methane Macroseeps

Martin Hovland1

61

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Because offshore macroseeps are rare (compared to the more common microseeps) and because they require spe-cial equipment for study, their characteristics, such as vari-ation in flux and composition over time, and their general importance for the local physicochemical environment are poorly constrained. In contrast to microseeps, macroseeps are acoustically and/or visually detected in the water col-umn. Dependent on water depth, they produce streams of bubbles that rise through the water column toward the sur-face (Leifer et al., 2004; Judd and Hovland, 2007).

Within the central and northern North Sea, there are three fairly well-studied macroseep locations: the Tommel-iten seep area (56°29.90′N, 2°59.80′E) (Hovland and Som-merville, 1985; Hovland and Judd, 1988; Niemann et al., 2005; Judd and Hovland, 2007; Wegener et al., 2008; Schneider von Deimling, 2010), the Scanner pockmark seeps (58°28.5′N, 0°96.7′E) (Hovland and Sommerville, 1985; Hovland and Judd, 1988; Hovland and Thomsen, 1989; Dando and Hovland, 1992; Hovland, 2007), and the Gullfaks seeps (61°10.1′N, 2°15.8′E) (Hovland and Judd, 1988; Judd and Hovland, 2007; Wegener et al., 2008). Al-though each is located in a different geologic setting, they all have one main aspect in common: they occur as continu-ous macromethane seeps. Except for the Scanner seeps, the two others have, over the last 20 years, been studied by multidisciplinary (including microbial) scientific surveys with some interesting and also globally valid results. Be-cause the mean sea level during the last glacial maximum (LGM) was about 120 m lower than at present (Jelgersma, 1979) and because the three seep locations occur at three different water depths, the seafloor sediments at these loca-tions geologically span the transition zones from terrestrial (Tommeliten, at ~75 m water depth [WD]) through inter-tidal (Gullfaks, 130 m WD) to submerged marine (Scanner, 160 m WD) zones.

Despite the growing interest in hydrocarbon seep sys-tems, many factors that modulate subsurface flux are poorly understood. Examples are the advective flux of pore fluid through the subsurface (Tryon et al., 1999), the formation of gas hydrates at seeps located at high latitudes or at water depths greater than approximately 300 m (Suess et al., 1999), the temporal variation of seeps, and the composition and mechanisms of associated anaerobic and aerobic meth-ane-oxidizing communities (Boetius and Suess, 2004). In addition, for a holistic understanding of their importance, one must study macroseeps using not only a multidisci-plinary viewpoint and tools but also at multiple scales, ranging from the regional (kilometers) to the microscale (millimeters and less) and over long, decadal spans. Some of the mentioned macroseeps occurring in shallow water (<200 m WD) may add significant amounts of methane to

the lower atmosphere and may add atmospheric methane, which can affect climatic (Hovland et al., 1993).

This chapter describes the three seep locations — Tom-meliten, Scanner, and Gullfaks — and their physical and geo-logical nature. It also summarizes the most pertinent geo-physical and geobiological results found there (Figure 1). Furthermore, a comparison is made with results from other macroseeps, in shallow and deep water (>200 m). Our analy-sis concludes with a general conceptual model, illustrating the most important characteristics of marine macroseepage and listing 14 pertinent aspects that may characterize them.

The Tommeliten seep areaGeologically, the Tommeliten concession block 1/9 is

located in the Greater Ekofisk and Norwegian salt structure trend of the North Sea (Figures 1 and 2). It is known for its numerous large underground diapiric salt piercement and salt pillow structures (Brewster and Dangerfield, 1984). At the so-called Tommeliten delta structure is a 3-km-wide, near-circular salt diapir, with its summit located about 1000 m below the seafloor (Hovland and Sommerville, 1985) (Figure 2).

Subsurface salt domes (salt diapirs) are notoriously leaky geologic megastructures and are often associated

Figure 1. General map, showing the location of the Gullfaks (G), Scanner (Sc), and Tommeliten (T) macroseeps (black dots) in the North Sea.

62 Hydrocarbon Seepage: From Source to Surface

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with surface seep manifestations on land and on the seafloor (Berryhill, 1987; Hovland and Judd, 1988; Hovland, 1990; Schmuck and Paull, 1993; Thrasher et al., 1996; Taylor et al., 2000; Geletti et al., 2008; Hovland, 2008). During the first geophysical predrilling site survey conducted at Tom-meliten in 1977, water-column anomalies were recorded on seismic and side-scan sonar records. They occur over the center of the buried salt dome in a ring-shaped area (Hov-land and Sommerville, 1985; Hovland and Judd, 1988). Such anomalies are also known as acoustic flares; they are caused by the ebullition of gas and by gas bubbles rising through the water column (Greinert et al., 2006; Judd and Hovland, 2007; Wegener et al., 2008; Schneider von Deim-ling, 2010) (Figure 3).

In 1983, Statoil mobilized a research cruise to perform a first-hand assessment of the seep features at Tommeliten. The vessel, Skandi Ocean, was equipped with towed side-scan sonar combined with a subbottom (high-resolution) profiler, a remotely operated vehicle (ROV) capable of ac-quiring gas samples, and a gravity coring system (Hovland and Judd, 1988; Judd and Hovland, 2007). The seafloor at Tommeliten has a mean depth of 75 m and is generally flat and even. It consists of an approximately 5-cm layer of fine-to-medium quartzite sand overlying stiff clay and marl

2 km

b)

1 s

2 s

3 s

Saltdiapir

GCS

20 km

56.5°N

a)

3°E

N

Norw

ay

U.K.

Figure 2. (a) The Tommeliten seeps (red arrow) in concession block 1/9 occur within the Greater Ekofisk area of the Norwegian sector, North Sea. (b) The Tommeliten delta structure, a subsurface salt diapir, as traced from an interpreted 2D seismic record (based on Hovland and Sommerville, 1985). Notice the gas-charged sediments (GCS) located above the summit of the diapir and the evidence of seep plumes (flares) in the water column above it. Two-way traveltime is given in seconds along the left margin.

a)

8 m

50 m

Seep

b)c)

Figure 3. (a) One of the many bubbling seeps at Tommeliten, as recorded on a penetration echosounder. Note the gas-charged subsurface sediments, which provide a strong backscatter (dark) acoustic signature. (b) A bubbling seep at Tommeliten, seen in ambient light conditions, June 1983 (Hovland and Judd, 1988). (c) A small bioherm located inside an eyed pockmark near the seep depicted on (a) and (b) (from Hovland and Thomsen, 1989).

Chapter 4: Characteristics of Marine Methane Macroseeps 63

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layers. An area of several square kilometers was surveyed with the towed system, and a much smaller area (0.25 km2) was surveyed visually with the ROV. A total of 22 actively bubbling seeps were observed. Extrapolating the whole seep area, about 100 m in diameter, resulted in an estimate that the area contained 120 individual bubbling seeps. The total methane output from this seepage field was estimated to be 24 m3 of methane per day at 75-m water depth (Hov-land and Sommerville, 1985; Hovland et al., 1993). These seeps were all concentrated within an area of about 0.06 km2 (Wegener et al., 2008).

The distribution of acoustic flares over the area has been persistent during all subsequent surveys at Tommeliten, con-ducted in 2001, 2002, and 2005 (Niemann et al., 2005; We-gener et al., 2008). According to the original geophysical site survey of 1977, this seep location, identified visually, is only one of perhaps 10–20 more such seep areas centred above the delta structure. Although only two of these seep areas have been investigated, the other seep areas occur some hundred meters apart, located in an approximately 2-km-wide ring above the summit of the salt diapir (Figure 3).

In 2005 and 2006, hydroacoustic flares at Tommeliten were mapped using a 38-kHz single-beam echosounder (Schneider von Deimling, 2010); a new patch of gas seep-age was identified a couple hundred meters farther north-west. A specially designed acoustic lander was placed for detailed acoustic detection and monitoring of single bubble streams. The objective was to discover if the seeps varied in flux over time. The mechanically scanning acoustic trans-ducer (180 kHz) was placed 3 m over the flat seafloor and scanned a sector of 63° horizontally; it had 21 beams verti-cally, each 3° wide. The sectors were scanned at ranges varying from 13 to 63 m (Schneider von Deimling, 2010). A total of 52 bubble streams were monitored simultaneous-ly with this device over a continuous period of 36 hours.

This unique time series revealed that the bubble streams were not as continuous as previously suggested by visual ob-servations. There was actually a wide range of gas-release patterns, with some very short (only 50-minute-long) seep events. However, most of the gas seeps were active for more than 70% of the observed time. The venting clearly exhibited tidal control, with peak production during the second quarter of the tidal pressure cycle, i.e., when the pressure on the sea-floor drops the fastest (Schneider von Deimling, 2010).

Description of seeps, topography, carbonates, and associated fauna

As can be seen from the first photographs taken with the ROV in ambient light at 75 m water depth in June 1983

(Figure 3b and 3c), the bubble streams emit from single-point holes in the flat seafloor. Each bubbling point source is a couple of centimeters wide (Figure 3). In adjacent regions, they are farther apart. One of the experiments covered the bubbling holes with sand (Hovland and Sommerville, 1985). This procedure halted ebullition for about 1.5 min-utes. The stream reestablished again, slowly at first, with large bubbles and some sand coming to surface; subse-quently, the stream was fully reestablished. This observa-tion means there are distinct conduits in the underlying strata through which gas (and interstitial water) transport takes place (Hovland and Judd, 1988).

The ebullition at Tommeliten is easily detectable with acoustic systems. However, these midwater flares are caused not only by various-sized bubbles but also possibly by high concentrations of dissolved methane in the water. Thus, Niemann et al. (2005) report up to two orders of magnitude higher concentrations (500 nanomoles [nmol]) of methane within the acoustic plumes, compared to the background methane concentration (5 nmol). Other possibilities for the formation of large acoustic flares are discussed later in this chapter.

In general, a consequence of the hydrogen sulfide trans-ported with escaping gas and interstitial water to the seafloor surface from deeper strata is the formation of sul-fur-oxidizing bacteria, notably the filamentous Beggiatoa, Thiothrix, and Thioploca sp. (Dando and Hovland, 1992). They normally occur over patches of the seafloor where the subsurface sediments are charged with reduced gases. They also occur close to gas outlets and on the underside of rocks (often carbonates) brushed by venting gas bubbles (Brooks et al., 1979; Hovland and Thomsen, 1989). These bacteria utilize chemical energy from sulfide oxidation to fix carbon dioxide into organic matter (Nelson et al., 1989). The graz-ing of macrofauna on bacterial mats has been observed (Stein, 1984) and is demonstrated at the Gullfaks seeps (Hovland, 2007).

During several of the ROV surveys at Tommeliten, “fluffy” white particles resembling snowflakes, up to sev-eral centimeters across, were observed floating and drifting in the water column up to 10 m above the seafloor surface. The visibility at Tommeliten is often remarkably high, es-pecially during summertime, because of relatively shallow water (even at 75 m, daylight penetrates to the bottom) and very few suspension sediments (the seafloor is covered by quartzite sand without mud). The drifting “snowflakes” may actually be bacterial flocks, originating from the abun-dant seafloor mats. They may also form in the water column as a result of the high concentrations of methane. However, the latter possibility seems very unlikely because the mats on the seafloor most probably represent the Beggiatoa sp.


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and are easily thrown up into the water column by ebullition (Niemann et al., 2005; Wegener et al., 2008).

As discussed by Hovland (2002), bacterial mats form-ing on the seafloor can effectively seal up the seep location and prevent ebullition for long periods of time. A short sur-vey in 2001 documented this opinion when the ROV’s ma-nipulator arm disturbed small (typically 0.5-m-wide) mats, resulting in relatively large (up to 5-cm-wide) bubbles issu-ing from below the mats. Therefore, the establishment of a bacterial cover over a seep location is suspected to repre-sent the first step in a methane-derived authigenic carbonate (MDAC). This is because their formation requires a certain microenvironment and is otherwise difficult to establish un-less there is a stagnant pool of anoxic gas in contact with the subsurface sediments (Hovland, 2002). Niemann et al. (2005) confirm there is rarely any seepage of gas from bac-terial mats at Tommeliten.

Although the first visual inspection and sampling cam-paign at Tommeliten was conducted in 1983, it was not until a survey in 2001 that carbonate crusts were discovered there, despite the previous reporting of bioherms and eyed pock-marks (Hovland and Sommerville, 1985; Hovland and Judd, 1988; Hovland and Thomsen, 1989) at the seep location. In 2001, the ROV’s manipulator arm stabbed into the base of one of the bioherms and the center of an eyed pockmark. The surface was clearly hard and consisted of MDAC (Hov-land et al., 1985b; Judd and Hovland, 2007). Niemann et al. (2005) report that the carbonate crusts range in size from decimeters to meters, and they are densely covered by sev-eral species of anthozoa and other sessile macrofauna typi-cal of hardgrounds. Hovland and Thomsen (1989) also dis-cuss the higher abundance of sessile invertebrates that live inside the eyed pockmarks and on the bioherms, compared to the surrounding seafloor (Figure 3). The animals living at these locations are markedly different from those living on the surrounding areas characterized by sandy sediments. Niemann et al. (2005) report a higher density of demersal fish in the vicinity of the crusts and bacterial mats than in areas away from the seep-related features at Tommeliten. It is thus concluded that the macroseeps attract fish.

Geochemistry, microbiology, and gas origin

Interesting geochemical results were acquired during sediment sampling and analysis of sediments taken at on- and off-seep locations. Here, the results from core stations 1904 (off seep) and 1860 (on seep), reported by Niemann et al. (2005), cores are reviewed. Table 1 shows the sedi-ment layers cored at the two locations.

In Table 1, the on-seep sediments have a thinner gas-charged silt/clay layer than the off-seep core due to updip-ping layers (doming) at the seep site. The geochemical re-sults show a distinct zoning of the anaerobic methane consumption processes and associated sulfide production in the cores. In core 1904 (off seep), the methane content is low throughout (<0.1 µmol). However, in core 1860 (on seep), methane concentrations reach supersaturation in the silty clay zone and in the MDAC-bearing sediment layer, where the methane had a concentration of 2.5 mmol. In the stiff marl zone, the methane concentration is low (<0.2 mmol), on seep and off seep. This means the horizon-tal distribution of gas (i.e., advection) is only possible in the permeable silty clay layer. Transport through the stiff marl therefore only occurs through distinct conduits, such as near-vertical cracks and fissures.

Although the governing biochemistry of anoxic oxida-tion of methane (AOM) is still not fully understood, we know it occurs with sulfate as the terminal electron acceptor and that it is the dominant biogeochemical process in gassy sediments. Its net reaction can be described according to equation 1 (Hinrichs and Boetius, 2002):

CH SO HCO HS H O4 342

2+ → + +− − − (1)

At Tommeliten, the AOM zone is defined as the zone where there is a distinct dip in methane and sulfate concen-trations as well as a peak in sulfide concentration. This oc-curred very near the surface, i.e., in zone 3, the sand and MDAC-bearing layer and the overlying sand. In contrast to a typical sulfate-methane transition zone (SMTZ), sulfate was not depleted in the methane-rich silt/clay zone 2, possibly because of advective influx of sulfate-rich seawater into the sediment, driven by ebullition and permeable gas/porewater transport (Tryon et al., 1999; Niemann et al., 2005). The sul-fate concentrations declined slightly in the AOM zone, from 28-mmol seawater values to 23-mmol off seep (1904) and 15-mmol on seep (1860). In both cores, there was a distinct sulfide peak just above the horizon in which methane declined

Table 1. Sediment zones in one off-seep core (1904) and one on-seep core (1860). Depths are in centimeters below the seafloor.

Zone Sediment Core 1904 (cm below seafloor)

Core 1860 (cm below seafloor)

3 Sand/MDAC 10–40 10–40

2 Gassy silt 240–175 200–125

1 Stiff marl 350–240 300–200


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with concentrations of 3.1 mmol off seep (1904, at 135 cm below seafloor [bsf]) and 2.1 mmol on seep (1860, at 55 cm bsf), respectively (Niemann et al., 2005).

The MDACs at Tommeliten contained 13C-depleted ar-chaeal lipids, indicating previous gas seepage and AOM ac-tivity. There were high amounts of sn-2 hydroxyarchaeol relative to archaeol and low abundances of biphytanes in the crusts, evidencing that anaerobic methane-oxidizing ar-chaea (ANME) of the phylogenetic cluster ANME-2 were the potential mediators of AOM at the time of carbonate formation.

A relatively high diversity of sulfate-reducing bacteria was found at Tommeliten (Wegener et al., 2008). Three se-quences of the uncultured seep SRB1 (ANME-2 partners) and 29 sequences of Desulfobulbus, known as the bacterial partner of ANME-3 (documented at the Haakon Mosby mud volcano in the Barents Sea, Niemann et al., 2006), were found. Single ANME-3 cells and a few aggregates oc-curred in the Tommeliten seep sediments, although their abundance was too low to analyze for potential bacterial partners. In addition, three sequences of relatives of Desul-fobacterium anilni were retrieved. These oxidize different aromatic hydrocarbons, such as naphthalene or xylene, and could have a function in hydrocarbon degeneration at the site. In addition, eighteen sequences related to Thioalkali-vibrio and Thioploca were found at Tommeliten (Wegener et al., 2008). These findings could probably be linked to the fact that the seeping gases at Tommeliten consist not only of methane but also small amounts of ethane, propane, butane, and pentane (Hovland and Sommerville, 1985; Hovland and Judd, 1988). Thus, the gases originate deep within the sediment column, from the thermogenic zone. This origin is confirmed by the stable carbon isotope val-ues of the bubbling methane at Tommeliten. Thus, occlud-ed methane from shallow-surface sediment samples has a stable isotope range, varying from −26.7‰ to −47.7‰ Pee Dee Belemnite (PDB), whereas the sampled bubbling free gas only ranges from −45.1‰ to −45.6‰ PDB (Hovland and Sommerville, 1985), which is typical for thermogenic methane.

The study by Niemann et al. (2005) concludes that all upward-migrating methane is consumed within the sedi-ments, except for a few locations with active ebullition, i.e., venting to the water column. This occurs at the crest of marl domes, where the free gas circumvents (bypasses) microbi-al consumption because of an effective subsurface conduit system through the marls and upper sandy sediments. So far, only two out of 10–20 seep fields at Tommeliten have been studied in detail. Therefore, the area is potentially a much larger producer of macroseeped methane than has been realized and lends itself to further investigation.

The Scanner pockmark macroseepsGeologically, the Scanner active pockmark field occurs

in the Witch Grund area of the North Sea, not far from the giant Forties oil and gas field. It is located at 150–170-m water depths and has a marine and glacial marine (soft clay) setting (Figures 4–6). There is an up to 25-m-thick succes-sion of marine sediments on top of an underlying subglacial till (boulder clay) (Hovland and Judd, 1988; Judd and Hov-land, 2007).The seepage in this area was first discovered during a routine drilling-hazard site survey conducted with the vessel Geo Scanner. The geophysical data collected dur-ing the site survey were released in a commercial publica-tion by Geoteam in 1984 (Hovland and Sommerville,

1E

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Figure 4. The actively seeping Scanner pockmarks occur in the Witch Ground Basin, seen in this geomorphology map to have a relatively thick (25-m) deposit of Weichselian marine and glaciomarine soft sediments, which are prone to pock-mark formation. The outer perimeter of these soft sediments is shown by the thick blue line. The highest density of pock-marks is shown by a high density of circles (inner zone of the basin) (right). The yellow, stippled line is the inferred location of a relict LGM beachline (Hovland and Dukefoss, 1981).

DIFF

GCL

AS

200 m100 ms

Figure 5. A sparker (1-kHz) seismic section across the Scan-ner pockmark (modified from Hovland and Sommerville, 1985). Clear indications of free-gas migration and accumula-tion are seen below the active pockmark: DIFF = diffractions (caused by strong local impedance contrasts in the sediments), GCL = gas-capping layer, AS = acoustic shadow.


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1985). Subsequent seafloor mapping in the area surround-ing the 900-m-long, 450-m-wide, 22-m-deep active Scan-ner pockmark has disclosed the presence of several other large active pockmarks. The two largest are Scanner and Scotia (Judd and Hovland, 2007).

Description of seeps

The seeps were first noticed as acoustic flares on hull-mounted echosounder and towed side-scan sonar data. Dur-ing an ROV-based survey conducted by Statoil in 1985, they were visually localized with the vessel Lador and ROV Solo. However, compared to the acoustic flares, the bubble streams issuing from the Scanner pockmark were disap-pointingly small and feeble (Figure 6). Only three bubble streams were found inside the pockmark; one of them, adja-cent to a protruding MDAC block, was sampled (Figure 7). The maximum gas production (by bubble streams) was esti-mated to be 1 m3/day from the Scanner pockmark (Hovland and Sommerville, 1985).

During one ROV survey line across the active Scanner pockmark in 1985, the Solo ran at a constant depth of about 130 m across the active pockmark (Figure 6). During this run, the ROV-mounted side-scan sonar recorded some dif-fuse noise on both sides of the vehicle that looked like small parcels of water with contrasting density or acoustic reflec-tivity (caused by change of impedance). The Lador followed the Solo, running the hull-mounted 38-kHz echosounder. The Lador’s echosounder recording shows the ROV beneath the vessel but also a strong, large acoustic flare centered on the pockmark (Figure 6). During the horizontal survey tran-sect through the water above the pockmark, no bubbles were seen on the video screen in the water column in front of the ROV. Also, Wegener et al. (2008) observed a relatively large acoustic flare over the Scanner pockmark, which reached to about 80 m below the sea surface.

To explain the apparent mismatch between feeble gas seepage and the large hydroacoustic plume, we suggest the plume (flare) is caused not only by rising bubbles but also by high concentrations of methane and/or hydrogen. One other possibility has been suggested: Ebullition and gas bubble clouds rising through the water column create a weak sound, i.e., noise that could be picked up as flares or weak reflections by echosounder and side-scan sonar trans-ducers. Either of these suggestions may be likely, but only careful acoustic and chemical studies will be able to deter-mine their validity.

Carbonates and fauna

The Scanner pockmark consists of a dense series or cluster of unit pockmarks (Judd and Hovland, 2007; Hov-land et al., 2010), which becomes evident when the ROV moves from the outside over the outer rim of the pockmark.

100 m

10 ms

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ROVtrack

Figure 6. A unique single-beam echosounder record acquired over the Scanner pockmark by the vessel Lador as the ROV Solo surveyed the pockmark at a constant depth below surface (~130 m) (see text for details).

Figure 7. Images from the bottom of the Scanner pockmark. (a) Note the rugged seafloor with MDAC rocks and two fish (wolf fish on the left, red fish in the middle). (b) The white funnel on the left is used to sample a relatively weak bubbling seep, located adjacent to the large MDAC seen to the right. The rock is clearly suspended by a central pillar and has been partly eroded from below by the action of seepage. Thick sed-iments located on top of this rock clearly demonstrate heavy sedimentation into the pockmark.


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The landscape is undulating as the ROV descends down the gentle slope to the pockmark bottom, 22 m below the surrounding seafloor. Because the seafloor consists of soft clay (mud), the ROV operation calls for careful naviga-tion. Too much use of ROV-thruster energy renders the seawater murky, with a visibility of less than 50 cm. The only visible marine life on the descent, across the undulat-ing landscape, consist of sea anemones, occurring in den-sity ranging from about 1 per 10 m to several per meter, and a few fish gliding by (Sebastes sp. and Brosme sp.). The life at the bottom is different; there, carbonate crusts, normally associated with fish, sponges, and sea anemones (Figure 7), exist. The wolf fish is also common at the bot-tom of the pockmark.

Dando et al. (1991) have studied the occurrence of bi-valves inside the Scanner pockmark, finding the thyasirid bivalve Thyasira sarsi, confined to hydrocarbon-enriched sites in the North Sea. They also have found the dead bi-valve Lucinoma borealis in carbonate-cemented aggre-gates inside the pockmark. No such shells have been found in control samples from the seafloor outside the pockmark. At least one of these bivalves is dependent upon sulfidic sediments associated with methane seeps (Dando and Hovland, 1992).

There is abundant occurrence of carbonate crusts and rubble inside the Scanner pockmark — not surprising be-cause the pockmark, according to detailed studies of sedi-mentation, has been active over the last 13,000 years and had its most prolific (erosive) period immediately after de-glaciation (Fichler et al., 2005). The ebullition activity has clearly eroded the fine-grained sediments from portions of these rocks, so they are partly exposed (Figure 5). They vary in shape and size from small, often round rocks formed by very fine-grained (muddy) claystone to large meter-sized slabs of coarser-grained material and shell hash (Hovland and Judd, 1988) (Figure 7).

The Gullfaks seep areaThe large, concrete, gravity-based Gullfaks A platform

is placed on the North Sea Plateau (110 m water depth). The plateau was subaerially exposed during the LGM low-stand sea level (Figure 8). The other two large concrete plat-forms, Gullfaks B and C, are placed farther to the east, with Gullfaks B located on an ancient beach (120–190 m) depos-ited during the LGM (Dekko and Rokoengen, 1978; Hov-land and Dukefoss, 1981). The largest of the platforms, Gullfaks C, is even farther east, at a water depth of 220 m, on the gentle slope where pockmarks occur below the beach (Judd and Hovland, 2007).

Geologic setting and gas migration

The Gullfaks Jurassic hydrocarbon reservoirs are charged by hydrocarbons generated in the Oseberg Kitchen to the south, filling and spilling from one field to another. Gullfaks is the last in this succession and is therefore fully charged and slightly overpressured. However, the seal is leaky (Karlsson, 1986), and the Gullfaks area has a long history of hazardous shallow gas accumulations and seepage (Figures 9–11). Thus, in 1980, exploration well 34/10-10 at

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Figure 8. A sketch representing a geoprofile across the northern North Sea, across the (left) Gullfaks and (center) Troll fields. The two fields are illustrated by their concrete gravity base platforms, placed in the 1980s and 1990s. The Troll A platform is still the largest man-made structure to be moved. Crystalline mainland Norway is seen on the right. The arrows indicate migration routes for light hydrocarbons originating from deeper reservoirs and source rocks. Intermediate gas storage occurs in shallow gas reservoirs as indicated.

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Figure 9. A 1-kHz sparker seismic line across the relict beach and down the upper portion of the western slope leading down to the Norwegian trough. On the plateau to the left, there is sand and gravel on the seafloor; the sediments become finer and finer to the right (east). The pockmarks start about 2 km from the beach, which has updipping bedding. Signs of shal-low gas are seen on this record: acoustic turbidity, bright spots, including high-amplitude/low-frequency reflections, and acoustic shadows. See also Judd and Hovland (2007, p. 38).


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Figure 10. At the Gullfaks field in the northern North Sea are three large concrete platforms: A, B, and C. Although they are not shown in this block diagram, their locations are indicated on the seafloor. Between locations A and B is an elongated area of seep-age. The bubbles rise through the water column and make a hydroacoustic plume (flare). Notice the ancient beach location on the shoulder of the North Sea plateau. Water depths are indicated with contours. Seabed pockmarks occur deeper than 190 m, where the seafloor consists of soft, silty clays.


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Gullfaks had a blowout from a reservoir located only 230 m below the seafloor.

Prior to installation of these three large platforms, during the mid-1980s, some of the seepage structures were doc-umented (Hovland and Judd, 1988). They include gas-charged sediments (acoustic turbidity, bright spots, and en-hanced reflectors) seen on shallow seismic records (Figure 9), slabs of MDAC found on the seafloor in their formation positions, and inside pockmarks. Pockmarks only occur at water depths greater than 200 m, i.e., where fine-grained sediments (silty clays) have been deposited in the western slope of the Norwegian trench (Figures 9 and 10). Several macroseeps are associated with the relict beach deposit, the gas following near-surface updipping sediment layers. They were found during a pipeline route survey as spurious vertical noise on multibeam echosunder data.

Analysis of gases taken from borehole samples dem-onstrates that the acoustic turbidity recorded widespread at Gullfaks is caused by gas — primarily methane. The top surface of the acoustic turbidity (the gas front) has a marked topography that is partly independent of the sedi-ment layering (Hovland, 1983; Hovland and Irwin, 1989). Gas, originating from depth, migrates upslope through the surficial sediments of the western slope, periodically or continuously escaping to the seabed pockmarks through vertical zones of columnar disturbance and causing pock-marks downslope of the relict beach at Gullfaks. However, near the very top of the slope, the gas migrates updip

through the coarse (and more permeable) sediments depos-ited near the relict beach zone. Gas also migrates laterally to the beach zone from beneath the plateau to the west where there are no pockmarks. Once reaching the beach zone, the gas migrates to the seabed because of the coarse-ness of the sediments. But there are no pockmarks, pre-sumably because the gas can percolate through the large pore spaces.

During the European Metrol project, this information was shared by Statoil with German researchers, and the Heincke seep area was named after the German research vessel Heincke, which, in 2003 and 2005, conducted exten-sive studies focusing on microbiology at the main seep site.

Description of the Heincke seep area

The gas at the Heincke macroseep location occurs on a flat area measuring 0.1 km2 (Figure 10). It has no topograph-ic expression, probably because the seabed consists only of relatively coarse sand and gravel. Extensive filamentous sul-fur-oxidizing bacteria (Beggiatoa sp.) are found as white mats on the seafloor (see Figure 5 in Hovland [2007]).

The lack of seabed topographic expression of the Hei-ncke seeps is quite extraordinary because most macroseeps have surface relief manifestations, either depressions or carbonate buildups (Hovland and Judd, 1988; Judd and Hovland, 2007). In general, the gas at the Heincke seeps continuously streams out at discrete, single-point bubble lo-cations in the gravelly sediments. Also, here the ROV’s manipulator arm was used to cover up a stream with gravel and sand. Even though the stream stopped bubbling for up to a minute, it reestablished at exactly the same location. This indicates that the surrounding seafloor may be capped, possibly by carbonate-cemented sediments, as indicated in Figure 11a (Hovland, 2002).

The subsurface holes where ebullition occurs are gen-erally invisible on the seabed surface because they are cov-ered by bacterial mat or sandy/gravelly sediments. The Heincke seep is located in a heavily fished area along the shoulder of the North Sea plateau, which is clearly indicat-ed by the trawl-board marks through the bacterial mats (see Figure 4 in Hovland [2007]). The largest bacterial mats cover up to 50-m-wide seafloor patches, where they occur in varying thickness and density. Wegener et al. (2008) re-port that the sediments recovered from the bacterial mat field at Heincke degassed strongly, releasing methane bub-bles. Beneath a sediment layer of only 1–5 cm, the beige color gave way to blackish sediments, i.e., from oxidized surface sediments to reduced conditions with the presence of free sulfide (also indicated by a strong smell of hydrogen

Figure 11. Two block diagrams indicating seep features at Gullfaks. (a) The seafloor at the Heincke seep, indicating pre-cipitated methane-derived authigenic carbonate (MDAC) oc-curring along updipping bedding planes in the ancient beach deposit. (b) MDAC also “grows” inside pockmarks, such as in PM204 located near the Gullfaks C location (see Figure 10 and also Judd and Hovland [2007]).


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sulfide) and iron sulfide precipitation. Ferromagnetic min-erals such as magnetite or greigite were found in the re-duced sediments, indicating a connection between methane and iron geochemistry (Wegener et al., 2008). Because the total organic carbon content was very low in the sampled sediments (only 0.17%), the sulfate reduction is assumed wholly coupled to methane oxidation.

Although no megafauna were observed by Wegener et al. (2008), a few fish and a remarkable observation of trophic bypass were witnessed when two hermit crabs were seen eating and competing over pieces of the Beggiatoa mat disturbed by the ROV’s manipulator arm (see Figure 6 in Hovland [2007]).

Flares at the Heincke seeps

Hydroacoustic flares can be found in an area of about 0.5 km2, where they extend up to 120 m above the seafloor (Wegener et al., 2008). Two hydrocaoustic flares are indicat-ed on Figure 10. Flares from the Heincke seeps have also been published by Wegener et al. (2008), who show images of the parametric shallow seismic Innomar SES1000 system, which combines a high-frequency and a low-frequency pulse. On such systems, the flare appears relatively wide. However, modern seafloor bathymetry surveys rely on multibeam echo-sounding, where erratic (spurious) reflections in the water col-umn are efficiently rooted out by automatic software routines.

After the advent of such systems during the late 1980s, it became much more difficult to locate gas seeps through the seafloor and associated plumes in the water column. However, several suppliers of multibeam echosunder sys-tems now offer software that takes care of reflections within the water column. For example, Figure 12 shows an image

above the Heincke seeps using the Kongsberg EM710 multibeam echosounder, with acoustic reflections from several flares in the water column.

Pockmarks and carbonates at Gullfaks

Numerous pockmarks occur downslope of the beach deposit at Gullfaks (Figure 10).

The nearest pockmarks to the Heincke macromethane seeps occur in water depths of about 190–200 m, farther downslope and to the east (Hovland, 1983) (Figure 10).

Pockmark PM204, occurring about 1 km basinward of the relict beach at the Heincke seep, was investigated visu-ally and sampled during the Lador survey in 1985 (Hovland and Judd, 1988). It is 100 m wide and 120 m long, elongat-ed along slope and along the prevailing bottom-current di-rection (running from north to south). At the southern and deepest end of the pockmark, numerous (up to 20) large fish were found facing northward into the current (see Figure 13). This was so unusual that the ROV Solo was directed toward this location; more fish were found hiding inside the “fish hollow,” a unique 1-m-wide cave. Large blocks of clay oc-cur in the opening of the cave. These are suspected to have fallen down and broken loose from the throat of the cave during gas eruptions. The blocks are full of small holes, suspected to have formed by expanding gas bubbles — per-haps by depressurized porewater (causing free gas to come out of solution) trapped within the clay. On first sight, the blocks were suspected to consist of MDAC, but prodding them with the ROV manipulator arm proved they consisted of relatively soft, cohesive clay. However, about 60 m far-ther north inside the same pockmark, an MDAC nodule of 0.5-m width and some small bacterial mats were found but without active gas venting.

Geochemistry, microbiology, and gas origin

The Heincke seeps

In sediment cores retrieved from the bacterial-mat-cov-ered area at the Heincke seeps, methane oxidation and sul-fate reduction rates were measured by Wegener et al. (2008), using replicate subsamples of bulk sediments. The methane oxidation rate ranged from 0.01 to 0.18 µmol g−1 dry weight/day. The sulfate reduction rates ranged from 0.05 to 0.30 µmol g−1 dry sediment/day. The integrated methane oxidation rates averaged 12.5 mmol m−2/day, and the integrated sulfate reduction rate was 18.5 mmol m−2/day — comparable to measurements at other seep sites such

Figure 12. Gas-flare detection with a modern multibeam echosounder system (Kongsberg EM710 hull mounted). The flare of the Heincke seep is depicted here (see text for details).


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as Hydrate Ridge (Treude et al., 2003) and the Håkon Mos-by mud volcano (Niemann et al., 2006). A total gas flux of 76 tonnes/per year of gas emitted to the water column for the Heincke seep area was estimated; based on this value, Wegener et al. (2008) found that the sediment microbial community consumes at least 16% of the total gas flux. However, because this is a single observation, it is totally unknown how the fluxes and consumptions vary over time.

Wegener et al. (2008) used fatty acids to trace the spe-cific microbial groups at work at the Heincke seeps. They

found a diverse group of sulfate reducers present. The fatty acid sn-2 hydroxyarchaeol was threefold more abundant than archaeol, indicating a dominance of ANME-2 popula-tions. In fact, the Heincke seeps area is the first site at which abundant single cells of ANME-2a and ANME-2c have been detected. In total, the cell numbers in surface sedi-ments were high, with up to 7.9 × 109 single cells per milli-liter sediment in the uppermost 10 cm, which is higher than previously reported for nonseep sandy sediments in the North Sea (Wegener et al., 2008).

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Figure 13. Pockmarks occurring downslope of the relict beach at Gullfaks. Two pockmarks are shown on the lower deep-tow boomer (3.5-kHz) record at the bottom. Pockmark PM204 at the right is imaged above-right as a 3D block diagram. It includes many smaller pockmarks, some of which are unit pockmarks. At the left on the record is pockmark PM222. A crenulated gas-charged sediment front is located about 20 ms (TWT) beneath the seafloor. The gas causes enhanced reflections. The red horizontal bars in the sediments below PM204 indicate the increasing concentration of methane gas with depth (see text). (upper left) Two photos from the fish hollow, the upper showing the approach toward this unique cave. Up to 20 large fish are seen hovering here, facing the bottom current sweeping from the right (north). The lower image depicts the fish hollow, which occurs in the southern (deepest) end of PM204. It is suspected to be a gas-eruption hole that may have periodic eruptions and intermittent microseepage (based on Hovland and Judd, 1988).


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Several questions remain to be answered in relation to seeps such as the Heincke seep. One is an old question (Hov-land et al., 1985a; Hovland and Thomsen, 1989; Dando and Hovland, 1992): “To which extent do visible, not to say dif-fusive and invisible, (micro) seeps of hydrocarbons contrib-ute to the total carbon cycle in the North Sea?” Another im-portant question is “To what extent such seeps contribute to the total atmospheric methane and carbon dioxide content?” (Hovland et al., 1993; Judd et al., 2002). A third question is “Can they be used for general hydrocarbon exploration?” (Thrasher et al., 1996). Although all of these questions have been addressed, only ongoing and future quantitative re-search and more fieldwork can positively contribute to an-swer them.

Pockmark PM204

A 2.3-m gravity core was taken inside the deepest por-tion of pockmark PM204. Analysis of the interstitial (oc-cluded) gases showed that methane was dominant, but the higher hydrocarbon gases (ethane, propane, butane, and pentane) were also present. The methane concentration is highest at 2.3-m depth (10,000 ppm) and decreases toward the seabed (Figure 13). The relative increase in the higher hydrocarbon gases indicates that methane is escaping or, more likely, is quickly utilized by bacteria. This depletion is also reflected in the carbon isotope data; ratios range be-tween −44.3‰ (a thermogenic signature) at 2.3-m depths to the very low value of −90.6‰ (microbial) near the surface. In spite of this very low δC13 value, we conclude that the hydrocarbons originate from deep source rocks and the Gullfaks reservoir.

In contrast to the occluded methane in these pock-mark sediment samples, the adsorbed methane at 1.5 and 2.3 m have a typical thermogenic isotope signature. Re-sults from exploration drilling at Gullfaks have shown a great variation in the δC13 signatures of methane. At reser-voir depth (2890 m below sea level), the δC13 value is −44.3‰ PDB to about −60‰ at 440 m below the seafloor and −73.9‰ PDB about 10 m below the seafloor (Hovland and Irwin, 1989).

Other pertinent methane macroseeps

Shallow water (<200 m)

Along the southern coastline of the Santa Barbara Ba-sin, California, is a long, linear trend of seepage. It runs par-allel to an adjacent megageologic structure, the Murray

transform fault line, that comes ashore in the bay of Los Angeles with the well-known La Brea tar pits (continuous terrestrial crude-oil seeps). At Coal Oil Point, a seep field has been studied intensely because of its good accessibility (Leifer et al., 2010) — a natural laboratory for the study of seep processes. The spatial distribution of the seeps here re-lates to anticlines and faults as well as to fault-damage zones (Hovland and Judd, 1988; Hornafius et al., 1999). The most vigorous seeps seem to be associated with inter-secting faults, critically stressed faults, and fault hanging walls (Leifer et al., 2010).

The Brian seep is located at 10-m water depth in a pe-ripheral area relative to the most vigorous seeps of the area (Hornafius et al., 1999; Kinnaman et al., 2010). Over a sea-floor patch covering approximately 200 m2, there is a re-lease of about 450 moles of CH4 per day, originating from 68 persistent gas vents in addition to 23 intermittent vents. Over a span of 33 repeat surveys with scuba divers conduct-ed over 6 months, eight persistent vents were monitored. The results revealed a substantial temporal variability from each vent. The flux varied by more than a factor of four (Kinnaman et al., 2010).

The carbonate concretions found at Brian Seep consist largely of quartz sand held together by calcite. They form at the interface between the sediment and the underlying bed-rock and are distinctly different from the deepwater MDAC described for the North Sea seeps. At the Brian Seep, there is a seasonal variation in sand cover over the subsurface bedrock. During summer, the bedrock is covered by sand; but during winter, it is exposed. The proposed mechanism of carbonate formation at Brian is that they form during pe-riods of sufficient sediment coverage in which anaerobic oxidation of methane is favored. Thus, the precipitation oc-curs at a sufficient distance from active venting for the mo-lecular and isotopic composition of seep gas to be masked by carbonate alkalinity generated from anaerobic methane oxidation (Kinnaman et al., 2010).

Deep water (>200 m)

Because of the stability envelope of methane hydrates, it was long thought impossible for methane bubbles to form in water deeper than about 300 m until Mereweather et al. (1985) reported bubbles of oil or hydrate-coated methane bubbles rising at a speed of 12–18 cm/s from 2000-m water depths in the Guaymas Basin, Gulf of California. Clennell et al. (1999) explain why this can occur; it has to do with the depletion of water (drying out) of subsurface conduits and the continuous supply of new methane from a deep (thermogenic or basinal) source of methane.


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The REGAB site, off West Africa

One of the world’s deepest locations of gas ebullition to have been studied in detail is the REGAB site, located at 3160-m water depth on the Congo-Angola, West Africa, continental margin (Ondréas et al., 2005; Olu-Le Roy et al., 2007). Inside a giant 800-m-wide pockmark (amalgamated from numerous smaller ones) are exposed gas hydrates and prolific methane ebullition from the midportion of the pock-mark. The researchers found a surprisingly extensive spa-tial heterogeneity of microbes and fauna within the active pockmark and determined the distribution of three symbi-ont-bearing species: mytilidae and vesicomyidae bivalves and large Siboglinidae polychaetes.

Most live seep fauna were found along the southwest–northeast axis of the pockmark, which crosses the central section, where gas hydrates, bacterial mats, and MDAC buildups were discovered. The ebullition of methane was strongest from mussel beds and adjacent to tubeworm clus-ters: “The ‘active’ section of the pockmark consisted of large Escarpia southwardae bushes, beds of modiolid mus-sels (Bathymodiolus sp.), and clusters of vesicomyid clams. E. southwardae bushes formed a dense cover (up to 20 m by 30 m) and were generally associated with small concre-tions or carbonate pavements. Bathymodiolus sp. beds cov-ering one to several square meters (up to 50 m by 60 m) were observed in small depressions, with or without visible concretions” (Olu-Le Roy et al., 2007).

In areas with ebullition, the oxygenated zone in surfi-cial sediments was very thin (1.5–3.5 mm). Methane con-centrations near chemosynthetic taxa were the unique vari-able among those tested, which explains why the densities of species at the sampling sites and methane levels de-creased from the center to the outside of vesicomyid clus-ters and generally increased with vesicomyid density. None of the bubbling methane from the REGAB pockmark is ex-pected to have made its way up to the surface of the ocean because of the depth of the seafloor.

Seeps in the Gulf of Mexico (GOM)

In strong contrast to expectations, Solomon et al. (2009) have found considerable methane fluxes to the at-mosphere originating from deepwater (550–600 m) hydro-carbon seeps in the GOM. They show that bubble size, up-welling flows, and the presence of surfactants (oil) inhibited bubble dissolution, and methane oxidation was negligible. Using ROVs, they find several other surprising aspects:

1) Bottom waters near seeps (bubble plumes) have CH4 concentrations ranging from 124–3660 nmol.

2) The CH4 concentration in the plumes decreases by 80%–99% from seafloor to 350-m water depth and is more-or-less constant between 350 m and 80 m but then increases up to 1609 nmol near the sea surface.

3) Using an open-system oxidation model, they demon-strate only 0%–37% of the input flux is oxidated.

4) The plumes have upward transport velocities of 10–50 m/s, which are possible only if they are mediated by high bubble fluxes and a broad bubble-size distribu-tion, extending to large, commonly oil-coated bubbles.

5) “As a result of the sharp and large density difference between plume fluids and ambient sea water at this depth, the plume cannot support the deeper fluid, caus-ing detrainment of plume fluids enriched in aqueous CH4 into the base of the mixed layer. This causes a build-up of CH4 concentrations as the CH4 flux into the mixed layer is higher than the air-sea flux” (Solomon et al., 2009).

Discussion

Significance of pockmarks

The increased use of high-resolution multibeam sys-tems for seafloor mapping has led not only to pockmarks being recognized and mapped worldwide but also to the distinction between various types of pockmarks (Judd and Hovland, 2007; Hovland et al., 2010; Pinet et al., 2010; Weibull et al., 2010). Even though it is very rare to find pockmarks directly associated with macroseepage, as in the Scanner and REGAB examples described here, the pock-marks — and especially the smallest ones, the unit pock-marks — represent foci of ongoing active or fossil (now relict) seepage (Figure 14).

The pockmarks are always associated with fluid flow in some way, wherein the fluids (gas and/or liquids) originate from any depth in the subsurface (Judd and Hovland, 2007). Even though the first to discover and name pockmarks, Lew King and Brian MacLean (1970) suggest them to be related solely to hydrocarbon-prone areas. The occurrence of pock-marks in areas underlain by metamorphic basement rocks (Pinet et al., 2010) clearly demonstrates that thermogenic fluids and derivatives (biogenic methane) are not the only fluids responsible for these morphological features (Kelley et al., 1994). Any fluid can be responsible for pockmarks, ranging from groundwater to deeply sourced CO2, CH4, or locally sourced fluids of biogenic origin associated with the degradation of recently buried, organic-rich material.

Pinet et al. (2010) demonstrate that a nonrandom (lin-ear-train) distribution of pockmarks can represent a key


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feature to interpret the origin of fluids and the mechanism responsible for fluid migration. They point out the impor-tance of interpreting pockmarks and other surface and near-surface migration manifestations at the sedimentary-basin scale because it may help to develop conceptual models ex-plaining the creation and maintenance of migration path-ways within the basin.

Carbonate constructions

MDAC nodules, crusts, and layers occur as important manifestations of micro- and macroseepage. Thus, they oc-cur inside pockmarks and on mud mounds, and they are as-sociated with mud volcanoes (Judd and Hovland, 2007; Weibull et al., 2010). They are formed at the surface, often above columnar zones of acoustically attenuated and dis-rupted reflections, so-called acoustic chimneys or pipes (Bünz and Mienert, 2004; Ligtenberg, 2005; Mazzini et al., 2006; Solomon et al., 2009). Because they often cause great

impedance contrast, they are often recorded as strong (en-hanced) reflections and can be used to identify fluid-migra-tion pathways in sedimentary basins. The distributon of MDACs can be crucial for understanding the spatial distri-bution of fluid seeps at the seabed as well as for discovering a potential source of origin of the leaking fluids (Cartwright et al., 2007; Weibull et al., 2010).

Summary of characteristic aspectsTo conclude this study, I identify and summarize the 14

most characteristic aspects of marine macroseeps identified so far (Figure 15). Future work with macroseeps worldwide will probably find even more aspects caused by seeps.

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Figure 14. Most pockmarks, at least in the North Sea, are not associated with continuous ebullition (macroseepage). The lower image shows a normal pockmark located about 100 km south of Gullfaks. It overlies an area of gas-charged sediment, seen here as enhanced reflections (ER). The shallowest patch of ER lies directly beneath the deepest portion of the pockmark and is interpreted to represent a shallow reservoir of gas that intermittently drains to the water column as ephemeral bubble streams (upper image). Notice also the lack of acoustically layered sediments on the right side of the pockmark, indicating that it is infilling in this part. This also means the pockmark is migrating toward the left, where the periodic ebullition and erosion are inferred to occur. GR = gas reservoir; GL = gas layer.

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Figure 15. A conceptual drawing showing the characteristics of methane macroseeps: (1) bubble streams (ebullition), (2) hydroacoustic flare, (3) relatively high CH4 levels in water and porewater, (4) visual and chemical aureole, (5) chemical and temperature anomalies, (6) topographical effects, (7) MDAC development, (8) (same location as 1 and 7) bacterial mats and blooms, (9) upwelling seawater, (10) sea-surface slicks, (11) attraction of fish and other macrofauna, (12) nutrients on surface/birds feeding. Not all of these effects occur at all methane macroseeps (based on Hovland et al., 1984).


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(1) Visual ebullition through seafloor holes. — This is the hallmark of a marine methane macroseep and as such is a prerogative for naming it a macroseep. If ebullition is not seen over a certain timespan (years), it is not a macroseep. However, when macroseeps expend their reservoir of sub-surface gas, whatever way it may have formed, they may turn into microseeps.

(2) Hydroacoustic flares. — If bubbles are emitting from the sea- or lake floor, then there will also be acous-tically detectable columnar midwater reflections. This is because the impedance contrast between gas and water is so great that the reflection will be strong at most seismic frequencies except low ones, where the wavelength is too large for bubble detection. There may be one exception from the rule of rising bubbles: when methane bubbles from a macroseep occur at depths and temperatures well within the stability envelope of methane hydrates. If the seep is feeble and the small bubbles are coated with hy-drates, then they may not rise very high (tens of meters?) from the seafloor before they drift horizontally and dissolve into the surrounding water.

(3) Methane concentration anomalies. — Because methane bubbling through the sea- or lake floor is at sat-uration level, it normally will be at higher concentrations than the ambient water. The methane will therefore im-mediately be dissolved in the surrounding water and may cause a strong methane concentration gradient, with high-est concentration adjacent to the stream of bubbles, reduc-ing outward in a radial pattern. Because the rising plume of bubbles is influenced by currents, this methane concentra-tion anomaly will be highest downcurrent. Within the sub-surface sediments, the same will occur and there will be a concentration gradient in the porewater surrounding the conduits transporting methane through the sediments. The gradient will be dependent on the porosity and permeability of the sediments.

(4) Visual and chemical aureole. — Surrounding the seep location where the subsurface conduit(s) breaks through to the water column, there is often a visible aureole, a circular zone of influence. It is caused by chemical and/or biological reactions and processes induced by the seeping action and by concentration gradients of various composi-tions transported upward. The aureole is thus a manifesta-tion of seepage.

(5) Chemical, temperature, and/or biological anoma-lies. — Strong gradients in composition, temperature, or biological species in the water column, especially downcur-rent of seeps (e.g., pH, eH, CO2, O2, CH4, sulfate, sulfide, bacteria, archaea) are common aspects of seepage because the transport of gas from deep below the ground can include the transport of heat, water, and other components. Seepage

can therefore manifest itself by chemical, temperature, and biological anomalies in the water column above and in the porewater system below ground.

(6) Topographic effects. — Topographic features such as depressions, craters (pockmarks), and mounds are per-haps the most common manifestations of focused fluid flow (seepage) through the seafloor. The features are caused by local erosion, accretion, or a combination of both (i.e., eyed pockmarks, pockmarks with bioherms and/or MDAC struc-tures).

(7) MDAC development. — The development of MDAC structures, nodules, pinnacles, mounds, crusts, etc., is caused by the inorganic and/or biologically medi-ated aragonite and/or calcite (CaCO3) precipitation at seep location. Precipitation (crystallization) occurs as a conse-quence of the supersaturation of water-dissolved CaCO3, caused by temperature changes, pH changes, and/or other physicochemical changes at seep sites. The process relies on a cryptic microenvironment, isolated from circulating seawater; so the carbonate cementation mostly occurs in the subsurface sediments surrounding the conduit (seepage feeder channel).

(8) Bacterial mats. — As a consequence of strong chemical gradients at seep locations (e.g., reduced ver-sus oxic fluids), bacteria flourish at such sites. The most common bacterium found at marine methane seep sites the world over is Beggiatoa sp. It is dependent on the anoxic-oxic seawater boundary and forms at the oxylim-nion, producing white natural sulfur within its fibrous cells. This and many other types of bacteria can produce thick mats on the seafloor. These mats can be torn and suspended into the water column, where they can form organic “snow.”

(9) Upwelling of seawater. — When bubbles rise by the action of buoyancy through the water column, they cause turbulence in their wake. If ebullition of gas is significant, this turbulence is strong enough to draw water from the surrounding seawater column into the upward-rising gas bubble stream. Thus, an upwelling of bottom water may occur, which again gives rise to temperature and chemical anomalies in the water column.

(10) Downwelling (circulation) of seawater into the ground. — Because conduits leading gas bubbles to the seabed cause hydraulic action (pressure pulses) within the subsurface conduit system, negative pressure gradients will cause seawater to entrain into the ground. This down-welling of seawater into the porewater system at seep loca-tions may cause chemical and temperature anomalies in the subsurface microenvironment.

(11) Sea-surface effects. — The action of methane mac-roseepage may also be detectable on the sea surface, mainly


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because of the upwelling effect and by the consequential transport of temperature, chemicals, and nutrients (bacte-ria and other organisms/organic particles). Seeps can also cause disturbances in the surface capillary wave patterns due to changes in surface currents surrounding the upwell-ing area.

(12) Sea-surface slicks and seabirds feeding. — Down-current of the seepage and upwelling locations, there may be slicks (water devoid of capillary waves) due to the en-trainment of oil on rising bubbles. In addition, there may be birds feeding on organic particles carried to the surface by the upwelling. Thus, slicks and feeding seabirds may repre-sent manifestations of seeps.

(13) Attraction of fish and other macrofauna. — Be-cause seeps may disturb the ambient layering of nutrients and organisms in the water column, seepage may attract fish from other locations. These effects may also cause the development of sessile colonies of filter feeders and other invertebrate organisms (bioherms) downstream of the seep locations. Thus, the hydraulic theory for deepwater corals is explained by this process (mainly from microseeps; Hov-land, 2008).

(14) Anomalies in methane concentration in the lower atmosphere. — On some occasions, the seafloor seepage of methane provides higher concentrations of methane in the near-surface seawater. Any such anomalous seawater con-centration will cause the entrainment of methane into the lower atmosphere. Thus, methane concentration anomalies in the lower atmosphere may be regarded as manifestations of submarine seepage.

This 14-point summary of the characteristics of marine methane macroseeps provides a preliminary list of what to look for when searching for seeps. As more research is done in the future, especially with respect to temporal variation of flux and the effects on the surrounding biological sys-tems, more items may be added. However, remember that not all of these elements occur at all seeps. This is what makes seepage hunting so stimulating and interesting. There may ac-tually be some seeps that are manifested by only one or two of these items.

AcknowledgmentsStatoil ASA is thanked for the release of data. The

crews on the Normand Tonjer, Scandi Ocean, Lador, Edda Fonn, and Acergy Viking are thanked for their professional work at the North Sea seep locations over a period of nearly 30 years. Portions of this study were part of the Metrol proj-ect of the fifth framework program of the European Com-mission (see www.metrol.org).

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Characteristics of Marine Methane Macroseeps