Modern biogenic gas-generated craters (sea-floor “pockmarks”) on the Bering Shelf, Alaska

Nelson, H. Roice; Thor, Devin R.; Sandstrom, Mark W.; Kvenvolden, Keith A.

doi:10.1130/0016-7606(1979)90<1144:mbgcsp>2.0.co;2

Cited by 64 publications

(24 citation statements)

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“…These crater-like depressions in softsediments are now recognised as a common feature across oceans, including the Barents Sea (Harrington 1985), the equatorial West African continental slope (Pilcher & Argent 2007), the Bering Sea (Nelson et al 1979) and the North Sea (Hovland 1984). They range in size from less than 1 m to over 4 km in diameter, with depression depths of 1 m to over 200 m, and occur in densities of up to 1340 km -2 (Nelson et al 1979). They do not exhibit strongly raised rims, and their internal slopes range from 2 to 35°with conical to flat centres.…”

Section: Introductionmentioning

confidence: 97%

Benthic ecology of pockmarks in the Inner Oslofjord, Norway

Webb¹,

Barnes²,

Gray³

2009

Mar. Ecol. Prog. Ser.

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Despite pockmarks being one of the most widespread small-scale topographic features of the seabed, almost nothing is known of their influence on fauna. Here we present the first ecological study of fjordic pockmarks, focusing on the contrast between macrofauna inside and outside of these craters. We report an analysis of macrofauna from 27 pockmarks of the Inner Oslofjord, Norway. Five replicate grab samples were collected from each of 3 pockmarks at 3 sites plus a further control (non-pockmark) sample at each site. A single grab sample was collected from an additional 3 pockmarks at 6 sites and 6 control locations. We compared macrofaunal assemblages inside and outside of pockmarks and found important but subtle differences to those on non-pockmarked substrata. The fauna of pockmarks were typical of a disturbed fjord environment with a dominance of small opportunistic taxa, such as pioneer bivalves and polychaetes. The position of sites in the fjord gradient drove the most obvious faunal differences, but contrary to expectations, we could find no influence of pockmarks on the composition of the fauna at any taxonomic level. This makes them very unusual amongst marine topographic features, which usually have considerable influence on the nature of benthic communities. However, we found that pockmarks do significantly alter the abundances of key species and, as such, we suggest that the presence of pockmarks in the Oslofjord has a considerable cumulative influence on densities and populations of benthic organisms. KEY WORDS: Benthic community · Pockmarks · Oslofjord · Environmental factorsResale or republication not permitted without written consent of the publisher

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Section: Introductionmentioning

confidence: 97%

Benthic ecology of pockmarks in the Inner Oslofjord, Norway

Webb¹,

Barnes²,

Gray³

2009

Mar. Ecol. Prog. Ser.

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“…García-Gil et al, 2002;Maestro et al, 2002), continental shelves (e. g. King and MacLean, 1970;Josenhans et al, 1978;Hovland, 1982;Hovland et al, 1984;Fader, 1991), continental slopes (e.g. Platt, 1977;Nelson et al, 1979;Hovland, 1981;Hovland and Judd, 1988;MacDonald et al, 1990; Baraza and Ercilla, 1996;Dimitrov and Woodside, 2003;Pilcher and Argent, 2007), and abyssal plains (e. g. Cronin et al, 1997;Çifçi et al, 2002). Where the vented liquids and gases have been analyzed, the major constituents are usually methane, and pore water with CO 2 and small amounts of other gases (Vogt et al, 1994), and for this reason these features are important as (1) potential indicators of subsurface petroleum deposits (Hovland and Judd, 1988), (2) offshore geologic hazards (Sills and Wheeler, 1992), (3) contributors to global warming through greenhouse gas (methane) release (Lashof and Ahuja, 1990;Judd and Hovland, 1992), and (4) sources of enhanced biological productivity (Levy and Lee, 1988).…”

Section: Introductionmentioning

confidence: 97%

Occurrence of pockmarks on the Ortegal Spur continental margin, Northwestern Iberian Peninsula

Jané

Maestro

Ercilla

et al. 2010

Marine and Petroleum Geology

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“…Acoustically detected gas-charged sediments have also been documented geochemically (Nelson et al, 1979;Hovland and Sommerville, 1985;Hovland et al, 1987).…”

Section: Gases Stored In Soft Sedimentsmentioning

confidence: 99%

The Formation of Pockmarks and Their Potential Influence on Offshore Construction

Hovland

1987

Doboku Gakkai Ronbunshu

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Pockmarks are craters formed in the soft seabed by gas and in some cases liquid expulsion. They were first described on the Scotian Shelf in 1970 (King and MacLean, 1970) and have since been mapped in a range of shallow seas including the North Sea and the Arabian Gulf. Pockmarks range in size from less than one metre to about 200m across and up to 20m deep.There are assumed to be three phases of pockmark development 1) The pressure build-up phase, whereby gas accumulates and builds up local pressure below the seabed.2) The eruption phase, where by gas, liquids and solids are ejected into the water colum, and 3) The post eruption phase, which can either be a dormant phase or one where gas is continually seeping through the pockmark floor. Triggering of pockmark eruptions are discussed. They may be triggered by earthquakes or seabed pressure perturbations caused by tidal or gravity waves or, in deep water, by a combination of tidal waves and low atmospheric pressure and storm waves.Pockmarks are of concern in relation to seabed construction. Pockmark avoidance and crossing with pipelines is discussed together with methods of artificial gas drainage away from construction sites.

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Modern biogenic gas-generated craters (sea-floor “pockmarks”) on the Bering Shelf, Alaska

Cited by 64 publications

References 0 publications

Benthic ecology of pockmarks in the Inner Oslofjord, Norway

Benthic ecology of pockmarks in the Inner Oslofjord, Norway

Occurrence of pockmarks on the Ortegal Spur continental margin, Northwestern Iberian Peninsula

The Formation of Pockmarks and Their Potential Influence on Offshore Construction

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