Methane and minor oil macro-seep systems — Their complexity and environmental significance
Highlights
► They can alter the topography of the seafloor. ► They introduce allochtonous minerals (including nutrients) to the seafloor and water column. ► Most of the dozen identified environmental effects of seeps are positive for the ecology in the ocean and lakes. ► The most derogatory effect is perhaps added carbon to the atmosphere.
Introduction
The continental margins are unique in that they include the most productive waters of the world and support over 90% of global fish catches (Pauly et al., 2002). They span the globe and range in depth from the photic zone (0 to ~ 200 m) to parts of the aphotic and the ocean mesopelagic zones (200–1000 m). Because of the depth range (0–500 m), the associated hydrocarbon seeps occur both in the photic and aphotic zones, and also within the stability range of methane gas hydrates. When studying the impact of seepage on the continental shelf's seafloor and ecology, it is therefore very difficult to discern between true seep (allochtonous) effects and the normal continental water mass and topographical/current (autochthonous) effects, including up-welling of water masses, on the near-field and far-field biosystems.
In this communication, we describe a number of known environmental effects of seepage that we rate as significant for various reasons. Although the release of methane into the atmosphere, is obviously rated as a negative environmental effect (climate factor), most of the other effects seem to be harmless, or even positive with regard to the local and general marine and lacustrine ecology. The same can also be said about some natural offshore oil seeps. Lately, the seepage of methane and heavier hydrocarbons through the seafloor and through lake beds have achieved increasing scientific focus for various reasons (Niemann et al., 2005, Whelan et al., 2005, Judd and Hovland, 2007, Jensen et al., 2008a, Jensen et al., 2008b, Wegener et al., 2008, Hovland et al., 2010, Plaza-Faverola et al., 2010, Valentine et al., 2010, Wessels et al., 2010, Brothers et al., 2011, Dondurur et al., 2011, Hammer et al., 2011, Krylova et al., 2011, Pape et al., 2011, Redmond and Valentine, 2011, Reitz et al., 2011, Solomon et al., 2011, Batang et al., 2012). One of the reasons is that scientists and the general public are becoming more concerned about the carbon cyclus, in general, and the radiative (greenhouse) gases in particular (Hovland et al., 1993, Kennett et al., 2000, Mienert et al., 2005, Etiope et al., 2008, Westbrook et al., 2009, Canet et al., 2010, Brett et al., 2011). Historically, only specialized branches, such as the marine acoustics industry, military, and fisheries, were concerned with gases and oil emanating from and migrating through the water column (Link, 1952, Landes, 1973, Hovland and Judd, 1988, Brooks et al., 1991, Thrasher et al., 1996, Anderson et al., 1998, Fichler et al., 2005, Dembicki and Samuels, 2007).
Offshore macro-seeps are relatively rare compared to the more common micro-seeps (e.g., Judd and Hovland, 2007). Furthermore, they require special equipment for detailed study, such as remotely and autonomous operated vehicles (ROVs and AUVs), plus aricraft and satellites for oil on the water surface (Thrasher et al., 1996, MacDonald et al., 2002). This means that their characteristics with respect to changes over time, i.e., variation in flux and composition as well as the impact on the local physicochemical environment, are currently poorly sampled and understood. In contrast to micro-seeps, macro-seeps are acoustically and/or visually detected in the water column. Dependent on water depth, they produce streams of bubbles and droplets that rise through the water column towards the surface (Leifer et al., 2004, Judd and Hovland, 2007, Valentine et al., 2010). Within the central and northern North Sea, there are three, fairly well-studied macro-seep locations that have been studied over several years: the Tommeliten seep area (56°29.90′N, 2°59.80′E) (Hovland and Sommerville, 1985, Hovland and Judd, 1988, Niemann et al., 2005, Judd and Hovland, 2007, Wegener et al., 2008, Schneider von Deimling et al., 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, Judd and Hovland, 2007), and the Gullfaks seeps (61°10.1′N, 2°15.8′E) (Hovland and Judd, 1988, Hovland, 2007, Judd and Hovland, 2007, Wegener et al., 2008). Although each of them are located in different geological settings, they have one main aspect in common: – they occur as long-lasting macro-methane seeps, some of which have heavier hydrocarbons associated with them.
Apparently, the first suggestion that such seepage may induce a positive effect on marine biosystems has been that it may cause an increase in the total nutrients and primary production levels in the seawater column and within the seafloor (Hovland et al., 1985a, Hovland et al., 1985b), suggesting that seepage can play a very important role for the North Sea fish stock (Judd and Hovland, 1992). The main basis for this inference was the remarkable find, in 1983, of 38 different meiofaunal species on one 10 kg carbonate sample (methane derived authigenic carbonate, MDAC) at the 25/7 pockmark micro-seep location (Hovland and Judd, 1988, Hovland and Thomsen, 1989). The inference was also founded on observed fish and numerous patches of fluffy, white bacterial mats at locations where reduced fluids seep through the seafloor.
Previous listings of the environmental effects of macro-seepage on the environment have included only a handful of effects (Dando and Hovland, 1992), and is listed as being:
- 1)
Changes in the topography
- 2)
Changes in the physical (i.e., sedimentological and mineralogical) composition of the seafloor
- 3)
Changes in the chemical composition of the seafloor
- 4)
Development of hardgrounds at seeps
- 5)
Changes in species composition due to the seeps.
Since then, several studies have shown how the added carbon source from beneath the seafloor not only feeds into the chemosynthetic community, but also benefits higher trophic animals (e.g., Reilly et al., 1996, MacDonald et al., 2002). This has also been proven at the Håkon Mosby mud volcano (HMMV), where meio- and macro-faunal organisms such as copepods, amphipods, and pycnogonids consumed recycled organic matter from symbiont-hosting and pure chemosynthetic species living directly on sulphur and methane oxidizing bacteria thereby demonstrating methane seep associated lifestyles partly unlinked the photic zone driven carbon flux (Decker and Olu, 2010).
The inference that marine seeps could also affect the atmosphere was realized and discussed by Hovland and Judd (1988), which was later elaborated on (Hovland et al., 1993), and finally more-or-less proved by MacDonald et al. (2002). Based on results from innumerable surveys and studies performed over the last 25 years, the objective of this communication is to highlight how macro-seeps not only impacts the geology, but also the chemistry, acoustics and not least, the biology of ocean and lakes. Furthermore, our aim is to stimulate awareness on these effects and to widen the list of documented significant environmental effects of macro-seeps. Although we will use results from numerous seep locations around the world (e.g., Judd and Hovland, 2007, and later publications), it is the study of the three seep locations in the North Sea, that forms the main basis for our new assessment (Fig. 1). Furthermore, a comparison has been made with results from other macro-seeps, both in shallow and deep water (> 200 m) (Fig. 2). Our analysis concludes with a general holistic conceptual model, illustrating the most important characteristics of macro-seepage. Thus, we list a total of 12 multidisciplinary aspects, suggesting that future seep studies will best be carried out with a high degree of complexity borne in mind.
Section snippets
The Tommeliten seep area
The so-called Tommeliten δ-structure is a 3 km wide, near-circular salt diapir with its summit located about 1000 m below the seafloor (Hovland and Sommerville, 1985). Sub-surface salt domes (salt diapirs) are notoriously leaky geological mega-structures, and are often associated with surface seep manifestations, both terrestrially and in the marine environment (Berryhill, 1987, Hovland and Judd, 1988, Hovland, 1990, Scmuck and Paull, 1993, Thrasher et al., 1996, Heggland, 1998, Taylor et al.,
Other pertinent methane macro-seeps
In this chapter, important new insights from the following regions, including seeps and a blowout will be reviewed, as they add some new factors to our list of significant effects of seeps, in general:
- 1)
The Santa Barbara Seeps (e.g. Hornafius et al., 1999, Kinnaman et al., 2010)
- 2)
The REGAB deep-sea pockmark seeps (e.g. Olu-Le Roy et al., 2007)
- 3)
Lake Baikal (e.g. Leifer et al., 2011)
- 4)
Deep-water GoM seeps (e.g. Solomon et al., 2011)
- 5)
The Deepwater Horizon (Makondo) blowout (e.g. Redmond and Valentine, 2011
Discussion and conclusions
To sum up what the significant effects of methane macro-seeps in the North Sea are, we may turn the question round and ask, what if there were no macro-seeps, such as the ones we have described? This may actually be easier to answer, than just listing our findings again. Without seepage in the Scanner area for example, there would most probably only be a flat, featureless sediment surface, like the one that remains in the area, between all the pockmarks there (Hovland and Judd, 1988, Judd and
Acknowledgements
Statoil ASA is thanked for the release of data. The crews on the ‘Normand Tonjer’, ‘Skandi Ocean’, ‘Lador’, ‘Edda Fonn’, and ‘Acergy Viking’ are thanked for their professional work at the North Sea seep locations. Two anonymous reviewers are thanked for their useful and constructive comments.
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