Methane seeps in the ocean are much more important to life in the ocean than we previously have suspected.
Sketch showing the effects of methane (and light oil) seeps in the ocean.
In this sketch, the following legend applies: 1) ebullition of bubbles, 2) hydroacoustic flares, 3) methane concentration anomalies, 4) aureoles (visual, chemical, mineralogical, biological), 5) topographical effects, 6) MDAC development, 7) bacterial mats, 8) upwelling seawater, 9) downwelling water / entrainment, 10) slicks and nutrients on surface /birds feeding, 11) attraction of fish and other macro-fauna, 12) methane anomalies in atmosphere. Note, that not all of these effects occur at all methane macro-seeps that have been described so far.
Discussion and conclusions With numerous macro seeps and abundant gas-charged sediments and an apparently reliable and steady flux of methane and other nutrients to the water column, there are the following beneficial (significant) effects: 1) A rugged terrain, which undoubtedly leads to induced turbulence in the bottom currents, 2) Hard-rock surfaces with many nooks and crannies, for benthic organisms to utilize for attachment and fish and other organisms to utilize for shelter 3) Cryptic micro-environments for many types of microorganisms to utilize for primary production 4) Flow of allochtonous chemicals, some of which will act as ‘fertilizers’ for primary producers 5) Plenty of authigenically precipitated (inorganic) carbonate (calcite and aragonite) for boring organisms to utilize and extract.
There are still several questions remaining to be answered in relation to seeps like the Heincke seep. One is an old question: to which extent does visible, not to say diffusive and invisible (micro) seeps of hydrocarbons contribute to the total carbon cycle in the regional area? Another important 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 if the seeps can be used for general hydrocarbon exploration (Thrasher et al., 1996). Although all of these questions have been addressed before, it is only ongoing and future quantitative and holistic research and more fieldwork that can positively contribute to answer them.
The increased use of high-resolution multibeam systems for seafloor mapping has led, not only to pockmarks being recognized and mapped worldwide, but also to the distinction between various types of pockmarks (Pinet et al., 2010; Judd and Hovland, 2007; Hovland et al., 2010; Weibull et al., 2010). Even though it is very rare to find pockmarks directly associated with macro-seepage, as in the Scanner and REGAB examples, the pockmarks, and especially, the smallest ones, the unit-pockmarks, represent foci of ongoing active micro-seepage. Pockmarks are generally associated with any kind of fluid flow, where 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) suggested them to be solely related to hydrocarbon-prone areas, the occurrence of pockmarks in areas underlain by metamorphic basement rocks (Pinet et al., 2010; Brothers et al., 2011) and hydrothermal activity, clearly demonstrates that thermogenic fluids and derivatives (biogenic methane) are not the only fluids responsible for these morphological features (Kelley et al., 1994). Thus, the fluids responsible for pockmarks may be any fluid, 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.
Most of the significant effects of methane macro-seeps we have described and discussed herein, are, on a large scale, regarded as being positive, or beneficial to the marine and lacustrine environment. This is because the allochtonous input from the substratum may be regarded as causing a general fertilization effect. Furthermore, this fact has also been recognised by the paleontologists, some of which have seeked to find seep-related carbonates in ancient sedimentary rock, as guides for the finding of spectacular animal remains, from large animals that utilized the seep-related organisms (Hammer et al., 2011).
However, there is one great potentially negative effect: the climate response to variations in methane from oceanic and lacustrian seepage (Hovland et al., 1993; Kennett et al., 2000; Westbrook et al., 2009). However, Reeburgh (2007, 2011) and also others, have recently pointed out the extremely efficient way in which AOMs manage to utilize methane advecting (migrating) upwards to the seafloor, before it should enter the water column. He calls it a ‘stealth process’ (Reeburgh, 2011), as it is a process impossible to measure, and he concludes, saying: ”Several recent high-profile climate modeling papers have fallen into the ‘stealth process’ trap by failing to explicitly consider microbial oxidation.” (Reeburgh, 2011, p. 1702).
Although some of the deep-water coral reefs (DWCR) occurring world-wide have been shown to occur adjacent to methane macro-seeps (Hovland, 2008), there is still no firm documentation that they are directly dependent on either macro- or micro-seeps. But, the intensified study of microorganisms in the water and sediments surrounding and beneath such reefs, demonstrates the presence of seep-related microorganisms in their immediate surroundings (Jensen et al., 2008, 2010). They seem to be ‘bathing’in nutrient rich water containing similar primary producers as those observed blooming in the Deepwater Horizon blowout plume, mentioned previously. Thus, based on Reeburgh’s notion of stealth processes, instead of looking for methane oxidizers in the water column, we should be looking for (and we are now finding) microorganisms utilizing other types of complex organic and inorganic substances caused by seepage, such as ethane, propane, etc.
Most studies of the ocean floor are local by nature. Over the last 30 years, therefore, we have learnt much about many different locations, where detailed mapping and investigations have been conducted. However, what we see as lacking, is an integration of this knowledge on a broad scale: – What are the effects of allochtonous (exotic) material seeping up to the seafloor and into the water column, in the long run, and on a broad scale? This may represent a typical ‘emerging science’, as discussed by Brett et al. (2011), where they point to the necessity for clarified definitions within the “allochtonous versus autochthonous framework”, with respect to organism nutrient uptake in the marine and lacustrine environments.
Summary of characteristics of methane macro-seeps To conclude this holistic study of macro-seepage, we list a dozen of the most environmentally significant aspects of marine macro-seeps identified so far (see Fig. 7). It is expected that future work at macro-seeps, world-wide, will find even more aspects that are caused by seeps.
Visual ebullition through seafloor holes This is the hallmark of a marine methane macro-seep, and as such is prerogative, for naming it a macro-seep. This means that if ebullition is not seen over a certain time-span (years) it is not a macro-seep. However, as macro-seeps expend their reservoir of sub-surface gas, whatever way it may have formed, they may turn into micro-seeps.
Hydroacoustic ‘flares’ If there are bubbles emitting from the sea- or lake-floor, then there will also be acoustically detectable columnar mid-water reflections. This is because the impedance contrast between gas and water is so high, that the reflection will be strong at most seismic frequencies, except for low frequencies, where the wave-length is too large for bubble detection. However, there is one exception from this aspect, i.e., when methane bubbles from a macro-seep occur at depths and temperatures that are well within the stability envelope of methane hydrates. If the seep is feeble, and the small bubbles are coated with hydrates, they may not rise very high (tens of metres?) from the seafloor before they drift horizontally and dissolve into the surrounding water. But, for vigorous seeps and large bubbles, hydrate skins tend to enhance bubble survival in the water column.
Methane concentration anomalies Because free methane bubbling through the sea- or lake-floor is at saturation level, it will be at higher concentrations than in the ambient water. The methane will therefore immediately start to dissolve in the surrounding water and may cause a strong methane concentration gradient, with highest concentration adjacent to the stream of bubbles and reducing outwards in a radial aureole pattern. Because the rising plume of bubbles is influenced by currents, this methane concentration anomaly will be highest down-current (Fig. 6). Within the sub-surface sediments, the same will occur, and there will be a concentration gradient in the pore-water surrounding the conduits transporting methane through the sediments. This gradient will be dependent on the porosity and permeability of the sediments.
Aureoles: visual, chemical, temperature, and/or biological anomalies Strong gradients in composition, temperature, or biological species in the water column, especially down-current of seeps (e.g., pH, eH, CO2, O2, CH4, sulphate, sulphide, bacteria, archaea, etc) are common aspects of seepage. Due to the rather deep origin of the gas, heat, water, and other components are transported upwards. Seepage can, therefore, manifest itself by chemical, temperature, and biological anomalies in the water column above, and in the pore-water system below ground. Surrounding the seep-location, where the sub-surface conduit(s) break 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 compositions transported upwards. The aureole is, thus, a manifestation of seepage. Thus, seeps induce spatial heterogeneity in the microbial activity and faunal zonation on the seafloor.
Topographical effects Topographical features, such as depressions, craters (pockmarks), and mounds are perhaps the most common manifestations of focused fluid flow (seepage) through the seafloor. The features are caused either by local erosion, accretion, or a combination of both (i.e., ‘eyed pockmarks’, pockmarks with bioherms and/or MDAC structures).
MDAC development The development of MDAC structures, nodules, pipes, pinnacles, mounds, crusts, etc., is caused either by the inorganic and/or biologically mediated aragonite and/or calcite (CaCO3) precipitation at seep locations. Precipitation (crystallization) occurs as a consequence of the super-saturation of water-dissolved CaCO3, caused by temperature changes, pH-changes, AOM-activityand/or other physicochemical changes at seep sites. As the process relies on a cryptic micro-environment, isolated from circulating seawater, the carbonate cementation mostly occurs in the sub-surface sediments surrounding the conduit (seepage feeder channel).
Bacterial mats As a consequence of strong chemical gradients at seep locations (e.g., reduced vs oxic fluids), bacteria flourish at such sites. The most common visible bacterium found at marine methane seep sites, the world over, is the Beggiatoasp. It is dependent on the anoxic-oxic seawater boundary and forms at the oxylimnion and produces white natural sulphur within its fibrous cells. This and also many other types of bacteria can produce thick mats on the seafloor, which can be torn and suspended into the water column where they can represent organic “snow”.
Up-welling of seawater When bubbles rise under influence of buoyancy through the water column, they will cause turbulence in their wakes. If there is a significant ebullition of gas, 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.
Entrainment (down-welling) of seawater into the ground Because conduits leading gas bubbles to the seabed cause hydraulic action (pressure pulses) within the sub-surface conduit system, negative pressure gradients, will cause seawater to entrain into the ground (O’Hara et al., 1995) called down-welling of seawater into the pore-water system at seep locations. This action may cause chemical and temperature anomalies in the sub-surface micro-environment.
Sea-surface slicks and sea-birds feeding The action of methane macro-seepage may also be detectable on the sea surface, mainly because of the up-welling effect and also by the consequential transport of temperature, chemicals, and nutrients (bacteria, and other organisms/organic particles). Seeps can also cause disturbances in the surface capillary wave patterns, due to changes in surface currents surrounding the up-welling area. Down-current of the seepage and up-welling locations, there may be slicks (water devoid of capillary waves) due to the entrainment of oil on rising bubbles. In addition, there may be birds feeding on organic particles (such as pieces of bacterial mats) carried to the surface by the up-welling. Thus, slicks and feeding sea-birds may also represent manifestations of seeps.
Attraction of fish and other macro-fauna Because seeps may disturb the ambient layering of nutrients and organisms in the water column, seepage is expected to attract fish from other locations. These effects may also cause the development of sessile colonies of filter-feeders and other invertebrate organisms (bioherms) down-stream of the seep locations. Thus, the ‘hydraulic theory’ for deep-water corals is explained by this process (manly from micro-seeps, e.g.,Hovland, 2008).
Anomalies in methane concentration in lower atmosphere On some occasions, the seafloor seepage of methane provides higher concentrations of methane in the near-surface seawater. Any such anomalous seawater concentration will cause the entrainment of methane into the lower atmosphere. Thus, also methane concentration anomalies in the lower atmosphere may be regarded as manifestations of sub-marine seepage.
This holistic 12-point summary of the most important environmental effects of marine and lacustrine methane macro-seeps provides a preliminary list of what to look for when searching for seeps. As more research is conducted, especially with respect to temporal variation of flux, and the effects on the surrounding biological and physicochemical systems, it is expected that more items may be added. It should, however, be borne in mind, that not all of these elements occur at all seeps or at a given time. This is what makes seep-hunting so fascinating. There may actually be some seeps that are only manifested by only one or two of the listed aspects.
Abstract The two main observations characterising marine and lacustrine methane macro-seeps are ebullition through holes in the sea- or lake-bed, and hydroacoustic flares in the water column. The paper reviews multi-year, multi-scale, and multi-discipline results from three seep locations in the North Sea and combines the knowledge with recent seafloor and water column results from seeps in the Santa Barbara basin, California, a seep off West Africa, seeps in the Gulf of Mexico, and in Lake Baikal, Russia. We have identified a total of 12 characteristics of methane and minor oil macro-seeps that are not only geological in nature, but also biological and geochemical. These are shown to impact the marine environment in different ways, not least in benefactory manners, as primary producers (mainly bacteria and archaea) tend to bloom during seepage. Therefore, the seepage is inferred to have a fertilizing effect on both the seafloor and the water column, which may be of broad ecological and biological significance. The study concludes with a holistic conceptual seep-model which is expected to be of interest to a broad range of researchers in the fields of oceanography and limnology.