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.
Read the abstract and full article here:
http://www.sciencedirect.com/science/article/pii/S0025322712000564
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.
Now visit this exciting site for further study:
http://www.living-petrol.blogspot.no/
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.
Read the abstract and full article here:
http://www.sciencedirect.com/science/article/pii/S0025322712000564
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.
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