In transform margins, such as the one generated by the
lateral movement between the Pacific and the North
American plates along the San Andreas fault zone, thick
sediment sequences may be exposed by the sideways plate
motion along the fault planes (Legg et al., 1989). These
cold seeps have received less attention than those in convergent or passive margins. However, a good example of
transform-margin seeps is those found along the San
Clemente fault, approximately 100 km southwest of San
Diego (Torres et al., 2002).
In this transform margin, the turbidite sand layers of the
Navy Fan deposits support migration of subsurface fluids,
which vent at the locations where the sediment package is
cut by fault-induced escarpments. Both methane anomalies in the water column, as well as the presence of chemosynthetic organisms along the fault traces, delineate the
zones of active methane venting. Fluid flow in these settings is driven primarily by the development of a hydraulic
head and the presence of high-permeability pathways,
similar to conditions that drive groundwater flow commonly studied in onshore and offshore aquifers (Torres
et al., 2002).
Passive margins are those not marked by a strike-slip
fault or a subduction zone. They occur around the Atlantic,
Arctic, and western Indian Oceans and define the entire
coasts of Africa, Greenland, northern Europe, and Australia. Because of their extensive distribution, the nature of
the seeping fluids and mechanisms that control flow are
quite large. Mass transport, which is common in these margins, provides an overriding compressional component to
the normal faulting regime typical of passive-margin tectonics. In addition, dehydration reactions and gas generation all create overpressures that drive flow. Flow is
channeled along highly permeable lithologies, such as ash
layers and turbidite sands through normal faulting.
The first cold seep on a passive margin was discovered
by Charles Paull along the Florida Escarpment (Paull
et al., 1992). Here, groundwater migrates through carbonate aquifers and discharges in the Gulf of Mexico at a
depth of 3,200 m. Since then a large number of seep localities have been identified on the northern margin of the
gulf. Fluid flow is associated with salt dome intrusions
within these (>10 km) thick sediment packages, from
which the thermogenic oil and gas accumulations are currently targeted for oil and gas production. In this margin,
migration conduits supply hydrocarbon materials that
migrate ~6–8 km toward the sediment surface. More
recently, cold seeps were discovered along the Mexican
margin of the gulf, which are characterized
by the presence of thick oil coatings and lavalike asphalt
flow structures on the seafloor (McDonald et al., 2004).
Because of the thermogenic nature of these seeps, it is
possible to image the naturally created oil slicks at the
sea surface, using satellite data. Ian MacDonald and collaborators have analyzed images from space that reveal
the presence of oil slicks across the north-central Gulf of
Mexico in water depths greater than 1,000 m (McDonald
et al., 2000).
Fate of methane and cold seep biomes
If all methane out of the sedimentary reservoir were to
reach the atmosphere, this could have significant impact
on the Earth’s climate, because methane is a strong greenhouse gas. Fortunately, microorganisms consume a very
large fraction of the sedimentary methane under anaerobic
conditions. If released to the water column, methane is further consumed by aerobic methane oxidation. This combined microbial filter, reviewed by William Reeburgh
(2007), is key in regulating methane flux from the sediments to the ocean and potentially the atmosphere.
Within the sediments, a metabolic pathway whereby
methane was oxidized by sulfate was originally proposed
based on the chemical composition of the pore water
(Reeburgh, 1976; Barnes and Goldberg, 1976). This postulate was later confirmed by observations of a microbial
consortium that oxidizes methane to bicarbonate while
reducing sulfate to hydrogen sulfide (Boetius and Suess,
2004). The hydrogen sulfide generated in this process
rises with the ascending fluids to the seafloor, where it is
metabolized by Beggiatoa that aggregate forming the bacterial mats typically found at cold seeps (Figure 2). In
these complex ecosystems, there are multiple interactions
between microbes and macrobiota that involve symbiosis,
heterotrophic nutrition, and geochemical feedbacks.
Bivalves (mytilids, vesicomyids, lucinids, and thyasirids)
are nourished by symbiotic bacteria that derive their
energy from methane and hydrogen sulfide, and constitute
prominent members of cold seep fauna. Vestimentiferan
tube worms, also common at cold seep locations, extract
hydrogen sulfide through their roots, which is also metabolized by symbionts. Pogonophorans, cladorhizid
sponges, gastropods, shrimp, and crabs are also abundant
at some locations (Levin, 2005).
Authigenic minerals
In addition to hosting unique communities, chemical reactions between the highly reduced seeping fluids and the
oxidant-rich seawater lead to precipitation of characteristic authigenic minerals. Calcium carbonate (CaCO 3 ) is
formed as a by-product of methane oxidation, which provides a bicarbonate source characterized by enrichment in
12
C. These methane-derived carbonates are ubiquitous
near cold vent sites throughout the globe and constitute a
significant carbon sink. The dominant mineral phases are
aragonite and high-magnesium calcite (Bohrmann et al.,
1998).
Microbially mediated carbonate formations have been
observed to rise above the seafloor by up to 90 m in height.
In the Cascadia margin, for example, carbonate
chemoherms display a pinnacle-shaped morphology with
steep flanks and are known to have been active for at least
270 ky (Teichert et al., 2005). Their irregular structure and
high porosity reflect vigorous outflow of methane-rich
fluids. In anoxic bottom waters, such as those found in
the Black Sea, microbial consortia fed by methane bubble
emissions at the seafloor support the buildup of towerlike
120
COLD SEEPS
lateral movement between the Pacific and the North
American plates along the San Andreas fault zone, thick
sediment sequences may be exposed by the sideways plate
motion along the fault planes (Legg et al., 1989). These
cold seeps have received less attention than those in convergent or passive margins. However, a good example of
transform-margin seeps is those found along the San
Clemente fault, approximately 100 km southwest of San
Diego (Torres et al., 2002).
In this transform margin, the turbidite sand layers of the
Navy Fan deposits support migration of subsurface fluids,
which vent at the locations where the sediment package is
cut by fault-induced escarpments. Both methane anomalies in the water column, as well as the presence of chemosynthetic organisms along the fault traces, delineate the
zones of active methane venting. Fluid flow in these settings is driven primarily by the development of a hydraulic
head and the presence of high-permeability pathways,
similar to conditions that drive groundwater flow commonly studied in onshore and offshore aquifers (Torres
et al., 2002).
Passive margins are those not marked by a strike-slip
fault or a subduction zone. They occur around the Atlantic,
Arctic, and western Indian Oceans and define the entire
coasts of Africa, Greenland, northern Europe, and Australia. Because of their extensive distribution, the nature of
the seeping fluids and mechanisms that control flow are
quite large. Mass transport, which is common in these margins, provides an overriding compressional component to
the normal faulting regime typical of passive-margin tectonics. In addition, dehydration reactions and gas generation all create overpressures that drive flow. Flow is
channeled along highly permeable lithologies, such as ash
layers and turbidite sands through normal faulting.
The first cold seep on a passive margin was discovered
by Charles Paull along the Florida Escarpment (Paull
et al., 1992). Here, groundwater migrates through carbonate aquifers and discharges in the Gulf of Mexico at a
depth of 3,200 m. Since then a large number of seep localities have been identified on the northern margin of the
gulf. Fluid flow is associated with salt dome intrusions
within these (>10 km) thick sediment packages, from
which the thermogenic oil and gas accumulations are currently targeted for oil and gas production. In this margin,
migration conduits supply hydrocarbon materials that
migrate ~6–8 km toward the sediment surface. More
recently, cold seeps were discovered along the Mexican
margin of the gulf, which are characterized
by the presence of thick oil coatings and lavalike asphalt
flow structures on the seafloor (McDonald et al., 2004).
Because of the thermogenic nature of these seeps, it is
possible to image the naturally created oil slicks at the
sea surface, using satellite data. Ian MacDonald and collaborators have analyzed images from space that reveal
the presence of oil slicks across the north-central Gulf of
Mexico in water depths greater than 1,000 m (McDonald
et al., 2000).
Fate of methane and cold seep biomes
If all methane out of the sedimentary reservoir were to
reach the atmosphere, this could have significant impact
on the Earth’s climate, because methane is a strong greenhouse gas. Fortunately, microorganisms consume a very
large fraction of the sedimentary methane under anaerobic
conditions. If released to the water column, methane is further consumed by aerobic methane oxidation. This combined microbial filter, reviewed by William Reeburgh
(2007), is key in regulating methane flux from the sediments to the ocean and potentially the atmosphere.
Within the sediments, a metabolic pathway whereby
methane was oxidized by sulfate was originally proposed
based on the chemical composition of the pore water
(Reeburgh, 1976; Barnes and Goldberg, 1976). This postulate was later confirmed by observations of a microbial
consortium that oxidizes methane to bicarbonate while
reducing sulfate to hydrogen sulfide (Boetius and Suess,
2004). The hydrogen sulfide generated in this process
rises with the ascending fluids to the seafloor, where it is
metabolized by Beggiatoa that aggregate forming the bacterial mats typically found at cold seeps (Figure 2). In
these complex ecosystems, there are multiple interactions
between microbes and macrobiota that involve symbiosis,
heterotrophic nutrition, and geochemical feedbacks.
Bivalves (mytilids, vesicomyids, lucinids, and thyasirids)
are nourished by symbiotic bacteria that derive their
energy from methane and hydrogen sulfide, and constitute
prominent members of cold seep fauna. Vestimentiferan
tube worms, also common at cold seep locations, extract
hydrogen sulfide through their roots, which is also metabolized by symbionts. Pogonophorans, cladorhizid
sponges, gastropods, shrimp, and crabs are also abundant
at some locations (Levin, 2005).
Authigenic minerals
In addition to hosting unique communities, chemical reactions between the highly reduced seeping fluids and the
oxidant-rich seawater lead to precipitation of characteristic authigenic minerals. Calcium carbonate (CaCO 3 ) is
formed as a by-product of methane oxidation, which provides a bicarbonate source characterized by enrichment in
12
C. These methane-derived carbonates are ubiquitous
near cold vent sites throughout the globe and constitute a
significant carbon sink. The dominant mineral phases are
aragonite and high-magnesium calcite (Bohrmann et al.,
1998).
Microbially mediated carbonate formations have been
observed to rise above the seafloor by up to 90 m in height.
In the Cascadia margin, for example, carbonate
chemoherms display a pinnacle-shaped morphology with
steep flanks and are known to have been active for at least
270 ky (Teichert et al., 2005). Their irregular structure and
high porosity reflect vigorous outflow of methane-rich
fluids. In anoxic bottom waters, such as those found in
the Black Sea, microbial consortia fed by methane bubble
emissions at the seafloor support the buildup of towerlike
120
COLD SEEPS