Reference Work Entry

Encyclopedia of Geobiology

Part of the series Encyclopedia of Earth Sciences Series pp 278-290

Cold Seeps

  • Robert G. JenkinsAffiliated withFaculty of Education and Human Sciences, Yokohama National University

Synonyms

Diffusive seep; Ground water seep; Hydrocarbon seep; Methane seep; Oil seep. The terms “seep” and “seepage” can be used as alternates

Definition

Fluid flow containing reduced compounds, e.g., methane or hydrogen sulfide (see entry Sulfur Cycle ), with almost no temperature anomalies compared with ambient seawater gushing out from or reaching very close proximity to the seafloor.

Introduction

Active water circulation systems can be found at seafloor level and beneath the seafloor along the plate boundaries such as mid-ocean ridges and trenches. It is well known that at ocean ridges, hydrothermal fluid containing reduced compounds and minerals vent out from the seafloor. In contrast to the hydrothermal vents, focused flow containing reduced compounds, e.g., methane and hydrogen sulfide, gushing out from the seafloor can also be found along active and passive continental margins and around diapirs of serpentine and salt rock (halite) with no or less temperature anomalies compared to the ambient seawater. This fluid flow is called a cold seep.

The first cold seep was found at the Florida Escarpment off Oregon in 1984 by the submersible Alvin (Paull et al., 1984). The Alvin found a large number of alleged tube worms and white clams which were thought to be highly endemic to hydrothermal vents. It turned out that there are fluid flows which contain reduced compounds and which support chemosynthesis-based life, similar to hydrothermal vents (Kulm et al., 1986).

Examples of the reduced compounds contained in cold seeps are methane and hydrogen sulfide. These compounds lead to extensive biogeochemical reaction when the fluid encounters oxic interstitial water which has penetrated into sediments from seawater. The reaction is mainly mediated by microbes as a way of obtaining chemosynthetic energy to synthesize their organic compounds. Primary production based on the chemosynthesis found in cold seeps results in the formation of characteristic life forms similar to those which can also be found in hydrothermal vents.

The important role played by methane in global warming is due to the fact that the greenhouse effect that results from it is more than 20 times that of carbon dioxide (Kvenvolden, 1988). Hence if we think of cold seeps as a pipe connecting the deep subsurface and the oceans, we can understand their importance in terms of global climate change and biogeochemical cycles at the sediment–water interface.

Distribution

Cold seeps are widely distributed along the active and passive continental margins of the world’s oceans (Figure 1; Sibuet and Olu, 1998; Levin, 2005). The depth range of a cold seep varies from shallow water less than 15 m (Montagna and Spies, 1985) to deep water more than 7,400 m in the Japan Trench (Fujikura et al., 1999).
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Cold Seeps. Figure 1

Distribution of modern and ancient cold seeps and hydrothermal vents. (After Campbell, 2006.)

The total number of cold seeps reported to date represent a minimum, because finding a cold seep is significantly more difficult than finding a hydrothermal vent. The difficulties of identification result from the natural characteristics of cold seeps which are difficult to observe by the use of a submersible or remote operated vehicle (ROV).

Natural characteristics of cold seeps

Cold seeps vary in accordance with their natural characteristics. Most cold seeps are very calm, i.e., the low seepage intensity and low fluid flow rate has no effect on the sea floor. The almost total absence of any temperature anomaly in relation to the ambient seawater also makes it difficult to distinguish the flow by visual observation. The occurrence of chemical anomalies is found over too narrow an area and the anomalies usually occur only in the sediment. Thus, this kind of calm cold seep, called a diffusive seep , is rather difficult to find on the basis of geological and geochemical signatures which exist only beneath the sea floor. One good indicator pointing to the existence of a cold seep is the occurrence of chemosynthesis-based life obtaining energy from the reduced compounds contained in the seep fluids (Sibuet and Olu, 1998). Such biota often comprises large bivalves and/or worm tubes which are clearly visible (Figure 2). However, seep-associated infaunal animals (i.e., animals living only in the sediments) and microorganisms are potentially difficult to detect.
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Cold Seeps. Figure 2

Cold seep in the Kuroshima Knoll, Okinawa, southwestern Japan. A dense colony of bathymodiolian bivalves, housing chemosynthetic microbes, covered the sea floor. Gushing out of gaseous bubbly methane can be seen. (by courtesy of the Japan Agency for Marine-Earth Science and Technology).

Direct measurements of cold seep fluid chemistry started around the end of the last century (e.g., Tryon and Brown, 2001, 2004). The results show that the fluid pattern and rate of flow are more complex than was previously thought to be the case. Although most cold seeps are calm, there are some effusive and explosive cold seeps. When the saturation level of dissolved methane in sea water is exceeded, the excess methane forms gas bubbles (Figure 3) or gas hydrate (Figure 4) depending on the physical conditions. The bubbly gaseous methane can be found by visual observation (Figure 2) and also by hydroacoustic sounders, like a fisheries sonar (Figure 5; Paull et al., 2007; Aoyama and Matsumoto, 2009). The acoustic sounder makes it possible to see the plume due to the relatively low density of the methane bubbles compared to the ambient sea water. In waters off Niigata, Sea of Japan, methane bubble streams venting out from the sea floor at about 900 m water depth produce a plume reaching to about 300 m below the sea surface (Figure 5; Aoyama and Matsumoto, 2009).
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Cold Seeps. Figure 3

Collecting methane gas bubbles at a cold seep. ROV Hyper-dolphin collected the bubbly methane gas venting out from the cold seep in the waters off Joetsu, Sea of Japan. Putative shell-like gas hydrate covers each gas bubble. (by courtesy of the Japan Agency for Marine-Earth Science and Technology).

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Cold Seeps. Figure 4

Outcrop observation of gas hydrate in the waters off Joetsu, Sea of Japan. (by courtesy of the Japan Agency for Marine-Earth Science and Technology).

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Cold Seeps. Figure 5

Methane plumes observed by acoustic sounder at the Umitaka spur, off Joetsu, Sea of Japan. The plumes rose up from the sea floor at depths ranging from ca. 900 m to ca. 300 m. The horizontal axis indicates time. The vehicle obtaining the echogram ran from north to south. (Modified from Aoyama and Matusmoto (2009) (Patent no. US 7539081 B2, Seabed resource exploration system and seabed exploration method by Chiharu Aoyama).

Cold seeps associated with a mud volcano or serpentine diapir show that the flow gushes out together with muddy sediment or brecciated serpentine rocks. It may also happen that methane erupts from the seafloor due to a pressure increase in the sediments caused by an accumulation of methane gas. Such an eruption forms chaotic structures of strata, like landslides, and crater-like topographically depressed areas, so-called pock marks . The size of a pock mark may vary from a few meters to a few hundred meters in diameter and from 1 m to 80 m in depth (Gay et al., 2006).

Pathway and migration of fluid flow

Generally, reduced compounds increase with depth in marine sediments. These reduced compounds can be oxidized by oxidants such as oxygen, nitrate, and sulfate and usually occur at a shallow depth under the sea floor. Although these reactions do also occur elsewhere below the sea floor, substantial reaction occurs in cold seeps because of the high amounts of reduced compounds brought up to the sea floor from the deep subsurface. Accumulation and migration of reduced compounds are required to form a cold seep.

The pore water containing reduced compounds favors migration through physically weak paths, e.g., faults, and permeable sediments. Tectonic stresses, e.g., plate subduction or diapirs, induce the formation of such pathways. Thus, occurrences of cold seeps are largely dependent on the tectonic background of the area concerned.

Cold seep pathways take the form of a fault or faulted anticline, salt or serpentine diapirs, or a cut bank of strata. These structures can be revealed by seismic profiles, direct observations of the outcrops in the ocean, and drill core samples with chemical profiles of the pore water (Figure 6; Moore et al., 1990, 2001).
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Cold Seeps. Figure 6

Interpreted seismic reflection obtained from accretionary prism, off Oregon. Rigid arrows indicate inferred flow paths for fluids. Direct measurements of bedding from the submersible Alvin correlate well with the seismic reflection lines. (After Moore et al., 1990.)

Iodine radioisotopes (see entry Isotopes, Radiogenic ) have recently been identified as a useful tool to determine the age of the strata from which the methane originated, as the iodine will be released at the same time as methane is generated from organic matter, and the diffusion rates of methane and iodine are very similar (Fehn et al., 2003). For example, pore water from a cold seep in the Japan Sea passed through and probably originated from strata which sedimented during 30–20 Ma (Tomaru et al., 2007). Faults associated with cold seeps occur particularly on the landward wedge of subduction zones, e.g., the Barbados accretionary wedge, the Cascadia margin and the Japan Trench. Faults created by tectonic stress due to subduction of plates play the role of physically weak pathways for the fluid flow. It may also happen that faults cut through a methane reservoir formed under impermeable strata, and the methane then migrates upward through the fault. Indeed, the direction formed by Calyptogena (bivalve) colonies, which mostly live in cold seeps and/or hydrothermal vents, is coincident with a theoretical stress fracture pattern caused by subduction tectonics (Ogawa et al., 1996).

The main driving forces of the migration of pore water containing reduced compounds are density differences and the compression of pore water by increasing pressure within the sediments. The buoyancy of gaseous and dissolved methane and light hydrocarbons in pore water is the main driving force for upward migration. Gaseous methane, the main hydrocarbon gas in the fluid flow, has a strong buoyancy, for example, the density of gaseous methane beneath 3–4 km of water is 200–300 kg m−3 compared with 1,024 kg m−3 for seawater (Clennell et al., 2000). Water in which methane is dissolved is also lighter than methane-free water. Thus, the pore water containing gaseous and dissolved methane potentially tends to migrate upward within permeable sediments. Entrapment of the fluid usually occurs where impermeable sediments are found. When strata composed of impermeable sediment are tilted, the fluid migrates within the permeable sediment in a lateral-upward direction directly beneath the impermeable strata. Thus, the hinge zone of an anticline is suitable for the accumulation of methane contained in pore water. Methane-rich fluids may further migrate upwards through faults that cut through the impermeable strata. The fluid also gushes out from outcrops where there is a break in the strata caused by erosion.

Increasing pressure arising from compaction due to continuous loading of sediments or tectonic stress squeezes out pore water containing reduced compounds into overlying strata. The increasing pressure causes not only pore water migration but also mud diapirs. This is why mud diapirs (or mud volcanoes ) are often associated with methane or other reduced compounds.

Heating of the pore water is the other factor causing upward fluid migration. Heating occurs as a result of geothermal activities in the deep subsurface. After heating by geothermal activity in the deep subsurface, the temperature of the fluid drops during upward migration. Thus, the fluid temperature at sea floor level shows no or fewer anomalies in contrast to hydrothermal vents.

Migration of reduced compounds is also caused by diffusion. Although diffusion can be considered as a factor much less important than buoyancy and pressure increase, it is important for the penetration of oxidants from the sea water into sediments.

Chemistry of fluid flow

Major reduced compounds in the fluid flow in cold seeps are methane and other light hydrocarbons, hydrogen sulfide, ammonia, reduced iron, barium, and hydrogen (e.g., Tunnicliffe et al., 2003; Tryon and Brown, 2004). Methane is the most important by virtue of the fact that it is the most common hydrocarbon gas in marine sediments. Methane has a pronounced greenhouse effect and plays an important role both as an energy source and as a carbon source for microbes; furthermore, the anaerobic oxidation of methane (AOM) coupled with sulfate reduction produces hydrogen sulfide which acts as a major energy source for chemosynthesis-based ecosystems.

Elements contained in the fluid flow change with the depth of sediments, because the generation and consumption occur at different depths. Concentrations of reduced compounds generally increase in line with the depth of the sediments. Drastic changes in the chemical profile occur at shallower depths where the reduced compounds encounter oxidants. These reactions take place mainly within sediments, but it sometimes happens that the reduced compounds discharge directly into the sea water column. In such a case, the oxidation reactions take place within the water column.

The drastic chemical changes in cold seeps are generally responsible for substantial biological activities. Methane, the major reduced compound in a cold seep, is generated at a relatively deep subsurface level under anaerobic conditions. When a fluid flow containing methane migrates toward the sea floor and encounters oxidants such as oxygen and sulfate (see entry Sulfur Cycle ) which penetrated into the sediment from oxic sea water, intensive biogeochemical reactions occur. The most important and extensive of these is AOM which occurs where the methane and sulfate coexist in anaerobic conditions. Following the AOM, oxidation of hydrogen sulfide, formation of pyrite (see entry Iron Sulfide Formation ), and precipitation of carbonate minerals occur. When the fluid flow gushes out into well-oxygenated conditions, aerobic oxidation of methane ( Methane Oxidation (Aerobic) ) occurs.

The origin of methane

There are several pathways for the generation of methane of which biogenic and thermogenic processes are the most important. In the case of the former, the methane is produced directly by microbes, and in the case of the latter, the methane is produced through the thermal degradation of organic compounds in the deep subsurface. Methane that is produced neither from organic material nor through organic processes can be found mainly along the edge of hydrothermal vent fields and above serpentine diapirs.

Biogenic methane

Biogenic methane is produced by microbes. The methanogenic microbes (methanogens) belong to the phylum Euryarchaeota within the domain Archaea. Methanogens are obligatory anaerobes, meaning that they live in an anoxic environment which usually occurred below the zone of sulfate reduction. The main substrates linked to the generation of methane are simple carbon-containing molecules such as carbon dioxide , formate , acetate , methanol , and methylamine (Whiticar, 1999). The two simple pathways of methanogenesis , i.e., carbonate reduction and acetate fermentation, are expressed as \( {\rm{CO}}_2 + 8{\rm{H}}^ + \to {\rm{CH}}_4 + 2{\rm{H}}_2 {\rm{O}} \) and \( {\rm{CH}}_3 {\rm{COOH}} \to {\rm{CH}}_4 + {\rm{CO}}_2 \), respectively. Archaeal methanogenesis promotes large fractionations of carbon isotopes. The stable carbon isotopic composition of methane produced by carbonate reduction generally ranges from −60 to −80‰ (vs. PDB), while the methane produced by the fermentation process ranges from −50 to −60‰ (vs. PDB; Whiticar, 1999). The methane is mostly a unitary hydrocarbon product of the biological process, whereas the thermogenic process explained below produces methane as well as other hydrocarbons such as ethane and propane . However, the recent suggestion that ethane and propane are also produced biologically (Hinrichs et al., 2006) upset the long-held concept of the origin of light hydrocarbons larger than methane, which were thought to be generated by the thermal degradation of organic matter only. The biogenic process is carried out at a relatively shallow depth compared to where thermogenic methane is produced. This is due to the presence of appropriate thermodynamic conditions, i.e., not too high a temperature and pressure, to permit microbial activities.

Thermogenic methane

The methane generated through the degradation of sedimentary organic matter as a result of thermodynamic conditions is called thermogenic methane (Kotelnikova, 2002). The rising temperature and pressure of sediments that accompany the increasing depth accelerates the degradation of organic compounds. Thus, the production of thermogenic methane generally increases in proportion to the depth of the sediments, usually more than a few hundred meters below the seafloor. Contrarily, a high temperature inhibits microbial activities at a deep subsurface level, thus it follows that the dominant methane in the deep subsurface is thermogenically generated. This process produces not only methane but also ethane, propane, and other light hydrocarbons. There are fewer isotopic effects on carbon and hydrogen from methane produced by thermal maturation compared to biogenic methane. Thus, the carbon isotopic composition of thermogenic methane ranges from −20 to −50‰ (vs. PDB; Whiticar, 1999).

Abiogenic methane

Methane produced from inorganic matter through an inorganic process is called abiogenic methane. Such methane is thought to be generated through the processes of cooling and degassing of mafic igneous rocks and magma and the process of serpentinization of ultramafic rocks. Although the exact mechanism of abiogenic methane formation is still not fully understood, some abiogenic methane formation processes have been revealed (e.g., Horita and Berndt, 1999; Proskurowski et al., 2008). We can now say that abiogenic methane is probably the dominant methane source in hydrothermal vent systems especially along mid-ocean ridges and above serpentine diapirs.

Identification of the origin of methane

Looking specifically at the methane in cold seeps, it has been thought that this originated from the processes of biogenesis and/or termogenesis. Most abiogenic methane will be found at hydrothermal vents and not at cold seeps except in the case of cold seeps associated with serpentine diapirs. Means of allowing us to estimate the origin of methane are a combination of analyses of the carbon isotopic compositions of the methane and the relative abundance of ethane and propane compared to the amount of methane included in fluid seepage (Schoell, 1988). Specifically, extremely negative carbon isotope values of methane, generally less than −50‰ (vs. PDB), with a high methane/(ethane + propane) ratio indicate that the methane was generated by a biogenic process. In contrast, less negative carbon isotopic composition, generally −20‰ to −50‰ (vs. PDB), with a relatively low methane/(ethane + propane) ratio indicates that the methane was generated by a thermogenic process.

Anaerobic oxidation of methane (AOM)

AOM is a key reaction at cold seeps. Although several probable reactions have been proposed (Valentine, 2002), the details of the AOM reaction are not yet clearly understood. What we can say is that the net reaction of the AOM is expressed as CH4 + SO4 2−→ HCO3 + HS + H2O. Thus, the relative amounts of both sulfate and methane regulate the activity of AOM. This reaction occurs widely in oceans because large amounts of sulfate are contained in sea water whereas the reaction will not occur in freshwater environments such as ponds, lakes, rivers, and rice fields because freshwater lacks sulfate (Whiticar, 1999). It has recently been recognized that AOM by sulfate reduction is carried out by consortia of anaerobic methane-oxidizing archaea and sulfate-reducing bacteria (Elvert et al., 1999; Boetius et al., 2000; Orphan et al., 2001, 2004). The anaerobic methane-oxidizing archaea (ANME) are found in three different phylogenetic clusters belonging to the Euryaechaeota, i.e., ANME-1, -2, and -3. It has also been revealed by genome analysis that methane is oxidized under anoxic conditions by means of a “reverse methanogenesis” process (Hallum et al., 2004). The ecology of each of these archaeal lineages (ANME-1, -2, and -3) differs from that of the others. Thus, the differences reflect their distribution pattern in cold seeps (Elvert et al., 2005; Knittel et al., 2005).

An important function of the AOM reaction is that it prevents the escape of methane, which has a pronounced greenhouse effect, from sediments into the ocean and the atmosphere. The carbon derived from methane is transformed into bicarbonate or carbonate ions. An increase in the ions leads to an increase in alkalinity and thus to the subsequent precipitation of carbonate minerals given the existence of calcium or other appropriate cations (explained below).

Oxidation of sulfides

The hydrogen sulfide (see entry Sulfide Mineral Oxidation ), a byproduct of the AOM, is used for subsequent reactions. One is the oxidation of sulfides mediated by sulfur-oxidizing bacteria under (sub)oxic conditions, and the other is the formation of pyrite (see entry Iron Sulfide Formation ) in anoxic conditions. Sulfur-oxidizing (thiotrophic) bacteria, such as Beggiatoa (qv), Thiothrix , and Thioploca , use the hydrogen sulfide as an electron donor and oxidize it by oxygen. The final product of the reaction is sulfate. Most sulfur-oxidizing bacteria are chemolithoautotrophs, but some can use organic compounds for their cell synthesis. The thiotrophic bacteria sometimes form white mats that cover the sea floor and some may live as symbiotic bacteria in animal tissues (e.g., in gills).

Aerobic oxidation of methane (qv)

The aerobic oxidation of methane (see entry Methane Oxidation (Aerobic) ) is performed by methane-oxidizing (methanotrophic) bacteria, whereas archaea are responsible for AOM. When the methane is released directly into an oxic water column, it will be aerobically oxidized by these methanotrophs. However, in normal marine environments, methane is mostly oxidized anaerobically by AOM within the sediments. Aerobic oxidation of methane becomes important when methane vents out vigorously into oxic sea water.

Authigenic mineral precipitation in cold seeps (qv “microbial mineralization” and “microbially induced sedimentary structures”)

One result of the substantial biological reactions referred to above is the precipitation of several minerals. The minerals most commonly found in cold seeps are carbonate such as aragonite, high magnesian calcite, and low magnesian calcite. The precipitation of these carbonates occurred because of increasing alkalinity resulting from AOM. When AOM occurs at sea-floor level or above the sea floor in euxinic waters, reef-like carbonate build-ups may be formed on the sea floor, as in the Black Sea (Michaelis et al., 2002). Under an oxic water column, the reaction generally occurred within the sediment, thus the carbonate formed beneath the sea floor. At a shallower depth below the sea floor, aragonite and high magnesian calcite can be formed, whereas low magnesian calcite and dolomite are frequently found relatively deeper within the sediments (e.g., Takeuchi et al., 2007). Methane-derived carbonate bodies are sometimes exposed on the sea floor because of surface erosion. Then the carbonate may function as a hard substrate for sessile organisms or carbonate borers.

Pyrite is another common authigenic mineral found in cold seeps. Hydrogen sulfide produced as a result of AOM may combine with reduced iron to form monosulfide, i.e., a precursor of pyrite. Subsequent reaction of the precursor with elemental sulfur or sulfides promotes the formation of pyrite. It is commonly the case that the pyrite crystal aggregation forms a framboidal shape, a so-called framboidal pyrite (for details see Chapter Iron Sulfide Formation ). The sulfur isotopic composition of pyrite inherits the composition of the hydrogen sulfide source and indicates whether or not the hydrogen sulfide derived from bacterial sulfate reduction (Peckmann and Thiel, 2004).

Relationship to methane gas hydrate

Cold seeps are frequently associated with gas hydrate (Figure 4) where the environment has appropriate physical conditions for gas hydrate formation. The main component of the gas hydrate is methane but carbon dioxide, ethane, and other gaseous molecules are also present. Gas hydrates form in low temperature − high pressure environments where the appropriate concentrations of methane exist (Zhang, 2003). Such environments are usually distributed in the deep sea around continental margins and in permafrost areas on land. The hydrate is one of the largest methane reservoirs in the world. It has been estimated that more than 2,100 Gt of methane are stored in gas hydrates.

Formation of a mass of methane gas hydrate usually occurs within sediments whereas methane rapidly escapes and does not readily accumulate in water columns. If a substantial amount of methane, e.g., bubbly methane gas, is released into a water column, gas hydrate may form as a thin shell on the surface of a gas bubble (Figure 3). The depth limit of gas hydrate formation and occurrence in the sediments is usually determined by the local geothermal gradient. The gas hydrate itself may act as an impermeable barrier within the sediment. In this case, the methane gas, which has migrated from the deep subsurface, is trapped and accumulates beneath the gas hydrate layer.

The importance of the gas hydrate for cold seeps is that (1) pore water surrounding the methane gas hydrate has an oversaturated level of methane, and (2) the rapid dissociation of methane gas hydrate may result in an episodic increase of cold seeps and sometimes an eruption of methane. When methane-undersaturated water penetrates the sediment and comes into proximity with the gas hydrate, the gas hydrate dissociates so as to create a physical equilibrium between the pore water and the gas hydrate. When the pore water, after taking up a substantial amount of methane, migrates further upward, a continuous methane supply is established between the methane gas hydrate reservoir and the sea floor.

Episodic emission of methane because of the dissociation of methane gas hydrate occurs, for example, in cases where the sea level decreases at the beginning of a glacial period or increasing sea water temperature because of global warming (e.g., Judd et al., 2002). The dissociation of gas hydrates may cause instability of the sea floor inducing landslides (Buffett, 2000). Such an underwater landslide results in additional lowering of pressure and may produce further dissociation of gas hydrate. It should be noted that the meaning of the existence of methane gas hydrate is that the area has large potential for the generation and/or accumulation of methane.

Because the density of gas hydrate is lower than that of sea water, the gas hydrate itself tends to rise upwards. Thus, an insufficient mixture of sediment in the gas hydrate and/or insufficient loading by overlain sediments causes the release of a mass of gas hydrate into a water column.

Ecosystem established at cold seeps – chemosynthesis-based ecosystems

Cold seeps can be considered as chemosynthesis-based ecosystems that support a large amount of life in deep seas (Figures 2 and 7; Sibuet and Olu, 1998). The main primary producers of the ecosystem found in cold seeps are chemoautotrophic microbes, such as sulfur-oxidizing bacteria. For the sulfur-oxidizing bacteria, hydrogen sulfide has the role of an electron acceptor, oxygen that of an electron donor, and carbon dioxide that of a carbon source. Such microbes that metabolize via the chemical reaction of inorganic compounds with an inorganic carbon source are called chemolithoautotrophs. An ecosystem based on chemoautotrophy is called a chemosynthesis-based ecosystem. This kind of ecosystem is a counterpart to the photosynthetic ecosystem found on the surface of the Earth. Chemosynthetic ecosystems establish themselves especially in areas where light does not penetrate such as the deep sea and the subsurface below the sea floor. Within these settings, chemosynthesis-based ecosystems particularly flourish at cold seeps and hydrothermal vents because these are environments where large amounts of reduced compounds encounter oxic waters.
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Cold Seeps. Figure 7

Chemosynthesis-based community composed of a dense accumulation of Bathymodiolus (mytilid bivalves) from Kuroshima Knoll, off Okinawa, southwest of Japan. (by courtesy of the Japan Agency for Marine-Earth Science and Technology).

With chemosynthetic microbes as a basis, heterotrophic microbes and animals flourish in cold seeps as members of the chemosynthetic ecosystem. In order to flourish in a oxygen-poor cold seep environment which is instead rich in toxic substances such as hydrogen sulfide, the animals found there often have a detoxification system (Felbeck et al., 1985). Most animals living in cold seeps house chemosynthetic bacteria, such as sulfur-oxidizing bacteria and methane-oxidizing bacteria, and obtain organic matter from those symbionts. These symbiotic chemosynthetic bacteria are called as chemosymbiotic bacteria. The animals living in cold seeps are generally endemic at a higher taxon level and diverge from the animals belonging to photosynthetic ecosystems. Major animals living in a cold seep environment are vesicomyid, mytilid, thyasirid, lucinid, and solemyid bivalves, provannid, acmaeid and neomphalid gastropods, and vestimentiferan tube worms. Crabs, sea urchins, sponges, and barnacles are occasionally found as minor faunal elements. Nectobenthic animals, i.e., animals that can live near the sea floor, such as hydrothermal vent shrimps, are relatively fewer in number at cold seeps than in hydrothermal vent fields. This is because the reduced compounds, their energy source, are often consumed already within the sediments, and only occasionally appear above the sea floor. In contrast, infaunal animals, i.e., animals that live in the sediment, are more abundant in cold seeps than in hydrothermal vents, because the cold seeps are usually covered by rich soft sediments whereas fresh (recently formed) hard igneous rocks are exposed at hydrothermal vent fields.

The biotic composition of cold seeps is controlled by physicochemical factors including the quantity of reduced compounds such as methane and hydrogen sulfides. One good example of the biotic elements controlled by methane accumulations is seen at Hydrate Ridge (Cascadia Margin, off Oregon). Beggiatoa (sulfur-oxidizing bacteria) mats, Calyptogena (vesicomyid bivalve), and Acharax (solemyid bivalve) all flourish in this cold seep area in a zoned distribution. A large concentration of hydrogen sulfide was detected in the Beggiatoa zone, decreasing toward the Acharax zone through the Calyptogena zone (Sahling et al., 2002). A similar regulation of distribution caused by the concentration of hydrogen sulfide occurred at the species level. Two species of Calyptogena (bivalve), C. kilmeri and C. pacifica, coexisted in a cold seep but they were roughly segregated. The C. kilmery lived where there was a much higher concentration of hydrogen sulfide and the other lived where the concentration of hydrogen sulfide was much lower (Barry et al., 1997).

The biotic component that lives in cold seep environments looks similar to those found in other reduced environments such as hydrothermal vents, sunken vertebrate carcasses, and driftwood (see Chapter Whale and Wood Falls ). Actually, the components are mostly the same at a higher taxonomic level (i.e., genus or family level), although they are restricted to living in a single type of habitat at the species level (Peek et al., 1997), except for vertebrate carcasses (Smith and Baco, 2003). Twenty species found in a cold seep environment also live on/around vertebrate carcasses. Thus, vertebrate carcasses may play a role as a dispersal stepping stone prior to living in a cold seep at least for some species. It is generally thought that the macro organisms in hydrothermal vent environments have evolved from cold seep species (Hecker, 1985; McLean 1985; Craddock et al., 1995). In addition, molecular data indicate that mytilid bivalves living in a cold seep environment adapt to such a reduced environment via whale carcasses as an evolutionary stepping stone (Distel et al., 2000; Smith and Baco, 2003).

Ancient cold seeps

Recognition of ancient cold seeps

Numerous ancient cold seeps dating from various ages have been found in a variety of tectonic settings (Figures 1 and 8; see review in Campbell, 2006). The identification of ancient cold seeps depends on revealing features which indicate the presence of methane and other hydrocarbons which escaped from the sea floor. In order to make these features evident, a comprehensive approach based on geological, geochemical, and paleontological techniques is a primary requirement.
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Cold Seeps. Figure 8

Late Cretaceous cold seep deposit (above) and dense accumulation of lucinid bivalves associated with the carbonate (below) from South Dakota, USA. The carbonate rock was induced by anaerobic oxidation of methane at a late Cretaceous active cold seep in the Western Interior Seaway, located in the eastern part of what is today the Rocky Mountains in northwestern America.

It is well known that carbonate minerals are forming in modern cold seep environments as a result of increased alkalinity due to AOM (Ritger et al., 1987; Aharon, 2000). Methane-derived carbonates are potentially imprinted with environmental information concerning the location where they were formed. Shapes, textures, and imprinted chemical signatures of modern cold seep deposits can be seen as a good analog facilitating the recognition of ancient cold seeps. A conduit pipe such as a carbonate chimney (Takeuchi et al., 2007), although they are generally formed under the sea floor in contrast to chimneys at hydrothermal vents, is a good example indicating the route of fluid. Cold seep carbonates often have chaotic fractured structures. The fracture is usually ductile and/or accompanied by brittle sediment deformations. The deformations are probably due to active fluid venting. Microbially induced carbonate cements, e.g., clotted fabrics and isopachous fibrous cements, are frequently found in the carbonate. The influence of methane and microbial activity can be detected by further biogeochemical analysis (Peckmann and Thiel, 2004).

The carbon isotopic compositions of the carbonate reflect the values of the carbon source. Methane, the main gaseous component of cold seeps, generally shows negative carbon isotopic compositions in the case of thermogenic and biogenic methane (see above). The carbon isotopic compositions of thermogenic and biogenic methane commonly range from −30 to −50‰ and −50 to −80‰ (vs. PDB), respectively, and are thus distinguished from other carbon sources in the marine environment. These values will be imprinted at roughly the same level in the carbon isotopic compositions of the methane-derived carbonate. Considering that the δ13C-values of carbonate are often somewhat higher than the values of the source methane because of the admixture of other carbon sources (Peckmann and Thiel, 2004), values ca. lower than −30‰ (vs. PDB) suggest that a major portion of the carbonate carbon is derived from methane.

Lipid biomarkers provide information on ancient microbial activities. It has been revealed that AOM is mediated by microbial consortia composed of archaea and sulfate-reducing bacteria (Orphan et al., 2002). Cell membrane lipids produced by those microbes have a strong propensity to be preserved in sediments (Peckmann and Thiel, 2004). Analyzing the carbon isotopic composition of each of these biomarkers (see entry Biomarkers (Organic, Compound-Specific Isotopes) ) tells us whether or not the microbes used methane as the carbon source for their biosynthesis. The carbon isotopic compositions of lipids derived from methanotrophic microbes sometimes reach as low as −120‰ or even lower. On the basis of this information, the presence of isotopically depleted lipid biomarkers derived from archaea will be a good indicator for the occurrence of AOM (Elvert et al., 2005).

Oldest cold seep

Ancient cold seeps with chemosymbiotic macrofossils have been traced back to at least Silurian (Barbieri et al., 2004). As it is difficult to find and recognize ancient cold seeps without macrofossils, there is practically no record of older cold seeps. However, a possible cold seep occurring after the severe glacial period around 600 Ma, i.e., the period known as “Snowball Earth,” has recently been proposed. The evidence for a cold seep related to the dissociation of methane gas hydrate is the strongly negative carbon isotopic composition of the “cap carbonate” from south China (Jiang et al., 2003). However, cap carbonates on a glacial deposit formed around 600 Ma are widely distributed throughout the world. If all the cap carbonates were formed under the influence of dissociating methane gas hydrate, numerous cold seeps would have appeared at that time. It is easy to imagine that widespread cold seeps existed in the Precambrian because biogenic methane already existed at 3.5 Ga (Ueno et al., 2006). The apparent lack of cold seeps in the Precambrian, apart from the cap carbonate, is probably due to their having been overlooked because of a lack of associated macro fossils.

Changes in cold seeps through time

The number of cold seeps varies through the Phanerozoic. In the Paleozoic, there are only five ancient cold seep localities. In spite of the fact that a stratigraphic change in carbon isotope ratios shows a global negative shift at the Permian/Triassic boundary, probably caused by the release of a large amount of methane into the ocean and the atmosphere, probably from methane gas hydrate, there are no records of ancient cold seeps from the Permian and Triassic. However, the number of records of ancient cold seeps in post-Triassic marine sediments has increased. More than 15 localities have been found in the Mesozoic and in an abundance of localities, more than a dozen, from the Cenozoic (Campbell, 2006). The cause of the drastic increase in cold seeps since the late Mesozoic might be due to an increase of organic flux into sediments resulting from increasing planktonic productivity and diversity during the Mesozoic Era (Tappan and Loeblich, 1973; COSOD II, 1987) leading to increasing generation of methane in marine sediments.

The biotic components of cold seeps changed through the Phanerozoic (Campbell and Bottjer, 1995; Little and Vrijenhoek, 2003). Mollusks are the most dominant organism in modern cold seeps, despite the dominance of brachiopods in the Paleozoic and mid-Mesozoic. The brachiopods were replaced by mollusks, mainly by bivalves, in the late Mesozoic (Campbell and Bottjer, 1995; Kiel and Peckmann, 2008). The last abundant occurrence of brachiopods in cold seeps is found from the Campanian, late Cretaceous (ca. 80 Ma), from the Omagari, Hokkaido, Japan (Hikida et al., 2003; Kaim et al., 2010). Vesicomyid and bathymodiolian bivalves, which flourished in modern cold seeps, entered the cold seep environment during the Eocene (Ca. 40 Ma; Kiel, 2006; Amano and Kiel, 2007; Amano et al., 2008). It should be noted that the fossil record of vesicomyid bivalves has recently been reexamined and its origin changed from Cretaceous to Eocene (Amano and Kiel, 2007; Amano et al., 2008; Kiel et al., 2008). Most living molluscan genera which lived in ancient cold seeps originated in the late Mesozoic to Paleogene (see basic data in Kiel and Little, 2006 and updated data in Amano et al., 2007; Jenkins et al., 2007; Campbell et al., 2008; Kaim et al., 2008; Kiel et al., 2008; Kaim et al., 2009).

Impact on climate change

Judd et al. (2002), indicate that the input of greenhouse gases from the sea floor into the world’s oceanic and atmospheric systems may be an important component of the atmospheric carbon budget, because the greenhouse potential of methane is more than 20 times that of carbon dioxide. A catastrophic release of methane may therefore produce global climate changes. Such an event due to the rapid escape of methane into the atmosphere happened at the Paleocene–Eocene boundary (ca. 55.5 Ma). At that time, the rapid dissociation of methane hydrate and subsequent release of methane into the atmosphere resulted in global warming and an increase of 6°C in the sea surface temperature (Dickens et al., 1995). A similar methane release outburst is thought to have occurred at the end-Permian forming one of the biggest known biodiversity crises (see entry Critical Intervals in Earth History ) (Ryskin, 2003; Retallack and Krull, 2006). Kennet et al. (2002) proposed that the periodic glacial-interglacial cycle during Quaternary was controlled by the decomposition of methane gas hydrate and the subsequent release of methane gas into the atmosphere. Evidence of these methane releases comes mostly from carbon isotopic anomalies which were recorded in carbonate and organic material. The release of isotopically depleted biogenic methane induces negative carbon isotope excursions because of the transfer of 13C-depleted carbon from methane to carbon dioxide.

Summary

Cold seeps have been the object of considerable attention because of their role as a connecting pipe between the lithosphere, hydrosphere, and biosphere on the Earth. Reduced compounds contained in cold seep fluid result in substantial biogeochemical reactions. The establishment of a cold seep requires specific geological and biological background conditions, such as tectonic stress to make a route for the fluid, sedimentary organic matter supply to provide a source of methane and other hydrocarbons, microbial and/or geothermal activities to form the hydrocarbons, and microbial activities accounting for the oxidation of reduced compounds. Revealing the processes and mechanisms has largely progressed during the last 2 decades. For example, it was revealed at the time of the transition from the twentieth to the twenty-first century that anaerobic methane-oxidizing archaea and sulfate-reducing bacteria are responsible for the anaerobic oxidation of methane. However, there are still many questions about cold seeps that remain unresolved, e.g., the long-term behavior of cold seeps, response to climate change and details of various biological processes and mechanisms. Gaining an understanding of cold seeps is a primary requirement because a cold seep provides a condensed version of geobiochemical processes related to methane. Thus, the knowledge that we can obtain from the research on cold seep may help us to understand a much wider range of geobiological phenomena taking place on Earth.

Furthermore, methane, the main component contained in cold seep fluids, has a potentially large effect on global warming. It is evident from several geological records through the Phanerozoic that dissociation of methane gas hydrate has drastic effects in terms of global climate change. Methane is also attractive to human beings because of its large potential as a future energy source. In fact, the methane amount stored in gas hydrate beneath the sea floor and permafrost is more than 2,100 Gt. However, as noted above, methane has a tremendous potential to bring about global climate change on the Earth. Thus, any form of intervention in the natural gases in marine sediments should be carefully carried out on the basis of sufficient knowledge of what will happen when the methane is released from the sea floor. Adopting this stance implies a very strong requirement to reveal the entire range of processes and mechanisms within a cold seep.

Cross-references

Alkalinity

Anaerobic Oxidation of Methane with Sulfate

Archaea

Biomarkers (Molecular Fossils)

Biomarkers (Organic, Compound-Specific Isotopes)

Biosignatures in Rocks

Calcite Precipitation, Microbially Induced

Cap Carbonates

Carbon Isotopes

Carbonate Environments

Carbonates

Dolomite, Microbial

Hydrogen

Hydrothermal Environments, Terrestrial

Iron Isotopes

Iron Sulfide Formation

Isotopes and Geobiology

Methane, Origin

Methane Oxidation (Aerobic)

Microbialites, Stromatolites, and Thrombolites

Mud Mounds

Pore Water

Pyrite Oxidation

Sulfate-Reducing Bacteria

Sulfide Mineral Oxidation

Thiotrophic Bacteria

Whale and Wood Falls

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