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The production of methane, hydrogen, and organic compounds in ultramafic-hosted hydrothermal vents of the mid-atlantic ridge.

Konn C, Charlou JL, Holm NG, Mousis O - Astrobiology (2015)

Bottom Line: Considering the vast number of these fields discovered or inferred, hydrothermal fluxes represent a significant input of H2 and CH4 to the ocean.More systematic experiments as well as integrated geochemical approaches are needed to disentangle hydrothermal geochemistry.This understanding is of prime importance considering the implications of hydrothermal H2, CH4, and organic compounds for the ocean global budget, global cycles, and the origin of life.

View Article: PubMed Central - PubMed

Affiliation: 1Ifremer, Unité Géosciences Marine, Laboratoire de Géochime et Métallogénie, F-29280 Plouzané, France.

ABSTRACT
Both hydrogen and methane are consistently discharged in large quantities in hydrothermal fluids issued from ultramafic-hosted hydrothermal fields discovered along the Mid-Atlantic Ridge. Considering the vast number of these fields discovered or inferred, hydrothermal fluxes represent a significant input of H2 and CH4 to the ocean. Although there are lines of evidence of their abiogenic formation from stable C and H isotope results, laboratory experiments, and thermodynamic data, neither their origin nor the reaction pathways generating these gases have been fully constrained yet. Organic compounds detected in the fluids may also be derived from abiotic reactions. Although thermodynamics are favorable and extensive experimental work has been done on Fischer-Tropsch-type reactions, for instance, nothing is clear yet about their origin and formation mechanism from actual data. Since chemolithotrophic microbial communities commonly colonize hydrothermal vents, biogenic and thermogenic processes are likely to contribute to the production of H2, CH4, and other organic compounds. There seems to be a consensus toward a mixed origin (both sources and processes) that is consistent with the ambiguous nature of the isotopic data. But the question that remains is, to what proportions? More systematic experiments as well as integrated geochemical approaches are needed to disentangle hydrothermal geochemistry. This understanding is of prime importance considering the implications of hydrothermal H2, CH4, and organic compounds for the ocean global budget, global cycles, and the origin of life.

No MeSH data available.


Related in: MedlinePlus

Modified after Bradley and Summons (2010). Ranges of δ13C and δD detected in methane produced by a variety of sources. “Autotrophic” and “heterotrophic” is microbial methane. “Thermogenic” refers to cracking of biologically derived oils, while “geothermal” refers to cracking of high-molecular-weight organic compounds. The remaining are values observed at several locations where abiotic methane formation has been suggested: Canadian Shield gases (including Kidd Creek), the Oman ophiolite, Zambales ophiolite. The dark surface represents the range of values (from Table 2) measured for methane in fluids from the Rainbow, Lost City, Logatchev 1 and 2, Ashadze 1 and 2 ultramafic-hosted vent fields.
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f3: Modified after Bradley and Summons (2010). Ranges of δ13C and δD detected in methane produced by a variety of sources. “Autotrophic” and “heterotrophic” is microbial methane. “Thermogenic” refers to cracking of biologically derived oils, while “geothermal” refers to cracking of high-molecular-weight organic compounds. The remaining are values observed at several locations where abiotic methane formation has been suggested: Canadian Shield gases (including Kidd Creek), the Oman ophiolite, Zambales ophiolite. The dark surface represents the range of values (from Table 2) measured for methane in fluids from the Rainbow, Lost City, Logatchev 1 and 2, Ashadze 1 and 2 ultramafic-hosted vent fields.

Mentions: The possible contribution to the methane budget in fluids from ultramafic-hosted vents may be multiple, as mentioned before and discussed in a review by McCollom (2008). Isotopic considerations appear compulsory to unravel the origin of CH4. Several authors have suggested that abiogenic methane is, or was, produced in various geological settings such as crystalline rocks, ophiolites, gas seepages, fumarolic discharges, and ultramafic-hosted hydrothermal systems on land, based on carbon and hydrogen isotope measurements (Sherwood Lollar et al., 1993, 2006; Fiebig et al., 2004, 2007, 2009; Hosgormez et al., 2008; Taran et al., 2010b; Etiope et al., 2011; Suda et al., 2014). The consensus is that it is formed via the Sabatier and FTT pathways (Reactions 1 and 2). In ultramafic-hosted fluids collected up to now along the MAR, the δ13C(CH4) value is found to be −11.9 ‰ at Lost City, −17.8 ‰ at Rainbow, −10.2 ‰ at Logatchev 1, −6.1 ‰ at Logatchev 2, −12.3 ‰ at Ashadze 1, and −8.7 ‰ at Ashadze 2 (Table 2). These results combined with the δD(CH4) fall into a range that is neither thermogenic nor biogenic as previously found in other hydrothermal fluids and shown in Fig. 3 (Schoell, 1988; Welhan, 1988; Charlou et al., 1993b, 1996b, 2000). The same trend is observed when considering δ13C(CH4) versus the CH4-to-higher-hydrocarbons ratio (C1/C2+) (Table 2). The results for Rainbow, Lost City, Logatchev 1 and 2, and Ashadze 1 are consistent with typical values for unsedimented MOR systems, whereas Ashadze 2 has a lower C1/C2+ ratio and is closer to the Zambales ophiolite values (McCollom, 2008). Only an abiogenic contribution may account for these observations. The variability in δ13C(CH4) observed here and also reported in the literature is probably due to fractionation at different T and P conditions. Nevertheless, there is a clear abiogenic contribution to the CH4 budget in hydrothermal fluids issued from ultramafic environments, which is supported by experiments (Horita and Berndt, 1999; Horita, 2001; Lazar et al., 2012; Cao et al., 2014).


The production of methane, hydrogen, and organic compounds in ultramafic-hosted hydrothermal vents of the mid-atlantic ridge.

Konn C, Charlou JL, Holm NG, Mousis O - Astrobiology (2015)

Modified after Bradley and Summons (2010). Ranges of δ13C and δD detected in methane produced by a variety of sources. “Autotrophic” and “heterotrophic” is microbial methane. “Thermogenic” refers to cracking of biologically derived oils, while “geothermal” refers to cracking of high-molecular-weight organic compounds. The remaining are values observed at several locations where abiotic methane formation has been suggested: Canadian Shield gases (including Kidd Creek), the Oman ophiolite, Zambales ophiolite. The dark surface represents the range of values (from Table 2) measured for methane in fluids from the Rainbow, Lost City, Logatchev 1 and 2, Ashadze 1 and 2 ultramafic-hosted vent fields.
© Copyright Policy - open-access
Related In: Results  -  Collection

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getmorefigures.php?uid=PMC4442600&req=5

f3: Modified after Bradley and Summons (2010). Ranges of δ13C and δD detected in methane produced by a variety of sources. “Autotrophic” and “heterotrophic” is microbial methane. “Thermogenic” refers to cracking of biologically derived oils, while “geothermal” refers to cracking of high-molecular-weight organic compounds. The remaining are values observed at several locations where abiotic methane formation has been suggested: Canadian Shield gases (including Kidd Creek), the Oman ophiolite, Zambales ophiolite. The dark surface represents the range of values (from Table 2) measured for methane in fluids from the Rainbow, Lost City, Logatchev 1 and 2, Ashadze 1 and 2 ultramafic-hosted vent fields.
Mentions: The possible contribution to the methane budget in fluids from ultramafic-hosted vents may be multiple, as mentioned before and discussed in a review by McCollom (2008). Isotopic considerations appear compulsory to unravel the origin of CH4. Several authors have suggested that abiogenic methane is, or was, produced in various geological settings such as crystalline rocks, ophiolites, gas seepages, fumarolic discharges, and ultramafic-hosted hydrothermal systems on land, based on carbon and hydrogen isotope measurements (Sherwood Lollar et al., 1993, 2006; Fiebig et al., 2004, 2007, 2009; Hosgormez et al., 2008; Taran et al., 2010b; Etiope et al., 2011; Suda et al., 2014). The consensus is that it is formed via the Sabatier and FTT pathways (Reactions 1 and 2). In ultramafic-hosted fluids collected up to now along the MAR, the δ13C(CH4) value is found to be −11.9 ‰ at Lost City, −17.8 ‰ at Rainbow, −10.2 ‰ at Logatchev 1, −6.1 ‰ at Logatchev 2, −12.3 ‰ at Ashadze 1, and −8.7 ‰ at Ashadze 2 (Table 2). These results combined with the δD(CH4) fall into a range that is neither thermogenic nor biogenic as previously found in other hydrothermal fluids and shown in Fig. 3 (Schoell, 1988; Welhan, 1988; Charlou et al., 1993b, 1996b, 2000). The same trend is observed when considering δ13C(CH4) versus the CH4-to-higher-hydrocarbons ratio (C1/C2+) (Table 2). The results for Rainbow, Lost City, Logatchev 1 and 2, and Ashadze 1 are consistent with typical values for unsedimented MOR systems, whereas Ashadze 2 has a lower C1/C2+ ratio and is closer to the Zambales ophiolite values (McCollom, 2008). Only an abiogenic contribution may account for these observations. The variability in δ13C(CH4) observed here and also reported in the literature is probably due to fractionation at different T and P conditions. Nevertheless, there is a clear abiogenic contribution to the CH4 budget in hydrothermal fluids issued from ultramafic environments, which is supported by experiments (Horita and Berndt, 1999; Horita, 2001; Lazar et al., 2012; Cao et al., 2014).

Bottom Line: Considering the vast number of these fields discovered or inferred, hydrothermal fluxes represent a significant input of H2 and CH4 to the ocean.More systematic experiments as well as integrated geochemical approaches are needed to disentangle hydrothermal geochemistry.This understanding is of prime importance considering the implications of hydrothermal H2, CH4, and organic compounds for the ocean global budget, global cycles, and the origin of life.

View Article: PubMed Central - PubMed

Affiliation: 1Ifremer, Unité Géosciences Marine, Laboratoire de Géochime et Métallogénie, F-29280 Plouzané, France.

ABSTRACT
Both hydrogen and methane are consistently discharged in large quantities in hydrothermal fluids issued from ultramafic-hosted hydrothermal fields discovered along the Mid-Atlantic Ridge. Considering the vast number of these fields discovered or inferred, hydrothermal fluxes represent a significant input of H2 and CH4 to the ocean. Although there are lines of evidence of their abiogenic formation from stable C and H isotope results, laboratory experiments, and thermodynamic data, neither their origin nor the reaction pathways generating these gases have been fully constrained yet. Organic compounds detected in the fluids may also be derived from abiotic reactions. Although thermodynamics are favorable and extensive experimental work has been done on Fischer-Tropsch-type reactions, for instance, nothing is clear yet about their origin and formation mechanism from actual data. Since chemolithotrophic microbial communities commonly colonize hydrothermal vents, biogenic and thermogenic processes are likely to contribute to the production of H2, CH4, and other organic compounds. There seems to be a consensus toward a mixed origin (both sources and processes) that is consistent with the ambiguous nature of the isotopic data. But the question that remains is, to what proportions? More systematic experiments as well as integrated geochemical approaches are needed to disentangle hydrothermal geochemistry. This understanding is of prime importance considering the implications of hydrothermal H2, CH4, and organic compounds for the ocean global budget, global cycles, and the origin of life.

No MeSH data available.


Related in: MedlinePlus