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Initial indications of abiotic formation of hydrocarbons in the Rainbow ultramafic hydrothermal system, Mid-Atlantic Ridge
Earth and Planetary Science Letters 191 (2001) 1^8 www.elsevier.com/locate/epsl
Initial indications of abiotic formation of hydrocarbons in the Rainbow ultrama¢c hydrothermal system, Mid-Atlantic Ridge Nils G. Holm a; *, Jean Luc Charlou b b
a Department of Geology and Geochemistry, Stockholm University, SE-106 91 Stockholm, Sweden De¨partement Ge¨osciences Marines, IFREMER Centre Brest, P.O. 70, 29280 Plouzane¨ Cedex, France
Received 18 December 2000; accepted 12 June 2001
Abstract In the presence of water, olivine of ultramafic rock is oxidized during the process referred to as `serpentinization'. Water in contact with the olivine is reduced to molecular hydrogen (H2 ) with the concomitant oxidation of Fe(II). The molecular hydrogen formed may be used as an energy source by lithotrophic bacteria, but may, at high temperature, also be combined with CO2 for the abiotic formation of organic compounds such as hydrocarbons and fatty acids through Fischer^Tropsch type (FTT) synthesis. Our analyses of fluids from the peridotite-hosted Rainbow hydrothermal field on the Mid-Atlantic Ridge indicate de novo synthesis of linear saturated hydrocarbons. The chain length of the hydrocarbons is between 16 and 29 carbon atoms. The discovery of FTT reactions in ultramafic hydrothermal systems on Earth provides an alternative pathway for the formation of early membranes and the origin of life. ß 2001 Elsevier Science B.V. All rights reserved. Keywords: synthesis; hydrocarbons; geothermal systems; magnetite; life origin; peridotites
1. Introduction The conditions for potential abiotic formation of organic compounds from inorganic precursors have great implications for our understanding of past and present global budgets as well as for the origin of life on Earth [1]. Much of the current discussion has focused on whether the early Earth's inventory of organic compounds was in-
* Corresponding author. E-mail address: [email protected] (N.G. Holm).
troduced from space or was a natural consequence of reactions taking place in a set of di¡erent types of environments on our planet. It is indeed of great interest to the ¢eld of geochemistry to determine plausible pathways for abiotic synthesis of organic compounds. It is, however, also of great importance to identify the natural settings that would be conducive for such reactions. In principle, hydrothermal systems o¡er a type of environment in which the thermal and geochemical activity has continued uninterrupted since the Earth's crust was formed more than four billion years ago. One of the major di¡erences between the modern and early Earth is that ultra-
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ma¢c volcanics, i.e. rocks low in SiO2 , were much more common during the Archean than they are today [2]. Therefore, the recent discovery of the Rainbow hydrothermal ¢eld in a setting of ultrama¢c peridotites of the Mid-Atlantic Ridge has attracted our attention from an abiotic synthesis and origin of life point of view, especially since the hydrothermal vents of the Rainbow systems have been shown to emit high concentrations of reduced gases such as H2 and CH4 [3]. Modern microbiological research seems to con¢rm the existence of a lithoautotrophic subsurface biosphere [4,5]. Unlike organoheterotrophs, which utilize organic matter, lithoautotrophs use the chemical energy stored in minerals and the carbon in carbon dioxide or bicarbonate (see Table 1 for de¢nitions). The concept of a `deep hot biosphere' [6,7], on the other hand, suggests the existence of a deep biosphere underneath the Earth's surface consisting of heterotrophic archaea and bacteria. Their sources of energy would be primordial hydrocarbons that accumulated in the Earth's interior during the accretionary phase of our planet. However, autotrophic organisms appear to outnumber heterotrophic ones at depth in igneous and volcanic rocks [8] and it has been possible to connect the isolation of autotrophic methanogenic archaea to raised concentrations of molecular hydrogen and the occurrence of ¢nely grained magnetite [8]. The most easily accessible source of energy in the lithosphere is the molecular hydrogen (H2 ) that is formed during oxidation of Fe(II) inherent in minerals. One of the most easily weathered Fe(II) minerals is olivine. This is a solid solution Table 1 De¢nitions of the various pre¢xes used to name microorganisms, based on their sources of carbon and of energy Pre¢x Carbon source Organic carbon CO2 Energy source Organic carbon Inorganic electron donor Sunlight
hetero auto organo litho photo
of the Mg mineral forsterite (Mg2 SiO4 ) with a minor contribution (10^20%) of the Fe(II) mineral fayalite (Fe2 SiO4 ). Olivine is particularly common in ultrama¢c rocks ( 6 45% SiO2 ), such as the peridotites of the upper part of the Earth's mantle. During `serpentinization' (transformation to serpentinite rock) of peridotite underneath the ocean £oor, Fe(II) in olivine may be oxidized, primarily to magnetite, with the reduction of water to molecular hydrogen (H2 ):
The overall reaction is associated with high pH, characteristic of deep aquifers such as the Columbia River basalt (pH 8^10.5) [8] and the Oman ophiolite (pH 10^12) [9]. Haggerty [10] has also reported pH 12.6 in ocean £oor serpentine mud containing fragments of peridotite at Conical Seamount in the Mariana Forearc, western Paci¢c. MacLeod et al. [11] have suggested that hydrothermal systems with alkaline solutions (up to pH 11) were common in the Archean due to the predominance of ma¢c rocks on the early Earth. During plate tectonic spreading processes on Earth, newly formed sea£oor normally consists of basalt lavas with a moderate content of olivine. However, because of deep fracturing at slow spreading ridges such as the Mid-Atlantic Ridge, sea£oor formation locally involves signi¢cant uplift of upper mantle and lower crustal material at the ridge axis, thus exposing ultrama¢c rocks. This leads to outcrops of partly serpentinized ultrama¢c mantle-derived peridotites and gabbros of the lower crust in many locations. The Fischer^Tropsch type (FTT) reactions are a group of mechanisms that have high potential for the synthesis of organic compounds in ultrama¢c rocks of oceanic crust. Szatmari [12] has published a hypothesis for the abiotic formation of hydrocarbons in petroleum reservoirs by FTT synthesis on magnetite and hematite catalysts.
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The carbon and hydrogen sources in his model would be CO2 derived from massive metamorphic or igneous decarbonation of subducted sedimentary carbonates and H2 generated by the serpentinization of shallow-mantle lithosphere and ophiolite thrust sheets, respectively. Commercial Fischer^Tropsch reactions are normally optimized for the synthesis of linear or straight-chain hydrocarbons from CO and H2 on metallic Fe, Ni, Co and Ru catalysts or oxide mixtures of Fe(III), such as magnetite, and Zn(II) at high temperature. Linear alcohols may be produced from CO and H2 in the presence of oxide mixtures of Cr(III) and Zn(II), whereas straight hydrocarbons are formed in the presence of oxides of Fe(III) and Zn(II). Formation of linear hydrocarbons and fatty acids has also been reported in laboratory experiments that use metal sul¢des as catalysts. Examples of such minerals would be molybdenum disul¢de [13], nickel sul¢de and tungsten sul¢de [14]. Linear fatty acids, in particular, are important in an origin of life context since they are necessary for the construction of biological membranes. Berndt et al. [15] have performed abiotic organic syntheses using H2 formed during serpentinization, based on results by Janecky and Seyfried [16]. H2 was formed from olivine present in their experimental setup. CO2 was present in the form of dissolved bicarbonate at 300³C and 500 bar. Formation of magnetite during the course of the experiment showed that Fe(II) in the fayalite was oxidized to Fe(III) leading to the generation of H2 . The data of Berndt et al. [15] suggested that magnetite's catalyzing e¡ect on FTT synthesis was not lost during reaction under high water pressures. In addition, Madon and Taylor [17] have shown that magnetite is much less susceptible than metallic iron to poisoning by H2 S. After 69 days of the experiment 1% of the CO2 initially present had been converted to hydrocarbon gases, primarily CH4 but also C2 H6 and C3 H8 . Berndt et al. [15] also reported that a much larger fraction ( s 75%) had been reduced to `carbonaceous' material. Later analyses have, however, shown that most of the reduced carbon was probably present in the form of formic acid [18]. Aqueous FTT experiments carried out by Simoneit and cowork-
3
ers [19,20], in the temperature window 150^250³C for 2^3 days, yielded lipid compounds ranging from C2 to s C35 . These compounds consisted of n-alkanols, n-alkanoic acids, n-alkyl formates, n-alkanals, n-alkanes, n-alkenes, and n-alkanones, i.e. all straight-chain molecules. The rationale for models of hydrothermal systems as sites for abiotic synthesis of organic compounds and the origin of life has been discussed extensively for about 20 years. The ¢rst widely distributed article proposing such ideas was published by Corliss et al. in 1981 [21]. A multi-authored book on the same subject was edited by Holm and published in 1992 [22]. In a series of papers Shock has presented the thermodynamic background of the metastability of organic compounds in hydrothermal systems that are bu¡ered with respect to the redox conditions. Such bu¡ering e¡ects can be obtained due to the establishment of equilibrium conditions between common minerals and gases in di¡erent mineral assemblages [23^26]. The redox conditions at the surface of the young basaltic ocean £oor down to about 300^1300 m below the sea£oor are bu¡ered, at the most, by the reducing conditions of the PPM mineral assemblage (pyrite (FeS2 ), pyrrhotite (FeS), and magnetite (Fe3 O4 )) [25]. There is, however, much evidence that the £uids reacting with basaltic crust are more oxidizing than this and that the redox conditions are set by plagioclase, epidote, quartz, magnetite, anhydrite, and pyrite (PEQMAP) [27]. In outcropping ultrama¢c rocks the redox conditions are likely to be bu¡ered to conditions much more reducing than the FMQ mineral assemblage, i.e. fayalite (Fe2 SiO4 ), magnetite, and quartz (SiO2 ). The redox conditions of ultrama¢c rocks do often approach those needed to stabilize native metals [15]. Experimental simulations of abiotic synthesis in hydrothermal systems have been relatively few. One successful approach to the hydrothermal formation of amino acids and peptide-like polymers was presented by Yanagawa and Kobayashi [28], although their experimental conditions were not constrained with respect to the redox conditions. Hennet et al. [29], on the other hand, synthesized amino acids under relatively mild hydrothermal conditions in the presence of the PPM redox bu¡-
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er, while the experiments in the presence of olivine by Berndt et al. resulted in the formation of primarily hydrocarbons [15]. It is well known to researchers on the origin of life that it is di¤cult to produce organic molecules from CO2 in abiotic synthesis experiments [30]. However, it is fairly easy from CO [31]. The FTT reactions are likely to proceed in hydrothermal systems of ultrama¢c settings because the reactants are heated at high temperature and pressure in the presence of minerals (magnetite) and native metals such as Fe3 Ni in the mineral awaruite [18]. The native Fe^Ni minerals are more common in ultrama¢cs such as peridotite than basalts. The results reported by Berndt et al. [15], Horita and Berndt [18] and Charlou et al. [3] suggest that the presence of Fe^Ni is crucial for the reduction of CO2 (via formate and CO) to CH4 and other reduced carbon compounds. These
are found at much higher concentrations in association with ultrama¢cs than elsewhere. 2. Methods The Rainbow hydrothermal ¢eld is situated at 36³14PN on the Mid-Atlantic Ridge (Fig. 1). Endmember £uids (364³C) of the ultrama¢c Rainbow hydrothermal system were sampled for the ¢rst time during the French Flores cruise in 1997. The samples analyzed in this study are identical to those for which H2 and CH4 data have been reported in the scienti¢c literature previously [3]. The £uids were collected in 1.2 l titanium samplers in a mini rosette system during dives of the research submersible Nautile. The bottles of the rosette were ¢lled horizontally. The water samples were rapidly transferred to both acid-
Fig. 1. The Rainbow hydrothermal ¢eld is situated in peridotites at 36³14PN on the Mid-Atlantic Ridge (courtesy Chris German).
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Fig. 2. GC-MS analysis of £uids (364³C) of the Rainbow hydrothermal ¢eld. The £uids were sampled by titanium samplers and collected in borosilicate glass vials. The chromatogram shows that C16 ^C29 linear hydrocarbons are present in the hot £uids.
cleaned Schott^Duran borosilicate glass vials and Nalgene0 polypropylene vials on board the R/V L'Atalante and stored in a coldroom. Subsequent analyses and comparisons between water samples stored in glass and polypropylene indicated that the walls of the polypropylene vials leak a certain amount of phthalic acid (plastic softener) to the samples, with the result that the analytical work was focused on water samples stored in borosilicate glass. Later, at IFREMER in Brest, France, dissolved organic compounds present in 50 ml £uid subsamples were concentrated onto Waters Oasis1 HLB (hydrophilic^lipophilic balanced) Sorbent units that had ¢rst been activated by acetonitrile and methanol. This procedure is simple and includes only two processing steps: (1) adsorption^concentration and (2) release of the adsorbed compounds. This minimizes the risk of procedural contamination. The sorbents were brought as carry-on luggage by air to Stockholm where the organic compounds were extracted using 0.5 ml acetonitrile. The recovery of these extracted organic compounds was unexpected
although postulated. Since they were the ¢rst to be collected from an ultrama¢c hydrothermal system they were analyzed primarily on a qualitative basis on a Fisons GC8000/MD800 GC^MS (gas chromatograph coupled to a mass spectrometer). Pure acetonitrile processed through Waters Oasis1 Sorbent units was used as the procedural blank. Fluids of Mid-Atlantic Ridge hydrothermal systems in basaltic settings have been collected and subsampled by the same procedure as used for the Rainbow £uids. The basaltic systems sampled were Menez Gwen, Lucky Strike, TAG, and Snake Pit (Fig. 1). The concentration methodology using HLB Sorbents has also been successfully applied for the concentration of 1^2 l of £uids at 240³C and 290³C from terrestrial hydrothermal systems in basaltic settings on Iceland. Such £uids show the presence of low concentrations (nmol range) of C8 ^C16 linear fatty acids ë . Bjarnason, unpublished (N.G. Holm and J.O data). In this case fatty acids shorter than C8 were expelled with the steam phase during phase
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separation at sampling. Traces of silanols originating from the stationary phase of the sorbent may be found in the ¢nal extract when £uids with low concentrations of organic compounds are analyzed and detected at high ampli¢cation. Other than that contaminants are not found in the hydrothermal samples when normal precautions as described above are taken. 3. Results Charlou et al. [3] reported the discovery of high temperature (364³C) £uids from the Rainbow hydrothermal ¢eld on the Mid-Atlantic Ridge. Shipboard analysis of the £uids showed high concentrations of H2 (13 mmol/kg, more than 40% of the total gas) and CH4 (2.2 mmol/kg), which is about one order of magnitude higher than in hydrothermal £uids of basaltic settings. The H2 - and CH4 -rich plume is associated with active fault zones that expose peridotite rocks. Our shorebased GC-MS analysis of the same samples indicated that the major class of organic compounds present are almost entirely linear saturated hydrocarbons. The chain length of the hydrocarbons was between 16 and 29 carbon atoms (Fig. 2) and can be compared to £uid inclusion studies of slow spreading Southwest Indian Ridge gabbros that have revealed the presence of C2 ^C5 hydrocarbons [32]. Compared to the Rainbow £uids much lower concentrations of organic compounds were detected in £uids collected from Mid-Atlantic Ridge hydrothermal systems in basaltic settings (Menez Gwen, Lucky Strike, TAG, and Snake Pit). 4. Discussion A hydrothermal £uid that cools from near magma chamber conditions to those of the ocean £oor passes from conditions at which CO2 is dominant to those at which CH4 should predominate at stable equilibrium. Such an equilibrium between CO2 and CH4 is, however, unlikely to occur below 500³C in submarine hydrothermal systems because of the rise of kinetic barriers
with decreasing temperatures. Therefore, CO2 will persist metastably at lower temperatures owing to this kinetic inhibition of the CH4 formation [25,26]. After crossing the equivalent line separating the CO2 or CH4 predominance ¢elds during the cooling process CO2 will also be unstable relative to many organic compounds such as hydrocarbons and fatty acids. Evidence from oil reservoirs indicates that kinetic pathways between CO2 and organic compounds exist in natural environments and permit the establishment of metastable equilibrium states. Since the redox conditions in basaltic rock are set by the relatively oxidizing PEQMAP mineral assemblage, their conditions for potential organic synthesis will be di¡erent and less likely to result in abiotic synthesis of organic compounds compared to the ultrama¢c peridotites that are bu¡ered down to conditions stabilizing native metals. The main reason for this is that the cross-over point between predominance of CO2 and CH4 in a system bu¡ered by a reducing assemblage occurs much closer to the lowest temperature where a stable equilibrium between CO2 and CH4 may be established, i.e. 500³C. As an example, while that cross-over occurs at about 250³C in PPM-bu¡ered environments the corresponding transformation happens closer to 400³C with the FMQ assemblage present [25,26]. Therefore, more activation energy is available to overcome di¡erent sets of kinetic barriers for the establishment of metastable equilibrium states in ultrama¢c hydrothermal environments compared to basaltic systems. A £uid temperature of 364³C in the Rainbow ¢eld would be close to optimal in this sense. In basaltic hydrothermal systems (such as Menez Gwen, Lucky Strike, TAG, and Snake Pit), however, the kinetic need for high activation energy is not as compatible with the thermodynamic requirement that the temperature has to decrease to less than 250³C before CO2 would be unstable and likely be transformed to organic compounds such as linear hydrocarbons. In essence, the geochemistry of slow spreading ridges on Earth with outcropping peridotites would thus allow both autotrophic and heterotrophic bacterial communities to co-exist. Abiotic reactions in this environment could provide the basis for carbon ¢xation of both lithoautotrophs
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and organoheterotrophs. At low temperatures molecular hydrogen is provided for lithotrophs due to serpentinization processes solely. At high temperatures, however, abiotic organic compounds such as hydrocarbons are produced in addition to H2 . Such organic matter would be readily available for consumption by organotrophic organisms. The absolute concentrations and quantities on a global scale of organic compounds produced abiotically on the contemporary Earth require additional exploration and analysis. In our view, the initial results presented in this study point to exciting geochemical and geomicrobiological future research opportunities in subsurface environments. Abiotic organic synthesis in ultrama¢c hydrothermal systems has several implications for our view of the origin of life on Earth. Thus far they provide the most potent environments for the initiation of chemical evolution eventually leading to Darwinian evolution. The chemistry of FTT reactions is also likely to promote the concentration and isolation of metabolic systems into separate entities. In contemporary cell membrane lipids, hydrophobicity is provided by hydrocarbon chains of fatty acids with chain lengths ranging from 12 to 20 or more carbons. It has previously been di¤cult to identify plausible sources of such chains in the prebiotic environment on Earth [33]. Deamer and Pashley [33] have remarked that such chain lengths are relatively abundant in carbonaceous chondrites and hypothesized that amphiphilic substances on Earth may have derived from meteoritic infall. The discovery of FTT reactions in ultrama¢c hydrothermal systems on Earth provides an alternative pathway for the formation of early membranes and the ¢rst compartmentalization of life's constituents. Acknowledgements This study was funded by grants from the Swedish Natural Science Research Council and the Crafoord Foundation.[AC]
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References [1] N.G. Holm, E.M. Andersson, Hydrothermal systems, in: A. Brack (Ed.), The Molecular Origins of Life; Assembling Pieces of the Puzzle, Cambridge University Press, Cambridge, 1998, pp. 86^99. [2] E.G. Nisbet, The Young Earth; An Introduction to Archean Geology, Allen and Unwin, Boston, MA, 1987. [3] J.L. Charlou, Y. Fouquet, H. Bougalt, J.P. Donval, J. Etoubleau, Ph. Jean-Baptiste, A. Dapoigny, P. Appriou, P.S. Rona, Intense CH4 plumes generated by serpentinization of ultrama¢c rocks at the intersection of the 15³20PN fracture zone and the Mid-Atlantic Ridge, Geochim. Cosmochim. Acta 62 (1998) 2323^2333. [4] K. Pedersen, The deep subterranean biosphere, Earth-Sci. Rev. 34 (1993) 243^260. [5] J.K. Fredrickson, M. Fletcher, Subsurface Microbiology and Biogeochemistry, Wiley-Liss, New York, 2001. [6] T. Gold, The deep hot biosphere, Proc. Natl. Acad. Sci. USA 89 (1992) 6045^6049. [7] T. Gold, The Deep Hot Biosphere, Springer, Heidelberg, 1999. [8] T.O. Stevens, J.P. McKinley, Lithoautotrophic microbial ecosystems in deep basalt aquifers, Science 270 (1995) 450^454. [9] C. Neal, G. Stanger, Hydrogen generation from mantle source rocks in Oman, Earth Planet. Sci. Lett. 66 (1983) 315^320. [10] J.A. Haggerty, Evidence from £uid seeps atop serpentine seamounts in the Mariana Forearc: Clues for emplacement of the seamounts and their relatioship to forearc tectonics, Mar. Geol. 102 (1991) 293^309. [11] G. MacLeod, C. McKeown, A.J. Hall, M.J. Russell, Hydrothermal and oceanic pH conditions of possible relevance to the origin of life, Origins Life Evol. Biosphere 24 (1994) 19^41. [12] P. Szatmari, Petroleum formation by Fischer-Tropsch synthesis in plate tectonics, Am. Assoc. Petrol. Geol. Bull. 73 (1989) 989^998. [13] H.H. Storch, N. Golumbic, R.B. Anderson, The FischerTropsch and Related Syntheses, John Wiley, New York, 1951. [14] F. Asinger, Para¤ns Chemistry and Technology (translated B.J. Hazzard), Pergamon, Oxford, 1968. [15] M.E. Berndt, D.E. Allen, W.E. Seyfried Jr., Reduction of CO2 during serpentinization of olivine at 300³C and 500 bar, Geology 24 (1996) 351^354. [16] D.R. Janecky, W.E. Seyfried Jr., Hydrothermal serpentinization of peridotite within the oceanic crust: Experimental investigations of mineralogy and major element chemistry, Geochim. Cosmochim. Acta 50 (1986) 1357^ 1378. [17] R.J. Madon, W.F. Taylor, Fischer-Tropsch synthesis on a precipitated iron catalyst, J. Catal. 69 (1981) 32^43. [18] J. Horita, M.E. Berndt, Abiogenic methane formation and isotopic fractionation under hydrothermal conditions, Science 285 (1999) 1055^1057.
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[19] T.M. McCollum, G. Ritter, B.R.T. Simoneit, Lipid synthesis under hydrothermal conditions by Fischer-TropschType reactions, Origins Life Evol. Biosphere 29 (1999) 153^166. [20] A.I. Rushdi, B.R.T. Simoneit, Lipid formation by aqueous Fischer-Tropsch-type synthesis over a temperature range of 100 to 400³C, Origins Life Evol. Biosphere 31 (2001) 103^118. [21] J.B. Corliss, J.A. Baross, S.E. Ho¡man, An hypothesis concerning the relationship between submarine hot springs and the origin of life on Earth, Ocean. Acta Spec. Publ. (1981) 59^69. [22] N.G. Holm, Marine Hydrothermal Systems and the Origin of Life, Kluwer, Dordrecht, 1992. [23] E.L. Shock, Organic acid metastability in sedimentary basins, Geology 16 (1988) 886^890. [24] E.L. Shock, Do amino acids equilibrate in hydrothermal £uids?, Geochim. Cosmochim. Acta 54 (1990) 1185^1189. [25] E.L. Shock, Geochemical constraints on the origin of organic compounds in hydrothermal systems, Origins Life Evol. Biosphere 20 (1990) 331^367. [26] E.L. Shock, Chemical environments of submarine hydrothermal systems, in: N.G. Holm (Ed.), Marine Hydrothermal Systems and the Origin of Life, Kluwer, Dordrecht, 1992, pp. 67^107. [27] W.E. Seyfried, Jr., K. Ding, Phase equilibria in subsea£oor hydrothermal systems: A review of the role of redox, temperature, pH and dissolved Cl on the chemistry of hot
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spring £uids at mid-ocean ridges, in: S.E. Humphris, R.A. Zierenberg, L.S. Mullineaux, R.E. Thomson (Eds.), Sea£oor Hydrothermal Systems; Physical, Chemical, Biological, and Geological Interactions, Geophysical Monograph 91, American Geophysical Union, Washington, DC, 1995, pp. 248^272. H. Yanagawa, K. Kobayashi, An experimental approach to chemical evolution in submarine hydrothermal systems, in: N.G. Holm (Ed.), Marine Hydrothermal Systems and the Origin of Life, Kluwer, Dordrecht, 1992, pp. 147^159. R.J.-C. Hennet, N.G. Holm, M.H. Engel, Abiotic synthesis of amino acids under hydrothermal conditions and the origin of life: a perpetual phenomenon?, Naturwissenschaften 79 (1992) 361^365. S.L. Miller, The endogenous synthesis of organic compounds, in: A. Brack (Ed.), The Molecular Origins of Life; Assembling Pieces of the Puzzle, Cambridge University Press, Cambridge, 1998, pp. 59^85. J.P. Ferris, Chemical markers of prebiotic chemistry in hydrothermal systems, in: N.G. Holm (Ed.), Marine Hydrothermal Systems and the Origin of Life, Kluwer, Dordrecht, 1992, pp. 109^134. D.S. Kelley, Methane-rich £uids in the oceanic crust, J. Geophys. Res. 101 (1996) 2943^2962. D.W. Deamer, R.M. Pashley, Amphiphilic components of the Murchison carbonaceous chondrite: Surface properties and membrane formation, Origins Life Evol. Biosphere 19 (1989) 21^38.
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Initial indications of abiotic formation of hydrocarbons in the Rainbow ultrama¢c hydrothermal system, Mid-Atlantic Ridge Nils G. Holm a; *, Jean Luc Charlou b b
a Department of Geology and Geochemistry, Stockholm University, SE-106 91 Stockholm, Sweden De¨partement Ge¨osciences Marines, IFREMER Centre Brest, P.O. 70, 29280 Plouzane¨ Cedex, France
Received 18 December 2000; accepted 12 June 2001
Abstract In the presence of water, olivine of ultramafic rock is oxidized during the process referred to as `serpentinization'. Water in contact with the olivine is reduced to molecular hydrogen (H2 ) with the concomitant oxidation of Fe(II). The molecular hydrogen formed may be used as an energy source by lithotrophic bacteria, but may, at high temperature, also be combined with CO2 for the abiotic formation of organic compounds such as hydrocarbons and fatty acids through Fischer^Tropsch type (FTT) synthesis. Our analyses of fluids from the peridotite-hosted Rainbow hydrothermal field on the Mid-Atlantic Ridge indicate de novo synthesis of linear saturated hydrocarbons. The chain length of the hydrocarbons is between 16 and 29 carbon atoms. The discovery of FTT reactions in ultramafic hydrothermal systems on Earth provides an alternative pathway for the formation of early membranes and the origin of life. ß 2001 Elsevier Science B.V. All rights reserved. Keywords: synthesis; hydrocarbons; geothermal systems; magnetite; life origin; peridotites
1. Introduction The conditions for potential abiotic formation of organic compounds from inorganic precursors have great implications for our understanding of past and present global budgets as well as for the origin of life on Earth [1]. Much of the current discussion has focused on whether the early Earth's inventory of organic compounds was in-
* Corresponding author. E-mail address: [email protected] (N.G. Holm).
troduced from space or was a natural consequence of reactions taking place in a set of di¡erent types of environments on our planet. It is indeed of great interest to the ¢eld of geochemistry to determine plausible pathways for abiotic synthesis of organic compounds. It is, however, also of great importance to identify the natural settings that would be conducive for such reactions. In principle, hydrothermal systems o¡er a type of environment in which the thermal and geochemical activity has continued uninterrupted since the Earth's crust was formed more than four billion years ago. One of the major di¡erences between the modern and early Earth is that ultra-
0012-821X / 01 / $ ^ see front matter ß 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 1 2 - 8 2 1 X ( 0 1 ) 0 0 3 9 7 - 1
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ma¢c volcanics, i.e. rocks low in SiO2 , were much more common during the Archean than they are today [2]. Therefore, the recent discovery of the Rainbow hydrothermal ¢eld in a setting of ultrama¢c peridotites of the Mid-Atlantic Ridge has attracted our attention from an abiotic synthesis and origin of life point of view, especially since the hydrothermal vents of the Rainbow systems have been shown to emit high concentrations of reduced gases such as H2 and CH4 [3]. Modern microbiological research seems to con¢rm the existence of a lithoautotrophic subsurface biosphere [4,5]. Unlike organoheterotrophs, which utilize organic matter, lithoautotrophs use the chemical energy stored in minerals and the carbon in carbon dioxide or bicarbonate (see Table 1 for de¢nitions). The concept of a `deep hot biosphere' [6,7], on the other hand, suggests the existence of a deep biosphere underneath the Earth's surface consisting of heterotrophic archaea and bacteria. Their sources of energy would be primordial hydrocarbons that accumulated in the Earth's interior during the accretionary phase of our planet. However, autotrophic organisms appear to outnumber heterotrophic ones at depth in igneous and volcanic rocks [8] and it has been possible to connect the isolation of autotrophic methanogenic archaea to raised concentrations of molecular hydrogen and the occurrence of ¢nely grained magnetite [8]. The most easily accessible source of energy in the lithosphere is the molecular hydrogen (H2 ) that is formed during oxidation of Fe(II) inherent in minerals. One of the most easily weathered Fe(II) minerals is olivine. This is a solid solution Table 1 De¢nitions of the various pre¢xes used to name microorganisms, based on their sources of carbon and of energy Pre¢x Carbon source Organic carbon CO2 Energy source Organic carbon Inorganic electron donor Sunlight
hetero auto organo litho photo
of the Mg mineral forsterite (Mg2 SiO4 ) with a minor contribution (10^20%) of the Fe(II) mineral fayalite (Fe2 SiO4 ). Olivine is particularly common in ultrama¢c rocks ( 6 45% SiO2 ), such as the peridotites of the upper part of the Earth's mantle. During `serpentinization' (transformation to serpentinite rock) of peridotite underneath the ocean £oor, Fe(II) in olivine may be oxidized, primarily to magnetite, with the reduction of water to molecular hydrogen (H2 ):
The overall reaction is associated with high pH, characteristic of deep aquifers such as the Columbia River basalt (pH 8^10.5) [8] and the Oman ophiolite (pH 10^12) [9]. Haggerty [10] has also reported pH 12.6 in ocean £oor serpentine mud containing fragments of peridotite at Conical Seamount in the Mariana Forearc, western Paci¢c. MacLeod et al. [11] have suggested that hydrothermal systems with alkaline solutions (up to pH 11) were common in the Archean due to the predominance of ma¢c rocks on the early Earth. During plate tectonic spreading processes on Earth, newly formed sea£oor normally consists of basalt lavas with a moderate content of olivine. However, because of deep fracturing at slow spreading ridges such as the Mid-Atlantic Ridge, sea£oor formation locally involves signi¢cant uplift of upper mantle and lower crustal material at the ridge axis, thus exposing ultrama¢c rocks. This leads to outcrops of partly serpentinized ultrama¢c mantle-derived peridotites and gabbros of the lower crust in many locations. The Fischer^Tropsch type (FTT) reactions are a group of mechanisms that have high potential for the synthesis of organic compounds in ultrama¢c rocks of oceanic crust. Szatmari [12] has published a hypothesis for the abiotic formation of hydrocarbons in petroleum reservoirs by FTT synthesis on magnetite and hematite catalysts.
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The carbon and hydrogen sources in his model would be CO2 derived from massive metamorphic or igneous decarbonation of subducted sedimentary carbonates and H2 generated by the serpentinization of shallow-mantle lithosphere and ophiolite thrust sheets, respectively. Commercial Fischer^Tropsch reactions are normally optimized for the synthesis of linear or straight-chain hydrocarbons from CO and H2 on metallic Fe, Ni, Co and Ru catalysts or oxide mixtures of Fe(III), such as magnetite, and Zn(II) at high temperature. Linear alcohols may be produced from CO and H2 in the presence of oxide mixtures of Cr(III) and Zn(II), whereas straight hydrocarbons are formed in the presence of oxides of Fe(III) and Zn(II). Formation of linear hydrocarbons and fatty acids has also been reported in laboratory experiments that use metal sul¢des as catalysts. Examples of such minerals would be molybdenum disul¢de [13], nickel sul¢de and tungsten sul¢de [14]. Linear fatty acids, in particular, are important in an origin of life context since they are necessary for the construction of biological membranes. Berndt et al. [15] have performed abiotic organic syntheses using H2 formed during serpentinization, based on results by Janecky and Seyfried [16]. H2 was formed from olivine present in their experimental setup. CO2 was present in the form of dissolved bicarbonate at 300³C and 500 bar. Formation of magnetite during the course of the experiment showed that Fe(II) in the fayalite was oxidized to Fe(III) leading to the generation of H2 . The data of Berndt et al. [15] suggested that magnetite's catalyzing e¡ect on FTT synthesis was not lost during reaction under high water pressures. In addition, Madon and Taylor [17] have shown that magnetite is much less susceptible than metallic iron to poisoning by H2 S. After 69 days of the experiment 1% of the CO2 initially present had been converted to hydrocarbon gases, primarily CH4 but also C2 H6 and C3 H8 . Berndt et al. [15] also reported that a much larger fraction ( s 75%) had been reduced to `carbonaceous' material. Later analyses have, however, shown that most of the reduced carbon was probably present in the form of formic acid [18]. Aqueous FTT experiments carried out by Simoneit and cowork-
3
ers [19,20], in the temperature window 150^250³C for 2^3 days, yielded lipid compounds ranging from C2 to s C35 . These compounds consisted of n-alkanols, n-alkanoic acids, n-alkyl formates, n-alkanals, n-alkanes, n-alkenes, and n-alkanones, i.e. all straight-chain molecules. The rationale for models of hydrothermal systems as sites for abiotic synthesis of organic compounds and the origin of life has been discussed extensively for about 20 years. The ¢rst widely distributed article proposing such ideas was published by Corliss et al. in 1981 [21]. A multi-authored book on the same subject was edited by Holm and published in 1992 [22]. In a series of papers Shock has presented the thermodynamic background of the metastability of organic compounds in hydrothermal systems that are bu¡ered with respect to the redox conditions. Such bu¡ering e¡ects can be obtained due to the establishment of equilibrium conditions between common minerals and gases in di¡erent mineral assemblages [23^26]. The redox conditions at the surface of the young basaltic ocean £oor down to about 300^1300 m below the sea£oor are bu¡ered, at the most, by the reducing conditions of the PPM mineral assemblage (pyrite (FeS2 ), pyrrhotite (FeS), and magnetite (Fe3 O4 )) [25]. There is, however, much evidence that the £uids reacting with basaltic crust are more oxidizing than this and that the redox conditions are set by plagioclase, epidote, quartz, magnetite, anhydrite, and pyrite (PEQMAP) [27]. In outcropping ultrama¢c rocks the redox conditions are likely to be bu¡ered to conditions much more reducing than the FMQ mineral assemblage, i.e. fayalite (Fe2 SiO4 ), magnetite, and quartz (SiO2 ). The redox conditions of ultrama¢c rocks do often approach those needed to stabilize native metals [15]. Experimental simulations of abiotic synthesis in hydrothermal systems have been relatively few. One successful approach to the hydrothermal formation of amino acids and peptide-like polymers was presented by Yanagawa and Kobayashi [28], although their experimental conditions were not constrained with respect to the redox conditions. Hennet et al. [29], on the other hand, synthesized amino acids under relatively mild hydrothermal conditions in the presence of the PPM redox bu¡-
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er, while the experiments in the presence of olivine by Berndt et al. resulted in the formation of primarily hydrocarbons [15]. It is well known to researchers on the origin of life that it is di¤cult to produce organic molecules from CO2 in abiotic synthesis experiments [30]. However, it is fairly easy from CO [31]. The FTT reactions are likely to proceed in hydrothermal systems of ultrama¢c settings because the reactants are heated at high temperature and pressure in the presence of minerals (magnetite) and native metals such as Fe3 Ni in the mineral awaruite [18]. The native Fe^Ni minerals are more common in ultrama¢cs such as peridotite than basalts. The results reported by Berndt et al. [15], Horita and Berndt [18] and Charlou et al. [3] suggest that the presence of Fe^Ni is crucial for the reduction of CO2 (via formate and CO) to CH4 and other reduced carbon compounds. These
are found at much higher concentrations in association with ultrama¢cs than elsewhere. 2. Methods The Rainbow hydrothermal ¢eld is situated at 36³14PN on the Mid-Atlantic Ridge (Fig. 1). Endmember £uids (364³C) of the ultrama¢c Rainbow hydrothermal system were sampled for the ¢rst time during the French Flores cruise in 1997. The samples analyzed in this study are identical to those for which H2 and CH4 data have been reported in the scienti¢c literature previously [3]. The £uids were collected in 1.2 l titanium samplers in a mini rosette system during dives of the research submersible Nautile. The bottles of the rosette were ¢lled horizontally. The water samples were rapidly transferred to both acid-
Fig. 1. The Rainbow hydrothermal ¢eld is situated in peridotites at 36³14PN on the Mid-Atlantic Ridge (courtesy Chris German).
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Fig. 2. GC-MS analysis of £uids (364³C) of the Rainbow hydrothermal ¢eld. The £uids were sampled by titanium samplers and collected in borosilicate glass vials. The chromatogram shows that C16 ^C29 linear hydrocarbons are present in the hot £uids.
cleaned Schott^Duran borosilicate glass vials and Nalgene0 polypropylene vials on board the R/V L'Atalante and stored in a coldroom. Subsequent analyses and comparisons between water samples stored in glass and polypropylene indicated that the walls of the polypropylene vials leak a certain amount of phthalic acid (plastic softener) to the samples, with the result that the analytical work was focused on water samples stored in borosilicate glass. Later, at IFREMER in Brest, France, dissolved organic compounds present in 50 ml £uid subsamples were concentrated onto Waters Oasis1 HLB (hydrophilic^lipophilic balanced) Sorbent units that had ¢rst been activated by acetonitrile and methanol. This procedure is simple and includes only two processing steps: (1) adsorption^concentration and (2) release of the adsorbed compounds. This minimizes the risk of procedural contamination. The sorbents were brought as carry-on luggage by air to Stockholm where the organic compounds were extracted using 0.5 ml acetonitrile. The recovery of these extracted organic compounds was unexpected
although postulated. Since they were the ¢rst to be collected from an ultrama¢c hydrothermal system they were analyzed primarily on a qualitative basis on a Fisons GC8000/MD800 GC^MS (gas chromatograph coupled to a mass spectrometer). Pure acetonitrile processed through Waters Oasis1 Sorbent units was used as the procedural blank. Fluids of Mid-Atlantic Ridge hydrothermal systems in basaltic settings have been collected and subsampled by the same procedure as used for the Rainbow £uids. The basaltic systems sampled were Menez Gwen, Lucky Strike, TAG, and Snake Pit (Fig. 1). The concentration methodology using HLB Sorbents has also been successfully applied for the concentration of 1^2 l of £uids at 240³C and 290³C from terrestrial hydrothermal systems in basaltic settings on Iceland. Such £uids show the presence of low concentrations (nmol range) of C8 ^C16 linear fatty acids ë . Bjarnason, unpublished (N.G. Holm and J.O data). In this case fatty acids shorter than C8 were expelled with the steam phase during phase
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separation at sampling. Traces of silanols originating from the stationary phase of the sorbent may be found in the ¢nal extract when £uids with low concentrations of organic compounds are analyzed and detected at high ampli¢cation. Other than that contaminants are not found in the hydrothermal samples when normal precautions as described above are taken. 3. Results Charlou et al. [3] reported the discovery of high temperature (364³C) £uids from the Rainbow hydrothermal ¢eld on the Mid-Atlantic Ridge. Shipboard analysis of the £uids showed high concentrations of H2 (13 mmol/kg, more than 40% of the total gas) and CH4 (2.2 mmol/kg), which is about one order of magnitude higher than in hydrothermal £uids of basaltic settings. The H2 - and CH4 -rich plume is associated with active fault zones that expose peridotite rocks. Our shorebased GC-MS analysis of the same samples indicated that the major class of organic compounds present are almost entirely linear saturated hydrocarbons. The chain length of the hydrocarbons was between 16 and 29 carbon atoms (Fig. 2) and can be compared to £uid inclusion studies of slow spreading Southwest Indian Ridge gabbros that have revealed the presence of C2 ^C5 hydrocarbons [32]. Compared to the Rainbow £uids much lower concentrations of organic compounds were detected in £uids collected from Mid-Atlantic Ridge hydrothermal systems in basaltic settings (Menez Gwen, Lucky Strike, TAG, and Snake Pit). 4. Discussion A hydrothermal £uid that cools from near magma chamber conditions to those of the ocean £oor passes from conditions at which CO2 is dominant to those at which CH4 should predominate at stable equilibrium. Such an equilibrium between CO2 and CH4 is, however, unlikely to occur below 500³C in submarine hydrothermal systems because of the rise of kinetic barriers
with decreasing temperatures. Therefore, CO2 will persist metastably at lower temperatures owing to this kinetic inhibition of the CH4 formation [25,26]. After crossing the equivalent line separating the CO2 or CH4 predominance ¢elds during the cooling process CO2 will also be unstable relative to many organic compounds such as hydrocarbons and fatty acids. Evidence from oil reservoirs indicates that kinetic pathways between CO2 and organic compounds exist in natural environments and permit the establishment of metastable equilibrium states. Since the redox conditions in basaltic rock are set by the relatively oxidizing PEQMAP mineral assemblage, their conditions for potential organic synthesis will be di¡erent and less likely to result in abiotic synthesis of organic compounds compared to the ultrama¢c peridotites that are bu¡ered down to conditions stabilizing native metals. The main reason for this is that the cross-over point between predominance of CO2 and CH4 in a system bu¡ered by a reducing assemblage occurs much closer to the lowest temperature where a stable equilibrium between CO2 and CH4 may be established, i.e. 500³C. As an example, while that cross-over occurs at about 250³C in PPM-bu¡ered environments the corresponding transformation happens closer to 400³C with the FMQ assemblage present [25,26]. Therefore, more activation energy is available to overcome di¡erent sets of kinetic barriers for the establishment of metastable equilibrium states in ultrama¢c hydrothermal environments compared to basaltic systems. A £uid temperature of 364³C in the Rainbow ¢eld would be close to optimal in this sense. In basaltic hydrothermal systems (such as Menez Gwen, Lucky Strike, TAG, and Snake Pit), however, the kinetic need for high activation energy is not as compatible with the thermodynamic requirement that the temperature has to decrease to less than 250³C before CO2 would be unstable and likely be transformed to organic compounds such as linear hydrocarbons. In essence, the geochemistry of slow spreading ridges on Earth with outcropping peridotites would thus allow both autotrophic and heterotrophic bacterial communities to co-exist. Abiotic reactions in this environment could provide the basis for carbon ¢xation of both lithoautotrophs
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and organoheterotrophs. At low temperatures molecular hydrogen is provided for lithotrophs due to serpentinization processes solely. At high temperatures, however, abiotic organic compounds such as hydrocarbons are produced in addition to H2 . Such organic matter would be readily available for consumption by organotrophic organisms. The absolute concentrations and quantities on a global scale of organic compounds produced abiotically on the contemporary Earth require additional exploration and analysis. In our view, the initial results presented in this study point to exciting geochemical and geomicrobiological future research opportunities in subsurface environments. Abiotic organic synthesis in ultrama¢c hydrothermal systems has several implications for our view of the origin of life on Earth. Thus far they provide the most potent environments for the initiation of chemical evolution eventually leading to Darwinian evolution. The chemistry of FTT reactions is also likely to promote the concentration and isolation of metabolic systems into separate entities. In contemporary cell membrane lipids, hydrophobicity is provided by hydrocarbon chains of fatty acids with chain lengths ranging from 12 to 20 or more carbons. It has previously been di¤cult to identify plausible sources of such chains in the prebiotic environment on Earth [33]. Deamer and Pashley [33] have remarked that such chain lengths are relatively abundant in carbonaceous chondrites and hypothesized that amphiphilic substances on Earth may have derived from meteoritic infall. The discovery of FTT reactions in ultrama¢c hydrothermal systems on Earth provides an alternative pathway for the formation of early membranes and the ¢rst compartmentalization of life's constituents. Acknowledgements This study was funded by grants from the Swedish Natural Science Research Council and the Crafoord Foundation.[AC]
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