Formation of H(2) and CH(4) by weathering of olivine at temperatures between 30 and 70 degrees C

Hydrocarbons such as CH4 are known to be formed through the Fischer-Tropsch or Sabatier type reactions in hydrothermal systems usually at temperatures above 100°C. Weathering of olivine is sometimes suggested to account for abiotic formation of CH4 through its redox lowering and water splitting properties. Knowledge about the CH4 and H2 formation processes at low temperatures is important for the research about the origin and cause of early Earth and Martian CH4 and for CO2 sequestration. We have conducted a series of low temperature, longterm weathering experiments in which we have tested the CH4 and H2 formation potential of forsteritic olivine. The results show low temperature CH4 production that is probably influenced by chromite and magnetite as catalysts. Extensive analyses of a potential CH4 source trapped in the crystal structure of the olivine showed no signs of incorporated CH4. Also, the available sources of organic carbon were not enough to support the total amount of CH4 detected in our experiments. There was also a linear relationship between silica release into solution and the net CH4 accumulation into the incubation bottle headspaces suggesting that CH4 formation under these conditions could be a qualitative indicator of olivine dissolution. It is likely that minerals such as magnetite, chromite and other metal-rich minerals found on the olivine surface catalyze the formation of CH4, because of the low temperature of the system. This may expand the range of environments plausible for abiotic CH4 formation both on Earth and on other terrestrial bodies. Background The CH4 detected in the Martian atmosphere [1-3] in 2004 raised the question whether or not the CH4 were formed biotically or abiotically. It was suggested by Krasnopolsky et al. [3] that microorganisms on Mars may have produced it. However, several abiotic processes may be responsible for the detected atmospheric CH4, such as volcanism, exogenous sources and serpentinization of ultramafic rocks [4-6]. There are too few hot spots present on Mars to account for the CH4 concentrations that were detected and volcanism is not likely to be the major source of CH4 on Mars. Neither are the exogenous sources, such as meteorites and comets, for the same reason. Oze and Sharma [4] have calculated reaction rates for olivine dissolution on Mars, using olivine chemical compositions found in the Martian Schergottite-Nakhlite-Chassigny (SNC) meteorites, a temperature of 25°C and varying pH. They came to the conclusion that dissolution of olivine is favorable in the subsurface of Mars at such low temperatures, both kinetically and thermodynamically, which means that serpentinization would be a potential source for CH4 detected on the Martian atmosphere. On the contemporary Earth, there are also CH4 seeps and plumes that are suggested to be of abiotic origin, at least to some extent [7-9]. Abiotically formed CH4 may provide carbon and energy for microorganisms in the deep subsurface biosphere and may serve as a precursor for forming longer hydrocarbons such as natural gas and oil. This process may be important for CO2 sequestration. Basaltic (45-52% SiO2) and ultramafic (<45% SiO2) hydrothermal systems as well as continental groundwaters host a vast number of bacterial and archaeal organisms [10,11] found at depths down to at least 800 meters below the seafloor (mbsf) [12] and in volcanic glass at depths down to 954 mbsf [13]. Microbial communities are also found in volcanic hot springs, in saline groundwaters at depths exceeding 2 km in igneous rocks, and in continental flood basalts [11]. Some microorganisms living in these environments are chemolithoautothrophs, i.e., they are autotrophic * Correspondence: [email protected] Department of Geological Sciences, Stockholm University, Sweden Full list of author information is available at the end of the article Neubeck et al. Geochemical Transactions 2011, 12:6 http://www.geochemicaltransactions.com/content/12/1/6 © 2011 Neubeck et al; licensee Chemistry Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. organisms that derive their energy from inorganic compounds such as H2 and CH4 emanating from rock-associated fluids and gases. An important question is to what extent microorganisms can use the chemical energy released exclusively from the alteration of olivine, one of the most common mineral in the Earth mantle [14-19]. This question bears upon the dynamics of contemporary subsurface microbial communities and the possibilities for such extreme environments to be modern analogues to early Earth ecosystems. Weathering of olivine is sometimes called serpentinization due to the formation of serpentine minerals as alteration products. Fluids associated with serpentinization hydrothermal vent systems such as Lost City in the Atlantic Ocean often show elevated concentrations of CH4 [7], which can be a product of H2 reacting with CO2 or CO, gases that can be found in hydrothermal systems. Hence the abiotic interaction between water and mafic minerals can result in formation of H2 and CH4 which both represent high quality electron donors for chemosynthetic organisms (e.g. hydrogenotrophic and methanotrophic microorganisms). The release of H2 from weathering of mafic minerals may be due to either formation through water reduction or release from the mineral itself. Freund et al. [20] suggest that nominally anhydrous minerals such as olivine, contains a considerable amount of H2 within its crystal structure in the form of hydroxyl anions (OH) or peroxy links released upon fracturing or heat. The formation of molecular H2 may also be coupled to the formation of magnetite (Eq. 1). In that reaction, ferrous iron is oxidized to ferric iron together with the reduction of water to H2. However, if the silica activity is high, serpentine or brucite will incorporate the iron into the crystal structure and thus prevent it from becoming oxidized [21] and thus prevent H2 formation. The Fischer-Tropsch (FT) reaction (Eq. 2) is widely known in the oil and petroleum industry as an abiotic, catalyzed reaction capable of producing CH4 and longer hydrocarbons such as petroleum, waxes and oils [22] from gaseous H2 and CO. The usual catalysts for that reaction are magnetite, Co and Ru oxides. The specific formation of CH4 from H2 and CO2 is also called the Fischer-Tropsch Type (FTT) or Sabatier reaction (Eq. 3). The FTT reactions are modified from the FT reaction in the way that the carbon source is CO2 instead of CO and the presence of water [23]. This reaction is often used to explain the presence of abiotic CH4 and other hydrocarbons in some natural systems on Earth [8]. The formation of CH4 in ultramafic natural systems is often thought to be the combination of the FTT reaction linked to the formation of H2 through the olivine hydration process [7,24]. Mg1.8Fe0.2SiO4 + 1.37H2O → 0.5Mg3Si2O5(OH)4 + 0.3Mg(OH)2 + 0.067Fe3O4 + 0.067H2 olivine serpentine brucite magnetite (1) (2n + 1)H2 + nCO → CnH(2n+2) + nH2O (2) CO2 + 4H2 → CH4 + 2H2O (3) FTT reactions are considered to be common in hydrothermal systems and ultramafic rocks and have also been the focus for research considering the abiotic formation of precursors of biologically critical molecules such as amino acids and lipids [7,8,17,25]. Berndt et al. [26] conducted olivine dissolution experiments based on the study of Janecky and Seyfried [27]. They wanted to explicitly study the CH4 forming processes coupled to olivine dissolution and serpentinization at 300°C and 500 bars. They could see a distinct increase in CH4 throughout the experiments and also an increase in other hydrocarbons such as C2H6 and C3H8. The catalyst present in their experiment was exclusively magnetite. Later, Horita et al. [28] confirmed the formation of CH4 through serpentinization, but also showed that magnetite is not the only and most efficient catalyst to form CH4 in an olivine dissolution environment. Instead, the presence of awaruite (Ni3Fe) increased the rate of formation severalfold. Since awaruite is a common associated mineral in ultramafic rocks [29], this approach was highly relevant. Another experiment made by McCollom et al. [30] with the purpose of investigating the formation of hydrocarbons through serpentinization of olivines and with no additional catalysts, showed continuous increase of CH4 throughout the experiment. The experiments were conducted under a pressure of 350 bars and 300°C. However, most of the CH4 (about 80%) found in these experiments was most likely not formed but was suggested to be released from fluid inclusions and carbon species within the olivine crystals. Another interesting observation in their experiments, though, was the need of fresh mineral surfaces in order to form CH4 which was probably due to partial oxidation of the surface. Instead of Ni-bearing catalysts, Foustoukos and Seyfried [23] used a mixture of Cr and Fe oxides (chromite, FeCr2O4) in an effort to produce hydrocarbons under hydrothermal conditions (390°C and 400 bars). Chromite is commonly associated with olivine-rich rocks and would therefore be part of a natural ultramafic hydrothermal system. The found CH4 concentrations were higher than earlier experimental efforts without the presence of Cr,Fe-bearing catalysts. It is now widely accepted that CH4 may be produced abiotically though serpentinization reactions at temperatures around 300°C. Previous studies regarding the FTT or Sabatier reactions often considered temperatures over 100°C. High temperatures promote faster reaction rates Neubeck et al. 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