Carbon capture and storage (CCS) provides a solution towards decarbonization of the 21global economy. The success of this solution depends on the ability to safely and 22 2 permanently store CO 2 . This study demonstrates for the first time the permanent 23 disposal of CO 2 as environmentally benign carbonate minerals in basaltic rocks. We 24 find that over 95% of the CO 2 injected into the CarbFix site in Iceland was mineralized 25 to carbonate minerals in less than two years. This result contrasts with the common 26 view that the immobilization of CO 2 as carbonate minerals within geologic reservoirs 27 takes several hundreds to thousands of years. Our results, therefore, demonstrate 28 that the safe long-term storage of anthropogenic CO 2 emissions through 29 mineralization can be far faster than previously postulated. 30 31The success of geologic CO 2 storage depends on its long-term security and public 32 acceptance in addition to regulatory, policy, and economical factors (1). CO 2 and brine 33 leakage through a confining system above the storage reservoir or through abandoned 34 wells is considered as one of the major challenges associated with geologic CO 2 storage 35 [e.g. (2, 3, 4)]. Leakage rates into the atmosphere of ≤0.1% are required to ensure 36 effective climate change mitigation [e.g. (5, 6)]. To avoid CO 2 leakage, caprock integrity 37 needs to be evaluated and monitored (7). Leakage risk is further enhanced by induced 38 seismicity, which may open fluid flow pathways in the caprock (8). Mineral 39 carbonatization (i.e. the conversion of CO 2 to carbonate minerals) via CO 2 -fluid-rock 40 reactions in the reservoir minimizes the risk of leakage and thus facilitates long-term 41 and safe carbon storage and public acceptance (9). The potential for carbonatization is, 42 however, limited in conventional CO 2 storage reservoirs such as deep saline aquifers, 43 and depleted oil and gas reservoirs in sedimentary basins due to the lack of calcium, 44 3 magnesium and iron rich silicate minerals required to form carbonate minerals (10, 11). 45An alternative is to inject CO 2 into basaltic rocks, which contain up to 25% by weight of 46 calcium, magnesium and iron. Basaltic rocks are highly reactive, and one of the most 47 common rock types on Earth, covering ~10% of continental surface area and most of the 48 ocean floor [e.g. (12, 13)]. 49 50The CarbFix pilot project was designed to promote and verify in situ CO 2 mineralization 51 in basaltic rocks for the permanent disposal of anthropogenic CO 2 emissions (14). Two 52 injection tests were performed at the CarbFix injection site near the Hellisheidi 53 geothermal power plant: Phase I: 175 tons of pure CO 2 from January to March 2012, and 54Phase II: 73 tons of a CO 2 -H 2 S gas mixture in June to August 2012, of which 55 tons were 55 CO 2 . Note that H 2 S is not only a major constituent of geothermal gases but also of CO 2 -56 rich sour gas. Since the cost of CCS is dominated by capture and gas separation, the 57 overall cost could be lowered substan...

Near-surface reaction of CO 2 -bearing fluids with silicate minerals in peridotite and basalt forms solid carbonate minerals. Such processes form abundant veins and travertine deposits, particularly in association with tectonically exposed mantle peridotite. This is important in the global carbon cycle, in weathering, and in understanding physical-chemical interaction during retrograde metamorphism. Enhancing the rate of such reactions is a proposed method for geologic CO 2 storage, and perhaps for direct capture of CO 2 from near-surface fluids. We review, synthesize, and extend inferences from a variety of sources. We include data from studies on natural peridotite carbonation processes, carbonation kinetics, feedback between permeability and volume change via reaction-driven cracking, and proposed methods for enhancing the rate of natural mineral carbonation via in situ processes ("at the outcrop") rather than ex situ processes ("at the smokestack"). 545 Annu. Rev. Earth Planet. Sci. 2011.39:545-576. Downloaded from www.annualreviews.org by University of British Columbia on 10/27/12. For personal use only.

Microbial abundance and diversity in deep subsurface environments is dependent upon the availability of energy and carbon. However, supplies of oxidants and reductants capable of sustaining life within mafic and ultramafic continental aquifers undergoing low-temperature water-rock reaction are relatively unknown. We conducted an extensive analysis of the geochemistry and microbial communities recovered from fluids sampled from boreholes hosted in peridotite and gabbro in the Tayin block of the Samail Ophiolite in the Sultanate of Oman. The geochemical compositions of subsurface fluids in the ophiolite are highly variable, reflecting differences in host rock composition and the extent of fluid-rock interaction. Principal component analysis of fluid geochemistry and geologic context indicate the presence of at least four fluid types in the Samail Ophiolite (“gabbro,” “alkaline peridotite,” “hyperalkaline peridotite,” and “gabbro/peridotite contact”) that vary strongly in pH and the concentrations of H2, CH4, Ca2+, Mg2+, NO3-, SO42-, trace metals, and DIC. Geochemistry of fluids is strongly correlated with microbial community composition; similar microbial assemblages group according to fluid type. Hyperalkaline fluids exhibit low diversity and are dominated by taxa related to the Deinococcus-Thermus genus Meiothermus, candidate phyla OP1, and the family Thermodesulfovibrionaceae. Gabbro- and alkaline peridotite- aquifers harbor more diverse communities and contain abundant microbial taxa affiliated with Nitrospira, Nitrosospharaceae, OP3, Parvarcheota, and OP1 order Acetothermales. Wells that sit at the contact between gabbro and peridotite host microbial communities distinct from all other fluid types, with an enrichment in betaproteobacterial taxa. Together the taxonomic information and geochemical data suggest that several metabolisms may be operative in subsurface fluids, including methanogenesis, acetogenesis, and fermentation, as well as the oxidation of methane, hydrogen and small molecular weight organic acids utilizing nitrate and sulfate as electron acceptors. Dynamic nitrogen cycling may be especially prevalent in gabbro and alkaline peridotite fluids. These data suggest water-rock reaction, as controlled by lithology and hydrogeology, constrains the distribution of life in terrestrial ophiolites.

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The increasing concentrations of CO2 in the atmosphere are attributed to the rising consumption of fossil fuels for energy generation around the world. One of the most stable and environmentally benign methods of reducing atmospheric CO2 is by storing it as thermodynamically stable carbonate minerals. Olivine ((Mg,Fe)2SiO4) is an abundant mineral that reacts with CO2 to form Mg-carbonate. The carbonation of olivine can be enhanced by injecting solutions containing CO2 at high partial pressure into olivine-rich formations at high temperatures, or by performing ex situ mineral carbonation in a reactor system with temperature and pressure control. In this study, the effects of NaHCO3 and NaCl, whose roles in enhanced mineral carbonation have been debated, were investigated in detail along with the effects of temperature, CO2 partial pressure and reaction time for determining the extent of olivine carbonation and its associated chemical and morphological changes. At high temperature and high CO2 pressure conditions, more than 70% olivine carbonation was achieved in 3 hours in the presence of 0.64 M NaHCO3. In contrast, NaCl did not significantly affect olivine carbonation. As olivine was dissolved and carbonated, its pore volume, surface area and particle size were significantly changed and these changes influenced subsequent reactivity of olivine. Thus, for both long-term simulation of olivine carbonation in geologic formations and the ex situ reactor design, the morphological changes of olivine during its reaction with CO2 should be carefully considered in order to accurately estimate the CO2 storage capacity and understand the mechanisms for CO2 trapping by olivine.

Serpentinization can generate highly reduced fluids replete with hydrogen (H2) and methane (CH4), potent reductants capable of driving microbial methanogenesis and methanotrophy, respectively. However, CH4 in serpentinized waters is thought to be primarily abiogenic, raising key questions about the relative importance of methanogens and methanotrophs in the production and consumption of CH4 in these systems. Herein, we apply molecular approaches to examine the functional capability and activity of microbial CH4 cycling in serpentinization-impacted subsurface waters intersecting multiple rock and water types within the Samail Ophiolite of Oman. Abundant 16S rRNA genes and transcripts affiliated with the methanogenic genus, Methanobacterium, were recovered from the most alkaline (pH > 10), H2- and CH4-rich subsurface waters. Additionally, 16S rRNA genes and transcripts associated with the aerobic methanotrophic genus, Methylococcus, were detected in wells that spanned varied fluid geochemistry. Metagenomic sequencing yielded genes encoding homologs of proteins involved in the hydrogenotrophic pathway of microbial CH4 production and in microbial CH4 oxidation. Transcripts of several key genes encoding methanogenesis/methanotrophy enzymes were identified, predominantly in communities from the most hyperalkaline waters. These results indicate active methanogenic and methanotrophic populations in waters with hyperalkaline pH in the Samail Ophiolite thereby supporting a role for biological CH4 cycling in aquifers that undergo low temperature serpentinization. Importance Serpentinization of ultramafic rock can generate conditions favorable for microbial methane (CH4) cycling, including the abiotic production of H2 and possibly CH4. Systems of low-temperature serpentinization are geobiological targets due to their potential to harbor microbial life and ubiquity throughout Earth's history. Biomass in fracture waters collected from the Samail Ophiolite of Oman, a system undergoing modern serpentinization, yielded DNA and RNA signatures indicative of active microbial methanogenesis and methanotrophy. Intriguingly, transcripts for proteins involved in methanogenesis were most abundant in the most highly-reacted waters that have hyperalkaline pH and elevated concentrations of H2 and CH4. These findings suggest active biological methane cycling in serpentinite-hosted aquifers, even under extreme conditions of high pH and carbon limitation. These observations underscore the potential for microbial activity to influence the isotopic composition of CH4 in these systems, information that could help in identifying biosignatures of microbial activity on other planets.