Effect of serpentinization on carbon speciation: an experiment with formic acid

<p>One of the principal theories about the origin of life is based on the abiotic reduction of carbon oxides to various organic molecules in hydrothermal systems. This synthesis is most favored in ultramafic environments undergoing hydrothermal alteration where the serpentinization reaction efficiently produces H<sub>2</sub>. Nevertheless, decades of hydrothermal experiments have hardly succeeded in producing abundant organic volatiles such as CH<sub>4</sub> and short-chain hydrocarbons. On another hand, natural observations have shown the occurrence of other abiotic compounds such as organic acids in fluids and carbonaceous matter (CM) within serpentinized rocks. But organic acids as carbon source and CM as product have not been investigated so far in experiments reproducing hydrothermal peridotite alteration. Here, we explored the effect of formic acid (HCOOH) on the serpentinization reaction and possible feedback effects on carbon speciation in both fluid and solid. We performed reactions at 300°C and 250 bar using peridotite powder (<40 microns) in the presence of  0.1 M formic acid. A temperature of 300°C has been shown to be optimal for olivine serpentinization, while formic acid should partly decomposed into H<sub>2</sub>, CO, and CO<sub>2</sub>. After 4 months, H<sub>2</sub>, CO, CO<sub>2</sub>, CH<sub>4</sub> and short-chain alkanes (mainly ethane) were measured in the fluid, and the powder was completely indurated. The solidified powder displayed a black and white layering perpendicular to fluid diffusion. Its analysis showed the advancement of the serpentinization reaction, and the incorporation of carbon compounds into the solid phase. XRD analysis indicated 70% of serpentinization. SEM-EDX observations showed peculiar texture with large and localized euhedral magnetite grains alternating with larger magnetite grains mixed with C-enriched areas of long chrysotile fibers. FT-IR measurement attested of the widespread formation of carbonaceous material in the solid. Liquid analyses are under progress. Those first results suggest that serpentine formation not only provides additional H<sub>2</sub> to the system, but also mineral surfaces that could play a role in the precipitation of carbonaceous material and carbon speciation in natural systems. The nature and formation mechanisms of this latter remain to be addressed but this opens new paths for abiotic organic synthesis under hydrothermal conditions. In addition to their implications as an abiotic carbon source for deep hydrocarbon degraders ecosystems, it could have important implications for the total carbon cycle.</p>

Reduction of CO2 with H2S in a simulated deep-sea hydrothermal vent system

CO2 can be reduced to organic molecules, such as formic and acetic acids in a yield of approximately 67% with metal sulfides catalysts, using H2S as a reductant and with SxOy2− as oxidative products in a simulated hydrothermal vent system. These results are significant for understanding abiotic organic synthesis from dissolved CO2 in deep sea hydrothermal vents.

Pathways for abiotic organic synthesis at submarine hydrothermal fields

Arguments for an abiotic origin of low-molecular weight organic compounds in deep-sea hot springs are compelling owing to implications for the sustenance of deep biosphere microbial communities and their potential role in the origin of life. Theory predicts that warm H2-rich fluids, like those emanating from serpentinizing hydrothermal systems, create a favorable thermodynamic drive for the abiotic generation of organic compounds from inorganic precursors. Here, we constrain two distinct reaction pathways for abiotic organic synthesis in the natural environment at the Von Damm hydrothermal field and delineate spatially where inorganic carbon is converted into bioavailable reduced carbon. We reveal that carbon transformation reactions in a single system can progress over hours, days, and up to thousands of years. Previous studies have suggested that CH4 and higher hydrocarbons in ultramafic hydrothermal systems were dependent on H2 generation during active serpentinization. Rather, our results indicate that CH4 found in vent fluids is formed in H2-rich fluid inclusions, and higher n-alkanes may likely be derived from the same source. This finding implies that, in contrast with current paradigms, these compounds may form independently of actively circulating serpentinizing fluids in ultramafic-influenced systems. Conversely, widespread production of formate by ΣCO2 reduction at Von Damm occurs rapidly during shallow subsurface mixing of the same fluids, which may support anaerobic methanogenesis. Our finding of abiogenic formate in deep-sea hot springs has significant implications for microbial life strategies in the present-day deep biosphere as well as early life on Earth and beyond.