Experimental investigation of single carbon compounds under hydrothermal conditions

Abstract

The speciation of carbon in subseafloor hydrothermal systems has direct implications for the maintenance of life in present day vent ecosystems and possibly the origin of life on early Earth. Carbon monoxide is of particular interest because it represents a key reactant during the abiotic synthesis of reduced carbon compounds via Fischer-Tropsch-type processes. Laboratory experiments were conducted to constrain reactions that regulate the speciation of aqueous single carbon species under hydrothermal conditions and determine kinetic parameters for the oxidation of CO according to the water water-gas shift reaction (CO2 + H2 = CO + H2O). Aqueous fluids containing added CO2, CO, HCOOH, NaHCO3, NaHCOO, and H2 were heated at 150, 200, and 300°C and 350 bar in flexible cell hydrothermal apparatus, and the abundance of carbon compounds were monitored as a function of time. Variations in fluid chemistry suggest that the reduction of CO2 to CH3OH under aqueous conditions occurs via a stepwise process that involves the formation of HCOOH, CO, and possibly CH2O, as reaction intermediaries. Kinetic barriers that inhibit the reduction of CH3OH to CH4 allow the accumulation of reaction intermediaries in solution at high concentrations regulated by metastable equilibrium. Reaction of CO2 to form CO involves a two-step process in which CO2 initially undergoes a reduction step to HCOOH which subsequently dehydrates to form CO. Both reactions proceed readily in either direction. A preexponential factor of 1.35 x 106 s-1 and an activation energy of 102 KJ mol-1 were retrieved from the experimental results for the oxidation of CO to CO2. Reactions rates amongst single carbon compounds during the experiments suggests SCO2 (CO2 + HCO3- + CO3=), CO, SHCOOH (HCOOH + HCOO-), and CH3OH may reach states of redox-dependent metastable thermodynamic equilibrium in subseafloor and other hydrothermal systems. The abundance of CO under equilibrium conditions, which in turn may influence the likelihood for abiotic synthesis via Fischer-Tropsch-type processes, is strongly dependent on temperature, the total carbon content of the fluid, and host-rock lithology. If crustal residence times following the mixing of high-temperature hydrothermal fluids with cool seawater are sufficiently long, reequilibration of aqueous carbon can result in the generation of additional reduced carbon species such as HCOOH and CH3OH and the consumption of H2. The present study suggests that abiotic reactions involving aqueous carbon compounds in hydrothermal systems are sufficiently rapid to influence metabolic pathways utilized by organisms that inhabit vent environments.