Simulating Serpentinization as It Could Apply to the Emergence of Life Using the JPL Hydrothermal Reactor

Abstract

The molecules feeding life's emergence are thought to have been provided through the hydrothermal interactions of convecting carbonic ocean waters with minerals comprising the early Hadean oceanic crust. Few laboratory experiments have simulated ancient hydrothermal conditions to test this conjecture. We used the JPL hydrothermal flow reactor to investigate CO₂ reduction in simulated ancient alkaline convective systems over 3 days (T = 120°C, P = 100 bar, pH = 11). H₂-rich hydrothermal simulant and CO₂-rich ocean simulant solutions were periodically driven in 4-h cycles through synthetic mafic and ultramafic substrates and Fe>Ni sulfides. The resulting reductants included micromoles of HS⁻ and formate accompanied possibly by micromoles of acetate and intermittent minor bursts of methane as ascertained by isotopic labeling. The formate concentrations directly correlated with the CO₂ input as well as with millimoles of Mg^{2 +} ions, whereas the acetate did not. Also, tens of micromoles of methane were drawn continuously from the reactor materials during what appeared to be the onset of serpentinization. These results support the hypothesis that formate may have been delivered directly to a branch of an emerging acetyl coenzyme-A pathway, thus obviating the need for the very first hydrogenation of CO₂ to be made in a hydrothermal mound. Another feed to early metabolism could have been methane, likely mostly leached from primary CH₄ present in the original Hadean crust or emanating from the mantle. That a small volume of methane was produced sporadically from the ¹³CO₂-feed, perhaps from transient occlusions, echoes the mixed results and interpretations from other laboratories. As serpentinization and hydrothermal leaching can occur wherever an ocean convects within anhydrous olivine- and sulfide-rich crust, these results may be generalized to other wet rocky planets and moons in our solar system and beyond.