Experimental setups. For experiments investigating the pH and redox gradient driven CO2 reduction mediated by different minerals and the effect of thermal gradient, we used a two-chamber glass reactor with separated water jackets and capacity of 250 mL in each chamber (Figure ). For simulating the vent-ocean interface, a vent analog fluid and an early ocean analog were used as electrolytes. Natural pyrite pieces or Fe pieces painted with a mineral ink were mediating the interface in contact of both fluids simultaneously. Moreover, an external electron-conductive wire connected both pieces immersed in the electrolytes. An ion-exchange membrane (Nafion 212), pretreated in 0.1 M NaOH, separated the two chambers for ionic equilibration during experiments.