Hydrothermal Fe fluxes during the Precambrian: Effect of low oceanic sulfate concentrations and low hydrostatic pressure on the composition of black smokers

Modern mid-ocean ridge hydrothermal systems typically release vent fluids to the ocean with dissolved H2S in excess of Fe. These fluids are the consequence of high-temperature interactions between sulfate-rich seawater and mid-ocean ridge basalt at conditions near the critical point for seawater. The precipitation of FeS and FeS2 during venting titrates most of the Fe from the fluid, significantly reducing the net flux of Fe to the open ocean. Here we suggest that hydrothermal fluids emanating from Precambrian seafloor systems older than ∼1.8 Ga had Fe / H2S ratios >> 1, and with fH2 higher than today, because seawater lacked its primary oxidant, dissolved sulfate ion. This predominance of Fe over H2S would have promoted the establishment of an iron-rich deep ocean and the deposition of banded iron formations (BIF). Accordingly, the end of BIF deposition at ∼1.8 Ga was the result of the buildup of sulfate in seawater from oxidative weathering, and its return at 750 Ma the result of reductions in seawater sulfate concentrations during Snowball Earth episodes, enhanced by elevated Fe concentrations during depressurization of hydrothermal systems by large eustatic sea-level falls. Moreover, Precambrian chemosynthetic vent communities may have been based on H2 synthesis rather than on H2S oxidation, as they largely are today.