Abiotic hydrogen (H) sources and sinks near the Mid-Ocean Ridge (MOR) with implications for the subseafloor biosphere.

Free hydrogen (H) is a basal energy source underlying chemosynthetic activity within igneous ocean crust. In an attempt to systematically account for all H within young oceanic lithosphere (<10 Ma) near the Mid-Ocean Ridge (MOR), we construct a box model of this environment. Within this control volume, we assess abiotic H sources (∼6 × 10 mol H/y) and sinks (∼4 × 10 mol H/y) and then attribute the net difference (∼2 × 10 mol H/y) to microbial consumption in order to balance the H budget. Despite poorly constrained details and large uncertainties, our analytical framework allows us to synthesize a vast body of pertinent but currently disparate information in order to propose an initial global estimate for microbial H consumption within young ocean crust that is tractable and can be iteratively improved upon as new data and studies become available. Our preliminary investigation suggests that microbes beneath the MOR may be consuming a sizeable portion (at least ∼30%) of all produced H, supporting the widely held notion that subseafloor microbes voraciously consume H and play a fundamental role in the geochemistry of Earth's ocean-atmosphere system.