Formation and fluxes of natural hydrogen in the crust and upper mantle

Molecular hydrogen (H2) is a fundamental component of planetary evolution and an important energy source for microbial life. It is now understood that natural mechanisms, spanning geological and biological processes, can produce high concentrations of hydrogen in natural fluids. Quantifying the processes that modulate natural hydrogen concentrations is necessary not only for conceptualising the distribution of life on Earth and elsewhere in the universe, but also for identifying settings where natural hydrogen may potentially accumulate to complement industrial hydrogen production. However, uncertainties persist in assessing these natural fluxes. In this review, we explore the biological and geological processes that can generate natural hydrogen. Compared to previous summary efforts, we include in our updated inventory hydrogen fluxes from biological processes, metamorphic degassing, and subduction zones. By integrating recent advances in quantifying hydrogen generation and transportation in geological environments, we demonstrate that significant concentrations and fluxes of hydrogen can arise in a plethora of settings worldwide, contributing towards a total abiotic production rate of 40 to 64' Mt H2 yr-1. We also highlight that geological environments characterised by high hydrogen production may be associated with high microbial hydrogen consumption (e.g., oceanic sediments). However, large uncertainties regarding the residence time of hydrogen within geological settings remain, and future research endeavours should aim to ascertain the long-term behaviour of hydrogen stored in the deep Earth to assess the viability of natural hydrogen as a renewable energy source.