Radiolytic Hydrogen Production in the South Pacific Subseafloor Basaltic Aquifer

Abstract:

Hydrogen (H₂) is produced in geological settings by dissociation of water due to radiation from natural radioactive decay of uranium (²³⁸U, ²³⁵U), thorium (²³²Th) and potassium (⁴⁰K). To quantify the potential significance of radiolytic H₂ as an electron donor for microbes within the South Pacific subseafloor basaltic aquifer, we calculate radiolytic H₂ production rates in basement fractures utilizing measured radionuclide concentrations in 42 basalt samples from IODP Expedition 329. The samples are from three sites with very different basement ages and a wide range of alteration types. Major and trace element concentrations vary by up to an order of magnitude from sample to sample. Comparison of our samples to each other and to previous studies of fresh East Pacific Rise basalt suggests that between-sample variation in radionuclide concentrations is primarily due to differences in initial (pre-alteration) concentrations (which can vary between eruptive events), rather than to alteration type or extent. Local maxima in radionuclide (U, Th, and K) concentrations produce ‘hotspots’ of radiolytic H₂ production; calculated radiolytic rates differ by up to a factor of 80 from sample to sample. Fracture width also greatly influences H₂ production. Due to the low penetration distance of alpha radiation, microfractures are ‘hotpots’ for radiolytic H₂ production. For example, radiolytic H₂ production rates normalized to water volume are 170 times higher in 1μm-wide fractures than in 10cm-wide fractures.