Abstract
The radiolysis of porewaters by uranium, thorium, and potassium in
mineral grains is a recognised source of molecular hydrogen in rock- and
sediment-hosted fluids. This radiolytic hydrogen is of
geomicrobiological interest as a potential energy source (electron
donor) for microbial metabolism, especially in energy-limited settings
such as the marine deep biosphere or the subsurface of Mars. Previous
efforts to predict the production of radiolytic hydrogen from columns of
rock and sediment have tended to rely upon analytic models that cannot
account for the attenuation of mineral radiation by grains larger than
~30 microns. To address this, we have developed a Monte
Carlo method to simulate the physics of mineral radiation and evaluate
the production of H2 as a function of mineral grain size
and radioisotope composition. The results confirm that grain size is a
major control on radiolytic H2 yield. For example, using
the standard geological classification of grain sizes, we find that clay
can produce up to an order of magnitude more H2 per unit
time than sand. The magnitude of this effect is illustrated using
compositional data from real geological units in order to demonstrate
the dependence of radiolytic hydrogen flux on natural radionuclide
concentration and bulk porosity.