Hydrological and geological controls for the joint evolution of
dissolved oxygen and iron in crystalline rocks
Abstract
Dissolved Oxygen (DO) plays a key role in reactive processes and
microbial dynamics in the critical zone. While the general view is that
oxygen is rapidly depleted in soils and that deeper compartments are
anoxic, recent observations showed that fractures can provide rapid
pathways for deep oxygen penetration, triggering unexpected
biogeochemical processes. As it is transported in the subsurface, DO
reacts with electron donors, such as $Fe^{2+}$ coming from
mineral dissolution, hence influencing rock-weathering. Yet, little is
known about the factors controlling the spatial heterogeneity and
distribution of oxygen with depth. Here we present analytical
expressions describing the coupled evolution of DO and $Fe^{2+}$
as a function of fluid travel time in crystalline rocks. Our model,
validated with reactive transport simulations, predicts a linear decay
of DO with time, followed by a rapid non-linear increase of
$Fe^{2+}$ concentrations up to an equilibrium state. Relative
effects of the reducing capacity of the bedrock and of transport
velocity are quantified through a Damkohler number, capturing key
hydrological and geological controls of $Fe^{2+}$ and DO
distributions in the subsurface. This framework is used to investigate
contrasted DO and $Fe^{2+}$ concentrations observed in two
crystalline catchments. These differences are explained by the Damkohler
number: one system is reaction-limited while the second is
transport-limited. We show that hydrological and geological drivers can
be discriminated by analyzing both O$_2$ and $Fe^{2+}$. These
findings provide a new conceptual framework to understand and predict
the evolution of DO in the subsurface, a key element in the critical
zone.