Page 122 - Geochemical Remote Sensing of The Sub-Surface

Spontaneous potentials and electrochemical cells 99

reactions that oxidise oxygen in water to 02 do not occur. Oxidising agents are therefore
likely to be restricted to oxygen and electrochemically-weaker oxidative species such as
Fe 3+, Mn 4+ and SO42. Geological materials more reducing than water do exist but the
natural reduction of hydrogen in water is rare in the zone of meteoric groundwater and
occurs only under exceptional circumstances (e.g., Barnes et al., 1978; Clark, 1987). As
a result, reducing agents are likely to be less reducing than H2(g) and could include
reduced aqueous sulphur species (e.g., HS), reduced sulphide minerals (e.g., pyrrhotite),
mafic or ultramafic minerals (e.g., ferrous olivines and pyroxenes) or their dissolved
products, and hydrogenous organic matter.
An electrochemical gradient such as occurs in the Earth's crust represents a field of
electrical potential in an electrolyte. This gradient induces the movement of ions (Fig. 3-
5) and results in an electrolytic charge transfer between deep and shallow areas
(Bolviken and Logn, 1975; Hamilton, 1998). Negative charge-carrying redox-active ions
tend to move upward toward more oxidising Eh conditions and positive charge-carrying
ions tend to move downward toward a more reducing environment. The ion migration is
analogous to movement of charge-carrying ions toward the electrodes of a voltaic cell.
In order for charge transfer to occur, ion movement must be accompanied by redox
reactions that attenuate some or all of the migrated species. All of the charge-carrying
species have a particular Eh range within which they are stable in groundwater, and a
particular Eh limit beyond which they are likely to become attenuated and thereby pass
on charge to other species (Fig. 3-5). As such, the reactions that transfer charge from the
migrating ions are likely to occur all down the gradient from deep in the crust to ground
surface. The movement of redox-inert species (such as Na § and CI) is also likely to
occur in the Earth's redox field, as it does in a voltaic cell, to prevent local charge
imbalances. However, this movement is in response to the migration of redox-active
species and the resultant redox reactions, and therefore does not cause the transfer of
electrical charge but rather results from it (Hamilton, 1998).
This electrical current that is inferred to exist between shallow and deep areas in the
Earth's crust must be subtle but ubiquitous. The upward movement of negative charge is
a kinetic process and counteracts, to some extent, the continuous supply of oxidising
agents to the shallow subsurface from the atmosphere. However, deep weathering
profiles in arid environments, in which the majority of mineralogical reducing agents
have been consumed, suggests that this process is not static but favours the long-term
consumption of mineralogical reducing agents.