Page 121 - Geochemical Remote Sensing of The Sub-Surface

98 S.M. Hamilton

deposits as compared to coarse-grained material. Generally, in older terrain with deep
weathering profiles, such as laterite, there is a greater availability of oxidising agents in
shallow areas because most reducing agents have already been consumed.
Finally, both temperature and pH variations can also affect redox reactions.
Significant horizontal temperature variations in the Earth are rare over short distances
but vertical temperature gradients are ubiquitous. However, the effect of temperature on
Eh is fairly small and, since vertical geothermal gradients are locally quite uniform, their
effects on redox reactions will not be considered here. On the other hand, pH does vary
significantly over short distances. Redox reactions that involve either H + or OH in either
the reactants or products are affected by pH, and this is the case in many, though not all,
natural redox reactions.
Surficial and bedrock processes control the local balance of oxidising and reducing
agents in the shallow subsurface. The result, in young surficial environments, is an
electrochemically inhomogeneous shallow subsurface with local redox gradients almost
everywhere. These represent fields of electrical potential (SPs) between the local sources
of oxidising and reducing agents along which ions have a tendency to move. This
movement of redox-active ions is consistent with the universal tendency of chemical
systems to approach maximum entropy. In the long term, and therefore in older deposits,
it results in increasing local homogenisation of redox conditions in the shallow
subsurface and a tendency for local conditions to approach the larger redox trend that
overprints all redox processes, i.e., the redox stratification of the Earth's crust.

Redox stratification in the Earth

An upward increasing redox stratification exists in the Earth's crust (Bass Becking et
al., 1960; Bolviken and Logn, 1975). This redox field results from the process of oxygen
re-supply by the atmosphere over-riding the general tendency toward redox homogeneity
(maximum entropy). It establishes an overall vertical gradient between the oxygenated
surface and mineralogical reducing agents deep in bedrock. Subject to the limitations
described, the upper limit of this Eh field is fixed by the electrical potential of oxygen at
the lowest geologically reasonable pH (around +1000 mV; Fig. 3-4). The lower limit is
usually considered to be the lower limit of water stability at the highest geologically
reasonable pH (around-400 mV; Fig. 3-4). Indeed, rocks from the lower crust and upper
mantle appear to have formed in Eh environments that are at or slightly below the Eh
stability field of water. This 1400 mV spread represents the maximum potential redox
differences to be expected due to naturally-occurring redox-active substances in the
Earth's crust. It is consistent with the vast majority of spontaneous potential
measurements, which are below 1500 mV (Sato and Mooney, 1960).
Almost any natural redox-active substance that can exist in the Eh stability field of
water could potentially contribute to this redox gradient. Natural terrestrial materials
more oxidising than oxygen are virtually non-existent and consequently natural redox