Page 120 - Geochemical Remote Sensing of The Sub-Surface

Spontaneous potentials and electrochemical cells 97

The principal oxidising agent on Earth is free oxygen (02(g)) , for which the only
significant terrestrial source is plant photosynthesis. The only appreciable source of
oxygen for the geological subsurface is the atmosphere, which contains 21% oxygen.
Since redox variability in shallow Earth materials cannot be due to variations in the
primary source of oxidising agents, it must be due to: (1) kinetic processes that limit the
transfer of oxygen into the subsurface; and/or (2) processes that control the consumption
of oxygen (i.e., that control the availability of reducing agents).
The primary limitations on the transfer of oxygen into the subsurface are its solubility
in water and its slow rate of aqueous diffusion. In the fully-saturated groundwater
environment below the water table, the concentration of oxygen has an upper limit from
which it can only decrease. Its solubility limits the concentration to a maximum of about
10 ppm at 25~ In contrast, the maximum concentration of gaseous oxygen in the
vadose zone is limited only by its molar ratio in the atmosphere and can therefore reach
210,000 ppm. This enormous disparity demonstrates why moisture content is the single
most important factor controlling the availability of oxygen in a geological environment
(it also demonstrates why subaqueous disposal of sulphidic mine tailings is so effective
in preventing their oxidation). In many subsurface environments, the water table
represents a sharp redox boundary between abundant oxidising agents above and
abundant reducing agents below. This is particularly true in young exotic overburden.
Most of the other surficial factors controlling local redox conditions involve
processes that control the availability of reducing agents. Chief amongst these is the
accumulation of organic matter. All forms of organic matter are reducing relative to
oxygen and most can be oxidised fairly quickly in Earth materials by microorganisms.
As such, the oxidation of organic matter is one of the primary consumers of oxygen in
the shallow subsurface. Typically, higher concentrations of organic matter in soils lead
to more reducing conditions, particularly in saturated environments. Also important is
the lability (i.e., availability to microorganisms as a source of metabolic energy) of the
organic matter. Well-humified peat is not as chemically reducing as methanogenic
organic muck in a perpetually submerged portion of a bog because the former has
already undergone oxidation of its most labile organic components.
The other major consumer of oxygen in the shallow subsurface is the oxidation or
weathering of mineral matter and particularly of metallic sulphides. Areas of unusually
active weathering, due either to large accumulations or to more reactive minerals,
typically result in an enhanced consumption of oxidising agents relative to surrounding
areas and to more negative redox conditions. The dispersal of the dissolved products of
weathering, such as Fe z+, can affect redox conditions at some distance from the source.
All forms of mechanical dispersion of rock material can affect the availability of
reducing agents. Continental glaciation results in the widespread deposition of relatively
unoxidised rock, till, clay and other drift materials over vast areas. Different deposit
types (e.g., sand, clay) resulting in different permeabilities in transported materials can
also lead to higher or lower water tables and variable rates of percolation of oxygenated
groundwater. This can result in a poorer availability of oxidising agents in fine-grained