to the atmosphere, and N and P to the hydrosphere is discussed in Sections 3.4.2
and 5.5.1 respectively.
The biosphere clearly influences rates of weathering reactions but there is
debate as to how much. Some estimates suggest the presence of biotic soils
enhance weathering rates by 100–1000 times over abiotic weathering rates. There
is also interest in whether microbial participation in weathering is simply a co-
incidental byproduct of metabolism. Is it possible that the microbes get something
for their trouble? If soil bacteria do gain something by colonizing specific mineral
substrates then the likely controls are nutrient element availability, absence of
toxic elements or capacity to help buffer microenvironmental pH, all parameters
that contribute to the success or failure of a microbial population. Recent work
shows that in some weathering systems specific silicate minerals are heavily col-
onized and broken down by bacteria, whereas other minerals are left untouched.
The colonized minerals contain the nutrient elements phosphorus (P) and iron
(Fe) (see Section 5.5.1), while the uncolonized minerals are typically aluminium-
rich but lacking nutrient potential. If bacteria are widely proven to select bene-
ficial minerals to weather, we will need to rethink the traditional view that mineral
weathering is mainly controlled by relative instability, as shown for example in
Fig. 4.14.
The debate surrounding weathering rates is important, since the consumption
of CO 2 by soil weathering reactions (Section 4.4.3) lowers atmospheric partial
pressure of CO 2 (pCO 2 ). Some researchers argue that, prior to the evolution of
vascular land plants some 400 million years ago, weathering rates may have been
much lower, giving rise to a higher atmospheric pCO 2 and enhanced greenhouse
warming (see Section 7.2.4). Others, however, believe that thin soils, stabilized
by primitive lichens and algae, covered the land surface billions of years before
the evolution of vascular plants. These primitive biotic soils may have been quite
effective in enhancing weathering rates, acting in a ‘Gaian’ way (see Section 1.3.3)
by consuming atmospheric CO 2 and lowering global temperatures. This cooling
effect may have helped improve the habitability of the early Earth for other
organisms.4.7 Wider controls on soil and clay mineral formation
In an average upper-crustal granodiorite, it is mainly feldspars that weather to
form clay minerals (eqns. 4.13 & 4.14). Since feldspars are framework silicates,
the formation of clay minerals (sheet silicates) must involve an intermediate step.
This step is not at all well understood although it has been proposed that fulvic
acids, from the decay of organic matter in soil, may react with aluminium to form
a soluble aluminium–fulvic acid complex, with aluminium in six-fold coordina-
tion. This gibbsitic unit may then have SiO 4 tetrahedra adsorbed on to it to form
clay mineral structures.
A minority of unweathered rock-forming silicates, for example the micas, are
already sheet silicates. It is not difficult to envisage that alteration could trans-104 Chapter Four