Environmental Engineering Reference
In-Depth Information
cycles, for which Earth's rock cycle provides major sources or sinks. Atmospheric
reservoirs of radiatively sensitive greenhouse and other anthropogenically disturbed gases
and aerosol components - CO
2
, CH
4
, CH
3
SCH
3
(dimethyl sulphide or DMS), SO
2
, SO
4
2−
,
H
2
SO
4
, H
2
S, NH
3
, NO
x
, etc. - are integrated with geomorphological processes. Traces of
these materials and their derivatives, in ice sheet and ocean sediment sinks, provide
valuable proxy evidence of past and present fluxes and concentrations. The hydrological
cycle and its component ocean/ice-sheet mass fluxes and reservoirs, revealed by
18
O/
16
O
isotope stratigraphy, form a useful example. Other elements are connected even more
directly with geological reservoirs. Volcanoes source S, N and C, whilst continental
denudation plays an active role in cycling O
2
, C, etc.
Earth's atmosphere and oceans are major reservoirs for N
2
and S respectively and they
also interact with geological processes. Biochemical weathering fixes and recycles
nitrates, and anthropogenic use of fertilizers has greatly increased biospheric N
2
fluxes.
Volcanic emissions add NH
3
and SO
2
to the atmosphere, augmented by even larger
anthropogenic fluxes from fuel combustion. Their combination with atmospheric H
2
O
and precipitation of acid derivatives increases weathering rates of minerals in soil and
materials of the built environment. Proxy records show exponential increases of both
gases in the atmosphere since 1800.
Chemical weathering is strongly influenced by climate but also helps to determine it.
Rocks contain 1,500 times the amount of global carbon stored in the atmosphere,
biosphere and hydrosphere combined. Since the quantity of atmospheric carbon is so
small, atmospheric and ocean CO
2
reservoirs are very sensitive to changes in chemical
weathering and volcanic emission rates. Weathering consumes 2 × 10
9
kg yr
−1
of CO
2
,
transferred as Ca(HCO
3
)
2
to the oceans. Marine organisms precipitate CaCO
3
to the sea
floor as carbonate mud and release CO
2
back to the atmosphere, maintaining a steady
state. The annual turnover of 4 × 10
11
kg of O
2
in geological
redox
processes is also in
steady state. However, both cycles can be disturbed. Tectonic uplift and glacio-eustatic
falls in sea level expose new continental land surfaces which may increase weathering
fifty fold in newly exposed areas.
Carbonation
could deplete the atmospheric CO
2
reservoir, leading to climatic cooling. There is also concern that the anthropogenic build-
up of atmospheric CH
4
, CO, N
2
O and SO
2
could reduce the oxidative capacity of the
atmosphere - but both sets of responses would no doubt set up further feedbacks.
operate at intergranular or intercrystalline discontinuities first, although larger-scale
weathering fronts open up along fractures.
CHEMICAL WEATHERING
Two important stages in the rock cycle haunt rocks at the land surface. Mineral stability
should be inversely proportional to the temperature at which it formed. This is mostly so,
with minerals crystallized from high-temperature melts in the greatest discomfort at the
low-temperature land surface (Figure 13.8). Siliciclastic rocks, formed from rock residues
in conditions of closest equilibrium to the land surface, are the most stable. Water is the
principal agent of chemical weathering, even though it is also essential to B-subduction
