Geoscience Reference
In-Depth Information
Tectonics, denudation and climate
NEW DEVELOPMENTS
Global climate change should induce spatial shifts in the impact of rivers, glaciers and wind on continental land surfaces
(see
Chapter 28).
Humid fluvial conditions should extend polewards as glaciers, ice sheets and permafrost are rolled
back, with arid belts expanding in tropical zones. The regional character and intensity of geomorphic processes and
sediment fluxes will change, although above-average warming is balanced elsewhere by regional cooling. The direction
and rates of geomorphic response are therefore less easy to predict, as are ecosystem responses which affect
weathering, soil-forming processes and sediment transfer rates. Climate change is thus a principal driver of future
land surface changes and their human impacts. How far, in turn, do land surface processes modulate or amplify
climate change through negative or positive feedbacks?
Reservoir models of global biogeochemical cycles, and hence the rock cycle, are incorporated into GCMs.
Geomorphological processes are integrated with atmospheric reservoirs of radiatively sensitive greenhouse and
anthropogenically disturbed gases and aerosols - 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. 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 (see
Chapter 23).
Other elements are connected
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. Biochemical
weathering fixes and recycles nitrates, and anthropogenic use of fertilizers has greatly increased biospheric N
2
fluxes.
Volcanic emissions of NH
3
and SO
2
to the atmosphere have been augmented by exponential increases from fossil
fuel combustion since
AD
1800. Combination with atmospheric H
2
O, and precipitation of its acid derivatives, increases
weathering rates of soil minerals and the built environment.
Chemical weathering is strongly influenced by climate but also helps to determine it. Rocks contain 1,500 times the
global carbon reservoirs of 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. A
balance is struck as marine organisms precipitate CaCO
3
to the sea floor as carbonate mud, releasing 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 but both cycles can be disturbed.
An uplift weathering hypothesis asserts that active Cenozoic tectonics accelerate denudation rates, drawing down
atmospheric CO
2
and cooling climate through weathering of the predominantly silicate crust by hydrolysis (Ruddiman
2001; Raymo et al.1986) ~ atmospheric CO
2
+ H
2
O
q
H
2
CO
3
; H
2
CO
3
+ (Ca)SiO
3
q
CaCO
3
+ SiO
2
+ H
2
O. (Brackets
around Ca indicate that many other silicate species are similarly vulnerable.) The chain of events started with rapid,
recent and extensive tectonic uplift of American and Eurasian cordilleran systems and high plateaux in Tibet, the
Bolivian Altiplano and east Africa. Considerable exposure of 'seismically cracked', unweathered rock, combined with
steep slopes, enhanced orographic precipitation and alpine glaciation to drive an exponential increase in chemical
weathering and general denudation rates. This is evidenced by sediment mass transfer rates five times the long-
term Cenozoic mean from the Tibetan plateau and Himalayan orogens during the last 10 Ma (Leeder 1999), and the
derivation of 80 per cent of Amazon basin chemical weathering products from just 10 per cent of its catchment, in
the Andes (Ruddiman 2001).
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 magmatization and the
formation of evaporites and most sedimentary rocks. H
+
(hydrogen) and OH
-
(hydroxyl) ions in water react with
other minerals, creating new species which are readily
