Page 64 - Fundamentals of Geomorphology

THE GEOMORPHIC SYSTEM 47

during glacial episodes at high latitudes (Partridge 1998). mantle and high runoff. In regions experiencing these
The uplift of the Tibetan Plateau and its bordering conditions, erosional processes are more likely to remove
mountains may have actively forced climatic change by weathered material, so exposing fresh bedrock to attack
intensifying the Asian monsoon (through altering sur- by percolating water. In regions of thick weathered man-
face atmospheric pressure owing to elevation increase), tle and shallow slopes, little water reaches the weathering
by creating a high-altitude barrier to airflow that affected front and little chemical weathering occurs. Interestingly,
the jet stream, and by encouraging inter-hemispherical steep slopes characterize areas of active uplift, which also
exchange of heat (Liu and Ding 1998; Fang et al. 1999a, happen to be areas of high precipitation and runoff. In
b).These forcings seem to have occurred around 800,000 consequence, ‘variations in rates of mountain building
years ago. However, oxygen isotope work on late Eocene throughgeologicaltimecouldaffectoverallratesofglobal
and younger deposits in the centre of the plateau suggests chemical weathering and thereby global mean tempera-
that this area at least has stood at more than 4 km for tures by altering the concentration of atmospheric CO 2 ’
about 35 million years (Rowley and Currie 2006). (Summerfield 2007, 105). If chemical weathering rates
Recent research shows that local and regional climatic increase owing to increased tectonic uplift, then CO 2 will
changes caused by uplift may promote further uplift be drawn out of the atmosphere, but there must be some
through a positive feedback loop involving the extrusion overall negative feedback in the system otherwise atmo-
of crustal rocks (e.g. Molnar and England 1990; Hodges spheric CO 2 would become exhausted, or would keep on
2006). In the Himalaya, the Asian monsoon sheds increasing and cause a runaway greenhouse effect. Nei-
prodigious amounts of rain on the southern flanks of ther has occurred during Earth history, and the required
the mountains. The rain erodes the rocks, which enables negative feedback probably occurs through an indirect
the fluid lower crust beneathTibet to extrude towards the effect of temperature on chemical weathering rates. It is
zone of erosion. Uplift results from the extrusion of rock likely that if global temperatures increase this will speed
and counterbalances the erosion, which reduces the land- upthehydrologicalcycleandincreaserunoff.Thiswill,in
surface elevation. Therefore, the extrusion process keeps turn, tend to increase chemical weathering rates, which
the front range of the Himalaya steep, which encourages will draw down atmospheric CO 2 and thereby reduce
heavy monsoon rains, so completing the feedback loop global mean temperature. It is also possible that varia-
(but see Ollier 2006 for a different view). tions in atmospheric CO 2 concentration may directly
Carbon dioxide is a key factor in determining mean affect chemical weathering rates, and this could provide
global temperatures. Over geological timescales (millions another negative feedback mechanism.
and tens of millions of years), atmospheric carbon diox- The idea that increased weathering rates associ-
ide levels depend upon the rate of carbon dioxide ated with tectonic uplift increases erosion and removes
input through volcanism, especially that along mid- enough carbon dioxide from the atmosphere to control
ocean ridges, and the rate of carbon dioxide withdrawal climate has its dissenters. Ollier (2004a) identified what
through the weathering of silicate rocks by carbonation, a he termed ‘three misconceptions’ in the relationships
process that consumes carbon dioxide. Given that carbon between erosion, weathering, and carbon dioxide. First,
dioxide inputs through volcanism seem to have varied weathering and erosion are not necessarily concurrent
little throughout Earth history, it is fair to assume that processes – erosion, especially erosion in mountainous
variations in global chemical weathering rates should regions, may occur with little chemical alteration of rock
explain very long-term variations in the size of the atmo- or mineral fragments. Second, in most situations, hydrol-
spheric carbon dioxide pool. So what causes large changes ysis and not carbonation is the chief weathering process –
in chemical weathering rates? Steep slopes seem to play weathering produces clays and not carbonates. Further-
a crucial role. This relatively new finding rests on the more, evidence suggests that chemical weathering rates
fact that weathering rates depend greatly on the amount have declined since the mid- or early Tertiary, before
of water passing through the weathering zone. Rates which time deep weathering profiles formed in broad
are highest on steep slopes with little or no weathered plains. Today, deep weathering profiles form only in