Written By : Richard John Hugget on Fundamental of Geomorphology
Since the 1990s, geomorphologists have come to realize that the global tectonic system and the world climate system interact in complex ways. The interactions give rise to fundamental changes in atmospheric circulation patterns, in precipitation, in climate, in the rate of uplift and denudation, in chemical weathering, and in sedimentation (Raymo and Ruddiman 1992; Small and Anderson 1995; Montgomery et al. 2001). The interaction of large-scale landforms, climate, and geomorphic processes occurs in at least three ways – through the direct effect of plate tectonic process upon topography (p. 108–15), through the direct effect of topography upon climate (and the effects of climate upon uplift), and through the indirect influence of topography upon chemical weathering rates and the concentration of atmosphere carbon dioxide.
Changes in topography, such as the uplift of mountain belts and plateaux, can influence regional climates, both by locally increasing precipitation, notably on the windward side of the barrier, and through the cooling effect of raising the ground surface to higher elevations (e.g.Ollier 2004a). Changes in topography could potentially have wide-ranging impacts if they interact with key components of the Earth’s climatic system. In southern Africa, uplift of 1,000 m during the Neogene, especially in the eastern part of the subcontinent, would have reduced surface temperatures by roughly the same amount as during glacial episodes at high latitudes (Partridge 1998). The uplift of the Tibetan Plateau and its bordering mountains may have actively forced climatic change by intensifying the Asian monsoon (through altering surface atmospheric pressure owing to elevation increase), by creating a high-altitude barrier to airflow that affected the jet stream, and by encouraging inter-hemispherical exchange of heat (Liu and Ding 1998; Fang et al. 1999a, b). These forcings seem to have occurred around 800,000 years ago. However, oxygen isotope work on late Eocene and younger deposits in the centre of the plateau suggests that this area at least has stood at more than 4 km for about 35 million years (Rowley and Currie 2006).
Recent research shows that local and regional climatic changes caused by uplift may promote further uplift through a positive feedback loop involving the extrusion of crustal rocks (e.g.Molnar and England 1990; Hodges 2006). In the Himalaya, the Asian monsoon sheds prodigious amounts of rain on the southern flanks of the mountains. The rain erodes the rocks, which enables the fluid lower crust beneathTibet to extrude towards the zone of erosion. Uplift results from the extrusion of rock and counterbalances the erosion, which reduces the landsurface elevation. Therefore, the extrusion process keeps the front range of the Himalaya steep, which encourages heavy monsoon rains, so completing the feedback loop (but see Ollier 2006 for a different view).
Carbon dioxide is a key factor in determining mean global temperatures. Over geological timescales (millions and tens of millions of years), atmospheric carbon dioxide levels depend upon the rate of carbon dioxide input through volcanism, especially that along midocean ridges, and the rate of carbon dioxide withdrawal through the weathering of silicate rocks by carbonation, a process that consumes carbon dioxide.Given that carbon dioxide inputs through volcanism seem to have varied little throughout Earth history, it is fair to assume that variations in global chemical weathering rates should explain very long-term variations in the size of the atmospheric carbon dioxide pool. So what causes large changes in chemical weathering rates? Steep slopes seem to play a crucial role. This relatively new finding rests on the fact that weathering rates depend greatly on the amount of water passing through the weathering zone. Rates are highest on steep slopes with little or no weathered mantle and high runoff. In regions experiencing these conditions, erosional processes are more likely to remove weathered material, so exposing fresh bedrock to attack by percolating water. In regions of thick weathered mantle and shallow slopes, little water reaches the weathering front and little chemical weathering occurs. Interestingly, steep slopes characterize areas of active uplift, which also happen to be areas of high precipitation and runoff. In consequence, ‘variations in rates of mountain building through geological time could affect overall rates of global chemical weathering and thereby global mean temperatures by altering the concentration of atmospheric CO2’ (Summerfield 2007, 105). If chemical weathering rates increase owing to increased tectonic uplift, thenCO2 will be drawn out of the atmosphere, but there must be some overall negative feedback in the system otherwise atmosphericCO2 would become exhausted, or would keep on increasing and cause a runaway greenhouse effect. Neither has occurred during Earth history, and the required negative feedback probably occurs through an indirect effect of temperature on chemical weathering rates. It is likely that if global temperatures increase this will speed up the hydrological cycle and increase runoff.This will, in turn, tend to increase chemical weathering rates, which will draw down atmospheric CO2 and thereby reduce global mean temperature. It is also possible that variations in atmospheric CO2 concentration may directly affect chemical weathering rates, and this could provide another negative feedback mechanism.
The idea that increased weathering rates associated with tectonic uplift increases erosion and removes enough carbon dioxide from the atmosphere to control climate has its dissenters. Ollier (2004a) identified what he termed ‘three misconceptions’ in the relationships between erosion, weathering, and carbon dioxide. First, weathering and erosion are not necessarily concurrent processes – erosion, especially erosion in mountainous regions, may occur with little chemical alteration of rock or mineral fragments. Second, in most situations, hydrolysis and not carbonation is the chief weathering process –weathering produces clays and not carbonates. Furthermore, evidence suggests that chemical weathering rates have declined since the mid- or early Tertiary, before which time deep weathering profiles formed in broad plains. Today, deep weathering profiles form only in the humid tropics. Third, Ollier questions the accepted chronology of mountain building, which sees Tibet, the highlands of western North America, and the Andes beginning to rise about 40 million years ago, favouring instead rise over the last few million years.
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