VARIABILITY OF SURFICIAL GEOFLUXES

Superimposed on the long-term trends of deglaciation and the precessional cycle is shorter-term variability inherent in the chaotic behavior of the climate system.

Temporal Variability

Although we have a general picture of the great changes that must have taken place in the fluxes of materials on the surface of the Earth during the change from the glacial age to the present interglacial age, we know very little about the shorter-term (decade to century to millennium) changes. A number of mass transport events can best be characterized as natural cataclysms, as documented and discussed by Baker (see Chapter 6, this volume). Flood events on a scale unknown in the course of human history characterized deglaciation. Massive diversions of rivers occurred as the glaciers receded and the land rose in isostatic response to the disappearing load of ice. Although the melting of ice sheets and the isostatic response to unloading were relatively slow, continuous processes, the diversion of rivers likely was rapid, taking place in the course of days to years or decades. Monsoonal circulation was stronger in the early Holocene when precession-controlled seasonal radiation contrasts reached a maximum in the Northern Hemisphere. The enhanced monsoons caused repeated large floods in southern Asia, central Africa, and northern Australia.

Continental fluxes are mostly linked to river runoff, but the variations in runoff are poorly documented. New data sources are needed to gain insight into temporal variability. For example, it might be possible to interpret varves in tropical lakes and other environments in terms of records of hydrologic variability. Better documentation of lake-level changes should be possible. Obtaining a record of runoff in the past could be very important for differentiating the natural variability of climate from anthropogenic effects.

Against a relatively constant background of erosion, transport, and deposition, large infrequent events result in step-function changes in material fluxes. Severe storms may breach the vegetation cover and expose previously stable slopes to erosion; deposition from streams overloaded with debris during extreme flood events may cause them to alter course; windstorms during periods of drought may strip the land of its soil; earthquakes may trigger landslides and submarine slumps; volcanic activity may result in massive ashfalls and mudflows that alter the character of the landscape. Although the everyday transport of materials is a significant part of the global total in any given span of time, some of the landscape features we observe are the result of extreme, even violent, infrequent events that are very poorly documented.

Spatial Variability

On short-term time scales, fluxes are mostly driven by small areas with very high rates. Mechanical and chemical weathering, eolian transport, and even glacial erosion all demonstrate the overriding significance of small areas in influencing the global rates. Because the fluxes are so strongly dependent on changes in small areas, we expect that they have a high temporal variability that is at present, with rare exceptions, undocumented and unknown.

Mechanical weathering is most rapid in areas of high relief, resulting from active tectonic processes. In the rugged Himalaya and Alps, average rates of denudation and uplift may be about equal and on the order of 1 m per thousand years (Menard, 1961; Schaer, 1979; Dodson and McClelland Brown, 1985). In the high-relief, densely forested island terrain of southeast Asia and in many other parts of the circum-Pacific region, transport depends on breaches in the vegetation cover. Once the binding root systems are



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