and (2) 10.5 percent is covered by ice caps, mostly Antarctica, which discharge about 2300 km3/yr of ice to oceans (Baumgartner and Reichel, 1975).

What is the influence of environmental factors on riverine transport on a global scale? This question was raised a long time ago for dissolved materials. Erikson (1960) and Garrels and Mackenzie (1971) tried to estimate the relative contributions of atmospheric inputs and of the weathering of major rock types. The question was readdressed by Meybeck (1987) through consideration of the chemical composition of waters from small monolithologic watersheds and of the relative abundance of a dozen rock types at the continental surface. Atmospheric inputs were also reassessed (Meybeck, 1983). The breakdown of major ions, Sr2+, and dissolved silica by rock weathering and atmospheric inputs is given in Table 4.4A. The influence of rare evaporitic outcrops (about 1.2 percent of the Earth's surface) on Ca2+, Na+, SO42-, Cl -, and Sr2+

FIGURE 4.6 Global evolution of total dissolved (Td) and particulate (Ts) transport rates of worldwide rivers. (A) Influence of basin relief on Td versus Ts relationship (• mountainous watersheds, others), compared to the Judson and Ritter (1964) variations for U.S. rivers. (B) Influence of basin morphology on the Ts/Td ration versus runoff q compared to relationships from (A) Langbein and Dawdy (1964) and (B) Leopold et al. (1964). (• river basin > 2.4 x 106 km2; + river basins from 0.5 x 106 km2 to 2.4 x 106 km2; other rivers).

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