budgets is evident. Although 62 percent of Ca²⁺ originate from carbonate rock weathering, 67 percent of HCO₃^- originates from soil and atmospheric CO₂.

The breakdown of river transport by geographic origins (Meybeck, 1979, 1982, 1988) is based on a typology of transport rates for a dozen morphoclimatic environments from tundra to mountainous wet tropics, defined by their average temperature (cold, temperate, desertic, tropical) and by their average runoff (five classes of runoff). It has been postulated that in these budgets, lithology is of secondary influence compared to runoff at scales on which these environments have been defined (10⁶ to 10⁷ km²). The relative importance of the four main climatic environments is presented in Table 4.4B. Because of its major contribution to the water budget, the tropical zone is the major source of silica and organic carbon. Given their relative area and water discharge, temperate regions contribute 60 percent more than tropical regions to the ionic budget: the hypothesis of equal distribution of rock types in morphoclimatic environments is effectively limited; most limestone outcrops are in temperate regions (Balazs, 1977).

In considering the global TSS budget to the oceans, Milliman and Meade (1983) found that Southeast Asia alone (from the Huang He to the Indus, 12 x 10⁶ km², islands included) is responsible for 35 percent of the global sediment discharge. Here, all major factors of high erosion rates (sedimentary rocks, volcanic ash in Indonesia, loess deposits in China, steep relief in Himalaya, monsoon climate) are combined with high sediment delivery ratio (steep slopes until river mouths in many cases). This rate (450 t/km²/yr) is twice that observed for the combined temperate and tropical zones (230 t/km²/yr).

When comparing the global riverine transport of major elements in dissolved and particulate states, the ratio of dissolved materials to total transport is highly variable, from 99 to 0.1 percent and the typical ranges are shown below:

Actually, these numbers may vary by nearly one order of magnitude, depending mainly on the amount of TSS. For example, the ratio of dissolved silica to total silica transport varies from 0.3 percent for regions of high mechanical erosion to 40 percent for lowlands.

The use of present-day river transport data for past geological times, particularly in ocean and climate modeling, should be undertaken cautiously. The following considerations need to be addressed: