Continent denudation

Rivers transport suspended sediments derived from the disintegration of basin surface layers. With reduced velocity, sediment is deposited in the river channel. The finest material is carried to the sea. It has been estimated that the average mechanical denudation rate for continents is 0.056 mm year (35). This is based on a total suspended load of 13.5 x 10 metric tons year (S). Presently, about two-thirds of the world s total suspended sediment load derives from Southern Asia and large Pacific Islands. Berner has estimated the increase in sediment loss in the U.S. and world since prehuman times to be approximately 200% (35). Current estimated erosion rate from the major land forms is provided in Table I. The relatively recent construction of large sediment trapping dams that normally caused sediment to be deposited in river valleys or transported to the ocean has drastically reduced sediment yields in great rivers. 

Fi2 0.69 Computed from combined rates of mechanical and chemical denudation of continents (2 X 10 tons/yr Garrels and Mackenzie, 1971) and a mean P content of crustal material of 0.1% (Taylor, 1964)... 

Garrels R.M. and Mackenzie F.T. (1971b) Gregor s denudation of the continents. Nature 231, 382-383. 

The change in character of sedimentation at the beginning of the Late Precambrian is explained by the fact that by this time a thick mantle of Middle Precambrian (Lower Proterozoic) sediments had arisen on the ancient continents. Subsequent denudation, erosion, and redeposition of these rocks, granitized to a considerable extent, contributed to further profound differentiation of matter. An increase in the role of the life activity of various organisms in sedimentation also was an important fact. 

Solids eroded from the continental surface account for about 80% of the material mass transport (Table 3). Most of the solid material flux, about 95% of it, is carried by water discharge and the remaining 5% are transported by winds. The total suspended material load of rivers and the dust load of the atmosphere include products of the bedrock erosion as well as materials eroded from clastic and biogenic sediments exposed on the continents. Equating the sum of the riverine and atmospheric transport fluxes of solids with the rate of continental erosion or mechanical denudation implies that a part of the eroded mass that is stored in clastic sediments on land remains constant (Fig. 4) ... 

Physical denudation rates of 80-800 mm ka 1 would cause erosion of the land mass to the present-day mean elevation of continents, 840 m, in a period of 106-107 years if there were no crustal uplift compensating for, or exceeding, the rates of erosion. 

Figure 5. Dissolution rates (R, mol cm " 2 s 1) of plicate and carbonate minerals in H20 near 25°C under laboratory conditions as a function of the (solution pH. Data sources listed in Tables 4 and 5. Global averages for chemical denudation of the continents, and soil denudation and formation rates are shown for near-neutral pH region.

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