Page 59 - Fundamentals of Geomorphology

42 INTRODUCING LANDFORMS AND LANDSCAPES

suspended sediment. On the other hand, in areas of will almost certainly be distorted by the effects of relief
high, year-round rainfall, such as the Congo basin, sedi- and rock type. Indeed, the particulate load (p. 72) data
ment loads are not necessarily high. In arid regions, low includeexceptionstotherule:someoftheirtropicalrivers
rainfall produces little river discharge and low sediment have high calcium concentrations, probably owing to the
yields; but, owing to the lack of water, suspended sedi- occurrence of limestone within the basin. Moreover, in
ment concentrations may still be high. This is the case explaining the generally low concentrations of calcium in
for many Australian rivers. The greatest suspended sed- sediments of tropical rivers, it should be borne in mind
iment yields come from mountainous tropical islands, that carbonate rocks are more abundant in the temperate
areas with active glaciers, mountainous areas near coasts, zone than in the tropical zone (cf. Figure 8.2).
and areas draining loess soils: they are not determined
directly by climate (Berner and Berner 1987, 183). As Climate and denudation
one might expect, sediments deposited on inner conti- Ignoring infrequent but extreme values and correcting
nental shelves reflect climatic differences in source basins: for the effects of relief, overall rates of denudation show a
mud is most abundant off areas with high temperature relationship with climate (Table 2.3). Valley glaciation is
and high rainfall; sand is everywhere abundant but espe-
cially so in areas of moderate temperature and rainfall and substantially faster than normal erosion in any climate,
though not necessarily so erosion by ice sheets. The wide
in all arid areas save those with extremely cold climates; spreadofdenudationratesinpolarandmontaneenviron-
gravel is most common off areas with low temperature; ments may reflect the large range of rainfall encountered.
and rock is most common off cold areas (Hayes 1967). The lowest minimum and, possibly, the lowest maximum
Large amounts of quartz, in association with high
ratios of silica to alumina, in river sediments indicate rates of denudation occur in humid temperate climates,
where creep rates are slow, wash is very slow owing to the
intense tropical weathering regimes. Work carried out dense cover of vegetation, and solution is relatively slow
on the chemistry of river sediments has revealed pat- because of the low temperatures. Other conditions being
terns attributable to differing weathering regimes in the same, the rate of denudation in temperate continental
(1) the tropical zone and (2) the temperate and frigid climates is somewhat brisker. Semi-arid, savannah, and
zones. River sands with high quartz and high silica-
to-alumina ratios occur mainly in tropical river basins tropical landscapes all appear to denude fairly rapidly.
Clearly, further long-term studies of denudational pro-
of low relief, where weathering is intense enough (or cesses in all climatic zones are needed to obtain a clearer
has proceeded uninterrupted long enough) to eliminate picture of the global pattern of denudation.
any differences arising from rock type, while river sands
with low quartz content but high silica-to-alumina ratios Chemical denudation
occur chiefly in the basins located in temperate and
frigid regions (Potter 1978). A basic distinction between The controls on the rates of chemical denudation are per-
tropical regions, with intense weathering regimes, and haps easier to ascertain than the controls on the rates of
temperate and frigid regions, with less intense weathering mechanical denudation. Reliable estimates of the loss
regimes, is also brought out by the composition of the of material from continents in solution have been avail-
particulate load of rivers (Martin and Meybeck 1979). able for several decades (e.g. Livingstone 1963), though
The tropical rivers studied had high concentrations of more recent estimates overcome some of the deficien-
iron and aluminium relative to soluble elements because cies in the older data sets. It is clear from the data
their particulate load was derived from soils in which in Table 2.2 that the amount of material removed in
soluble material had been thoroughly leached. The tem- solution from continents is not directly related to the
perate and arctic rivers studied had lower concentrations average specific discharge (discharge per unit area). South
of iron and aluminium in suspended matter relative to Americahasthehighestspecificdischargebutthesecond-
soluble elements because a smaller fraction of the solu- lowest chemical denudation rate. Europe has a relatively
ble constituents had been removed. This broad pattern low specific discharge but the second-highest chemical