Environmental Engineering Reference
In-Depth Information
caveats concern the short time scales and other uncertainties of measuring terrestrial
processes. Sediments in transit at one point of measurement may be detained elsewhere.
This is pertinent, for example, in the Karakoram and New Zealand's Southern Alps,
where river terrace sediments can be uplifted and recycled before reaching the coast!
Dissolved rock is more difficult to track, and biogenic processes extract minerals from
one part of the sediment cascade and redeposit them elsewhere later. Sediment loads in
northern Europe may be more indicative of the easier reworking of Pleistocene glacial
sediments than of current lowering. The restless state of Quaternary environments, the
emergence of hominids and increasing
anthropogenic
impact complicate assessments of
long-term rates. Only deep-ocean undisturbed terrigenous sedimentation produces
reliable figures (of approximately 10 m Ma
−1
) over long, radiometrically secure time
scales. Nevertheless, some gross figures are available. An average of recent estimates
suggests that 25-28 × 10
9
t a
−1
of terrigenous sediment is delivered to the oceans, in a
solid : dissolved ratio of 6:1. This is equivalent to 62-70 mm ka
−1
of surface lowering.
Allowing for isostatic recovery, net lowering of terrestrial surfaces amounts to 12-14 mm
ka
−1
. Global solid (suspended) sediment transfers are shown in Figure 13.2.
Figure 13.2
Global suspended sediment yield.
Source: After Walling and Webb, in Gregory and Walling (1973).
DENUDATION CYCLES AND CHRONOLOGY
Episodic uplift and the presence of large continental areas of low relative relief, even at
moderate altitudes (1-2 km), prompted the formulation of model
denudation cycles
and
denudation chronologies
to chart land surface development. The most enduring models
