Geoscience Reference
In-Depth Information
time-scales and major changes in sea-level. At the
last glacial maximum (LGM), 20,000 -18,000
years ago, sea-levels around the globe were
120 -130 m below present (Chappell et al. 1996)
because large volumes of ocean water were locked
up as ice in polar regions. Those physical areas
that now form continental shelves were thus
subject to subaerial processes and the lowstand
coastline was located to seaward of the present
continental shelves. Inundation of these areas
occurred during the post-glacial transgression,
which together with processes operating at the
modern sea-level highstand reworked the sub-
aerial deposits to various extents. Viewed simply,
the post-glacial sea-level rise was rapid until
around 8000 years ago, when it slowed to reach
its highest level around 6000 years ago. Con-
sequently, many regions of the world's contin-
ental shelves have been subject to their present
processes for only a few thousand years, and many
of the bathymetric, mineralogical and granulo-
metric features of continental shelves and their
sediments derive from the actions of past pro-
cesses that operated at different stages of relative
sea-level. Considerable regional variability occurs
in relative sea-level, because of the interactions
of 'global' sea-level, gravitational variations, tec-
tonics, hydro-isostacy and weather, operating at
different temporal and spatial scales.
in catchments and rivers, such as deforestation
and conversion to agriculture (e.g. Chapters 2
& 3). There are therefore wide variations in
the nature and volume of sediments delivered
to the shelves (Chapters 1 & 8). Some shelves
receive huge volumes of sediments, particularly
from those rivers draining steep tropical catch-
ments with relatively young geology, such as the
Indonesian Archipelago (Fig. 10.4). The Amazon
River produces an average of 1.2
10
9
tyr
−1
of
sediment, about 90% of which is silt and clay
(Dunne et al. 1988), and which represents an
average denudation rate of the catchment of
69 mm kyr
−1
. On relatively dry and old contin-
ents, such as Australia, the largest river in terms
of sediment delivery is the Burdekin. This delivers
3
×
10
6
tyr
−1
of sediment to the continental
shelf, the sediment comprising around 75% silt
and clay. Sediment yields equate to a catchment
denudation rate of 9 mm kyr
−1
(Belperio 1979;
Allen 1997). For some continental shelves that
receive relatively little river input, material
reworked from the shelf seabed by storms or
strong currents may form a significant com-
ponent of the overall shelf sediment budget.
×
10.2.2.2 Shelf
in situ
production
Continental shelves produce significant amounts
of 'new' sediment, mainly biogenically, but
also locally by inorganic precipitation in some
tropical environments. On some shelves and in
certain environmental conditions, authigenic
minerals are formed, such as the clay mineral
glauconite (Bornhold & Giresse 1985; Odin
1988). By far the most significant global con-
tribution is that of biogenic material, which
includes skeletal parts of animals and marine
plants, of various calcareous and siliceous com-
position, living and growing within the water
column and on the sea bed (Walker & James
1992; Wright & Burchette 1996). Significant
production of biogenic sediments needs a suit-
able regime of salinity, temperature and nutrients,
but also good light intensity within the water
column, because many primary producers are
phototrophic (such as green and red algae) or
are mixotrophic (many corals and large benthic
10.2.2 Sediment sources and characteristics
10.2.2.1 Fluvial/lithogenic material
Chemical and physical weathering of rocks
in river catchments leads to sediments being
carried by rivers to the sea. Globally, the total
load of silt delivered by rivers to the ocean has
been estimated to be 13.5
10
9
tyr
−1
(Milliman
& Meade 1983), to which bedload transport
adds 1-2
×
10
9
tyr
−1
. The combined rate of sedi-
ment supply represents an average denudation
of the world's catchments of 50 mm ky
−1
(Allen
1997), but there is great geographical variation
in rates of erosion and transport, controlled by
factors including the rate of uplift, climate, rock
type, topography and precipitation. Sediment
delivery also can be affected by human activities
×
