Geoscience Reference
In-Depth Information
Anthropogenic inputs/outputs
- contaminants
- sediment extraction
- sediment dumping
River/estuarine
supply
Erosion of
pre-existing
sediment
deposits
Zone of
deposition
Fig. 1.3
Schematic
diagram illustrating the
different potential natural
and anthropogenic
sources of sediment
into a nearshore marine
environment.
Longshore
transport
Onshore/
offshore
transport
Mixing/reworking
- physical
- biological
In situ (biogenic)
sediment production
in low-relief parts of the catchment thick soils
accumulate and the detrital grains become
highly altered chemically. Furthermore, sedi-
ment deposited in low-lying alluvial floodplains
undergoes additional chemical weathering and
alteration, further modifying their composition.
Consequently, within any individual environ-
ment sediments tend to be derived from a range
of source areas and weathering regimes, and these
both contribute to, and influence, the composi-
tion of the accumulating sediment (Fig. 1.3).
Over the time-scales considered in this topic, the
release of sediment from previously deposited
sedimentary sequences should, therefore, also
be regarded as a key sediment source.
evident across individual reef or platform envir-
onments and reflect subtle spatial variations in
marine environmental conditions (e.g. light and
wave energy). These influence the composition
of the reef community and, hence, the com-
position of the sediment substrate. In high-
latitude settings, biogenic carbonate production
may remain important but in many settings is
volumetrically 'swamped' by terrestrial inputs
of detrital minerals.
Carbonate deposits are also relatively common
within a range of intertidal, terrestrial and fresh-
water settings, and are associated primarily with
physico-chemically induced carbonate deposition.
On a localized scale these represent important
sources of carbonate to the sediment record. For
example, within intertidal and supratidal settings
that are characterized by high aridity and high
evaporation rates, the precipitation of evaporite
minerals such as gypsum (CaSO
4
.2H
2
O) and
anhydrite (CaSO
4
) is common. At present,
extensive evaporite-rich sediments occur in the
south-east Arabian Gulf and an excellent review
of their occurrence is given by Alsharhan &
Kendall (2003). The hydrology of such deposi-
tional settings has been reviewed by Yechieli &
Wood (2002).
In some freshwater fluvial and lacustrine set-
tings carbonate deposition also occurs and can
lead to the development of significant carbonate
bodies (in some cases these have been described
as forming 'freshwater reefs'; Pedley 1992).
1.2.2 Biogenic and organic sediments
In addition to detrital minerals, significant
amounts of sediment are derived from the
remains of skeletal carbonate-secreting organ-
isms. These form across a wide range of marine
environments (Schlager 2003), although marked
latitudinal variations occur both in the types
and rates of biogenic sediment production (Lees
1975; Carannante et al. 1988). Such production
peaks in the low latitudes and, in particular,
in the vicinity of coral reefs, where carbonate
sediments often represent the primary sedi-
ment constituents (see Chapter 9). Even in these
environments, however, marked variations in
the composition of sediment assemblages are
