Bottom Sediments
31
terrigenous clay brought by rivers (Neumann 1965; Morelock and Koenig
1967). In particular, the terrigenous components comprise the fine
sediments which cover the bottom of the GBR lagoon (Maxwell 1968) and
the bottom joining the outer slopes of the monsoon fringing reefs off the
coast of central Vietnam. The fine downy sediments consisting mostly of the
needle crystals of aragonite, glued with coral mucus, often cover bottom of
the patch-reef areas of some Pacific atolls at the leeward edge of the reef flat
(Thorp 1936). The soft aragonite and calcite oozes occupy large areas in the
reef zones around the Great Bahama Bank (Black 1933). They are formed
by the fine globules and crystals of skeletal lime material produced by
calcareous algae (Lowenstam 1955), and probably by the carbonates
sedimented from sea water under the influence of intensive photosynthesis
of reef bottom plant associations, followed by an increase in pH in water
columns up to pH 9 (Broecker and Takahashi 1966; Chave et al. 1972). The
sedimentation of fine carbonaceous material produced during this
chemogenic crystallization of calcium carbonate in water columns over the
activety functioning reef proceeds largely under the action of the filtering
and sedimentary feeding activity of animals. An important role is played by
the corals themselves. During their sedimentary feeding, they accumulate
the mineral particles on their surface, then glue them together with their
mucus and shed them off to the ground, thus avoiding siltation. During this
process, the soft fine carbonaceous sediments are formed. The same kind of
sediments are formed by filtering bivalves or by sedentary polychaetes.
Their dense populations produce a mass of pseudofeces enriched with
particles of the chemogenic calcyte. This materials form often the thickness
of the soft fine sediments, which are enriched with mucus and have a slimy
texture (Roy and Smith 1971).
A significant part of the calcareous particulated material formed on flats
during the processes of bioerosion, and which are accumulated there by
biofiltration from water columns, is then driven out to the lagoon (Smith et
al. 1971). The rate of sedimentation in the lagoons is therefore very high -
1-1.2 m 10- 3 years (Cloud 1962) and could sometimes reach 3 m (Bardach
1961). The medium and the fine sediments occupy often also the foot of the
outer reef slope. They are driven out by tidal currents from the flat and from
the lagoon through the channels, which cut the reef slopes (Fairbridge
1950). A significant amount of suspended carbonate material is driven out
from the lagoon by currents through the passes in the ring reefs of atolls
(Smith et al. 1971). The flow of this material from the reef down to its outer
slope could be evaluated as being up to 1-4kgm- 1 day-l. During storms it
could be much more (Hubbard et al. 1981a). The annual transport of
calcareous sediments from the reef to the fore-reef foot zone amounts to
100-1000 kg m -I. This material will reach even the bottom of the
surrounding deep-sea floor, which is evidenced by the presence of the
shallows-dwelling foram Homotrema in deep-sea sediments (Mackenzie et
al. 1965).
31
terrigenous clay brought by rivers (Neumann 1965; Morelock and Koenig
1967). In particular, the terrigenous components comprise the fine
sediments which cover the bottom of the GBR lagoon (Maxwell 1968) and
the bottom joining the outer slopes of the monsoon fringing reefs off the
coast of central Vietnam. The fine downy sediments consisting mostly of the
needle crystals of aragonite, glued with coral mucus, often cover bottom of
the patch-reef areas of some Pacific atolls at the leeward edge of the reef flat
(Thorp 1936). The soft aragonite and calcite oozes occupy large areas in the
reef zones around the Great Bahama Bank (Black 1933). They are formed
by the fine globules and crystals of skeletal lime material produced by
calcareous algae (Lowenstam 1955), and probably by the carbonates
sedimented from sea water under the influence of intensive photosynthesis
of reef bottom plant associations, followed by an increase in pH in water
columns up to pH 9 (Broecker and Takahashi 1966; Chave et al. 1972). The
sedimentation of fine carbonaceous material produced during this
chemogenic crystallization of calcium carbonate in water columns over the
activety functioning reef proceeds largely under the action of the filtering
and sedimentary feeding activity of animals. An important role is played by
the corals themselves. During their sedimentary feeding, they accumulate
the mineral particles on their surface, then glue them together with their
mucus and shed them off to the ground, thus avoiding siltation. During this
process, the soft fine carbonaceous sediments are formed. The same kind of
sediments are formed by filtering bivalves or by sedentary polychaetes.
Their dense populations produce a mass of pseudofeces enriched with
particles of the chemogenic calcyte. This materials form often the thickness
of the soft fine sediments, which are enriched with mucus and have a slimy
texture (Roy and Smith 1971).
A significant part of the calcareous particulated material formed on flats
during the processes of bioerosion, and which are accumulated there by
biofiltration from water columns, is then driven out to the lagoon (Smith et
al. 1971). The rate of sedimentation in the lagoons is therefore very high -
1-1.2 m 10- 3 years (Cloud 1962) and could sometimes reach 3 m (Bardach
1961). The medium and the fine sediments occupy often also the foot of the
outer reef slope. They are driven out by tidal currents from the flat and from
the lagoon through the channels, which cut the reef slopes (Fairbridge
1950). A significant amount of suspended carbonate material is driven out
from the lagoon by currents through the passes in the ring reefs of atolls
(Smith et al. 1971). The flow of this material from the reef down to its outer
slope could be evaluated as being up to 1-4kgm- 1 day-l. During storms it
could be much more (Hubbard et al. 1981a). The annual transport of
calcareous sediments from the reef to the fore-reef foot zone amounts to
100-1000 kg m -I. This material will reach even the bottom of the
surrounding deep-sea floor, which is evidenced by the presence of the
shallows-dwelling foram Homotrema in deep-sea sediments (Mackenzie et
al. 1965).