Geoscience Reference
In-Depth Information
9.2.2.1 Reef sediment transport
of biogenic components and physico-chemical
processes interact to trap and stabilize reef sedi-
ments. Particularly important in this respect
are marine seagrasses and green algae. The long
blades of seagrasses, such as
Thalassia
and
Syringodium
, locally reduce current speeds and
promote sediment settling (Scoffin 1970). In
the long-term, such processes can lead to the
development of carbonate mudbanks (Bosence
et al. 1985), although this will depend upon
local rates of carbonate mud production (Perry
& Beavington-Penney 2005). The dense rhizome
(root) networks associated with seagrasses also
facilitate substrate stabilization and binding.
Similar binding of sediment occurs around the
holdfasts of green algae such as
Halimeda
,
Penicillus
and
Udotea
(Scoffin 1970). Organic
binding of sediment also occurs in areas where
algal-mat communities develop (most commonly
in low-energy lagoon settings). Associated with
these mats are filamentous algae including the
cyanobacteria
Lyngbya
and
Schizothrix
, and the
chlorophyte
Enteromorpha
(Scoffin 1970), which
promote sediment adhesion and trapping, and in
turn enhance substrate stabilization. Binding of
substrates also occurs in areas subject to physico-
chemical and organically induced carbonate
cement precipitation (Scoffin 1987).
Sediment transport and deposition are deter-
mined by two main factors: (i) shear stress and
(ii) settling velocity. The former relates to the
velocities required to move or entrain sediment
particles of a specific size, the latter to the differ-
ence between the gravitational and buoyancy
forces acting on the particle. The relationship
between threshold and settling velocities is rela-
tively well established for quartz grains, which,
due to uniform densities, behave in a reasonably
predicable fashion dictated by grain size (see
Chapter 1). Grain transport and deposition in
carbonate sediments, however, are complicated by
differences in grain skeletal structure (and hence
density) and by grain size, shape and texture.
Grains with plate-like morphologies will settle
at a slower rate than block or rod-shaped grains
and hence such parameters influence grain trans-
port and deposition (Kench & McLean 1996).
These hydraulic controls have been well illus-
trated in a study of sediment transport in the
Cocos (Keeling) Islands (Kench 1997). Carbonate
sediment assemblages around the atoll can be
related to classes defined by settling velocities
and which delineate sediment transport path-
ways. Essentially two main sediment assemblages
are present: one dominated by reef-derived com-
ponents, and a second produced
in situ
within
the lagoon. The former occur within what has
been described as an 'active transport zone'
in which reef-derived sediments are selectively
transported by currents from shallow reef flat
areas into the channels entering the central
lagoon. Sediments in these areas are dominated
by faster settling grains (mainly larger fragments
of coral, coralline algae and the foraminifera
Amphistegina
sp.). Slower settling grains (smaller
coral grains,
Halimeda
and the foraminifera
Marginopora
sp.) are transported through the
reef flat channels and deposited along the lagoon
margins (Kench 1997).
9.2.2.3 Reef sediment reworking
In addition to sediment stabilization, significant
sediment reworking also occurs, much of which
can be attributed to wave and current action (see
section 9.2.2.1) and to grain diminution (see
section 9.2.1.1). Bioturbation also occurs, how-
ever, associated with surface feeders such as holo-
thurians and crabs, and subsurface organisms
such as shrimps. Holothurians, for example, are
estimated to ingest and excrete up to 250 g of
sediment per day. The excreted sediment not only
produces a highly homogenized surficial sediment
layer, but the ingestion process may also result
in chemical grain dissolution (Hammond 1981).
Extensive sediment reworking (with volumes of
sediment turnover in excess of 11 kg m
−2
yr
−1
)
may occur associated with infaunal organisms
such as the
Callianassa
shrimp (Bradshaw 1997).
9.2.2.2 Reef sediment trapping and stabilization
Although the physical properties of grains dic-
tate sediment entrainment and transport, a range
