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Such intense bioturbation results in preferential
sediment sorting and aeration of surface sedi-
ments, and creates a highly mobile surface layer
poorly conducive to colonization (Tudhope &
Scoffin 1984). Finer sediments expelled during
burrowing are also prone to resuspension and
transport (Roberts et al. 1981).
shelf settings. Detailed sedimentological descrip-
tions of the two former examples are found in
Tucker & Wright (1990).
Although clear patterns of cross-reef or cross-
shelf sediment facies therefore can be identified,
marked variations in grain assemblages are also
evident between reef settings. As outlined in
section 9.1.3, the extent of reef development
varies markedly between regions and is influenced
by spatial and latitudinal changes in marine
environmental parameters such as tempera-
ture, light penetration and aragonite saturation
state. These same limiting factors also influence
patterns and rates of reef sediment production
and are clearly illustrated in relation to shifts in
the types of skeletal and non-skeletal sediment
contributors. On a global scale these have been
related to latitudinal changes in temperature
and salinity (Lees 1975), although related shifts
in aragonite saturation state (Buddemeier 1997;
Fig. 9.3) are also important. Lees (1975) iden-
tified three distinct carbonate grain assemblages
that characterize different temperature and
salinity zones:
1 a chlorozoan assemblage (characterizing trop-
ical waters) dominated by corals and calcareous
green algae;
2 a chloralgal assemblage (characterizing sub-
tropical waters) in which corals and calcareous
green algae become progressively less common
and the dominant grain types are coralline red
algae, molluscs and foraminifera;
3 a foramol association (characterizing warm
temperate to cold waters) in which coralline red
algae, molluscs and bryozoans dominate.
Transitions between such assemblages there-
fore occur as environmental conditions become
progressively more marginal for coral survival
(Halfar et al. 2000).
9.2.3 Spatial variations in sediment accumulation
Spatial variations in the abundance and pro-
ductivity of reef sediment contributors, and the
processes of sediment transport and reworking,
produce distinct grain assemblages in different
reef areas. Across nearshore fringing or bank-
barrier reefs, local variations in sediment types
are evident between lagoon, reef crest, and
shallow and deep reef front environments. Each
subenvironment can be delineated on the basis
of grain assemblage (i.e. the relative abundance
of skeletal grain types) and texture. These reflect
not only initial grain inputs, but also grain
reworking and transport. Across the narrow
( c . 1 km wide) fringing reefs of north Jamaica,
clear patterns of sediment accumulation can be
identified (Fig. 9.10a), which broadly reflect
the abundance of sediment contributing groups
on the reef (Boss & Liddell 1987). Lagoon sedi-
ments are, for example, characterized by rela-
tively high abundances of Halimeda , benthic
foraminifera and molluscs, whereas shallow
reef-front sediments are dominated by coral,
coralline algae and encrusting foraminifera.
These patterns are representative of sediment
accumulation patterns across many narrow shelf
reefs. Distinct patterns of carbonate sediment
accumulation are also evident across larger car-
bonate shelf or platform environments. Reef
sediment assemblages (comparable to those
described above) occur where reefs are developed
along the seaward margins, but significant car-
bonate production can also occur across the
inner platform areas where oolitic sand bodies,
seagrass beds or green algal meadows develop
(Purdy 1963). Such large-scale patterns of car-
bonate sediment accumulation are evident across
the Great Bahama Bank (Fig. 9.10b), and the
land-attached Florida and Great Barrier Reef
9.2.4 Sources and characteristics of
mangrove sediments
Mangroves colonize a wide range of coastal
environments (see section 9.1.4) and the sedi-
ments that contribute to mangrove substrates
are derived from a range of sources. These can
be classified as either:
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