Geoscience Reference
In-Depth Information
Mechanical breakdown
Segments
(4-8 mm)
Broken segments
(1 mm)
)
Halimeda
Dust (1 µ
m)
Grit (250 µ
m)
Joints (64 mm)
Coral
256 64 16 4 1 250 63 15.6 3.9 0.9
mm µ m
Biological breakdown
Coral chips
Cliona sponge
boring
(< 63
µ
m)
Faecal carbonate
NB. Most particles are also
subject to further diminution
by microendolithic borers
(500-125
µ
m)
Parrotfish
Coral
fragments
(250-125 µ
m)
Faecal pelle ts
Diadema
urchin
(1 mm-250 µ
m)
Fig. 9.9 Mechanical and biological
breakdown of carbonate sediment
contributors and resultant sediment size
fractions produced. (Adapted from
Scoffin 1987.)
256 64 16 4 1 250 63 15.6 3.9 0.9
mm µ m
termed macroborers. In addition, a range of
microborers, including forms of cyanobacteria,
chlorophytes and fungi, facilitate substrate de-
gradation on a microscopic scale (Golubic et al.
1975; Perry 1998b). Although all boring activ-
ity results in framework degradation, not all
results in sediment production, because varying
proportions of the excavated material will be
directly dissolved. The most significant sediment
producers are the boring sponges. These sponges
excavate small fragments of coral skeleton, which
are subsequently expelled (Fig. 9.8), resulting
in the production of abundant fine-grained sand
(Fig. 9.9; Fütterer 1974). Once released into the
sediment carbonate grains are subject to fur-
ther physical, chemical and biological alteration
(Perry 2000).
Coral framework is not, however, the only
source of reef sediment. Skeletal carbonate sedi-
ment is also derived from infaunal and epifaunal
organisms as well as a wide array of calcifying
algae. Common infaunal and epifaunal sediment
contributors include bivalves and gastropods,
foraminifera and echinoids (Fig. 9.8; Swinchatt
1965). Skeletal sediments are also contributed
by calcareous epiphytic organisms (including
crustose coralline algae, foraminifera, serpulids
and bryozoans) that encrust seagrass blades and
the stems of calcifying algae (such as Udotea
and Penicillus ; Fig. 9.8; Land 1970; Nelson &
Ginsburg 1986). In addition, carbonate sediment
is produced during breakdown of carbonate-
secreting algal species. Some, such as Penicillus
and Udotea , release abundant carbonate needles
into the sediment (Neumann & Land 1975),
whereas others, such as Halimeda , produce heav-
ily calcified segments, which often subsequently
disintegrate into carbonate needles (Fig. 9.9).
Many calcifying algae have high turnover rates,
producing several standing crops per year, and
are thus significant components of reef sedi-
ment budgets (Neumann & Land 1975). Minor
 
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