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additional skeletal, but non-carbonate, sediments
derive from diatoms and sponge spicules (both of
which are formed from silica). Morphological
descriptions of these varied sediment contributors
are found in Scoffin (1987).
Production rates by different contributing
groups and processes vary markedly between reef
environments, thus dictating spatial variations
in rates of sediment production and accumula-
tion. At Kailua Bay (Oahu, Hawaiian Islands),
both total carbonate sediment production rates
and the production rates of specific sediment
contributors vary between environments. Near-
shore hardgrounds have the highest sediment
production rates (0.62 kg m −2 yr −1 ) with produc-
tion dominated by Halimeda (0.25 kg m −2 yr −1 ),
bioerosion-derived coral (0.13 kg m −2 yr −1 ) and
molluscs (0.18 kg m −2 yr −1 ). In contrast, sedi-
ment production rates in reef front sites average
0.41 kg m −2 yr −1 , of which 95% is derived from
coral bioerosion (Harney & Fletcher 2003).
calcium carbonate under high-energy conditions
(Ginsburg 1957).
9.2.1.3 Allochthonous sediments
Although the majority of sediment that accumu-
lates on and around coral reefs is carbonate,
there are significant external inputs of sediment
in some environments. This is particularly evid-
ent in areas where reefs develop close to sources
of fluvial sediment discharge (Fig. 9.6). Typic-
ally the proportion of terrigenous sediment will
decrease with distance from source (e.g. Acker
& Stearn 1990), although sediment accumula-
tion within individual coastal environments
commonly reflects not only local carbonate and
fluvially derived sediment inputs, but also sedi-
ment flux into and out of the environment.
A detailed sediment budget of a carbonate
embayment at Hanalei Bay, Hawaii, influenced
by terrigenous sediment input (Calhoun et al.
2002), found that significant proportions of fluvi-
ally derived suspended sediments were exported
offshore, and the bay was also subject to inputs
of carbonate sediment derived from adjacent
coastal areas. In this example, the total volume
of Holocene carbonate sediment that has accu-
mulated in the bay exceeds estimates of in situ
Holocene carbonate sedimentation, and the bay
is therefore acting as a net sink for sediment
produced in adjacent coastal areas. Increasingly
associated with terrigenous sediment inputs
are a range of dissolved and particulate con-
taminants linked to anthropogenic (industrial
or agricultural) discharges. These include heavy
metal and hydrocarbon contaminants and are
discussed in section 9.4.4.
9.2.1.2 Non-skeletal carbonate sediments
In addition to skeletal sediments, non-skeletal
grains also occur within reef and reef-related
sediments. These include carbonate muds, peloids
and ooids (Fig. 9.8). Carbonate silts (grain size
<
m) often comprise a volumetrically signi-
ficant component of lagoon sediments. Much
of this material comprises single aragonite crys-
tals derived from the breakdown of carbonate-
secreting marine algae, although some may also
result from direct precipitation from super-
saturated sea waters. Peloids are small, rounded
or elliptical grains characterized by a micro-
crystalline internal structure and have a range
of origins (Macintyre 1985). Some represent the
calcified remnants of faecal pellets, others frag-
ments of skeletal grains where the internal struc-
ture has been modified by microboring. Some
peloids are also believed to result from phases of
chemical precipitation around a central nucleus.
Ooids are also small rounded grains (typically
<
63
μ
9.2.2 Controls on coral reef sediment transport
and accumulation
Although local environmental factors influence
the composition and abundance of individual
sediment contributors, the accumulation of
carbonate sediment in reef environments is influ-
enced by a wide range of physical and biogenic
processes. These influence sediment transport,
reworking, trapping and stabilization.
1 mm diameter), but exhibit multiple concen-
tric lamellae that form a coating around a central
nucleus (often a peloid or skeletal fragment) and
result from physico-chemical precipitation of
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