Geoscience Reference
In-Depth Information
influence, as well as by the seasonality of rainfall
(Fig. 9.5). Substrate salinity levels are particularly
important and mangrove colonization is enhanced
where freshwater inputs, from either river or
groundwater sources, dilute salinity levels. At the
regional scale, different climatic regimes result in
variable fluvial runoff and substrate availability
(Tomlinson 1986). Such variable influences result
in complex and diverse sediment transport and
accumulation processes in different settings, and
the development of a diverse range of mangrove
sedimentary environments.
become progressively more marginal for coral
survival, and carbonate accumulation rates
insufficient for framework construction. This
state is illustrated by the reefs along the Natal
coast of South Africa (Riegl et al. 1995), where
coral communities colonize subtidal bedrock,
but framework accumulation does not occur
(Fig. 9.6c). Similar reductions in coral growth
potential can occur in lower latitude settings
(a 'pseudo-high-latitude effect'; Sheppard &
Salm 1988) where seasonal upwelling brings cool,
nutrient-laden waters to the surface, for example
the Gulf of Aden (Fig. 9.6d; Glynn 1993). At
these sites, framework development can be both
spatially and bathymetrically restricted. Such
examples illustrate the diverse range of environ-
mental settings in which coral reefs occur, and
which in turn influence patterns of reef sediment
accumulation.
9.1.3 Reef types and geomorphology
Reef structures are commonly described in terms
of their overall geomorphology and proximity to
adjacent landmasses. James & Macintyre (1985)
delineate five basic reef types: (i) fringing reefs,
(ii) bank-barrier reefs, (iii) barrier reefs, (iv) atolls
and (v) patch reefs. The extent of reef develop-
ment at individual sites is, however, highly vari-
able and reflects local environmental parameters
(see section 9.1.1). In warm, clear water and
low nutrient environments, reefs can form both
spatially and bathymetrically extensive struc-
tures. This state is exemplified by the fringing
reefs of north Jamaica, where coral growth and
framework development has occurred to depths
of 80
9.1.4 Mangrove types and geomorphology
Although mangroves are typified as forming
extensive swamps associated with shorelines and
estuaries that are accumulating sediment, they
actually occupy a diverse range of coastal, off-
shore (island) and fluvially influenced settings
(Thom 1982; Woodroffe 1992). These include:
1
alluvial plains - areas characterized by high
fluvial sediment accumulation, throughput or
discharge (Fig. 9.7a);
2
tidal plains - areas of high tidal range charac-
terized by strong bi-directional flow patterns
(Fig. 9.7b);
3
wave-protected coastlines - mangroves develop
along the landward sides of barrier islands and
beach ridges (Fig. 9.7c), and the shorelines of
protected lagoons (Fig. 9.7d);
4
coastal embayments and drowned valleys
(Fig. 9.7e);
5
carbonate-dominated coastal environments -
these include subtidal carbonate mudbanks, and
substrates associated with intertidal reef flats
(Fig. 9.7f ).
Although mangroves thus occur in a range
of geomorphological settings, they also occur
across a range of climatic settings. These span
arid, through subtropical to tropical shorelines.
m (Liddell & Ohlhorst 1988; Fig. 9.6a)
and where framework structures some 5-10
+
m
thick have developed during Holocene times
(Land 1974). In contrast, restricted coral com-
munities, with limited framework development,
occur in turbid, nearshore environments. Around
Inhaca Island, southern Mozambique, coral
communities are restricted by low light levels to
depths of
+
6 m (Perry 2003) and framework
development is replaced by unconsolidated coral
rubble, set within a carbonate:clastic sediment
matrix (Fig. 9.6b).
Marked variations in calcium carbonate
accumulation rates (and reef development) also
occur as mean sea-surface temperatures and
aragonite saturation state decrease (Fig. 9.3b;
Buddemeier 1997). As a result, reef framework
development typically decreases towards higher
latitude areas as environmental conditions
<
