(9.6 m diameter, 7009 Æ 41 cal years BP at centre)
reported from the Leizhou Peninsula, in the southernmost
part of Guangdong province in southern China (Yu et al.,
2009). Porites compressa microatolls 7 m in diameter
occur in Kaneohe Bay, Hawaii (Roy, 1970), and fossil
Porites microatolls 6 m in diameter occur at several
locations on the GBR (Hopley and Barnes, 1985; Hendy
et al., 2003). Large microatolls demonstrate continuity
of growth for several centuries, but most microatolls
are < 2 m in diameter. This probably reflects the relatively
low probability of extended survival in shallow reef environments where corals are most vulnerable to a range of
debilitating conditions. Microatolls on the northern GBR
surveyed by Scoffin and Stoddart (1978, p.105) averaged
0.5 m in diameter, although other than stating that they
“vary in size from a few centimeters to a few meters” no
statistical description of the variance was offered.
Microatoll formation
Corals develop into microatolls when prolonged emergence during low tides kills polyps on a colony’s upper
surface and constrains subsequent growth to its sides.
Tolerances to emergence vary with species; Goniastrea
and Platygyra typically grow higher than Porites when
they occur on the same reef flat. Other factors have been
argued as drivers of upper surface mortality and microatoll
formation, including excessive sedimentation (WoodJones, 1910), nutrient dynamics, and hydrodynamic stress
(Stoddart and Scoffin, 1979); however, the general symmetry developed by individual microatolls and the accordant morphologies developed by neighboring microatolls
are difficult to reconcile with these typically and unevenly
distributed stresses. Subaerial emergence is clearly the
main environmental parameter driving the formation of
microatolls in most shallow reef settings, where the absolute elevation of the upper limit to coral growth is a function of the duration of exposure and the intensity of
desiccating conditions, which are strongly influenced by
elevation relative to a tidally modulated water level.
Where microatolls grow in “open water” habitats freely
connected to the open ocean, the confining water level is
a tidal datum that can be linked to sea level. However,
where the ebbing tide is impeded as it drains off the reef,
the confining water level is moated above the open water
level (see Chapter Moating) and thus the upper limit to
coral growth is controlled by the height of the moat
sill, and is less precisely tied to sea level (Hopley and
Isdale, 1977).
The relationship between the upper limit to coral
growth and tidal levels may vary with exposure to factors
such as wavelet wetting, diurnal, and seasonal tidal patterns, but on the central GBR, where the mean tidal range
is 2–3 m, the tops of microatolls within the same field are
usually within Æ10 cm (Chappell et al., 1983). Smithers
and Woodroffe (2000) indicate that even greater uniformity is found where the tidal range is smaller, such as
at the microtidal Cocos (Keeling) Islands, although even
there microatoll elevations may vary by as much as
40 cm between different habitats (reef flat, intertidal passage, lagoonal) that experience different hydrodynamic
conditions. The tops of Porites microatolls in open-water
habitats on the GBR are usually close to the mean low
water spring (MLWS) tide level, but may extend to the
mean high water neap (MHWN) tide level in moated habitats (Scoffin and Stoddart, 1978). In the Cocos (Keeling)
Islands the tops of open water microatolls are typically
elevated approximately midway between MLWS and
MLWN.
The relatively flat top developed by coral microatolls is
referred to as the microatoll plane (Scoffin and Stoddart,
1978). This surface is rarely perfectly horizontal as the
water level which confines upward coral growth is seldom
absolutely constant through time. As water levels and the
upper limit to coral growth fluctuate over a microatoll’s
lifetime, complex microtopographies composed of concentric ridges and swales can progressively develop across
the microatoll plane. Hopley (1982) identified four common microatoll forms:
1. “Classical” microatolls are the basic discoidal kind,
with a relatively flat microatoll plane formed under relatively stable water level;
2. “Top hat” microatolls have an elevated centre and
lower outer rims. They form where the upper limit to
coral growth is abruptly lowered, for example, where
a storm breaches the sill of a pond that holds water over
the reef flat at low tide (Hopley and Isdale, 1977);
3. “Upgrown” microatolls have low centres encircled by
higher living rims. They form where the confining water
level has risen, as might occur if falling tides are moated
behind a rubble rampart deposited on a reef flat during
a storm, or where the reef is subsiding (possibly driven
by compaction, or tectonic subsidence);
4. “Multiple-ringed” microatolls with concentric undulations across the microatoll plane form when the upper
limit to coral growth has oscillated through time, either
episodically or dynamically, as may occur where
interannual variations in sea level forced by relatively
rapidly changing oceanographic and atmospheric conditions – such as ENSO – are experienced.
The progressive development of different microatoll surface morphologies driven by temporal changes in the
water level that limits upward coral growth is shown schematically in Figure 2.
Microatolls and sea level
Where the confining water level is an open water level,
this relationship means that microatolls, or collections of
microatolls, can be used to reconstruct sea-level histories
that spatially and temporally augment those available
from instrumental records (see Chapter Sea-Level Indicators for diagrams). Microatoll derived sea-level records
are generally of two main types: (a) records of mid-late
Holocene sea level constructed by comparing the elevations of radiometrically dated fossil microatolls to the
MICROATOLL
693
reported from the Leizhou Peninsula, in the southernmost
part of Guangdong province in southern China (Yu et al.,
2009). Porites compressa microatolls 7 m in diameter
occur in Kaneohe Bay, Hawaii (Roy, 1970), and fossil
Porites microatolls 6 m in diameter occur at several
locations on the GBR (Hopley and Barnes, 1985; Hendy
et al., 2003). Large microatolls demonstrate continuity
of growth for several centuries, but most microatolls
are < 2 m in diameter. This probably reflects the relatively
low probability of extended survival in shallow reef environments where corals are most vulnerable to a range of
debilitating conditions. Microatolls on the northern GBR
surveyed by Scoffin and Stoddart (1978, p.105) averaged
0.5 m in diameter, although other than stating that they
“vary in size from a few centimeters to a few meters” no
statistical description of the variance was offered.
Microatoll formation
Corals develop into microatolls when prolonged emergence during low tides kills polyps on a colony’s upper
surface and constrains subsequent growth to its sides.
Tolerances to emergence vary with species; Goniastrea
and Platygyra typically grow higher than Porites when
they occur on the same reef flat. Other factors have been
argued as drivers of upper surface mortality and microatoll
formation, including excessive sedimentation (WoodJones, 1910), nutrient dynamics, and hydrodynamic stress
(Stoddart and Scoffin, 1979); however, the general symmetry developed by individual microatolls and the accordant morphologies developed by neighboring microatolls
are difficult to reconcile with these typically and unevenly
distributed stresses. Subaerial emergence is clearly the
main environmental parameter driving the formation of
microatolls in most shallow reef settings, where the absolute elevation of the upper limit to coral growth is a function of the duration of exposure and the intensity of
desiccating conditions, which are strongly influenced by
elevation relative to a tidally modulated water level.
Where microatolls grow in “open water” habitats freely
connected to the open ocean, the confining water level is
a tidal datum that can be linked to sea level. However,
where the ebbing tide is impeded as it drains off the reef,
the confining water level is moated above the open water
level (see Chapter Moating) and thus the upper limit to
coral growth is controlled by the height of the moat
sill, and is less precisely tied to sea level (Hopley and
Isdale, 1977).
The relationship between the upper limit to coral
growth and tidal levels may vary with exposure to factors
such as wavelet wetting, diurnal, and seasonal tidal patterns, but on the central GBR, where the mean tidal range
is 2–3 m, the tops of microatolls within the same field are
usually within Æ10 cm (Chappell et al., 1983). Smithers
and Woodroffe (2000) indicate that even greater uniformity is found where the tidal range is smaller, such as
at the microtidal Cocos (Keeling) Islands, although even
there microatoll elevations may vary by as much as
40 cm between different habitats (reef flat, intertidal passage, lagoonal) that experience different hydrodynamic
conditions. The tops of Porites microatolls in open-water
habitats on the GBR are usually close to the mean low
water spring (MLWS) tide level, but may extend to the
mean high water neap (MHWN) tide level in moated habitats (Scoffin and Stoddart, 1978). In the Cocos (Keeling)
Islands the tops of open water microatolls are typically
elevated approximately midway between MLWS and
MLWN.
The relatively flat top developed by coral microatolls is
referred to as the microatoll plane (Scoffin and Stoddart,
1978). This surface is rarely perfectly horizontal as the
water level which confines upward coral growth is seldom
absolutely constant through time. As water levels and the
upper limit to coral growth fluctuate over a microatoll’s
lifetime, complex microtopographies composed of concentric ridges and swales can progressively develop across
the microatoll plane. Hopley (1982) identified four common microatoll forms:
1. “Classical” microatolls are the basic discoidal kind,
with a relatively flat microatoll plane formed under relatively stable water level;
2. “Top hat” microatolls have an elevated centre and
lower outer rims. They form where the upper limit to
coral growth is abruptly lowered, for example, where
a storm breaches the sill of a pond that holds water over
the reef flat at low tide (Hopley and Isdale, 1977);
3. “Upgrown” microatolls have low centres encircled by
higher living rims. They form where the confining water
level has risen, as might occur if falling tides are moated
behind a rubble rampart deposited on a reef flat during
a storm, or where the reef is subsiding (possibly driven
by compaction, or tectonic subsidence);
4. “Multiple-ringed” microatolls with concentric undulations across the microatoll plane form when the upper
limit to coral growth has oscillated through time, either
episodically or dynamically, as may occur where
interannual variations in sea level forced by relatively
rapidly changing oceanographic and atmospheric conditions – such as ENSO – are experienced.
The progressive development of different microatoll surface morphologies driven by temporal changes in the
water level that limits upward coral growth is shown schematically in Figure 2.
Microatolls and sea level
Where the confining water level is an open water level,
this relationship means that microatolls, or collections of
microatolls, can be used to reconstruct sea-level histories
that spatially and temporally augment those available
from instrumental records (see Chapter Sea-Level Indicators for diagrams). Microatoll derived sea-level records
are generally of two main types: (a) records of mid-late
Holocene sea level constructed by comparing the elevations of radiometrically dated fossil microatolls to the
MICROATOLL
693