Nina conditions and associated ENSO cycles, and it
has been demonstrated that microatolls in the Pacific
Ocean preserve smoothed records of these oscillations
across their upper surfaces (Woodroffe and McLean,
1990; Spencer et al., 1997; Woodroffe and Gagan,
2000). The records are smoothed because the record
is biologically mediated and only rises in sea level
below the coral growth rate ($1–2 cm a year for massive Porites) can be completely recorded. Furthermore, in mid-ocean settings changes in sea surface
temperature (SST) and salinity (SSS) related to ENSO
are documented by the geochemistry of microatoll
skeletons (see Chapter Palaeoclimate from Corals),
with high fidelity between adjacent corals (Woodroffe
and Gagan, 2000). Concerns that geochemical records
from microatolls may not represent oceanic water conditions appear allayed by studies that demonstrate
excellent correlations between these geochemical
records and instrumental SST datasets. Narrow stress
bands may also be deposited in microatoll skeletons
when conditions for coral growth become critical –
and these may document the occurrence of past severe
ENSO or other stress events.
(b) Tectonic movements and event prediction: Microatolls
respond to relative sea level and thus their surface morphologies reflect not only fluctuations in the sea surface
but also movements of the substrates on which they
grow (see Figure 2c). Microatolls in tectonically active
areas have been used to establish histories of subduction or uplift associated with earthquakes or more subtle interseismic crustal deformation processes (Taylor
et al., 1987; Zachariasen et al., 2000; Natawidjaja
et al., 2004, 2006, 2007; Briggs et al., 2006), to investigate spatial patterns of plate rupture during single
and multiple events (Konca et al., 2008), and to calculate the rates of tectonic stress accumulation, the frequency of past tectonic events, and the probability of
future tectonic hazards (Sieh, 2006; Sieh et al., 2008).
For example, Briggs et al. (2006) used GPS measurements of coral microatolls to establish abrupt
coseismic uplift of reefs at Nias by as much as 3 m during the Great 2005 Nias-Simeulue earthquake, which
followed decades of strain accumulation and submergence, documented by the development of “upgrown”
microatolls.
(c) Other environmental parameters: Coral microatolls
confirm that a reef has reached sea level and are critical
evidence in geomorphological investigations of reef
structure and growth. For example, fossil microatolls
across reef flats can reveal reef flat progradation histories (Smithers et al., 2006). Storm histories can also be
derived from microatolls, either from upper surface
patterns of moated microatolls where moat height or
integrity (and thus moated water level) is affected by
storms (Hopley and Isdale, 1977) or where storms
cause tilting of the microatoll plane (relative to the
pre-storm alignment), the timing of which can be
derived from the colony’s internal structure. Some
microatolls grow in areas where tidal conditions only
expose the coral during daylight at a particular time
of year. For example, on the Central GBR daytime low
spring tides only occur during the winter months, when
the sun is in the northern sky. As a result, higher growing
microatolls, such as those formed by Goniastrea and
Platygyra, develop tilted tops in which the northern,
more directly radiated section of the living rim is lower
than the “lee” side (Figure 1d). Buskirk et al., (1981)
reported similar microatolls with tops dipping towards
the sun from Vanuatu. This pattern is usually developed
across entire microatoll fields, and where storms
have affected these reefs, disturbance of this pattern is
conspicuous.
Summary
Coral microatolls are excellent sea-level indicators.
Advances in survey and radiometric dating technologies
that have improved both elevational and temporal resolution have further enhanced their utility. Furthermore, in
recent decades researchers have discovered that microatolls
yield excellent data on a broader range of environmental
phenomena including atmospheric, oceanic, and tectonic
processes. These high-resolution datasets augment more
recent and scattered instrumental records both spatially
and temporally, improving knowledge of the natural variability and significance of recent changes in these
parameters.
Bibliography
Abe, N., 1937. Ecological Survey of Iwayama Bay, Palao. Palao
Tropical Biological Station Studies, 1, 217–324.
Agassiz, 1895. A visit to the Bermudas in March 1894. Bulletin of
the Museum of Comparitive Zoology Harvard University, Vol.
26, pp. 209–281.
Briggs, R. W., Sieh, K., Meltzner, A. J., Natawidjaja, D., Galetzka, J.,
Suwargadi, B., Hsu, Y. J., Simons, M., Hananto, N.,
Suprihanto, I., Prayudi, D., Avouac, J. P., Prawirodirdjo, L.,
and Bock, Y., 2006. Deformation and slip along the Sunda
Megathrust in the great 2005 Nias-Simeulue earthquake.
Science, 311, 1897–1901.
Buskirk, R., Taylor, F., O
0 Brien, W., Maillet, P., and Gilpin, L.,
1981. Seasonal growth patterns and mortality of corals in the
New Hebrides (Vanuatu). 4th International Coral Reef Symposium, pp. 197–200.
Chappell, J., Chivas, A., Wallensky, E., Polach, H., and Aharon, P.,
1983. Holocene palaeo-environmental changes central to north
Great Barrier Reef inner zone. BMR Journal of Australian Geology and Geophysics, 8, 223–235.
Dana, J. D., 1872. Corals and coral islands. New York: Dodd and
Mean, 398 pp.
Darwin, C. R., 1842. The structure and distribution of coral reefs.
London: Smith, Elder and Co., 214 pp.
Goodwin, I. D., and Harvey, N., 2008. Subtropical sea-level history
from coral microatolls in the Southern Cook Islands, since 300
AD. Marine Geology, 253, 14–25.
Guppy, H. B., 1889. The Cocos-Keeling islands. The Scottish Geographical Magazine, 5, 569–588.
Hendy, E. J., Lough, J. M., and Gagan, M. K., 2003. Historical mortality in massive Porites from the central Great Barrier Reef,
MICROATOLL
695
has been demonstrated that microatolls in the Pacific
Ocean preserve smoothed records of these oscillations
across their upper surfaces (Woodroffe and McLean,
1990; Spencer et al., 1997; Woodroffe and Gagan,
2000). The records are smoothed because the record
is biologically mediated and only rises in sea level
below the coral growth rate ($1–2 cm a year for massive Porites) can be completely recorded. Furthermore, in mid-ocean settings changes in sea surface
temperature (SST) and salinity (SSS) related to ENSO
are documented by the geochemistry of microatoll
skeletons (see Chapter Palaeoclimate from Corals),
with high fidelity between adjacent corals (Woodroffe
and Gagan, 2000). Concerns that geochemical records
from microatolls may not represent oceanic water conditions appear allayed by studies that demonstrate
excellent correlations between these geochemical
records and instrumental SST datasets. Narrow stress
bands may also be deposited in microatoll skeletons
when conditions for coral growth become critical –
and these may document the occurrence of past severe
ENSO or other stress events.
(b) Tectonic movements and event prediction: Microatolls
respond to relative sea level and thus their surface morphologies reflect not only fluctuations in the sea surface
but also movements of the substrates on which they
grow (see Figure 2c). Microatolls in tectonically active
areas have been used to establish histories of subduction or uplift associated with earthquakes or more subtle interseismic crustal deformation processes (Taylor
et al., 1987; Zachariasen et al., 2000; Natawidjaja
et al., 2004, 2006, 2007; Briggs et al., 2006), to investigate spatial patterns of plate rupture during single
and multiple events (Konca et al., 2008), and to calculate the rates of tectonic stress accumulation, the frequency of past tectonic events, and the probability of
future tectonic hazards (Sieh, 2006; Sieh et al., 2008).
For example, Briggs et al. (2006) used GPS measurements of coral microatolls to establish abrupt
coseismic uplift of reefs at Nias by as much as 3 m during the Great 2005 Nias-Simeulue earthquake, which
followed decades of strain accumulation and submergence, documented by the development of “upgrown”
microatolls.
(c) Other environmental parameters: Coral microatolls
confirm that a reef has reached sea level and are critical
evidence in geomorphological investigations of reef
structure and growth. For example, fossil microatolls
across reef flats can reveal reef flat progradation histories (Smithers et al., 2006). Storm histories can also be
derived from microatolls, either from upper surface
patterns of moated microatolls where moat height or
integrity (and thus moated water level) is affected by
storms (Hopley and Isdale, 1977) or where storms
cause tilting of the microatoll plane (relative to the
pre-storm alignment), the timing of which can be
derived from the colony’s internal structure. Some
microatolls grow in areas where tidal conditions only
expose the coral during daylight at a particular time
of year. For example, on the Central GBR daytime low
spring tides only occur during the winter months, when
the sun is in the northern sky. As a result, higher growing
microatolls, such as those formed by Goniastrea and
Platygyra, develop tilted tops in which the northern,
more directly radiated section of the living rim is lower
than the “lee” side (Figure 1d). Buskirk et al., (1981)
reported similar microatolls with tops dipping towards
the sun from Vanuatu. This pattern is usually developed
across entire microatoll fields, and where storms
have affected these reefs, disturbance of this pattern is
conspicuous.
Summary
Coral microatolls are excellent sea-level indicators.
Advances in survey and radiometric dating technologies
that have improved both elevational and temporal resolution have further enhanced their utility. Furthermore, in
recent decades researchers have discovered that microatolls
yield excellent data on a broader range of environmental
phenomena including atmospheric, oceanic, and tectonic
processes. These high-resolution datasets augment more
recent and scattered instrumental records both spatially
and temporally, improving knowledge of the natural variability and significance of recent changes in these
parameters.
Bibliography
Abe, N., 1937. Ecological Survey of Iwayama Bay, Palao. Palao
Tropical Biological Station Studies, 1, 217–324.
Agassiz, 1895. A visit to the Bermudas in March 1894. Bulletin of
the Museum of Comparitive Zoology Harvard University, Vol.
26, pp. 209–281.
Briggs, R. W., Sieh, K., Meltzner, A. J., Natawidjaja, D., Galetzka, J.,
Suwargadi, B., Hsu, Y. J., Simons, M., Hananto, N.,
Suprihanto, I., Prayudi, D., Avouac, J. P., Prawirodirdjo, L.,
and Bock, Y., 2006. Deformation and slip along the Sunda
Megathrust in the great 2005 Nias-Simeulue earthquake.
Science, 311, 1897–1901.
Buskirk, R., Taylor, F., O
0 Brien, W., Maillet, P., and Gilpin, L.,
1981. Seasonal growth patterns and mortality of corals in the
New Hebrides (Vanuatu). 4th International Coral Reef Symposium, pp. 197–200.
Chappell, J., Chivas, A., Wallensky, E., Polach, H., and Aharon, P.,
1983. Holocene palaeo-environmental changes central to north
Great Barrier Reef inner zone. BMR Journal of Australian Geology and Geophysics, 8, 223–235.
Dana, J. D., 1872. Corals and coral islands. New York: Dodd and
Mean, 398 pp.
Darwin, C. R., 1842. The structure and distribution of coral reefs.
London: Smith, Elder and Co., 214 pp.
Goodwin, I. D., and Harvey, N., 2008. Subtropical sea-level history
from coral microatolls in the Southern Cook Islands, since 300
AD. Marine Geology, 253, 14–25.
Guppy, H. B., 1889. The Cocos-Keeling islands. The Scottish Geographical Magazine, 5, 569–588.
Hendy, E. J., Lough, J. M., and Gagan, M. K., 2003. Historical mortality in massive Porites from the central Great Barrier Reef,
MICROATOLL
695