for the interval after the last glacial maximum but they
have also been extended to earlier periods, particularly
the Last Interglacial. In the context of coral reefs from this
latter period, it is important to recognize that these isostatic processes also occur and that the elevations of Last
Interglacial sea levels will exhibit similar patterns of spatial variability.
Figure 5 illustrates the predicted spatial variability for
one region, the Japan Sea, that, in the absence of tectonics, is representative of most far-field regions. The results
are for two epochs, the time of the last glacial maximum
(LGM) and the time of the mid-Holocene highstand but
the spatial variability is similar for all other epochs.
Thus, even within relatively small regions sea level cannot be represented by a single curve. At the LGM the
predicted sea levels vary by $30 m, being significantly
shallower in the Korea Strait, for example, than in the
middle of the Japan Sea where it exceeds the ESL value.
Likewise, the Holocene highstand amplitudes are
predicted to occur up to 2 m above sea level in some
locations but not in northern Hokkaido or the Kurile
Islands.
Conclusion
The phenomenon of glacio-hydro isostasy means that
even in the absence of vertical tectonics, sea level at any
time is spatially variable, that it can be rising in one location while falling elsewhere. This variability is a consequence of the earth’s deformation under the changing
load and to the changes in the gravity field. The mapping
of this variability therefore provides information on both
the earth’s rheology and on the history of the ice sheets.
But it also needs to be known if sea level is used as
a reference for vertical tectonics or, for example, if reef
response to changing sea level is to be investigated. The
theory and implementation of the glacio-hydro-isostasy
theory is sufficiently well developed for it to provide
a sound basis for interpolating between the limited
sea-level observations for the time since maximum glaciation and for reconstructing paleo shorelines and water
depths.
Bibliography
Lambeck, K., and Chappell, J., 2001. Sea level change through the
last glacial cycle. Science, 292, 679–686.
Lambeck, K., Purcell, A., Johnston, P., Nakada, M., and Yokoyama, Y.,
2003. Water-load definition in the glacio-hydro-isostatic sea-level
equation. Quaternary Science Reviews, 22, 309–318.
Mitrovica, J. X., and Milne, G. A., 2003. On post-glacial sea level: I.
General theory. Geophysical Journal International, 154,
253–267.
Cross-references
Last Glacial Lowstand and Shelf Exposure
Sea Level Change and Its Effect on Reef Growth
Volcanic Loading and Isostasy
GLOBAL OCEAN CIRCULATION AND CORAL REEFS
John E. N. Veron
Coral Reef Research, QLD, Townsville, Australia
Synonyms
Global scale currents
Definition
Ocean circulation denotes ocean-scale current systems
that change seasonally but persist from one year to the
next.
Introduction
Reefs develop in most parts of the world where reefbuilding (zooxanthellate) corals grow and where the ocean
is warm. This geographic template is created by the
world’s great ocean currents (Figure 1). Currents therefore
play a key role in the geographic extent of reef formation
as well as in the dispersal of reef biota. They are also critical determinants of the pathways of species evolution.
The historical role of ocean circulations as drivers of
reef and faunal distributions is closely linked to other
drivers, not only temperature but also sea-levels and
the configuration of the continents. Two very different
scales – global and regional – operate simultaneously.
Global scales over geological time provide the template
of both coral reef distributions and zooxanthellate coral
distribution. Regional scales over millennia or less control
details of the distributions of corals and have a short-term,
ecologically driven influence on reef development (see
below).
Origins of modern circulation patterns
At the start of the Cenozoic Era, ocean circulations were
dominated by a circum-global equatorial path via the
Tethys Sea and Central American Seaway. There was no
circulation around Antarctica (the remnants of Gondwana)
and all oceans were warm and sluggish, with no strong latitudinal temperature gradients as we have today. This pattern persisted until South America rifted free of Antarctica
during the Late Eocene–Early Oligocene 40–35 million
years ago (mya), and Drake Passage opened. At this point,
for the first time since Gondwana formed, the Southern
Ocean began circulating around Antarctica, moving eastward in response to the direction of rotation of the Earth
and checked only by the coastlines of the three southern
continents.
The formation of the Antarctic Circumpolar Current
was the critical event that gave birth to modern circulation
patterns. Where the three continents of South America,
Africa, and (to a lesser extent) Australia project into its
path, cold water is deflected north along the western margin of each continent. The currents then move westward in
the tropics where they are warmed, and return south along
GLOBAL OCEAN CIRCULATION AND CORAL REEFS
497
have also been extended to earlier periods, particularly
the Last Interglacial. In the context of coral reefs from this
latter period, it is important to recognize that these isostatic processes also occur and that the elevations of Last
Interglacial sea levels will exhibit similar patterns of spatial variability.
Figure 5 illustrates the predicted spatial variability for
one region, the Japan Sea, that, in the absence of tectonics, is representative of most far-field regions. The results
are for two epochs, the time of the last glacial maximum
(LGM) and the time of the mid-Holocene highstand but
the spatial variability is similar for all other epochs.
Thus, even within relatively small regions sea level cannot be represented by a single curve. At the LGM the
predicted sea levels vary by $30 m, being significantly
shallower in the Korea Strait, for example, than in the
middle of the Japan Sea where it exceeds the ESL value.
Likewise, the Holocene highstand amplitudes are
predicted to occur up to 2 m above sea level in some
locations but not in northern Hokkaido or the Kurile
Islands.
Conclusion
The phenomenon of glacio-hydro isostasy means that
even in the absence of vertical tectonics, sea level at any
time is spatially variable, that it can be rising in one location while falling elsewhere. This variability is a consequence of the earth’s deformation under the changing
load and to the changes in the gravity field. The mapping
of this variability therefore provides information on both
the earth’s rheology and on the history of the ice sheets.
But it also needs to be known if sea level is used as
a reference for vertical tectonics or, for example, if reef
response to changing sea level is to be investigated. The
theory and implementation of the glacio-hydro-isostasy
theory is sufficiently well developed for it to provide
a sound basis for interpolating between the limited
sea-level observations for the time since maximum glaciation and for reconstructing paleo shorelines and water
depths.
Bibliography
Lambeck, K., and Chappell, J., 2001. Sea level change through the
last glacial cycle. Science, 292, 679–686.
Lambeck, K., Purcell, A., Johnston, P., Nakada, M., and Yokoyama, Y.,
2003. Water-load definition in the glacio-hydro-isostatic sea-level
equation. Quaternary Science Reviews, 22, 309–318.
Mitrovica, J. X., and Milne, G. A., 2003. On post-glacial sea level: I.
General theory. Geophysical Journal International, 154,
253–267.
Cross-references
Last Glacial Lowstand and Shelf Exposure
Sea Level Change and Its Effect on Reef Growth
Volcanic Loading and Isostasy
GLOBAL OCEAN CIRCULATION AND CORAL REEFS
John E. N. Veron
Coral Reef Research, QLD, Townsville, Australia
Synonyms
Global scale currents
Definition
Ocean circulation denotes ocean-scale current systems
that change seasonally but persist from one year to the
next.
Introduction
Reefs develop in most parts of the world where reefbuilding (zooxanthellate) corals grow and where the ocean
is warm. This geographic template is created by the
world’s great ocean currents (Figure 1). Currents therefore
play a key role in the geographic extent of reef formation
as well as in the dispersal of reef biota. They are also critical determinants of the pathways of species evolution.
The historical role of ocean circulations as drivers of
reef and faunal distributions is closely linked to other
drivers, not only temperature but also sea-levels and
the configuration of the continents. Two very different
scales – global and regional – operate simultaneously.
Global scales over geological time provide the template
of both coral reef distributions and zooxanthellate coral
distribution. Regional scales over millennia or less control
details of the distributions of corals and have a short-term,
ecologically driven influence on reef development (see
below).
Origins of modern circulation patterns
At the start of the Cenozoic Era, ocean circulations were
dominated by a circum-global equatorial path via the
Tethys Sea and Central American Seaway. There was no
circulation around Antarctica (the remnants of Gondwana)
and all oceans were warm and sluggish, with no strong latitudinal temperature gradients as we have today. This pattern persisted until South America rifted free of Antarctica
during the Late Eocene–Early Oligocene 40–35 million
years ago (mya), and Drake Passage opened. At this point,
for the first time since Gondwana formed, the Southern
Ocean began circulating around Antarctica, moving eastward in response to the direction of rotation of the Earth
and checked only by the coastlines of the three southern
continents.
The formation of the Antarctic Circumpolar Current
was the critical event that gave birth to modern circulation
patterns. Where the three continents of South America,
Africa, and (to a lesser extent) Australia project into its
path, cold water is deflected north along the western margin of each continent. The currents then move westward in
the tropics where they are warmed, and return south along
GLOBAL OCEAN CIRCULATION AND CORAL REEFS
497