3 – The Great Barrier Reef in Time and Space: Geology and Palaeobiology
23
size and extent. In the Moreton Bay example a few thousand years of still stand also led to the development of
sedimentary deposits (coastal plains and tidal deltas)
and their inshore environmental correlates (mangroves,
seagrasses etc.). This restricted back-barrier circulation
increased the estuarine nature of these environments
with negative consequences for mid-Holocene backbarrier coral communities.
In the Moreton Bay example many millions of tonnes
of coral carbonate was deposited throughout the entire
bay in sequences up to 8 m thick immediately after sea
level stabilised between 6 ka and 4 ka. So extensive were
these deposits they supported a dredge mining operation for over six decades. While corals are still found in
Moreton Bay today the reduced circulation and increasingly estuarine conditions experienced after 4 ky has
reduced their extent, growth and diversity.
These corals are of interest to the GBR context for
what they are not as much as for what they are. They
did not form ‘reefs’ according to the ‘wave resistant
structure of organic origin’ definition. Rather, the corals
were flourishing mounds or banks of corals in a backbarrier setting with open circulation. Unpublished
radiocarbon dates from Moreton Bay show these Acropora dominated coral communities (40 spp.) grew and
accumulated carbonate at rates of up to 5 m ky
1
, similar to those known from GBR reefs. The Moreton Bay
back-barrier model for coral communities is therefore a
diverse, fast acting and geologically significant vehicle
for corals over time. A significant difference between
these communities and true reefs is that if sea level had
continued to rise, these uncemented carbonate deposits
would most likely have been eroded away.
Further instances of coral communities from turbid
environments forming detrital mounds (as opposed to
‘true reefs’) have been documented from near-shore
reefs of the GBR, including Broad Sound and Paluma
Shoals. We feel it is helpful to differentiate these ‘coral
communities’ from ‘coral reefs’ because they provide
an alternative phase during the ‘life cycle’ of the GBR.
As sea levels rise and fall, barrier islands will form in
response to still stands providing for Moreton Bay-style
opportunities again and again. From a palaeoecological perspective they are a useful alternative to the high
sea level reef paradigm.
Having discussed the geological boundary conditions and some of the processes that frame the GBR in
space and time we can now better understand our notion of a single interglacial to interglacial ‘life cycle’ of
the GBR and also better scrutinise some of our assumptions about the system. A review of the drilling data
shows that the majority of framework growth associated with the current interglacial GBR grew between
9 ka and 4 ka—5 ka window at depths shallower than
30 m. If we assume a similar window for the previous
interglacial GBR then the extensive matrix of high sea
level platform reefs we know as the GBR was probably
active for about 10% of the ecological time between the
last two interglacials. Moreover, it may only have been
actively growing for about 5% of that time.
A model of the GBR in time and space also needs to
account for environmental gradients. Today, there is no
single locality that supports all of the roughly 400 coral
species found in the GBR region. Richest reefs are in the
far northern to central region. Areas like Princess Charlotte Bay, the Palm and Whitsunday Islands provide
important ecological space for ‘inshore’ or turbid water
coral communities. These communities collectively
contain most species present in the GBR coral fauna,
with only a small pool of species apparently restricted to
offshore reefs. There are, nevertheless, substantial differences in species’ abundance in respect of the major
environmental gradients, resulting in more or less characteristic community types across and along the GBR. In
particular, there are major differences in species composition between the wave washed, clear water reef crest
communities of the seaward slopes of outer barrier reefs
and their highly sheltered, turbid water, inshore counterparts, most notably those of the deeper reef slopes of
leeward sides of continental islands. These communities
are at opposite ends of the physico-chemical spectrum
and environmental gradients for the GBR.
So do wave resistant high-stand reefs adequately
represent a model for the ecological and geological
propagation of the GBR in time and space? Clearly,
only partially. If sea level were to fall by 10 m, 20 m and
then 30 m would the corals of the GBR, and the thousands of coral connected species, go charging out to the
Queensland Plateau to form a clear water ‘reef’? Again,
probably not. Just as understanding the workings of
23
size and extent. In the Moreton Bay example a few thousand years of still stand also led to the development of
sedimentary deposits (coastal plains and tidal deltas)
and their inshore environmental correlates (mangroves,
seagrasses etc.). This restricted back-barrier circulation
increased the estuarine nature of these environments
with negative consequences for mid-Holocene backbarrier coral communities.
In the Moreton Bay example many millions of tonnes
of coral carbonate was deposited throughout the entire
bay in sequences up to 8 m thick immediately after sea
level stabilised between 6 ka and 4 ka. So extensive were
these deposits they supported a dredge mining operation for over six decades. While corals are still found in
Moreton Bay today the reduced circulation and increasingly estuarine conditions experienced after 4 ky has
reduced their extent, growth and diversity.
These corals are of interest to the GBR context for
what they are not as much as for what they are. They
did not form ‘reefs’ according to the ‘wave resistant
structure of organic origin’ definition. Rather, the corals
were flourishing mounds or banks of corals in a backbarrier setting with open circulation. Unpublished
radiocarbon dates from Moreton Bay show these Acropora dominated coral communities (40 spp.) grew and
accumulated carbonate at rates of up to 5 m ky
1
, similar to those known from GBR reefs. The Moreton Bay
back-barrier model for coral communities is therefore a
diverse, fast acting and geologically significant vehicle
for corals over time. A significant difference between
these communities and true reefs is that if sea level had
continued to rise, these uncemented carbonate deposits
would most likely have been eroded away.
Further instances of coral communities from turbid
environments forming detrital mounds (as opposed to
‘true reefs’) have been documented from near-shore
reefs of the GBR, including Broad Sound and Paluma
Shoals. We feel it is helpful to differentiate these ‘coral
communities’ from ‘coral reefs’ because they provide
an alternative phase during the ‘life cycle’ of the GBR.
As sea levels rise and fall, barrier islands will form in
response to still stands providing for Moreton Bay-style
opportunities again and again. From a palaeoecological perspective they are a useful alternative to the high
sea level reef paradigm.
Having discussed the geological boundary conditions and some of the processes that frame the GBR in
space and time we can now better understand our notion of a single interglacial to interglacial ‘life cycle’ of
the GBR and also better scrutinise some of our assumptions about the system. A review of the drilling data
shows that the majority of framework growth associated with the current interglacial GBR grew between
9 ka and 4 ka—5 ka window at depths shallower than
30 m. If we assume a similar window for the previous
interglacial GBR then the extensive matrix of high sea
level platform reefs we know as the GBR was probably
active for about 10% of the ecological time between the
last two interglacials. Moreover, it may only have been
actively growing for about 5% of that time.
A model of the GBR in time and space also needs to
account for environmental gradients. Today, there is no
single locality that supports all of the roughly 400 coral
species found in the GBR region. Richest reefs are in the
far northern to central region. Areas like Princess Charlotte Bay, the Palm and Whitsunday Islands provide
important ecological space for ‘inshore’ or turbid water
coral communities. These communities collectively
contain most species present in the GBR coral fauna,
with only a small pool of species apparently restricted to
offshore reefs. There are, nevertheless, substantial differences in species’ abundance in respect of the major
environmental gradients, resulting in more or less characteristic community types across and along the GBR. In
particular, there are major differences in species composition between the wave washed, clear water reef crest
communities of the seaward slopes of outer barrier reefs
and their highly sheltered, turbid water, inshore counterparts, most notably those of the deeper reef slopes of
leeward sides of continental islands. These communities
are at opposite ends of the physico-chemical spectrum
and environmental gradients for the GBR.
So do wave resistant high-stand reefs adequately
represent a model for the ecological and geological
propagation of the GBR in time and space? Clearly,
only partially. If sea level were to fall by 10 m, 20 m and
then 30 m would the corals of the GBR, and the thousands of coral connected species, go charging out to the
Queensland Plateau to form a clear water ‘reef’? Again,
probably not. Just as understanding the workings of