The Great Barrier Reef
18
N ORIGINS OF THE GREAT BARRIER REEF
The history of the GBR is influenced by the postGondwanan continental drift history of the Australian
continent and repeated episodes of global environmental change associated with the late Tertiary and
Pleistocene ice ages. The ‘reefal’ GBR is a relatively
young geological structure that was slow to respond
to favourable environmental conditions early on. In
fact, the central Queensland continental shelf has
enjoyed warm tropical waters that could well have
supported coral growth for the past 15 million years
(My). However, the best evidence indicates that the
initiation of the GBR did not occur until around 600
thousand years ago (ka), and the regional province of
reef systems as we now know them probably did not
occur until around 365–452 ka. This is coincident with
Marine Isotope Stage (MIS) 11, perhaps the warmest
interglacial of the past 450 thousand years (ky), and
one with climatic conditions most similar to those we
are now experiencing. Some workers believe that the
‘switching-on’ of the GBR was related to the midPleistocene transition from 41 ky to 100 ky-long
climatic cycles, and to the development during MIS 11
of a marked highstand that enabled sustenance of both
a cyclone corridor and a reef tract along a relatively
wide and deeper water continental shelf (Fig. 3.1A).
Cores drilled through Ribbon Reef 5 have shown
that the GBR has been able to reestablish itself repeatedly during high sea level episodes associated with
major environmental fluctuations in sea level, temperature and CO 2 over the past several hundred thousand
years. Moreover, these reefs have maintained a similar
coral and algal species composition during their repeated formation (see section on Palaeoecology below)
(Fig. 3.1B, C).
N REEF GROWTH AND GLOBAL SEA
LEVEL CHANGE
The growth and decay of ice sheets in the northern
hemisphere were controlled by 10
4 - to 10
5
-year scale climate changes forced by natural cyclic changes in several parameters of Earth’s orbit (so called Milankovitch
cycles). These cycles influence the amount of sun energy
received by the Earth. They include obliquity (changes
in the angle of Earth’s axis of rotation with respect to
the sun), eccentricity (changes in the circularity of
Earth’s orbit around the sun), and precession of the
equinoxes (changes in the position of the Earth in its
orbit around the sun at the time of the equinox). The
cycles are 41 000, 100 000, and 23 000 years respectively.
During the last 500 000 years, global sea level underwent at least 17 such cycles of rise and fall. Average
rates of sea level change between glacial and interglacial intervals approached 5 m per thousand years with
the possibility of greater rates associated with Heinrich
events (abrupt climatic episodes associated with icerafted detritus during the last glacial). The magnitude
of sea level change from one interglacial to the next is
on the order of 120 m, a major repetitive 100 m
+ rhythm
to the late Pleistocene ice ages with which all marine
life contends (Fig. 3.2A).
The GBR is very similar to other reefs around the
world in having grown during rises in sea level, or
transgressions, associated with the deglaciation part
of the cycle. One of the best examples of transgressive
reef growth that has been clearly related to the oxygen
isotope record for the late Pleistocene occurs at the
Huon Peninsula, Papua New Guinea (PNG) (Fig. 3.2B,
C). In this remarkable tectonically active locality, ongoing uplift during the last several hundred thousand
years has left a record of transgressive reef terraces
like ‘bath rings’ along over 80 km of coast. Here, nine
transgressive reef growth phases are recorded between 125 ka and 30 ka. Overall the record of dated
transgressive reef growth episodes extends back to at
least 340 ka.
During rising seas, reefs can accumulate at rates
exceeding 10 m per thousand years. This involves a
huge bulk of cemented biological framework, principally coral and coralline algae, and even larger quantities of associated sediments. However, once reefs
reach sea level, or sea level rise slows and stabilises,
this growth slows. From here the interplay between
the growth of the bound biological framework, the
production of reef associated skeletal sediment and
their destruction by bioerosion and physical forces
becomes of critical importance to the maintenance of
reef growth.
18
N ORIGINS OF THE GREAT BARRIER REEF
The history of the GBR is influenced by the postGondwanan continental drift history of the Australian
continent and repeated episodes of global environmental change associated with the late Tertiary and
Pleistocene ice ages. The ‘reefal’ GBR is a relatively
young geological structure that was slow to respond
to favourable environmental conditions early on. In
fact, the central Queensland continental shelf has
enjoyed warm tropical waters that could well have
supported coral growth for the past 15 million years
(My). However, the best evidence indicates that the
initiation of the GBR did not occur until around 600
thousand years ago (ka), and the regional province of
reef systems as we now know them probably did not
occur until around 365–452 ka. This is coincident with
Marine Isotope Stage (MIS) 11, perhaps the warmest
interglacial of the past 450 thousand years (ky), and
one with climatic conditions most similar to those we
are now experiencing. Some workers believe that the
‘switching-on’ of the GBR was related to the midPleistocene transition from 41 ky to 100 ky-long
climatic cycles, and to the development during MIS 11
of a marked highstand that enabled sustenance of both
a cyclone corridor and a reef tract along a relatively
wide and deeper water continental shelf (Fig. 3.1A).
Cores drilled through Ribbon Reef 5 have shown
that the GBR has been able to reestablish itself repeatedly during high sea level episodes associated with
major environmental fluctuations in sea level, temperature and CO 2 over the past several hundred thousand
years. Moreover, these reefs have maintained a similar
coral and algal species composition during their repeated formation (see section on Palaeoecology below)
(Fig. 3.1B, C).
N REEF GROWTH AND GLOBAL SEA
LEVEL CHANGE
The growth and decay of ice sheets in the northern
hemisphere were controlled by 10
4 - to 10
5
-year scale climate changes forced by natural cyclic changes in several parameters of Earth’s orbit (so called Milankovitch
cycles). These cycles influence the amount of sun energy
received by the Earth. They include obliquity (changes
in the angle of Earth’s axis of rotation with respect to
the sun), eccentricity (changes in the circularity of
Earth’s orbit around the sun), and precession of the
equinoxes (changes in the position of the Earth in its
orbit around the sun at the time of the equinox). The
cycles are 41 000, 100 000, and 23 000 years respectively.
During the last 500 000 years, global sea level underwent at least 17 such cycles of rise and fall. Average
rates of sea level change between glacial and interglacial intervals approached 5 m per thousand years with
the possibility of greater rates associated with Heinrich
events (abrupt climatic episodes associated with icerafted detritus during the last glacial). The magnitude
of sea level change from one interglacial to the next is
on the order of 120 m, a major repetitive 100 m
+ rhythm
to the late Pleistocene ice ages with which all marine
life contends (Fig. 3.2A).
The GBR is very similar to other reefs around the
world in having grown during rises in sea level, or
transgressions, associated with the deglaciation part
of the cycle. One of the best examples of transgressive
reef growth that has been clearly related to the oxygen
isotope record for the late Pleistocene occurs at the
Huon Peninsula, Papua New Guinea (PNG) (Fig. 3.2B,
C). In this remarkable tectonically active locality, ongoing uplift during the last several hundred thousand
years has left a record of transgressive reef terraces
like ‘bath rings’ along over 80 km of coast. Here, nine
transgressive reef growth phases are recorded between 125 ka and 30 ka. Overall the record of dated
transgressive reef growth episodes extends back to at
least 340 ka.
During rising seas, reefs can accumulate at rates
exceeding 10 m per thousand years. This involves a
huge bulk of cemented biological framework, principally coral and coralline algae, and even larger quantities of associated sediments. However, once reefs
reach sea level, or sea level rise slows and stabilises,
this growth slows. From here the interplay between
the growth of the bound biological framework, the
production of reef associated skeletal sediment and
their destruction by bioerosion and physical forces
becomes of critical importance to the maintenance of
reef growth.