have been destroyed and regrown again on at least six
occasions on the outer shelf (Figure 11a, R1–R6) and four
occasions on the inner shelf (Figure 11b, R1–R4).
Coupled with the isotopic data from the ODP cores, it is
concluded that sea-level oscillations are the cause of the
repeated demise and subsequent reestablishment. Further,
the isotopic data also indicate that the initial establishment
of the reefs on the outer shelf was climate controlled, and
occurred on a rhodolith-dominated bank on the outer
shelf. Such subtropical red algal banks may have formed
in the way such banks occur on the south Queensland shelf
today.
The modern reef
The history of growth and development of the modern reef
has been defined largely by drilling through the reefs into
their Pleistocene substrate. Hopley et al. (2007) report 160
drill holes in 50 reefs, and 750 dates. These data indicate
that the amount of Holocene reef growth varies from 0 to
28 m (Davies and Hopley, 1983; Hopley et al., 2007).
Transects across the northern reefs east of Cooktown and
off Townsville indicate little difference in the thickness
of reef from west to east (Davies et al., 1985). Longitudinally, however, down the length of the Great Barrier
Reef, Hopley et al. (2007) report an increase in thickness
from Torres Strait to south of Cairns and then a gradual
decrease in thickness to the south. They, therefore, imply
a change in the depth of the Pleistocene substrate from
north to south and invoke either long or short-term subsidence to explain it. An additional process, hydroisostasy,
has also been at work to ultimately define systematic
variations in thickness (see Glacio-Hydro Isostasy, this
volume).
The composition of the Pleistocene substrate has been
shown to be reef framework on the mid- and outer shelf,
similar or identical to the modern reef. The Pleistocene
corals are, however, sometimes accompanied by thick coralline algal crusts and Halimeda grainstones as seen at
One Tree Reef and Ribbon Reef 5 (Marshall, 1983;
Webster and Davies, 2003). On fringing reefs, the Pleistocene is sometimes composed of mud, sand, and gravel
(Kleypas and Hopley, 1993). Soil horizons have been
reported from Fairfax and Fitzroy reefs in the southern
Great Barrier Reef, from Redbill, Hayman, and Davies
Reefs in the central region and from Ribbon Reef 5 in
the northern region. Soil development implies erosion of
the underlying limestone and it is surprising that so few
have been reported considering the number of holes
drilled. As the age of the Pleistocene substrate has been
established in the south at One Tree Reef (Davies and
Marshall, 1980; Marshall, 1983) and in the north at Ribbon Reef 5 (International Consortium for Great Barrier
Reef Drilling, 2001) as isotope stage 5 (around 125 ka),
then sea level is likely to have been þ5 m above present
sea level. Assuming the stage 5 reef grew to sea level, erosion of up to 33 m must have occurred on reefs in the
region (assuming no subsidence). Allowing for
subsidence of 10 m (rapid) in the 100 ka period since
exposure of the 125 ka reef, then erosion of up to 23
m will have occurred. The Pleistocene surface on which
the Holocene reef has grown is, thus, a much modified
(some would say karstified) earlier reef.
Holocene reef start-up
Holocene sea level in the Great Barrier Reef transgressed
the foundations of most Pleistocene platforms in the time
frame 7–10,000 years BP (Hopley et al., 2007). Start-up
growth occurred after transgression was complete, with
17 out of 22 reefs dated starting growth in the time frame
7,100–9,000 years BP. On the outer shelf start-up began at
6.6–8.6 years BP; in the mid-shelf it was around 9.1–6.9
years BP, while on the inner shelf it was around 7.8–6.3
years BP.
In the Great Barrier Reef, sea level changed from
a transgressive to a stillstand condition around 6,500 years
BP. The age at which reefs reached sea level broadly
depends on the depth of their foundations and these ages
are summarized in Hopley et al. (2007). Nearly 60% of
all reefs cored and dated reached sea level in the interval
5,000–6,500 years BP meaning that nearly 60% of the
reefs studied have been subject to a shallow water energy
regime for most of the time that they have been growing.
Growth, therefore, must reflect this.
The interactions between growth and sea level prompted
Davies and coworkers (Davies and Montaggioni, 1985;
Davies et al., 1985) and Neumann and Macintyre (1985)
separately, independently, and in different oceans to define
five strategies of reef growth in the Holocene:
Keep-Up Reef growth – reefs growing and keeping pace
with sea-level change, such that reef flats developed
early. Pleistocene foundations are relatively shallow.
Katch-Up 1 Reef growth – reefs which started to grow
well after inundation but then grew so quickly that they
were able to catch-up before sea level stabilized.
Katch-Up 2 Reef growth – reefs which started to grow
well after inundation but did not grow so fast so that
catch-up occurred after sea level had stabilized.
Katch-Up 3 Reef growth – reefs which did not start to
grow until after sea level had stabilized so that the
growth was into continually shallowing water.
Screwed-Up (Give-Up) Reef Growth – reefs which died
before reaching sea level. Neumann and Macintyre
(1985) considered that these were “drowned reefs,”
but Davies and coworkers considered other factors
could also effect death (turbidity, elevated nutrients,
and subsidence).
More than one growth strategy can occur on the same reef
as local environment factors affect the growth response
(Davies and Montaggioni, 1985; Davies et al., 1985). Further, the rates of growth are depth dependent; thus growth
approximates an “S” shaped curve – slow growth to start,
rapid growth as the reef grows into shallow water and slow
growth again as the reef is subject to high energy in shallow water. Plots of growth rates against paleo-water
GREAT BARRIER REEF: ORIGIN, EVOLUTION, AND MODERN DEVELOPMENT
523
occasions on the outer shelf (Figure 11a, R1–R6) and four
occasions on the inner shelf (Figure 11b, R1–R4).
Coupled with the isotopic data from the ODP cores, it is
concluded that sea-level oscillations are the cause of the
repeated demise and subsequent reestablishment. Further,
the isotopic data also indicate that the initial establishment
of the reefs on the outer shelf was climate controlled, and
occurred on a rhodolith-dominated bank on the outer
shelf. Such subtropical red algal banks may have formed
in the way such banks occur on the south Queensland shelf
today.
The modern reef
The history of growth and development of the modern reef
has been defined largely by drilling through the reefs into
their Pleistocene substrate. Hopley et al. (2007) report 160
drill holes in 50 reefs, and 750 dates. These data indicate
that the amount of Holocene reef growth varies from 0 to
28 m (Davies and Hopley, 1983; Hopley et al., 2007).
Transects across the northern reefs east of Cooktown and
off Townsville indicate little difference in the thickness
of reef from west to east (Davies et al., 1985). Longitudinally, however, down the length of the Great Barrier
Reef, Hopley et al. (2007) report an increase in thickness
from Torres Strait to south of Cairns and then a gradual
decrease in thickness to the south. They, therefore, imply
a change in the depth of the Pleistocene substrate from
north to south and invoke either long or short-term subsidence to explain it. An additional process, hydroisostasy,
has also been at work to ultimately define systematic
variations in thickness (see Glacio-Hydro Isostasy, this
volume).
The composition of the Pleistocene substrate has been
shown to be reef framework on the mid- and outer shelf,
similar or identical to the modern reef. The Pleistocene
corals are, however, sometimes accompanied by thick coralline algal crusts and Halimeda grainstones as seen at
One Tree Reef and Ribbon Reef 5 (Marshall, 1983;
Webster and Davies, 2003). On fringing reefs, the Pleistocene is sometimes composed of mud, sand, and gravel
(Kleypas and Hopley, 1993). Soil horizons have been
reported from Fairfax and Fitzroy reefs in the southern
Great Barrier Reef, from Redbill, Hayman, and Davies
Reefs in the central region and from Ribbon Reef 5 in
the northern region. Soil development implies erosion of
the underlying limestone and it is surprising that so few
have been reported considering the number of holes
drilled. As the age of the Pleistocene substrate has been
established in the south at One Tree Reef (Davies and
Marshall, 1980; Marshall, 1983) and in the north at Ribbon Reef 5 (International Consortium for Great Barrier
Reef Drilling, 2001) as isotope stage 5 (around 125 ka),
then sea level is likely to have been þ5 m above present
sea level. Assuming the stage 5 reef grew to sea level, erosion of up to 33 m must have occurred on reefs in the
region (assuming no subsidence). Allowing for
subsidence of 10 m (rapid) in the 100 ka period since
exposure of the 125 ka reef, then erosion of up to 23
m will have occurred. The Pleistocene surface on which
the Holocene reef has grown is, thus, a much modified
(some would say karstified) earlier reef.
Holocene reef start-up
Holocene sea level in the Great Barrier Reef transgressed
the foundations of most Pleistocene platforms in the time
frame 7–10,000 years BP (Hopley et al., 2007). Start-up
growth occurred after transgression was complete, with
17 out of 22 reefs dated starting growth in the time frame
7,100–9,000 years BP. On the outer shelf start-up began at
6.6–8.6 years BP; in the mid-shelf it was around 9.1–6.9
years BP, while on the inner shelf it was around 7.8–6.3
years BP.
In the Great Barrier Reef, sea level changed from
a transgressive to a stillstand condition around 6,500 years
BP. The age at which reefs reached sea level broadly
depends on the depth of their foundations and these ages
are summarized in Hopley et al. (2007). Nearly 60% of
all reefs cored and dated reached sea level in the interval
5,000–6,500 years BP meaning that nearly 60% of the
reefs studied have been subject to a shallow water energy
regime for most of the time that they have been growing.
Growth, therefore, must reflect this.
The interactions between growth and sea level prompted
Davies and coworkers (Davies and Montaggioni, 1985;
Davies et al., 1985) and Neumann and Macintyre (1985)
separately, independently, and in different oceans to define
five strategies of reef growth in the Holocene:
Keep-Up Reef growth – reefs growing and keeping pace
with sea-level change, such that reef flats developed
early. Pleistocene foundations are relatively shallow.
Katch-Up 1 Reef growth – reefs which started to grow
well after inundation but then grew so quickly that they
were able to catch-up before sea level stabilized.
Katch-Up 2 Reef growth – reefs which started to grow
well after inundation but did not grow so fast so that
catch-up occurred after sea level had stabilized.
Katch-Up 3 Reef growth – reefs which did not start to
grow until after sea level had stabilized so that the
growth was into continually shallowing water.
Screwed-Up (Give-Up) Reef Growth – reefs which died
before reaching sea level. Neumann and Macintyre
(1985) considered that these were “drowned reefs,”
but Davies and coworkers considered other factors
could also effect death (turbidity, elevated nutrients,
and subsidence).
More than one growth strategy can occur on the same reef
as local environment factors affect the growth response
(Davies and Montaggioni, 1985; Davies et al., 1985). Further, the rates of growth are depth dependent; thus growth
approximates an “S” shaped curve – slow growth to start,
rapid growth as the reef grows into shallow water and slow
growth again as the reef is subject to high energy in shallow water. Plots of growth rates against paleo-water
GREAT BARRIER REEF: ORIGIN, EVOLUTION, AND MODERN DEVELOPMENT
523