a base at around 300,000 years. Kroon and Alexander
(1993) interpret this signal to indicate a progressive
decoupling of surface and bottom water temperatures
and raised surface water temperatures.
(2) Between 75 and 145 m (Unit B above), the signal is
characterized by a much higher frequency as well as
high amplitude. Peerderman et al. (1993) interpret this
as dominated by the obliquity signal and extending
from isotope stage 9–19 at the Brunhes/Matayama
boundary. Davies and Peerdeman (1998) divided the
section into two parts (2–1) between 75 and 110 m
(isotope stages 9–13) characterized by low frequency
high amplitude and (2–2) between 110 and 143 m
(isotope stages 13–19) characterized by a much
higher frequency and also a high-amplitude signal.
These correspond with the division in packstones
thicknesses noted above.
(3) Betweeen 145 and 220 m (Unit C above), the signal is
characterized again by a much lower frequency(perhaps dominated by the eccentricity signal) and has
high amplitude and as such is similar to the section
above 75 m.
Peerdeman et al. (1993) consider the boundary at 75 m as
a critical event in the history of the margin. Using the difference between the planktonic O18 signal in the Site 820
core and the benthic O18 signal at the deep sea Site 677
(Shackleton and Hall, 1989), they defined sea surface
temperature variation for the northeast Australian margin
which showed a substantial rise in temperature of
around 3–5
C at around 75 m in the core (around isotope
stage 9). This temperature variation corresponds to a
change in frequency and amplitude of the isotope signal
arising from the change from obliquity to precessional
orbital cyclicity. Kroon and Alexander (1993) recognized
a similar temperature change at Site 819. Further, Jansen
et al. (1986) also recognized a fundamental change in isotope frequency at the 9–8 boundary as defining a change
from lower to higher sea surface temperatures leading to
the establishment of more interglacial conditions in the
period of isotope stages 1–8. At sites 820 and 819, the
boundary at 65 m is, therefore, a globally driven change
dated at around 300,000 years. Further, the changes defined
at the B1/B2 boundary, particularly the change from
prograding to aggrading geometries (dated as just prior to
465 ka) reflects a change from shallow to deeper water, coincident with major regional oceanographic reorganization in
the Southern Ocean (Kuijpers, 1989; Nelson et al., 1988).
The boundary may also mark the change from an
obliquity signal to the eccentricity signal seen in Unit A,
with Unit B1 representing the transitional warming.
Relations between Ribbon 5 core and ODP
site 820 and implications
The proposed relations and reconciliation of the two data
sets are shown in Figure 11a. The profile to the east of Ribbon 5 is one proven by profiling and submersible examination (Phipps et al., 1985). The fundamental change
from subtropical rhodoliths to tropical reef reflects a temperature change and just such a change is seen in the isotopic signal at Sites 820 at a depth of 65 m. The reef
section is thus equivalent to Unit A (isotope stages 1–8)
and the rhodolith section is equivalent to Unit B1 (isotope
stages 9–13). Initial reef establishment of the coral frameworks in the middle of the rhodolith section must, therefore, date at sometime after 465 ka while the firm
establishment of the main reef section above 96 m dates
from around 300,000 years. The six stacked reefs at Ribbon 5 correspond to sea-level highstand growth phases,
the first being either stages 8 or 9. The rhodolith section,
equivalent to B1, represents the transitional warming
suggested by the change in the obliquity signal noted
above. This would be equivalent to conditions operating
today to the south of the Great Barrier Reef on the south
Queensland shelf (Davies et al., 1991). Further the sealevel oscillations defined by the wachestone/packstone
alternations at Site 820 and representing warming and
cooling episodes, serve also to explain the reef intercalations within the rhodolith sections. Davies and Peerdeman
(1998) propose that the facies variations on that shelf and
their lateral migration during oscillating sea level, define
a model for the sub-reef rhodolith section at Ribbon 5
and explain the reef frameworks within that section.
Cross-shelf growth of the great barrier reef
Once established in the shelf edge zone, is it right to
assume that reefs quickly spread everywhere on the shelf?
The results of a second borehole at Boulder Reef
(Figure 11b) on the inner shelf and drilled during the Ribbon 5 campaign suggest otherwise. Four stacked reef
sections (R1–R4 in Figure 11b) sit on siliciclastic mud
and clay foundations at a depth of 34 m. The siliciclastics
extend to 86 m where drilling terminated; however, coralbearing units (R6 and R5) lacking a true framework occur
within this mud section and have been interpreted as lowdiversity coral communities reworked to form a rubble
(Webster and Davies, 2003). Their presence is, however,
significant. The base of the core has been tentatively dated
by ESR as 210 þ À40 ka, which suggests that the beginning of reef growth on the inner shelf was much later than
on the outer shelf. Two failed attempts (R5 þ R6,
Figure 11b) were followed by the establishment (R4) of
a reef structure, after which Boulder Reef was reestablished
on three later occasions. The two failed attempts at 55
m (R5) and 63 m (R6) and the beginning of the main reef
section (R4 at 34 m) occurred on siliciclastic foundations,
testifying to the fact that the inner shelf was a turbid environment during highstands, perhaps more turbid than
today. It is concluded, therefore, that the pre-reef environment was totally different on the inner shelf compared with
the outer shelf and that reef initiation was later on the inner
shelf compared with the outer. In both cases, however, reef
growth was a relatively recent phenomenon.
The drill core at Ribbon 5 and Boulder Reefs testify to
the youth of the Great Barrier Reef and to the fact that reefs
522
GREAT BARRIER REEF: ORIGIN, EVOLUTION, AND MODERN DEVELOPMENT
(1993) interpret this signal to indicate a progressive
decoupling of surface and bottom water temperatures
and raised surface water temperatures.
(2) Between 75 and 145 m (Unit B above), the signal is
characterized by a much higher frequency as well as
high amplitude. Peerderman et al. (1993) interpret this
as dominated by the obliquity signal and extending
from isotope stage 9–19 at the Brunhes/Matayama
boundary. Davies and Peerdeman (1998) divided the
section into two parts (2–1) between 75 and 110 m
(isotope stages 9–13) characterized by low frequency
high amplitude and (2–2) between 110 and 143 m
(isotope stages 13–19) characterized by a much
higher frequency and also a high-amplitude signal.
These correspond with the division in packstones
thicknesses noted above.
(3) Betweeen 145 and 220 m (Unit C above), the signal is
characterized again by a much lower frequency(perhaps dominated by the eccentricity signal) and has
high amplitude and as such is similar to the section
above 75 m.
Peerdeman et al. (1993) consider the boundary at 75 m as
a critical event in the history of the margin. Using the difference between the planktonic O18 signal in the Site 820
core and the benthic O18 signal at the deep sea Site 677
(Shackleton and Hall, 1989), they defined sea surface
temperature variation for the northeast Australian margin
which showed a substantial rise in temperature of
around 3–5
C at around 75 m in the core (around isotope
stage 9). This temperature variation corresponds to a
change in frequency and amplitude of the isotope signal
arising from the change from obliquity to precessional
orbital cyclicity. Kroon and Alexander (1993) recognized
a similar temperature change at Site 819. Further, Jansen
et al. (1986) also recognized a fundamental change in isotope frequency at the 9–8 boundary as defining a change
from lower to higher sea surface temperatures leading to
the establishment of more interglacial conditions in the
period of isotope stages 1–8. At sites 820 and 819, the
boundary at 65 m is, therefore, a globally driven change
dated at around 300,000 years. Further, the changes defined
at the B1/B2 boundary, particularly the change from
prograding to aggrading geometries (dated as just prior to
465 ka) reflects a change from shallow to deeper water, coincident with major regional oceanographic reorganization in
the Southern Ocean (Kuijpers, 1989; Nelson et al., 1988).
The boundary may also mark the change from an
obliquity signal to the eccentricity signal seen in Unit A,
with Unit B1 representing the transitional warming.
Relations between Ribbon 5 core and ODP
site 820 and implications
The proposed relations and reconciliation of the two data
sets are shown in Figure 11a. The profile to the east of Ribbon 5 is one proven by profiling and submersible examination (Phipps et al., 1985). The fundamental change
from subtropical rhodoliths to tropical reef reflects a temperature change and just such a change is seen in the isotopic signal at Sites 820 at a depth of 65 m. The reef
section is thus equivalent to Unit A (isotope stages 1–8)
and the rhodolith section is equivalent to Unit B1 (isotope
stages 9–13). Initial reef establishment of the coral frameworks in the middle of the rhodolith section must, therefore, date at sometime after 465 ka while the firm
establishment of the main reef section above 96 m dates
from around 300,000 years. The six stacked reefs at Ribbon 5 correspond to sea-level highstand growth phases,
the first being either stages 8 or 9. The rhodolith section,
equivalent to B1, represents the transitional warming
suggested by the change in the obliquity signal noted
above. This would be equivalent to conditions operating
today to the south of the Great Barrier Reef on the south
Queensland shelf (Davies et al., 1991). Further the sealevel oscillations defined by the wachestone/packstone
alternations at Site 820 and representing warming and
cooling episodes, serve also to explain the reef intercalations within the rhodolith sections. Davies and Peerdeman
(1998) propose that the facies variations on that shelf and
their lateral migration during oscillating sea level, define
a model for the sub-reef rhodolith section at Ribbon 5
and explain the reef frameworks within that section.
Cross-shelf growth of the great barrier reef
Once established in the shelf edge zone, is it right to
assume that reefs quickly spread everywhere on the shelf?
The results of a second borehole at Boulder Reef
(Figure 11b) on the inner shelf and drilled during the Ribbon 5 campaign suggest otherwise. Four stacked reef
sections (R1–R4 in Figure 11b) sit on siliciclastic mud
and clay foundations at a depth of 34 m. The siliciclastics
extend to 86 m where drilling terminated; however, coralbearing units (R6 and R5) lacking a true framework occur
within this mud section and have been interpreted as lowdiversity coral communities reworked to form a rubble
(Webster and Davies, 2003). Their presence is, however,
significant. The base of the core has been tentatively dated
by ESR as 210 þ À40 ka, which suggests that the beginning of reef growth on the inner shelf was much later than
on the outer shelf. Two failed attempts (R5 þ R6,
Figure 11b) were followed by the establishment (R4) of
a reef structure, after which Boulder Reef was reestablished
on three later occasions. The two failed attempts at 55
m (R5) and 63 m (R6) and the beginning of the main reef
section (R4 at 34 m) occurred on siliciclastic foundations,
testifying to the fact that the inner shelf was a turbid environment during highstands, perhaps more turbid than
today. It is concluded, therefore, that the pre-reef environment was totally different on the inner shelf compared with
the outer shelf and that reef initiation was later on the inner
shelf compared with the outer. In both cases, however, reef
growth was a relatively recent phenomenon.
The drill core at Ribbon 5 and Boulder Reefs testify to
the youth of the Great Barrier Reef and to the fact that reefs
522
GREAT BARRIER REEF: ORIGIN, EVOLUTION, AND MODERN DEVELOPMENT