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The Great Barrier Reef in Time and Space: Geology and Palaeobiology

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  • Coral Identification Capacity Building Program
17
3. The Great Barrier Reef in
Time and Space: Geology
and Palaeobiology
J. M. Pandolfi & R. Kelley
■  INTRODUCTION
Reefs in their many forms are found throughout the
fossil record and represent some of the earliest
structure-forming ecosystems on Earth. Since the
explosion of metazoans in the Cambrian around 540
Mya (million years ago) many groups of organisms
have formed ‘reef like’ features on the sea floor, mak-
ing reef communities difficult to singularly character-
ise. Following the greatest extinction of all time, the
Permo-Triassic event (251 Mya), scleractinian corals,
bivalve molluscs, and crustose coralline algae have
dominated the construction of wave resistant organic
carbonate structures on the planet—commonly called
reefs.
While the definition of just what is a ‘reefhas a
long and tortuous history of debate in the scientific
literature, there is no dispute about the importance of
the reef ecosystems of the coastlines, continental
shelves and ocean provinces of the tropical realm. In
this chapter we take a broad spatial and temporal
view of the largest of the world’s platform reef
provinces—the Great Barrier Reef (GBR). In particular
we look at the boundary conditions and mechanisms
that underpin the perpetuation of the GBR province
in space and time and how environmental change has
influenced the reef biota.
The last decade has seen an increasing apprecia-
tion of the importance of understanding the GBR from
a total system perspective. It has also seen the recog-
nition of the need for a temporal perspective in every
aspect of ecology, especially where it seeks to relate to
natural resource management. Understanding the
ecosystems we live in and exploit over medium term
time scales allows natural resource managers to plan
for ecological resilience that is key to sustainability.
But what about the long term view? Geological
evidence accumulated over the last 30 years shows
that the ‘reef’ part of the GBR is a relatively young
feature—less than 1 million years old. Because reefs
are built mainly during rising sea levels, and the high-
est sea levels are associated with interglacial periods,
much of our attention is attracted towards GBR reef
growth during the interglacial high sea level episodes
of the last 500 000 years. But the GBR, like reefs else-
where in the tropics, survived during lower sea level
stands as well. In this chapter we explore the life and
times of the GBR and the dynamic ecological response
of coral reefs to constant environmental change.
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The Great Barrier Reef
18
■  Origins Of the great Barrier reef
The history of the GBR is influenced by the post-
Gondwanan continental drift history of the Australian
continent and repeated episodes of global environ-
mental 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
(m.y.). 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 (k.y.), 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 mid-
Pleistocene transition from 41 k.y. to 100 k.y.-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 repeat-
edly during high sea level episodes associated with
major environmental fluctuations in sea level, tempera-
ture and CO2 over the past several hundred thousand
years. Moreover, these reefs have maintained a similar
coral and algal species composition during their re-
peated formation (see section on Palaeoecology below)
(Fig. 3.1B, C).
■  reef grOwth and glOBal sea
level change
The growth and decay of ice sheets in the northern
hemisphere were controlled by 104- to 105-year scale cli-
mate changes forced by natural cyclic changes in sev-
eral 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 under-
went at least 17 such cycles of rise and fall. Average
rates of sea level change between glacial and intergla-
cial intervals approached 5 m per thousand years with
the possibility of greater rates associated with Heinrich
events (abrupt climatic episodes associated with ice-
rafted 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, on-
going 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 be-
tween 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, princi-
pally coral and coralline algae, and even larger quan-
tities 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.
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Figure 3.1 A, Curve for the past 1.5 million years showing the change in frequency and amplitude of the climatic, and by
inference, sea level fluctuations after the mid-Pleistocene transition (MPT) at 0.9–0.6 Mya (MPT, a shift in the periodicity
of radiative forcing by atmospheric carbon dioxide that caused higher amplitude climate periodicities). Growth of the
GBR occurred after the MPT. (after Larcombe and Carter 2004.) B, Photographs from the Ribbon Reef 5 core showing the
major coral components of ‘Assemblage A’. These include robust branching corals of species from the Acropora humilis
group (AH), the Acropora robusta group (AR), Acropora palifera (AP), Stylophora pistillata (S), and Pocillopora (P). Much
of the coral framework is encrusted with coralline algae. C, Photographs from the Ribbon Reef 5 core showing the major
coral components of ‘Assemblage B1’ and ‘B2’. These include massive Porites (e.g. Porites cf lutea) (PO), encrusting
Porites (EPO), and massive faviids such as Favites (FA) and Plesiastrea versipora (PE). Again, extensive coralline algal rims
encrust much of the coral framework. (B and C from Webster and Davies 2003.)
(B)
(C)
(A)
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The Great Barrier Reef
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(A)
(C)
Figure 3.2 A, Sea level, temperature, and greenhouse gas fluctuations over the past 650 k.y. from the EPICA ice core from
Antarctica. B, View of the Pleistocene and Holocene raised reef terraces at Huon Peninsula, PNG. (Photo: R. Kelley.)
C, Sea level curve for the past 150 k.y. derived from Huon Peninsula, supplemented with observations from Bonaparte
Gulf, Australia (from Lambeck et al. 2002). D, Pleistocene reef terrace from the 125 ka reef at Exmouth, Ningaloo Western
Australia. (Photo: R. Kelley.)
■  THE GREAT BARRIER REEF AND GLOBAL
ENVIRONMENTAL CHANGE
The geology, geomorphology and age structure of the
GBR is described in detail in Chapter 2. While there is
evidence of Pleistocene age reef growth older than
140 ka, we will focus on what the more recent evidence
can tell us about the GBR ecosystem in time and space.
Here we discuss the GBR during its most recent ‘life
cycle’—from the previous to the current interglacial cy-
cle and spanning the last ice age.
The superbly exposed and documented record from
the Huon Peninsula, PNG, provides a template for ex-
pected expressions of transgression within the physical
GBR province. Specifically, we should find evidence of
reef growth leading to a still stand (i.e. when sea level has
ceased to rise or fall) in the previous (128–118 ka: 210 to
15 m a.s.l. (above sea level)) and the present (10 ka to
present: 215 to 0 m a.s.l.) interglacials (Fig. 3.2C). There is
extensive physical evidence from drill cores of reef
growth leading into both of these interglacials. Chapter 2
discusses the dating literature associated with the
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3 – The Great Barrier Reef in Time and Space: Geology and Palaeobiology
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Holocene transgression from the Last Glacial Maximum
(18 ka) to the current high sea level stand. There is also
radiometric evidence from the GBR that the Holocene
reef growth was superimposed upon relic Pleistocene
reef topography from the last interglacial age.
In other tectonically stable parts of the world this last
interglacial reef is well documented at about 12–6 m
a.s.l. For example, the last interglacial reef (125 ka, 5 m
a.s.l.) is emergent along the west Australian Ningaloo
coast, where it is extensively preserved in near desert
conditions (Fig. 3.2 D). Whereas the north Queensland
coast is well endowed with evidence of Pleistocene
shorelines in the form of beach rock, dunes, dune foun-
dations, and beach ridges (e.g. Cowley beach near Innis-
fail), there are relatively few occurrences providing
surficial expression of last interglacial reef framework
from the GBR (e.g. Stradbroke Island, Evan’s Head, Lord
Howe Island, Saibai Island in the Torres Strait, Digby Is-
land). One possible explanation for this is that the north-
east coast’s moist airflow from onshore tradewinds has
weathered and eroded the emergent 125 ka GBR reef be-
low the current high sea level such that they now only
exist as a base for Holocene reef growth. But perhaps the
last interglacial reef did not everywhere grow to the high
sea level. One recent explanation is that vertical move-
ments in the form of hydro-isostasy or tectonic lowering
are the major factors.
Because of the Milankovitch cycles discussed ear-
lier, sea level fluctuations are not confined to ‘glacial’
(i.e. Last Glacial Maximum [LGM] 18 ka) and ‘intergla-
cial’ (i.e. Last Interglacial [LI] 125 ka) periods; smaller
scale fluctuations are referred to as ‘stadial’ (temporary
ice advance) and ‘interstadial’ (temporary ice retreat)
times. Abundant studies carried out in tropical seas
correlate the growth of ‘wave resistant organic struc-
tures’ such as coral reefs with sea level transgressions.
We therefore might expect to see transgressive reef de-
posits developed during the smaller scale sea level
changes between the high stand LI and the low stand
LGM (Fig. 3.2C). But is there any evidence for GBR
reef-building during these lower sea levels?
The nature of low sea-stand reefs has been studied in
the Huon Gulf in Papua New Guinea. Here are found
similar rates of accumulations and coral communities that
were not unlike their high sea level stand counterparts
in the adjacent raised reef terraces of the Huon Penin-
sula, PNG. Regardless, reefs must be seen as dynamic
and fluid, reacting to sea level throughout the major and
minor Pleistocene fluctuations in sea level (Fig. 3.2A).
On the GBR, there is a history of investigation of the
terraces and positive-relief features on the continental
shelf and margin for evidence of lower sea levels.
‘Wave-cut’ terraces have been recorded in the southern
GBR at 2175 m and in the central GBR at 2113 m,
288 m and 275 m, where they were interpreted to cor-
respond to postglacial shorelines. Submerged reefs, ter-
races and notches have been consistently recorded on
single beam echo sounder transects across the southern
GBR shelf edge but so far there is insufficient evidence
on the spatial distribution of these features to make ac-
curate comparisons against sea level curves.
Recent investigations by marine geologists using
multibeam echo sounders have revealed that drowned
reefs extend for hundreds of kilometres along the GBR
outer shelf edge in 240 m to 270 m depth. They ap-
pear to be submerged ‘barrier reefs’ approximately
200 m wide and are comprised of two parallel ridges
of eroded limestone pinnacles (Fig. 3.3A). These
drowned shelf-edge reefs might be an important ar-
chive of past climate and sea level changes, and po-
tentially provide predictive tools for GBR coral
community response to future climate changes. It is
now also possible to map shelf depth palaeo-drainage
in greater detail than ever before (Fig. 3.3B). Very re-
cent work has extended the occurrence of these sub-
merged shelf-edge reefs as far south as the northern
end of the Swain Reefs.
In previous decades the inherent difficulty of re-
mote underwater exploration has restricted the useful-
ness of this work. A submarine terrace might represent
a constructional feature—an interstadial reef—but it
might also represent an erosional feature—a wave cut
cliff or bench. Modern acoustic techniques involving
multibeam sonar hold great promise for finally illumi-
nating the inter-reef and shelf-edge stories by combin-
ing high resolution 3-dimensional structure with an
ability to map its regional extent.
Despite the limitations of technology, four decades
of exploration combined with the new hydrographic
charts do tell us one thing. The interstadial GBR does
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22
not have as grand or extensive an expression as its
modern interglacial sibling in terms of accumulated or-
ganic carbonate features. There are some obvious fac-
tors that may ultimately explain this. The first is the sea
floor slope. Worldwide, continental shelves typically
have very shallow gradients from the coast to the shelf-
slope break where the gradient markedly increases.
Here, a one metre rise in sea level can result in kilome-
tres of shoreline displacement. During times of rapidly
rising sea level, rates of reef growth from 4 m k.y.21 to
10 m k.y.21 are common and environmental gradients
are shallow, broad and dynamic. This means that on a
shallow continental shelf environmental conditions
have the potential to change very rapidly both in a
‘turn on’ (increased oceanic circulation/reduced shore-
line terrigenous influence), and ‘turn off’ (decreasing
circulation, water depth, increasing sedimentation)
mode. By contrast, steeper gradients, seen in the steep
drop-offs on the GBR Ribbon Reefs and in atoll settings,
are more like dipsticks, where the shoreline recedes lit-
tle during sea level rise and environmental gradients
are steep, narrow and less dynamic.
A further consideration influencing reef develop-
ment is the effect of sea level when still stand is achieved.
Rivers that flow across the continental shelf during ice
ages have their floodplain sediments remobilised dur-
ing the next transgression. These materials are moved
inshore by the wave climate and end up, in the eastern
Australian case, coming onshore in spectacular dune
fields. Geological studies of a dune island barrier sys-
tem enclosing Moreton Bay, southern Queensland,
showed that when sea level rise stops, the onshore
movement of sediments into the near-shore sediment
profile slows and coastal dune building decreases in
(A)
(B)
(C)
Figure 3.3 A, Drowned shelf-edge reef at Grafton Passage. Recent investigations by marine geologists at the James Cook
University School of Earth and Environmental Sciences using multi-beam echo sounders have revealed drowned reefs that
extend for hundreds of kilometres along the GBR outer shelf edge in 240 m to 270 m depth. This submerged ‘barrier
reef’ near Grafton Passage is approximately 200 m wide. (Image: R. Beaman.) B, C, Palaeochannel near Cruiser Passage
North Queensland. During the last glacial maximum, sea level was over 100 m lower than today. During these times,
rivers deposited floodplain and channel sediments on the continental shelf and upper slope. (Image: R. Beaman.)
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3 – The Great Barrier Reef in Time and Space: Geology and Palaeobiology
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size and extent. In the Moreton Bay example a few thou-
sand 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 back-
barrier 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 opera-
tion for over six decades. While corals are still found in
Moreton Bay today the reduced circulation and increas-
ingly estuarine conditions experienced after 4 kya 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 back-
barrier setting with open circulation. Unpublished
radiocarbon dates from Moreton Bay show these Acro-
pora dominated coral communities (40 spp.) grew and
accumulated carbonate at rates of up to 5 m k.y.21, simi-
lar 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 palaeoecologi-
cal perspective they are a useful alternative to the high
sea level reef paradigm.
Having discussed the geological boundary condi-
tions and some of the processes that frame the GBR in
space and time we can now better understand our no-
tion of a single interglacial to interglacial ‘life cycle’ of
the GBR and also better scrutinise some of our assump-
tions about the system. A review of the drilling data
shows that the majority of framework growth associ-
ated with the current interglacial GBR grew between
9 ka and 4 ka—a 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 Char-
lotte 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 differ-
ences in species’ abundance in respect of the major
environmental gradients, resulting in more or less char-
acteristic community types across and along the GBR. In
particular, there are major differences in species compo-
sition between the wave washed, clear water reef crest
communities of the seaward slopes of outer barrier reefs
and their highly sheltered, turbid water, inshore coun-
terparts, 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 thou-
sands 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
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24
the modern GBR benefits from a wider whole-of-
system approach, grappling with the GBR in time and
space requires a broader conception of coral communi-
ties than just the clear water platform reefs where we
might prefer to go diving.
■  WHAT CAN WE LEARN FROM
PALAEOECOLOGICAL RESPONSES IN
ANCIENT REEFS?
Like tropical marine communities throughout the
world, the corals, coral communities and coral reefs of
the GBR are fundamentally influenced by their re-
sponse to climate change and associated environmen-
tal parameters, including magnitude and rates of sea
level change, CO2, temperature, and turbidity. There is
a growing recognition that the integration of palae-
oecological and climate data on the GBR provides
essential insight into how natural communities are as-
sembled and structured in the face of environmental
variability over extended periods of time. Given our
ability to discriminate among the various kinds of coral
and reef development on the GBR, we next consider
some examples of the ecological dynamics of coral reef
communities over long time frames.
Although less true for the GBR, many reef organisms
are sufficiently preserved in fossil sequences around the
world as to provide generic or even species level infor-
mation on community structure, including corals, mol-
luscs, echinoderms, coralline algae, and foraminifera.
We will discuss these examples to help illustrate what
the long term ecology of GBR reefs might have been.
In the Indo-Pacific, recent evolution of corals has
been rather slow, with less than 20% of new taxa
appearing in the past 2–3 Mya. In the Caribbean, only
two species have gone extinct in the past 125 k.y. As
such, palaeoecological patterns from Quaternary reefs
(past 1.8 to 2.6 Mya) can be investigated from what
are essentially modern faunas. For example, during
the last interglacial (128–118 ka), sea level was two to
six metres higher than present levels. This has left a
fossilised remnant reef in a large number of locations
through the tropics (Fig. 3.2D), giving global insight
into the ecological nature of reefs in the recent geo-
logical past.
One of the best archives for understanding the eco-
logical effects of sea level fluctuations on coral reefs is
contained in Pleistocene reef sequences from several tec-
tonically active sites around the world, the most famous
of which is the Huon Peninsula in PNG, where nine such
reefs were developed between 125 ka and 30 ka (Fig. 3.2B).
This series of coral reef terraces, formed by the interac-
tion between Quaternary sea level fluctuations and local
tectonic uplift, allows investigation of the assembly of
coral reefs during successive sea level rises. Here, eco-
logical trends over millennial time scales point to high
levels of persistence in community structure, regardless
of the magnitudes of change in environmental variables.
In the Caribbean, similar coral community structure
was noted among four reef-building episodes ranging in
age from 104 ka to 220 ka on Barbados. Remarkably, the
high similarity in community composition derived from
surveys of common species was also characteristic of
separate surveys targeting rare taxa. These studies point
to persistence in coral community structure over succes-
sive high sea level stand reefs that grew optimally during
rising sea level, and are consistent with the rare glimpses
we have of the GBR that also show that recurrent associa-
tions of coral reef communities are the norm (Fig. 3.4A).
Current concern over the deteriorating condition of
coral reefs worldwide has focussed intense attention
upon the relationship between past ‘natural’ levels of
disturbance and community change versus modern
human-induced agents of decline. To understand the
impact of humans our only recourse is to study the fossil
record. The uplifted Holocene reef at the Huon Peninsula,
PNG, age-equivalent to the GBR, has been studied to
determine the frequency of disturbance in fossil se-
quences with little or no human impacts. Rates of mass
coral mortality were far lower (averaging one in 500
years) than are presently being experienced in living
reefs (multiple events per decade) (Fig. 3.4B). Recovery
from disturbance was swift and complete, and the his-
tory of communities provides predictive power for the
nature of their recovery. The stark contrast between liv-
ing and fossil reefs provides novel insight to the abnor-
mally high disturbance frequencies now occurring.
But what happens when sea level falls or stands still,
and how do reefs respond to habitat reduction caused
by lowered or lowering sea level? Some spectacular
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Figure 3.4 A, Lithologic and biologic variation in the Ribbon Reef 5 core through the past 600 k.y. Ancient environments
and coral assemblages remained constant through a number of cycles during the growth and development of the GBR (from
Webster and Davies 2003). (Continued )
(A)
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The Great Barrier Reef
26
sequences of drowned coral reefs occur at significant
water depths in Hawaii and the Gulf of Papua. Although
the scale of resolution is much less for these drowned
reefs, the overall picture is one of community similarity
through large intervals of geological time. When sea
level falls and a new reef grows, there is a high degree of
predictability in the coral composition of the reefs.
■  EPILOGUE
Global environmental change has had a profound in-
fluence over the development of tropical coral reefs
since time immemorial, and their effects are no less
profound for the GBR. Coral growth culminating in
today’s GBR has been shaped by the natural variations
in sea level, temperature, CO2 and other climate varia-
bles that control light levels, rainfall, turbidity and
ocean acidification. It is no wonder then that the ecol-
ogy of coral and coral reef communities must be seen
as dynamic and fluid in their response to Pleistocene
sea level and climatic fluctuations.
During the last interglacial to interglacial cycle the
GBR has experienced large scale platform reef accretion
during two short intervals near the peak of each trans-
gression. However, for more extensive periods the GBR
has been doing ‘something else’, including intervals of
(B)
Figure 3.4 (Continued ) B, 14C dates of coral mass mortality along 27 km of the Holocene raised reef terrace from the
Huon Peninsula, PNG. Two widespread disturbance events were dated at 9100–9400 years BP (before present), and
8500 years BP. Isolated examples of coral mortality were observed in the Bonah River lagoon, and the Hubegong dive
site. Mortality events are shaded, and labelled with their likely cause where this has been deduced (‘ash’, associated with
a volcanic ash layer, or, ‘debris flow’, associated with a submarine debris flow). Shown here are the 2 sigma age ranges in
calendar years BP (from Pandolfi et al. 2006).
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27
restricted shelf edge fringing reef development on the
eastern slopes coupled with other non-reefal modes of
coral community exploitation of the southern regions of
the GBR province during the regressive and glacial in-
tervals. Further work into the comparative response of
coral communities and reef growth to these two end
member phases should provide important insight into
prediction of reef response to future climatic changes.
Now the GBR, like many coral reefs around the
world, is changing dramatically in response to human
interaction. But living ecological systems provide few
clues as to the extent of their degradation. The only
recourse into understanding the natural state of living
reefs is the fossil record. Our knowledge of past ecosys-
tems on the GBR contributes to formulating sound
approaches to the conservation and sustainability of
the GBR; specifically in ensuring that policy makers
and managers use geological contexts and perspectives
in setting realistic goals and measuring their success.
ADDITIONAL READING
Sea level change
Beaman, R. J., Webster, J. M., and Wust, R. J. A. (2008).
New evidence for drowned shelf edge reefs in the
Great Barrier Reef, Australia. Marine Geology 247,
17–34.
Chappell, J., Omura, A., Esat, T., McCulloch M., Pan-
dolfi, J., Ota, Y., and Pillans, B. (1996). Reconciliation
of late Quaternary sea levels derived from coral
terraces at Huon Peninsula with deep sea oxygen
isotope records. Earth and Planetary Science Letters
141, 227–236.
Lambeck, K., Esat, T. M., and Potter, E.-K. (2002). Links
between climate and sea levels for the past three
million years. Nature 419, 199–206.
Reef development
Larcombe, P., and Carter, R. M. (2004). Cyclone pump-
ing, sediment partitioning and the development
of the Great Barrier Reef shelf system: a review.
Quaternary Science Reviews 23, 107–135.
Pickett, J. W., Ku, T. L., Thompson, C. H., Roman, D.,
Kelley, R. A., and Huang, Y. P. (1989). A review of
age determinations on Pleistocene corals in eastern
Australia. Quaternary Research 31, 392–395.
Webster, J. A., and Davies, P. J. (2003). Coral variation in
two deep drill cores: significance for the Pleistocene
development of the Great Barrier Reef. Sedimentary
Geology 159, 61–80.
Reef systems
Cappo, M., and Kelley, R. (2001). Connectivity in the
Great Barrier Reef World Heritage Area – an over-
view of pathways and processes. In ‘Oceanographic
Processes of Coral Reefs: Physical and Biological
Links in the Great Barrier Reef’. (Ed. E. Wolanski.)
pp. 161–187. (CRC Press: New York.)
Hopley, D., Smithers, S., and Parnell, K. (2007).
‘The Geomorphology of the Great Barrier Reef:
Development, Diversity and Change.’ (Cambridge
University Press: Cambridge.)
Pandolfi, J. M., Tudhope, A., Burr, G., Chappell, J.,
Edinger, E., Frey, M., Steneck, R., Sharma, C., Yeates,
A., Jennions, M., Lescinsky, H., and Newton, A.
(2006). Mass mortality following disturbance in
Holocene coral reefs from Papua New Guinea.
Geology 34, 949–952.
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... Pleistocene divergences (0.27-0.6 MY) reported between A. tenuis cryptic taxa (Cooke et al., 2020), albeit with older estimates in the present study. The GBR is a geologically young structure that formed in the last 0.5 MY (Chadwick-Furman, 1996;Pandolfi & Kelley, 2008;Webster & Davies, 2003). Cyclical sea level changes during the late Pleistocene and the resulting redistribution of species ranges (Hewitt, 2000), however, likely promoted repeated SCs and periodic gene flow between coral populations. ...
Article
Full-text available
Genomic studies are uncovering extensive cryptic diversity within reef‐building corals, suggesting that evolutionarily and ecologically relevant diversity is highly underestimated in the very organisms that structure coral reefs. Furthermore, endosymbiotic algae within coral host species can confer adaptive responses to environmental stress and may represent additional axes of coral genetic variation that are not constrained by taxonomic divergence of the cnidarian host. Here, we examine genetic variation in a common and widespread, reef‐building coral, Acropora tenuis, and its associated endosymbiotic algae along the entire expanse of the Great Barrier Reef (GBR). We use SNPs derived from genome‐wide sequencing to characterise the cnidarian coral host and organelles from zooxanthellate endosymbionts (genus Cladocopium). We discover three distinct and sympatric genetic clusters of coral hosts, whose distributions appear associated with latitude and inshore‐offshore reef position. Demographic modelling suggests that the divergence history of the three distinct host taxa ranges from 0.5 – 1.5 million years ago, preceding the GBR’s formation, and has been characterised by low to moderate ongoing inter‐taxon gene flow, consistent with occasional hybridisation and introgression typifying coral evolution. Despite this differentiation in the cnidarian host, A. tenuis taxa share a common symbiont pool, dominated by the genus Cladocopium (Clade C). Cladocopium plastid diversity is not strongly associated with host identity but varies with reef location relative to shore: inshore colonies contain lower symbiont diversity on average but have greater differences between colonies as compared to symbiont communities from offshore colonies. Spatial genetic patterns of symbiont communities could reflect local selective pressures maintaining coral holobiont differentiation across an inshore‐offshore environmental gradient. The strong influence of environment (but not host identity) on symbiont community composition supports the notion that symbiont community composition responds to habitat, and may assist in the adaptation of corals to future environmental change.
... The mitochondrial results suggest that the clades evolved well before the emergence of the contemporary GBR (\12,000 yr ago). For the past million years, several glacial cycles with corresponding changes in sea level (e.g., approximately 130 m after the last ice age) caused dramatic changes in coral cover (Webster and Davies 2003;Pandolfi and Kelley 2008), creating new habitats that may have driven the genetic divergence that we now observe in O. doederleini. This supports the notion that the spatial overlap between the clades resulted from secondary contact, perhaps during reef emergence. ...
Article
Full-text available
Larval dispersal and limited knowledge of physical boundaries challenge our understanding of the processes that drive genetic divergence and potential speciation in the marine environment. Divergence, both within and between populations of marine taxa, is not uncommon, but spatial and temporal stability of observed genetic structure is not well known. Previously, we detected large genetic differences among populations of the cardinalfish species Ostorhinchus doederleini inhabiting adjacent coral reefs. Here, we determined the spatial and temporal persistence of these genetic structures over the course of ten consecutive generations. Using microsatellite markers, we detected large changes (genetic population distance, D est, ranged from 0.04 to 0.46) in the genetic structure in some years, but some reefs maintained the same populations for nearly all sampling years. As this species’ life span does not exceed 1 yr, persistence of distinct reef populations suggests natal homing. Mitochondrial identity based on two mtDNA markers corroborates the nuclear genetic evidence for genetic differences large enough to constitute different clades and even cryptic species in O. doederleini, which, based on gross morphology, was thought to be a single taxon. Habitat specialization was observed in one clade that exclusively inhabited reef lagoons, while all clades could be observed on reef slopes. We suggest that local habitat recognition combined with local population recognition and selection against hybrids can form barriers that maintain a cryptic species complex.
Chapter
The Cenozoic successions are integrated and analyzed here with respect to the dominant controlling factors present during deposition, namely tectonics, oceanography, climate, and influence of Antarctica. Middle Eocene–early Oligocene SA2 biogenic shelf sediments accumulated during a time of at first warm, but then gradually cooling ocean waters under a relative quiescent tectonic regimen. The climate was mostly humid subtropical with extensive temperate rainforests and fluvial activity that gradually waned in the post-Eocene. It is interpreted that the prolific nutrient elements delivered from land during the Eocene promoted extensive neritic biosiliceous deposition. The Oligocene -Miocene SA3 carbonate shelf was similar to that of today under a progressively warming climate and ocean waters such that in the mid-Miocene sedimentation was nearly photozoan. The comparatively quiet AAG had evolved into the Southern Ocean by the Oligocene resulting in a much more active hydrodynamic marine system. Antarctica had become ice covered and glacioeustacy promoted extensive m-scale carbonate cyclicity. The Plio-Pleistocene SA4 shaved shelf developed because of active tectonism that is continuing today and resulted in a different sedimentary system dominated by marginal marine and slope carbonate deposition.
Article
Full-text available
We present four new high-resolution multibeam bathymetry datasets from the shelf edge of the northern Great Barrier Reef (GBR). Analysis of these data, combined with Chirp sub-bottom profiles and existing submersible observation data provides a fresh insight into the detailed morphology and spatial distribution of submerged reefs and terraces at the shelf edge. An extensive and persistent line of drowned shelf edge reefs exist on the GBR margin in about 40 to 70 m. They appear as barrier reefs up to 200 m wide and comprising twin parallel ridges of rounded pinnacles. Subtle yet consistent terrace and step features lie between 78 and 114 m seaward of the shelf edge reefs in the southern study area. Submersible observations confirm that the drowned reefs now provide a favorable hard substrate for live soft corals and algae. They form a consistent and extensive seabed habitat that extends for possibly 900 km along the GBR shelf edge. The submerged reef and terraces features may reflect a complex history of growth and erosion during lower sea-levels, and are now capped by last deglaciation reef material.
Article
Full-text available
The frequency and intensity of disturbance on living coral reefs have been accelerating for the past few decades, resulting in a changed seascape. What is unclear but vital for management is whether this acceleration is natural or coincident only with recent human impact. We surveyed nine uplifted early to mid-Holocene (11,000-3700 calendar [cal] yr B.P.) fringing and barrier reefs along similar to 27 km at the Huon Peninsula, Papua New Guinea. We found evidence for several episodes of coral mass mortality, but frequency was < 1 in 1500 yr. The most striking mortality event extends > 16 km along the ancient coastline, occurred ca. 9100-9400 cal yr B.P., and is associated with a volcanic ash horizon. Recolonization of the reef surface and resumption of vertical reef accretion was rapid (< 100 yr), but the post-disturbance reef communities contrasted with their pre-disturbance counterparts. Assessing the frequency, nature, and long-term ecological consequences of mass-mortality events in fossil coral reefs may provide important insights to guide management of modern reefs in this time of environmental degradation and change.
Article
Full-text available
The oscillations between glacial and interglacial climate conditions over the past three million years have been characterized by a transfer of immense amounts of water between two of its largest reservoirs on Earth -- the ice sheets and the oceans. Since the latest of these oscillations, the Last Glacial Maximum (between about 30,000 and 19,000 years ago), approximately 50 million cubic kilometres of ice has melted from the land-based ice sheets, raising global sea level by approximately 130 metres. Such rapid changes in sea level are part of a complex pattern of interactions between the atmosphere, oceans, ice sheets and solid earth, all of which have different response timescales. The trigger for the sea-level fluctuations most probably lies with changes in insolation, caused by astronomical forcing, but internal feedback cycles complicate the simple model of causes and effects.
Article
A broad knowledge base is associated with the Great Barrier Reef (GBR) province from the earliest navigational survey vessels of the 1800s, subsequent scientific expeditions, and an expanding body of contemporary research literature from the physical, geological, ecological, and molecular sciences. This has been complemented by an important body of unpublished literature and personal observations collected from the public and reef users, making the GBR one of the most comprehensively investigated ecosystems on earth. Across these disciplines “connectivity” is a recurrent theme, and here we give an illustrated overview and examples of some types and scales of ecological connectivity spanning the GBR World Heritage Area, with an emphasis on fish life-history studies.
Article
The modern Great Barrier Reef (GBR) is part of the world's largest and best known mixed terrigenous-carbonate continental margin. The GBR shelf contains three shore-parallel sedimentary belts: an inner shelf zone of terrigenous sedimentation at depths of 0–22 m; a middle shelf zone of sediment starvation at depths of 22–40 m; and an outer shelf reef tract with its inner edge at ca. 40 m depth. These zones are controlled by the dynamics of northward, fair-weather, along-shelf drift, driven by southeasterly trade winds, and by the regular passage of tropical cyclones. Cyclones cause wind-driven north-directed middle shelf flows in excess of 130 cm/s, which erode the seabed, concentrate the sparse mobile sediment into sand ribbons, and advect suspended load onto the outer part of the nearshore terrigenous sediment prism and into inter-reef depocentres within the outer shelf reef tract. Cyclones largely control the input of new sediment into the GBR system, via river flooding, seabed erosion or reef breakage. They also help to control the partitioning and dispersion of the three main shore-parallel belts of sediment, and hence stratigraphic accumulation. Acting as a sediment pump, especially during interglacial highstands, cyclones have exerted great control on the development of the modern GBR province and its sediments by maintaining a broad shelf-parallel zone of episodically mobilised sediment and scoured seabed upon which coral reefs have been unable to form. Cyclones may also have partly controlled the timing of initiation of the first GBR at B0.6 mybp. Contrary to current models, GBR storm beds are most likely to be preserved intact close to the shoreline, and become coarser-grained away from the shoreline. For the central GBR, ''highstand shedding'' only applies to carbonate sediment at the scale of local reefs; system-wide, oceanographic controls cause high rates of carbonate sedimentation on the slope during both sea-level rise and highstand; concomitantly, terrigenous sediment accumulates fastest on the slope during sea-level rise, and slowest during sea-level lowstand and highstand.
Article
Variations in lithology and coral assemblages in drill cores from outer- and inner-shelf reefs are used to characterize the Pleistocene development of the Great Barrier Reef. Based on petrographic, isotopic and seismic characteristics, the outer-shelf core from Ribbon Reef 5 is divided into three sections: (1) a main reef section from 0 to 96 m is composed of six reef units, (2) a rhodolith section from 96 to 158 m is interbedded with two thin reef units and (3) a basal section from 158 to 210 m is composed of non-reefal skeletal grainstones and packstones. Two distinct coral assemblages identified in this core represent a shallow, high-energy community and lower-energy community. These two assemblages are repeated throughout the main reef section, with some units recording transitions between assemblages, and others composed of only a single assemblage. These coral assemblage data also correlate with transitions recorded by coralline algae. Using similar criteria, the inner-shelf core from Boulder Reef is divided into two sections: (1) an upper carbonate-dominated section from 0 to 34 m is comprised of four reef units and (2) an underlying mud section from 34 to 86 m is composed of siliciclastics and two thin, coral-bearing units. The four reef units in the upper section are dominated by a single coral assemblage representing a community typical of low energy, turbid environments. Taken together, these data indicate that: (1) reef growth on the inner shelf initiated later than on the outer shelf, (2) true reef ‘turn-on’ in outer shelf areas, as represented by the main reef section in Ribbon Reef 5, was preceded by a transitional period of intermittent reef development and (3) the repeated occurrence of similar coral assemblages in both drill cores indicates that the Great Barrier Reef has been able to re-establish itself, repeatedly producing reefs of similar composition over the last 500 ky, despite major environmental fluctuations in sea level and perhaps temperature.
Article
A major discrepancy between the Late Quaternary sea level changes derived from raised coral reef terraces at the Huon Peninsula in Papua New Guinea and from oxygen isotopes in deep sea cores is resolved. The two methods agree closely from 120 ka to 80 ka and from 20 ka to 0 ka (ka = 1000 yr before present), but between 70 and 30 ka the isotopic sea levels are 20–40 m lower than the Huon Peninsula sea levels derived in earlier studies. New, high precision U-series age measurements and revised stratigraphic data for Huon Peninsula terraces aged between 30 and 70 ka now give similar sea levels to those based on deep sea oxygen isotope data planktonic and benthic δ18O data. Using the sea level and deep sea isotopic data, oxygen isotope ratios are calculated for the northern continental ice sheets through the last glacial cycle and are consistent with results from Greenland ice cores. The record of ice volume changes through the last glacial cycle now appears to be reasonably complete.
A review of age determinations on Pleistocene corals in eastern Australia
  • J W Pickett
  • T L Ku
  • C H Thompson
  • D Roman
  • R A Kelley
  • Y P Huang
Pickett, J. W., Ku, T. L., Thompson, C. H., Roman, D., Kelley, R. A., and Huang, Y. P. (1989). A review of age determinations on Pleistocene corals in eastern Australia. Quaternary Research 31, 392-395.
The Geomorphology of the Great Barrier Reef: Development, Diversity and Change
  • D Hopley
  • S Smithers
Hopley, D., Smithers, S., and Parnell, K. (2007). 'The Geomorphology of the Great Barrier Reef: Development, Diversity and Change.' (Cambridge University Press: Cambridge.)