Transcript Great Barrier Reef - Home - Caltech: Geological and

The Great Barrier
Reef
Reef Formation Age, Reef
Building Processes and Current
Reef Situation
Great Barrier Reef Overview
• Located in the Coral Sea on
the coast of Queensland (NE
Australia)
• Largest Reef System in the
world consisting of 3,000
individual reefs surrounding
900 islands
• 2,600 km long with an area of
344,400 km2
• Consists of 400 species of
hard and soft corals and
supports 10 of thousands of
other species
Outline
• Formation Age
– Deep core estimate
– Cause of formation
• Building Processes
– Formation of Fringe Reefs
– Morphologies and Growth Rates of the
Great Barrier Reef
• Current Reef
– Study to determine relationships
between coral communities and reef
building
– Causes of deterioration
Reef Formation Age
• Previous age of formation unknown,
estimated from 1 Ma to less than 500 ka.
• Modern Reef system likely formed on a
succession of reefs from throughout the
Pleistocene
• New core holes drilled to find more exact
dates
Reef Formation Age
• Core Taken in Two Locations- younger
inner reef, older outer reef (inner reef is
largely ignored)
• Dates found using a combination of Sr
isotope stratigraphy,
magnetostratigraphy and spinresonance dating.
• Base of Section 1 dated at 770 +/- 280
ka
• Two samples in Section 2 dated at 600
+/- 280 ka
• One sample in Section 3 dated at 600
+/- 280 ka as well
• Reef Formation age is ~600 ka
Reef Formation Age
• Entirety of reef formation occurs
after the Matuyama-Brünhes
magnetostratigraphic change
• Reef formation begins at
approximately the same time as
the onset of large amplitude
“saw-tooth” 100 k.y. δ18O cycles
• The Authors believe this means
the Reef initiation is related to
the onset of full eccentricitydominated glacio-eustatic seal
level oscillation
Reef formation Age: Recap
• Isolated Reef
formation begins after
790 ka
• Full Reef Conditions
formed at ~600 ka
• Reef formation
possibly related to
100 k.y. oxygen
isotope variations
Reef Building Processes
• Reef formation begins with Fringing Reefs
• Fringing Reefs generally form on sediment or
bedrock near an island or coastline, but are
inhibited by an influx of freshwater and river
sediment
• Eventually enough reef is formed to become a
barrier reef (Evolution of Reef forms
hypothesized by Darwin)
• Great Barrier Reef a semi-unique mix of platform
reefs, fringing reefs and “almost” barrier reefs
that are a hybrid between fringing and barrier
types. The large extent of reefs creates “great
barrier”
Reef Building Processes
• Fringing Reefs form to fill accommodation
space, growing up to sea level then outward
• Where the reef intersects the sea surface a reef
flat is formed
Reef Building Processes
•There are two general reef building morphologies in the
Great Barrier Reef, one in which the reef hit sea level at an
early age and grew mostly laterally (A), and another in
which the reef grew below sea level for a time and formed
small islands with debris filled troughs between them upon
reaching sea level (B)
• In both cases reef
building occurs at a rate
of 1.7-11.8 mm/year
vertically to sea level and
laterally
Current Reef
• Study of relationship
between coral
communities and reef
development
• Why is there less
development at Broad
Sound (near 22°
South)
• ‘Natural’ vs. ‘Humaninduced’ degradation
• Developed reefs formed of both framework and
detrital elements into distinctive flats and slopes
• Least developed reefs are ‘incipient reefs’ with
lack reef flats
• In addition there are coral communities lacking
framework or detrital accumulations
Area of Study
• Reef development is stunted near Broad Sound
(22° S), a large shallow, silty embayment with no
major river running into it
• Broad Sound is the location of Eastern
Australia’s largest tidal range of ~10 m
• Tidal currents suspend fine bottom sediments,
preventing reef development
• Reefs to the north and south of the area are
beyond the influence of these tidal forces
Location of Study Sites
• Sites located in
four regions
• Islands classified
as inshore or
offshore based on
distance to land,
surrounding shelf
depth and distance
from major
channels
Methods
• Fringe reefs visited at a number of islands and both
exposed and sheltered sides were surveyed
• 10X20 m area within 5 m of reef flat surveyed in each
case
• Size and identity of each coral was recorded.
• Size was divided into 1-10, 11-50, 51-100, 101-300 and
greater than 300 cm classes
• Species was recorded in most cases, although genera
was recorded in some highly speciose cases
• Depth, mean annual tidal range, distance to mainland,
distance to nearest river, depth of surrounding shelf,
sheltered or unsheltered and island location were
recorded for each site
Analytical Methods
• Correlations between site descriptions and coral
distribution and abundance were found using canonical
correspondence analyses
• A variety of other sortings were used to find similarities
and differences between dissimilar locations
Results
• Regions 1, 2 and 4 had corals to depths of 12 m,
and region 3 had corals only to 4 m, which
corresponds to the increased turbidity and
shallowness of region 3
• Offshore reefs tended to exhibit more species
richness, higher colony density, higher soft coral
cover and lower macrophyte cover
Results
• Region 1 featured
diverse, dense and large
corals
• Region 2 featured diverse
but below average
density corals
• Region 3 featured small
corals with low densities
except for encrusting
corals
• Region 4 featured
generally low density
corals and was
dominated by Acropora
Comparison between
Regions 1 and 3 (22° S)
• Despite different
environments, both
region 1 and region 3 had
similar densities of
scleractinian and
alcyonarian corals and
soft corals
• The clear difference that
existed was that region 1
had more slow growing
corals and arborescent
corals and small corals
Light Color- Region 1
Dark Color- Region 3
Ordination of sites
• Locations of sites in
ordination and outlines of
each region
• Penrith Island is in Northern
corner of Region 3 and is
mostly reef flat resulting in
faunistic differences with
other regions
• Region 3 has an abundance
of Montipora and a lack of
Acropora, differentiating it
from the other regions
Ordination of Sites
Coral Communities
• Coral Communities
can be contoured to
show their ‘nested’
faunistic relationships
• There is increasing
diversity away from
the reef in some
cases
Environmental Correlations
• Depth, distance from mainland and esposure were
significantly correlated with specific species and genera
• Island location and mean annual tidal range correlated
significantly when size descriptors were added
• Distance from river and shelf depth never had better
than poor correlation
Discussion
• There is clear evidence of a link between coral
community structure and degree of reef development
• Broad Sound area is sparsely developed and consists of
encrusting and foliaceous growth forms of coral
• More developed areas tends to have large massive and
branching coral colonys.
Discussion
• Regions 1 and 2 are experiencing reef growth
due to high settlement densities and large
colony sizes, especially of massive and
branching corals
• Region 4 is also experiencing growth by
accumulation of rubble from staghorn Acropora
thickets
• Region 3 is experiencing little reef development
due to fast growing small corals which die before
growing large and do not grow densely. In
addition they are poorly cemented and more
easily eroded
Discussion
• Net growth is the sum of frame accretion,
sediment accretion and destruction
• Not only do large corals create larger
frames, they are likely to stay in site when
broken, resulting in more sedimentation
• Additionally, large corals have a smaller
surface area to volume ratio, making them
less prone to destruction
Discussion
• It is likely there has been poor reef
development in the Broad Sound area due
to a high sediment load
• The tidal currents associated with a large
tidal range are able to keep 89 mg/l of fine
sediment in suspension
• Sediment load can coat corals and prevent
growth and also limits sunlight available
for energy
Degradation
• Comparing past coral fauna in an area to those
currently present can be used to help determine
if current degradation is a change from past reef
building capacity
Human vs. Natural Reef
Degradation
• A mismatch if reef building capacity could be due to anthropogenic
causes or due to natural cycles
• In cases where human activity favors non-reef building coral
communities it is likely that human intervention is a cause for reef
degradation
• In the case of change that seems related to past cycles, natural
causes are a likely cause
• Neither of these cases can provide a sure distinction between
causes, and in some cases both causes may be present
References
• http://en.wikipedia.org/wiki/Great_Barrier_Reef
• http://www.reefhq.com.au/
• Van Woesik, R., Done, T.J., 1997. Coral communities
and reef growth in the southern Great Barrier Reef. Coral
Reefs 16,103–115.
• New constraints on the origin of the Australian Great
Barrier Reef: Results from an international project of
deep coring. Geology, Jun 2001; 29: 483 - 486.
• D.M. Kennedy, C.D. Woodroffe, 2001. Fringing reef
growth and morphology: a review. Earth-Science
Reviews 57 (2002) 255–277
100
50
0
1st
Qtr
3rd
Qtr
East
West
North
• A Kitten is your prize if you looked at the slides
after the presentation