The Great Barrier Reef

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The Great Barrier Reef

• Reef Formation Age, Reef Building Processes and
  Current Reef Situation

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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

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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

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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

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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
  spin-resonance 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

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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. d18O cycles
• The Authors believe this means the Reef
  initiation is related to the onset of full
  eccentricity-dominated glacio-eustatic seal level
  oscillation

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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

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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

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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

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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

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Current Reef

• Study of relationship between coral communities
  and reef development
• Why is there less development at Broad Sound
  (near 22 South)
• Natural vs. Human-induced degradation

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• 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

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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
  Australias 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

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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

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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

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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

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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

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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

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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

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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

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Ordination of Sites
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Coral Communities

• Coral Communities can be contoured to show their
  nested faunistic relationships
• There is increasing diversity away from the reef
  in some cases

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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

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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.

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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

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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

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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

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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

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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

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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 16,103115.
• 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) 255277

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• A Kitten is your prize if you looked at the
  slides after the presentation