Great Barrier Reef by wulinqing

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									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 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
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 eccentricity-
           dominated 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. „Human-
  induced‟ 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
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