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The Great Barrier Reef Reef Formation Age, Reef Building Processes and Current Reef Situation.

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Presentation on theme: "The Great Barrier Reef Reef Formation Age, Reef Building Processes and Current Reef Situation."— Presentation transcript:

1 The Great Barrier Reef Reef Formation Age, Reef Building Processes and Current Reef Situation

2 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 km 2 Consists of 400 species of hard and soft corals and supports 10 of thousands of other species

3 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

4 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

5 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

6 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. δ 18 O cycles The Authors believe this means the Reef initiation is related to the onset of full eccentricity- dominated glacio-eustatic seal level oscillation

7 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

8 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”

9 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

10 Reef Building Processes In both cases reef building occurs at a rate of mm/year vertically to sea level and laterally 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)

11 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

12 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

13 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

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15 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

16 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, , 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

17 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

18 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

19 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

20 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

21 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

22 Ordination of Sites

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

24 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

25 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.

26 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

27 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

28 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

29 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

30 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

31 References Van Woesik, R., Done, T.J., 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: D.M. Kennedy, C.D. Woodroffe, Fringing reef growth and morphology: a review. Earth-Science Reviews 57 (2002) 255–277

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


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