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BIOL3219, Sandie L01
The deep geological history of reef systems
| Question | Answer |
|---|---|
| Central focus of BIOL3219 (Sandie's lectures)? | Biogenic limestone (calcium carbonate) reef formations produced by living organisms; these formations act as a base and/or a living veneer. |
| Central focus of lecture 1 (the deep geological history of reefs)? | The carbonate‐producing organisms have varied through geological Ame, so modern reefs are different from those of geological past. |
| What were the earliest reef builders? | Cyanobacteria (blue-green algae) were the earliest reef builders from >3000 mya. They are still alive today. |
| What were the only reef builders in Precambrian period (4600mya - 570mya)? | Cyanobacteria were the only REEF builders during the Precambrian period. There were other "carbonate workers" but the structures built are by them not regarded as reefs. |
| What are cyanobacteria? | A division of microorganisms that are related to the bacteria but are capable of photosynthesis. Photosynthesising bacteria that form layered mats by trapping fine-grained carbonate sediment. |
| What are stromatolites? | A calcareous mound built up of layers of lime-secreting cyanobacteria and trapped sediment, found in Precambrian rocks as the earliest known fossils, and still being formed in lagoons in Australasia. Cyanobacterial mat + sediment = stromatolite |
| How is calcium carbonate (CaCO3) promoted in stromatolites? | Photosynthesis removes CO2 from the water within the mat, reducing the acidity (reducing the pH). |
| What does the geological record indicate? | The geological record provides context for the timing of changes in reef structure. The Phanerozoic period (since the start of the Cambrian, 560mya) had the first TRUE reef communities. |
| What goes the geologic record suggest about tropical reefs? | There have been SIX major successions of reef-building organisms during the Phanerozoic, with different species in each phase. |
| What happened after each of the FIVE mass reef extinctions? | There was a long period before other organisms could fill the environmental void. There was a absence of reefs after the extinctions, followed by incipient reefs and then fully developed reefs. |
| What were the first 3/6 major successions of reef-building organisms during the Phanerozoic? | in the Cambrian, 543mya: ARCHAEOCYATHID and ALGAE in the Devonian, 400mya: STROMATOPOROIDS; TABULATE AND RUGOSE CORALS in the Permian, 290mya: CALCIC SPONGES AND ALGAE |
| What were the last 3/6 major successions of reef-building organisms during the Phanerozoic? | in the Triassic, 230mya: SCLERACTINIAN CORALS AND CALCIC SPONGES in the Cretaceous, 100mya: RUDIST CLAMS AND SCLERACTINIAN CORALS in the Cenozoic, 30mya: SCLERACTINIAN CORALS |
| The six major successions in the Phanerozoic can be broken down into four distinct what? | These 6 successions can be collapsed into FOUR distinct cycles of reef-building through the Phanerozoic, each separated by a significant extinction event. |
| What are three interesting points about Phanerozoic Reefs? | Greater diversity compared with Pre-Cambrian reefs. They are TRUE reef communites. Most, but not all, extinctions appear to coincide with mass marine extinctions. |
| What was reef cycle #1: Cambrian Reefs? | The first true reefs, which appeared in the Lower Cambrian (530 mya). They were characterised by Archaeocyath Sponges. - very brief (10-15 my) but spectacular history (NOW EXTINCT) - diversified into 100s of species; very successful in terms of numbers |
| What happened by the late Cambrian? | The first corals had appeared. |
| What was reef cycle #2: Ordovician‐Silurian‐Devonian reefs? | In the Ordovician, tabulate and rugose corals became common, and stromatoporoid sponges appeared. Along with algae, they remained the main reef builders through the Silurian and Devonian periods. This cycle was from 500 to 350 mya. |
| What is the Mid-Devonian Peak? | Occurred during reef cycle #2. • the largest, and latitudinally most widespread, of the Phanerozoic reefs • times of exceptional high sea‐levels and widespread epicontinental shallow seas • 26 m. long peak of coral-sponge reef growth |
| What was the temperature like during the MDP? | It coincided with the warmest global temperatures known for the Phanerozoic, i.e., with a "supergreenhouse" climate mode well above Holocene interglacial norms. |
| Where is there evidence of a mid-Devonian reef in Australia? | Canning Basin, Western Australia. |
| When did reef cycle #2 collapse? | These communities all collapsed by the late Devonian. |
| What is reef cycle #3: Late Devonian to Permian? | In the late Devonian, there was widespread collapse of reef-building communities but during the upper-middle Permian there was a return of more massive reef builders. |
| What happened by the upper-middle Permian? | By the middle‐upper Permian, there was a shit from glacial climates to warm climates. This period was characterised by a return to more massive reef builders, including tabulate and rugose corals. |
| What were the major organisms in the Carboniferous reefs? | Through the entire Carboniferous, no large, massive reef‐building organisms existed! Instead, Carboniferous reefs were dominated by diverse, small organisms, especially algae-bryozoan-crinoid‐sponge and microbial assemblages. |
| What are some examples of Carboniferous reefs? | eg. ‘WaulsorAan’ mud mounds -‐ limited to early Carboniferous strata where storms not severe eg. Bahamas sand bank |
| What organisms typically occupied a Permian reef? | Typical Permian reef with abundant tabulate and rugose coral, plus sponges, molluscs and calcareous algae. |
| When did the tabulate and rugose corals become extinct? | At end of the Permian (250 mya), tabulate & rugose coral became extinct |
| What is reef cycle #4: Triassic to Modern | The early Triassic (~250 mya) saw the origin of the first modern scleractinian (hard) corals. |
| What is interesting about the first scleractinian corals? | • filled ecological niche previously held by rugose and tabulate corals • probably not closely related to those (independent origin) • probably formed symbiosis with algae soon ater first appearance |
| When did the first scleractinian corals become extinct? | At end of the Triassic (250 mya), the first scleractinian corals went extinct! |
| What happened during the Jurassic period? | In the Jurassic, corals had to start from near zero ater the end-‐Triassic extinction. Other Jurassic reef-builders included sponges. |
| How did the peak Mesozoic development of reefs occur? | The Jurassic and on into the Cretaceous supported the peak Mesozoic development of reefs – including the proliferaAon of modern colonial scleractinian corals. |
| What reef settings were apparent during the Mesozoic? | A variety of reef settings apparent in this period: coral reefs, siliceous sponge reefs, bivalve reefs, microbial reefs, or mixtures of all of these. Today, these have formed widespread and important hydrocarbon reservoir rocks (fossil fuel sources). |
| When were shallow marine reef environments in equatorial latitudes were dominated by rudist bivalve reefs? | From the Late Jurassic to the Late Cretaceous. The Cretaceous reef-builders were rudistid bivalves of different growth forms (along with scleractinian corals). |
| When were rudist reefs especially abundant? | Rudist reefs were especially abundant during a period of extra salty, extra warm seaway known as the Supertethys Ocean. But the Cretaceous‐Tertiary mass extinction event caused the demise of these rudist-dominated reefs. |
| What are probably the most well-developed scleractinian reefs in geological history? | Present-day Cenozoic reefs are probably the most well-developed scleractinian reefs in geological history. They are comprised of stony corals and coralline algae. |
| What are major features of modern scleractinian corals? | Invertebrate animals (anthozoan cnidarians) that are often colonial. Reef-builders; deposit CaCO3 as aragonite (cf tabulate and rugose corals deposited CaCO3 as calcite). |
| Why are modern scleractinian corals so successful? | Symbiosis with zooxanthellae renders them hugely successful hermatypic (reef‐building) corals. |
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