1.3.1.4 Palaeoenvironments, Palaeogeography and Basin Analysis
Modern and ancient reefs are good monitors of environmental and geological settings and change. We will discuss various aspects of these in subsequent sections. In this introduction, just try to imagine to visit a modern coral island beach and you have already realized a good portion of the environmental necesseties of reefs which help reconstruction former palaeogeographies and sedimentary basins.
Let's base our considerations on modern reefs; we will later discuss many examples of differences in environmental demands for ancient reefs.
Distribution of modern reefs
Fig: Let's see again our reef map of modern tropical reefs. Try to understand this distribution. Could you explain it (if it were a test, oops, but it isn't!)? If you read below demands of modern reefs, you should be able to.
Water temperature: warm surface waters, normally not much cooler than 18-20°C winter isotherme. Modern tropical reefs are therefore mostly situated between about 20°N and 20°S, but not where cold, nutrient rich waters are upwelling (i.e. not at the western continental margins which are influenced by the cold, nutrient rich currents such as the California, Canary, Humboldt, or Benguela Streams. The warm water facilitates the secretion of the calcareous skeleton, since waters are strongly enriched (more exactly: even supersaturated) in respect of Ca2+ and HCO3-.
Optimum mostly 26-27°C; maximum range often 20-29°C, Persion Gulf 13-38° (special development); above 30-31° stress (coral bleaching, see reef hazards section)
Clear waters: modern reefs in general do not occur where waters are turbid, because photosymbiontic corals need a lot of light. So they do not grow e.g. off the Amazon delta. There are however a couple of examples which will be discussed in this course. There are many important adaptations towards sedimentation (or more exactly: towards protection from sedimentation) which makes fossil reefs important indicators of sediment influx patters and rates.
Shallow waters: The light-dependance of most modern reefs allows restricts their growth to well-lit waters. Peak coral density is mostly shallower than 10 meters; highest diversity is often between 9-18 meters. This is strongly dependant on water turbidity.
Oligotrophic waters: Oligotrophic, i.e. extremely nutrient-poor waters are necessary for growth of modern coral reefs. We will see the effects of elevated nutrients in the reef hazards section. This is why tropical coral reefs do not grow in upwelling areas.
Modern reefs are not really widespread in terms of coverage. Only about 0,2% of the oceans (or 15% of tropical shelves) are covered by reefs, however, if corresponding lagoones and other platform areas are considered, the coverage is much larger. Only 15% of all modern reefs grow in the Caribbean (and parts of the South Atlantic), the rest grows in the Indopacific. The Great Barrier Reef which is said to be more than 2000 km long actually consists of at least 3000 individual reefs.
(## update of numbers may be necessary, see Veron 1995)
Other possibilities of reef-based interpretations:
wave energy (see ecology chapter)
wave direction (see ecology chapter)
submarine morphology: different reef types may grow at a shallow outer shelf break, in a slope break position. Reefs tend to grow on preexisting morphologies which may help detecting palaeotectonic or palaeohalokinetic activity.
calculation of basin subsidence: shallow water reefs can normally cope up with and compensate basin subsidence. Reefs normally do not compact so that reef and carbonate platform thicknesses, together with age indicators (e.g. fossils) allow a good estimate of subsidence patterns. Uplift patterns can also be detected by palaeokarstic features.
Reconstruction of sea-level change: similarily, sea-level fluctuations can be easily detected using reef and carbonate settings. Stay tuned for our architecture and sea-level sections in this course.
High-resolution climatic reconstructions: Corals often develop annual growth rings which include a lot of information on ambient sea-water characteristics at their time of formation. We will put a major focus on this in our ###-section.
Exceptions:
The above characteristics are valid for modern tropical coral reefs. Both in the modern world and in the fossil record there are a lot of exceptions to this, so that reef characteristics will have to be thoroughly analyzed before using them for conclusions and interpretations as listed above. Some examples:
Modern reefs:
some tropical coral reefs can withstand or are even adapted to elevated terrigeneous sediment runoff and elevated nutrient values (e.g. Abrolhos reefs off Brazil, some caribbean reefs of Panamá). These reefs often are better models for ancient reefs than modern tropical oligotrophic reefs. See ## section for detailed treatment.
There are a lot of deep and cold water coral meadows in modern seas, such as the North Atlantic. These coral meadows can be excessive and may be also termed reefs. These corals are non-photosymbiontic and depend on elevated nutrient values. We will discuss them in the ## section.
There are also other, non-coraliferous modern reefs such as sponge reefs, oyster reefs, serpulid reefs or stromatolite reefs. We will also have a brief look at these.
Modern shelves:
Modern shelves are quite exceptional, i.e. they do not serve very well as fossil examples. Their exceptional character is due to the Pleistocene frequent and extensive sea-level change and rapid holocene sea-level rise. Many reefs had only short-termed stable conditions, especially short residence times in the same water depth. This explains why many are small. Many reefs were drowned or underwent subaerial exposure and karstic dissolution. On the other hand, the sea-level fluctuations had often positive feedbacks on reef growth, see the #sea-level section of this course. Also, upwelling of cold waters is stronger nowadays than during many parts of Earth History, which is another effect of our post-ice age situation. The exceptions can be summed up this way:
- narrow shelves
- dominance of steep rocky coasts, often causing strong siliciclastic influx into tropical marginal seas.
- many drowned or 'frozen' pleistocene reefs
- short residence time in same water level (Pleistocene sea level drop frequently 70 metres and more.
- Pleistocene extinction much higher in Atlantic (which is smaller and more open to the poles, especially the North Pole) than in the Pacific. Pacific was partially sheltered from melting polar ice by the Himalaya barrier.
Genus
PacificSpecies Number
AtlanticAcropora > 200 3 Porites 30 6
Fossil reefs: Reefs have had a continuous, though punctuated evolution during Earth History. So environmental characteristics for reefs vary largely through Earth History. Characteristics of modern reefs cannot be simply transferred to ancient times. This would cause a lot of errors (and does probably cause a lot of "dry holes" in petroleum exploration). A few examples, which will be discussed in more detail in the # Fossil Reef section.