To the previous page of the Jurassic Reef Park?

I am a bit in a hurry and the sun is shining outside. I will come back later but first let me have a look at the conclusions.

Why were those reefs so different?

As we have seen earlier, many reef have existed during the Jurassic Period which had no resemblance to modern reefs. Coral reefs grew during the Jurassic as they do today, but even these had certain differences. Siliceous sponge reefs and microbial reefs only have poor and very rare modern representatives not comparable with the Jurassic ones.

What is the reason for these differences? In order to discuss this we should briefly recap which factors defined the occurrence and differences of Jurassic reefs. These were:

  1. Water depth: Jurassic coral reefs grew in shallow waters (as do their modern counterparts), whereas mixed coral-siliceous sponge reefs and pure microbial crust reefs grew in deeper waters.
  2. Amount of influx of sand and clay (i.e., the sedimentation rate). Whenever sedimentation rate was too high, reefs could not grow. If sedimentation rate was tolerable, reefs composed of only a few organisms adapted to the still relatively poor life conditions could develop. Under conditions of zero sedimentation, much more different organisms could live in the reef. Moreover, microbial crusts thrived during such conditions which greatly contributed to the stability and upwards growth of the reefs.
  3. Oxygen concentration and frequency of oxygen fluctuations. (Probably such fluctuations in oxygen concentration were linked to fluctuations in nutrient concentration). If oxygen depletion events were frequent, no corals or siliceous sponges could live. However, the tolerant microbial crusts remained which under these conditions built up entire reefs without the participation of other organisms.

In Fig. 57 these factors are plotted against each other. Can you figure out which reef type corresponds to which area? To test your knowledge simply click into the colored areas.

please load image by clicking into the icon, Fig.57, 7 kb

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So why are most of these reefs types not existing today? Sedimentation rate is an important factor even for our modern reefs and essentially influences occurrence and composition patterns of reef organisms. But why are there no more reefs in deeper tropical waters? Why have the microbial crust reefs almost completely disappeared from modern seas? And why can modern coral reefs grow even up to the shallowest waters, although destructive force of storms is particularly strong there?

To answer these questions we should have a closer look at the global ecological conditions during the Jurassic Period.

Fig. 58 present a reconstruction of the globe for the Late Jurassic Epoch. Continents were still assembled to the supercontinent Pangaea, although its northern part started rifting apart. This is highlighted by the narrow seaways of the young Atlantic and the Mesozoic "mediterranean" sea known as the Tethys Sea. Since there were no polar ice caps and since ocean basins had a lower volume than today, sea level was higher than today by at least 100 to 150 meters. Many continental areas were therefore covered by shallow seas. Shelf break is positioned today at depths of 60 to 150 meters (or deeper), and is followed seawards by a steep continental slope. During the Late Jurassic the shelf break was at least 100 meters deeper. This resulted in very low slope gradients even in deeper waters (but shallower than 100 to 150 meters), so that reefs (in this case siliceous sponge reefs) could thrive in waters of several tens of meters depth (possibly 50 to 80 meters).
The high (and continuously rising) sea level had additional effects: most of river-derived sediments from the continental mainland was trapped in estuaries and coastal swamps (whose development is greatly favored by the rising sea level). Sediment was therefore not bypassed to deeper parts of the shallow shelf seas, where it oppresses reef growth as happening in modern seas.

Fig. 58, Jurareefs global, 8 kb

Global distribution of Late Jurassic coral reefs which demanded warm temperatures shows that during this time episode warm surface waters were widely distributed and even reached high palaeolatitudes.

There are many indicators (for insiders: calcareous green algae, certain sediment particles such as grapestones and ooids) which clearly demonstrate that Late Jurassic coral reefs occurred in warm ocean waters just as modern tropical coral reefs. Global distribution of these reefs shows that during the Late Jurassic warm surface waters were widely distributed across the globe and even reached high paleolatitudes (for example southern Chile and Argentina). This was an effect of the temperature buffer capacity of oceans and shelf seas. The high sea level caused a balanced maritime climate where latitudinal differences in temperatures were much more moderate than today and a warm climate characterized major areas of the Late Jurassic globe. A side effect of such a 'greenhouse-type' climatic leveling was that wind systems and the linked water current systems were less constant than today. Water exchange was therefore probably much lower and regional oxygen depletion could occur much easier than today. We all know modern algal blooms leading to oxygen depletion from semi-enclosed sea basins such as the Northern Sea or the Adriatic sea. We must assume that under the above scenario of the Late Jurassic such oxygen depletion was not only a local but a regional problem. This problem was particularly pronounced when additional sea-level rise of shorter duration added an additional climate-buffer component to the already well equilibrated Late Jurassic climate. These were times were circulation systems of shelf seas could collapse regionally, causing the death of oxygen demanding reefs or the transformation of such reefs to pure microbial crust reefs which tolerated oxygen depletion (we will come back to this in some more detail in the next chapter).

Figs. 59 and 60 compare the environmental demands and their limits (we call this the 'reef windows') of modern tropical reefs and Jurassic reefs. Let's summarize the major differences:

  1. Due to the much lower sea level of today much more sediment is transported from the continental mainland to the shelf seas where it often is transported to the somewhat deeper parts because it cannot be stored in coastal areas due to the relative rarity of estuaries and swamps. Therefore modern reefs cannot grow in waters deeper than about 40 to 50 meters. During the Jurassic these depths could be nicely occupied by siliceous sponge reefs due to the lower influx of sediment and the lower inclination of the sea floor. Both was an effect of the much higher sea level.
  2. Because of the high sediment influx modern reefs preferably grow in extremely shallow, high-energy waters. This is also where illumination is best for the light-dependant reef corals. However, the occupation of these high-energy settings was only possible after the evolutionary development of (a) corals with a high regeneration potential (such as modern staghorn corals) and of (b) organisms which can bind and stabilize reef debris even under such high-energy conditions. In modern reefs this is performed by cementing calcareous red algae, the coralline algae. Both types of organisms did not yet exist during the Jurassic. The abundant microbial crust of Jurassic reefs were normally not effective enough to bind reef debris in high-energy settings.
  3. The high sea level of the Jurassic world caused a leveling of climatic contrasts which in turn resulted in restricted water circulation and associated frequent events of regional oxygen depletion. Because of this instable situation, oxygen depletion could even occur in fairly shallow waters. This happened particularly when a intra-Jurassic sea level rise additionally buffered the climatic contrasts and further reduced water exchange. Microbial reefs adapted to such situations could then even thrive in fairly shallow waters.

Abb. 59, Shelf section modern, 13 kb

Abb. 60, Shelf section Jurassic, 13 kb

Figs. 59 and 60: The shelf position of reefs from the modern and the Late Jurassic world. Most important reef factors are indicated. See text for more details.


Last changes 19. Nov. 98 by Reinhold Leinfelder

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