In this final chapter we want to briefly focus on whether Jurassic reefs could help us to learn more about the uncertain future of our global ecosytem:
A look at the global distribution of Late Jurassic coral reefs highlights the wide distribution of warm waters and hence of equilibrated, warm climate across the earth during that Ancient time. (You may wish to return to the corresponding Fig. 58 for recap.) Such an equlibration of climatic differences was caused by the equilibrating effects of a high sea level which buffered the climate.
Sea level was - with some to and fro (we'll come back to this in a minute) - generally rising during the entire Jurassic Period. Especially on the northern shelf of the Tethys Ocean it is obvious that as a consequence reefs and related carbonate platforms became increasingly frequent (Fig. 61). To some extent, the rising sea level triggered an evolutionary increase of species richness of reef organisms. More important were, however, two other effects of sea-level rise: (1) the consequent increase of available space ('accomodation space') for reefs and carbonate platform and (2) the continuous pushing-back of sand and clay sediments towards the mainlands.
Fig. 61: On the northern shelf of the Tethys both coral and siliceous sponge reefs became more frequent in the course of the Jurassic. This is, to some extent, due to evolutionary increase in the number of available reef organisms but the main reason was the continuous increase of space suitable for reef growth, all of which is related to the general sea-level rise.
Besides this long-term, general sea-level rise, short-term sea level fluctuations (with frequences between 50.000 and 1 million years) also had an important influence on the temporal occurrence pattern of reefs (Fig. 62): During relative stillstand of sea level or even falling sea-level much more mainland-derived clay and sand could reach the seas, mostly due to an increase in the slope gradient from river springs to sea level. In this scenario reefs could only grow in the very shallow waters where constant wave agitation prevented settlement of suspended clay. During episodes of sea-level rise, clay and sand was largely trapped in coastal swamps and estuaries which formed as a consequence of continuous flooding of coastal areas. Frequently reefs and carbonate platforms could then fully or partially occupy the increasing space created by the ongoing sea level rise.
Most interestingly, there are a few examples from the Jurassic where the development of coral and sponge reefs was largely inhibited despite rapid (short-term) sea-level rise. These were the times with a strong component of additional climatic leveling causing depletion of oxygen even in fairly shallow waters, so that only microbial crust reefs adapted to such situation could grow (as mentioned earlier).
Fig.62: The influence of short-term sea-level fluctuations on reefs from the Jurassic. See text for detailed explanation.
Why could this shallow-water oxygen depletion happen in the course of (geologically spoken) short-term sea level rise? Fig. 63 shows the time-correlation of sedimentary successions deposited during the Late Jurassic in Portugal, Spain, southwestern France and southern Germany. It is quite obvious that reefs and limestones either occurred exclusively during times of sea level rise or rapidly expanded laterally during these episodes. (These time-episodes are marked with a green horizontal bar.) The reefs comprised coral reefs (in the shallow water) and siliceous sponge reefs (in deeper waters).
Only once (during the time-span shown in Figure 63) strange things have happened: 143,5 million years ago there was another rise of sea level (yellow horizontal bar) and again reefs occurred. This time, however, reefs were mostly represented by microbial crust reefs adapted to oxygen depletion which now even grew in the shallow water. The other reef types disappeared almost completely. Why was this sea-level rise accompanied by strong oxygen depletion? (However, we must not think of an episode of general shallow-water oxygen depletion lasting for several 100.000s of years. It is more likely that very frequently, probably every summer or every couple of years such oxygen depletion occurred, similar to what happens with algal blooms in today's Northern Sea or the Adriatic Sea.) Was it a coincidence or was it related to the sea-level rise?
Fig.63: Growth episodes of reefs from the Late Jurassic in Portugal, Spain, France and southern Germany. Growth episodes are clearly related to sea level rises (green horizontal bars). 143,5 millions of years ago, strong oxygen depletion occurred along with a sea-level rise (yellow horizontal bar), causing the frequent occurrence of microbial reefs. This episode coincides with a time-span where formation of limestones was strongly suppressed due to crustal movements and related influx of sand and clay into the shelf seas. See text for more details.
It is clearly visible that this rise of sea-level was positioned within a time-interval where hardly any formation of limestones took place. The reason for this was that a great amount of clay and sand particles was transported from the mainland into the shelf seas as a consequence of an increase in erosion of accentuated hinterland morphologies created by crustal movements. (These movements were related to the origin of the young Atlantic seaway.) It is therefore very plausible that the lack of formation of limestones and reefs was responsible for the collapse of climate and ocean circulation. But how can a cessation of limestone deposition and reef growth cause this all?
To answer this, let's first have a look at how reefs and other marine limestone deposits are created. Calcium carbonate (which is limestone) is almost exclusively precipitated by marine organisms such as corals, calcarous algae, clams, mussels, cockles and snails, even though their calcareous hardparts are often only preserved as minute fragments (hence calcareous sand and calcareous mud). This explains why influx of mainland derived clay and sand can cause the collapse of limestone production, simply because many organisms cannot tolerate this (for instance, clay may clog respiratory organs, bury hard substrates necessary for settlement of larvae, or may decrease the depth of light penetration).
However, if environmental factors allow the production of calcium carbonate by these organisms, its precipitation follows this equation (simplified), known as the carbon dioxide equilibrium (Fig. 64):
Fig.64: The carbon dioxide equilibrium
The atmospheric greenhouse gas carbon dioxid is partly dissolved in sea water and together with dissolved calcium may precipitate as calcium carbonate, hence limestone. This process is facilitated by warm water temperatures; of paramount importance are, however, the photosynthetic activity of plants as well as the direct formation of calcium carbonate shells and skeletons of marine invertebrate organisms. The most effective producers of calcium carbonate are reef organisms such as corals and calcareous algae. This means that reef growth continuously removes carbon dioxide from the sea water . To maintain equilibrium it will be refilled by atmospheric carbon dioxid, a process which causes a constant depletion of this greenhouse gas from the atmosphere during times of rapid and widespread reef growth. It is true that respiration of reef organisms contributes to the production of carbon dioxide which is directly or indirectly expelled into the atmosphere and remains active in the open carbon cycle. However, a larger amount of carbon dioxide is withdrawn for very long geological episodes (hundreds of millions to billions of years) from the connected atmosphere-ocean system because it is deposited as limestone in the Earth's crust (SCI-NOTE).
Fig. 65 should demonstrate how important this mechanism is for maintaining a given climate: If sea level rises (say, by the diminishing of ocean basin volume due to plate tectonics) the global area of water cover and, together with this, the rate of evaporation of water increases. Water vapor is a weak but widely distributed natural greenhouse gas, and a rise in the atmospheric concentration of water vapour will cause a certain initial increase of atmospheric temperature. At the same time, the rising sea level creates more 'accomodation'-space for reefs and carbonate platforms which normally will rapidly expand and counteract the effects of initial rise of atmospheric temperature by increasingly removing atmospheric carbon dioxide according to the process outlined above. This will keep atmospheric temperatures within normal ranges. If, however, the growth of reefs and carbonate platforms is suppressed by other reasons such as the increase in crustal activity, as it happened in our Jurassic example 143,5 million years ago, a positive feedback circle may be activated: The initial rise in atmospheric temperature related to higher water evaporation will heat up the surface waters of shelf seas and oceans, which as a consequence will not be able to dissolve as much carbon dioxide as before so that part of the gas will be discharged into the atmosphere. This will again heat up the atmosphere, which then will further warm up the surface waters, which as a consequence will have to expel even more carbon dioxide to the atmosphere, and so on and so on......
This vicious circle will eventually result in a strong increase of global temperature and in the slow-down of oceanic circulation systems. One of the consequences could be shallow water oxygen depletion as we have seen in our Jurassic example.
Fig.64: The carbon cycle during rises of sea level
Thank God that this all has happened many million years ago! We will certainly not face such catastrophes. Or will we?
Naturally, we do not live in the time of the Late Jurassic. Our modern climate is not directly comparable with the Jurassic one. The danger of natural collapse of climate and oceans was certainly higher during the 'greenhouse' time of the Late Jurassic than today. But there are obviously some striking parallels:
Sea level and global temperate do rapidly rise today. However, the rise is still weaker than should be expected by the enormous production of atmospheric carbon dioxide by man. Responsible, at least partially, for this appear to be the remaining reefs and associated carbonate platforms which act as climatic buffers. If we keep on destroying these powerful climatic regulators, the greenhouse effect may suddenly increase to unexpected dimensions, once the previously discussed vicious circle starts working. Needless to describe which catastrophic effects shelf sea oxygen depletion would have for the fishing industry, how rapidly Bangla Desh, the Netherlands, Northern Germany, the Mississippi area and many other regions would be flooded and which disastrous effects changing or fading climatic zones would have for global nutrition.
For all that, the ecosystem Earth has many control mechanisms to confine the dangerous effects of our often inconsiderate behaviour, such as the growth of reefs or rain forests which can partially compensate excess carbon dioxide. However, we should be on the watch out not to one day (and maybe soon) get a catastrophic response for our short-sighted way of acting and our unlimited believe in technology. Maybe you think about it next time you delight yourself by snorkeling through a - still - phantastic coral reef.
JURASSIC REEF PARK, Version 1.01 e, 25. May 1996
© Reinhold Leinfelder
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