Climatic control of Oxfordian coral reef distribution in theTethys Ocean
Including a comparative survey of Recent coral communities (Indian Ocean) and a new method of coral morphometrics based on fractal dimensions
Bertrand MARTIN-GARIN
PhD summary
Summa
Introduction
Why do so many coral species coexist in the tropics (Veron 1995)? The starting point of our discussion is the observation that generic diversity of corals is strongly correlated with latitudes. Also the abundance of coral reefs strongly depends on the physico-chemical parameters of the environment. The following central question arises: Which parameters of the physico-chemical environment are limiting to coral diversity and reef formation? Recent coral reefs are heavily perturbed by ongoing climatic changes. According to Kleypas et al. (1999), and Harriot and Banks (2002), the main limiting factors attributed to climate are the following: seawater temperature, insolation (light availability), aragonite saturation of ambient water, and nutrient input. According to Leinfelder (1993), most of these parameters already controlled coral growth in the geological past (Insalaco et al. 1997; Insalaco 1998; Dupraz and Strasser 1999, 2002) on the northern Tethys shelf (western Europe) especially during Oxfordian and Kimmeridgian times (Late Jurassic). All these parameters do not only control the growth of coral reefs and the diversity of coral communities, but also affect the skeletal morphology of corals. Almost all the species of zooxanthellate corals exhibit a wide range of morphological variability. As a result, identification of Recent and fossil coral species has become a task for specialists and remains a source of continuous confusion in the taxonomic literature (Veron 1995).
Methods
In this study, we compile the distribution data of both Oxfordian coral genera and coral reefs gathered from personal fieldwork carried out in England, Lorraine, Burgundy, the French Jura Mountains, the Swiss Jura, and Morocco (3500 polished slabs and 300 thin-sections) supplemented by the results retrieved from 97 publications and PhD theses available to us.
Diversity indices (Simpson, Shannon et Hill) of coral genera were calculated based on 3805 specimens identified. The changing geographic distribution of the reef corals during the Oxfordian, a time span of 6 million years in the Late Jurassic, is compared with Recent coral distribution As a result we obtained a scenario of astounding faunal migrations during the Oxfordian which provoke a new reflection on the Uniformitarian Principle and on the future of Recent coral communities. Is the past a key for the future?
Stable isotope studies were carried out on shells of reef-dwelling brachiopods and oysters to evaluate the impact of the climate changes on coral communities during the Oxfordian.
To better understand the factors controlling the growth of Recent reefs and to make use of the results obtained in fossil reef environments, coral communities of the Recent fringing reef at Saint Leu (Réunion Island, Indian Ocean) were studied. The same indices of diversity used for the Oxfordian coral genera were employed in order to establish a zonation and to identify the controlling factors of this reef.
In addition, a new morphometrical method is presented to quantify and characterize coral corallites using the Richardson Plot and the Kaye’s notion of fractal dimensions (see Kaye 1994). A Jurassic coral species (Aplosmilia spinosa) and five Recent coral species are compared using the Box-Counting Method. As defined by Mandelbrot (1967), lines have a dimension of one, surfaces a dimension of two and solid bodies a dimension of three. The fractal dimension of a rough, jagged line may be any real number between one and two.
The Box-Counting Method enables us to characterize by their fractal dimensions, the morphologies of each species at the calicular and septal levels. Fractal dimension, which is characteristic of the morphology i.e. the overall structure of the corallite (calicular level), is defined as the structural fractal dimension (δs). On the other hand, the fractal dimension, which is useful for the description of the texture or fine details at the septal level, is defined as the textural fractal dimension and is named by the symbol δt. Thus, it is possible to determine differences between species of Montastraea and to tackle the high phenotypic plasticity of Montastraea annularis. The use of fractal dimensions versus conventional methods (e.g. measurements of linear dimensions with a calliper, landmarks, Fourier analyses) to explore a rugged boundary object is discussed.
Results and discussion
Distribution of Oxfordian versus Recent coral reefs
Until recently, the Jurassic was thought to have been a period characterized by a predominantly warm and equable climate. During the Oxfordian (a time span of 6 million years in the Late Jurassic) the distribution of tropical coral reefs was limited to about 35°N and near to 25°S. However, in Middle Oxfordian time, coral reefs were abundant only at higher latitudes and almost entirely missing near the equator. During that time the area of maximum reef development had shifted poleward to a belt lying between 20°N and 35°N leaving hardly any coral formations at the lower inner tropical latitudes. This reef distribution completely diverges from the present-day situation. After demise towards the end of Middle Oxfordian time, the low-latitude reefs recovered during the Late Oxfordian accompanied by a southward migration of reef corals in the northern hemisphere. As suggested by stable isotope and palynological data, the faunal migration can be correlated with a significant rise in seawater temperature during the Middle Oxfordian. This study compares the latitudinal variation of Oxfordian coral communities through time with the coral distribution observed in a modern reef province. It appears that the faunal migration found in the Oxfordian may perhaps serve as a predictive model for the fate reserved to modern coral reefs in the near geological future. These might also have to shift to higher latitudes forced by the ongoing global warming process that already leads to coral bleaching and general reef degradation in the present equatorial belt.
Generic diversity of corals and Oxfordian water temperatures
During the Middle and Late Oxfordian, the generic diversity of scleractinian corals continuously increased during each of several successive reef-building events recorded from the northern Tethyan (England, Lorraine, Swiss and French Jura, and Burgundy) and northeastern Atlantic shelves (Morocco). The different reef-building events are clearly identifiable by the generic compositions of their corals.
At each of the outcrop areas, the ‘first reef-building event’ (corresponding to the Plicatilis and Transversarium Chrons in England, the early Transversarium Chron in Lorraine, the Swiss and French Jura, and to the late Transversarium and Bifurcatus Chrons in Burgundy) is characterized by a predominantly heterotrophic mode of nutrition and by low coral diversity at seawater temperatures below 20.5°C. The ‘second reef-building event’ recorded from the end of the Transversarium Chron in Lorraine, the French and Swiss Jura, and from the end of the Bifurcatus and early Bimammatum Chrons in Burgundy showed a greater richness of coral genera in a balanced hetero-phototrophic nutrition mode at seawater temperatures between 22°C and 28°C. This ‘second reef-building event’ is not known from England. During the ‘third reef-building event’ corresponding to the Bifurcatus and Bimammatum Chrons in Lorraine and the Swiss Jura, a heterotrophic nutrition mode prevailed during considerable siliciclastic input into the otherwise pure carbonate system. Coral diversity was lowered and seawater paleotemperatures decreased again below 20.5°C.
In the Recent, coral reefs are a gauge for the health of the planet and for the equilibrium of world climates. A slight rise or fall of seawater temperatures will result in degradation of reefs and in a decrease of the generic diversity of their corals (e.g. Harriott and Banks 2002). This same gauge existed already during the climatic changes of the Oxfordian, when the temperature fluctuations of the seawater controlled coral diversity and thus, as a consequence, also the growth of the coral reefs themselves.
Climatic control and temporal evolution (Middle and Late Oxfordian) of a coral-microbialite reef
Middle to Upper Oxfordian reefs of a shallow marine carbonate platform located in northeastern France show important facies changes in conjunction with terrigenous contents. The Pagny-sur-Meuse section shows coral-microbialite reefs that developed both in pure carbonate limestones and in mixed carbonate-siliciclastic deposits. Phototrophic coral associations dominated in pure carbonate environments, whereas a mixed phototrophic / heterotrophic coral fauna occurred in more siliciclastic settings. Variations in terrigenous input and nutrient content, rather related to climatic conditions than to water depth and accumulation rate, were major factors controlling the development of reefs and their taxonomic composition.
Main controlling factors and zonation of a Recent coral reef. Does the diversity decrease with depth?
The fringing reef of Saint Leu at Réunion Island (Indian Ocean, France) is particularly exposed to human impact and to the extreme meteorological conditions caused by tropical cyclones. Nevertheless, the reef shows a high coral diversity. The water is normally very clear. Using the perpendicular transect method, quantitative studies on scleractinian colonies were carried out from the back-reef with reef flats to the fore-reef slope. The coral associations are found in four ecological zones corresponding to the following major environments: ‘back-reef area’, ‘spurs-and-grooves zone’, ‘upper platform’ and ‘lower platform’. Seaward from the beach towards the surf zone, the coral cover increases in the very shallow (1 to 2m deep) ‘back-reef area’, but decreases again sharply in the surf zone where the wave activity is strongest. Then, the coral cover increases again on the fore-reef slope down to –20 meters of water depth followed by a decrease towards the ‘lower platform’. In comparison, the coral diversity is quite moderate within the ‘back-reef area’ and in the surf zone. In the fore-reef the coral diversity increases progressively and surprisingly attains a maximum on the ‘lower platform’ at a water depth of –30 meters. Some scleractinian colonies usually encountered in shallow waters (‘back-reef area’, upper fore-reef slope), were also observed below on the ‘upper platform’ and/or the ‘lower platform’. Apparently, the limiting factors, which control coral growth in this reef, are wave activity and light availability.
Coral associations and zonation of a Late Jurassic coral reef from Morocco
A quantitative study of the Upper Jurassic coral associations of Cape Ghir (Atlantic High Atlas, Morocco) revealed highly diverse coral assemblages characterizing three reef environments, each of them dominated by one of the following genera: Dimorpharaea, Microsolena and Stylina. A fourth assemblage is characterized by nerinean gastropods and stromatoporoids. Combined GPS surveys, 3D simulation, and facies distribution studies permitted to understand the geometry of this coral reef within a particular tectonic setting. The reef became installed on top of a tilted block of Jurassic age subsequently folded into an east-west trending anticline near the village of Tighert, exhibiting a 5° to 10° northward dip of its northern flank near the lighthouse of Cape Ghir. We suggest that the different fossil assemblages encountered in the field belong to one and the same fossil reef tract (within a unique facies model). The previously reported hypothesis of two successive reef horizons representing different biochrons is abandoned.
The decrease of the coral cover with depth is consistent with the observations made for the modern fringing reef on Réunion Island. Nevertheless, the Oxfordian coral reef in Morocco presents a very low diverse coral association in the deeper environment of the fore-reef, while the diversity of the Recent Réunion reef is maximum in this zone.
Fractal dimensions to quantify the morphologic variability of corals
Fractal dimensions supply considerable advantages in coral morphometrics. (1) Outlines and fine structures of coral corallites are thoroughly analyzed and quantified at different levels of observation (calicular and septal). Thus, they provide results as accurate as the best quantitative methods presently available for delimitation of coral taxa. (2) The Counting-Box Method is straightforward to apply and easily reproducible for Recent and fossil scleractinian corallites. (3) Statistical analyses are simplified, because only two parameters are necessary to characterize coral corallites: the structural fractal dimension (δs) and the textural fractal dimension (δt).
The preliminary studies presented here, are a first step towards the revision of the systematics of the Upper Jurassic species of the genus Aplosmilia and need to be extended to other fossil or Recent species. The method may be applied to a surprisingly wide range of issues: (1) to scientific disciplines such as evolution, biology, biostratigraphy or ecology, (2) to the morphology of organisms, and (3) to general coral research at different scales, from microscopic to remote sensing.
References
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