Fossil sponges
2/21/12

It paints a breathtaking scene: Wallonia covered by a tropical ocean, submerged in azure blue waters populated by exotic flora and fauna. This was precisely the case some hundreds of millions of years ago in the Frasnian stage. This is a period which fascinates the geologist Anne-Christine Da Silva, who was recently awarded the Edmond de Selys Longchamps prize for two of her articles (1) in which she studied stromatoporoids, fossil sponges.

The journey through geological time is unusual because it takes place vertically. Geological archives (from the Cambrian period, some 545 million years ago, through to the Recent period) are effectively located right beneath our feet. By analyzing the strata which comprise sedimentary rocks, geologists travel through millions of years of our planet's evolution. From the most recent strata through to the oldest and from the uppermost to the most deeply buried. Geologists observe, scrutinize, and categorize these structures; sifting through the numerous fossils imprisoned within to identify the environment in which the rock was formed. Piled on top of one another in different layers, witnesses to different environments, they turn to rock through the effects of changes in pressure and temperature. In Belgium, 99% of which is covered in sedimentary rocks, the early interest in geology (as far back as the 18th century, the rocks were studied by experienced geologists) has led to the international recognition of our rocks. Frasnian, Tournaisian, Visean, Namurian, and Dinantian - many different terms derived from the names of well-known towns in our lands and which are used today by the world's entire scientific community. Regardless of where they are found, "Frasnian" refers to rocks dating to between 376 and 382 million years ago.

Stromatoporoid

Study the past, anticipate the future

During the Frasnian stage, as a result of tectonic plate movement, Wallonia was covered by a tropical ocean, located around 30° latitude south. The average temperature was around thirty degrees. A lagoon-like area sat alongside a barrier reef. The environment was similar to that which can be found today on the coast of Australia or the Bahamas. If this conjures up idyllic images, the Belgian Frasnian stage attracts researchers for other reasons. "Understanding the evolution of the planet and events which affected the surface of the earth several millions of years ago is crucial in order to better evaluate future changes, in particular in terms of an increase in CO2 levels. In the past, CO2 reached much higher levels than it has today and it is interesting to see what impact this had upon the flora and fauna, among other things. The Frasnian stage, for example, is a period which experienced global warming with a tenfold increase in CO2 levels - much more significant than the rise which we face today - followed by a brutal cool-down period which led to a major extinction of more than 75% of marine species. The overall view that we have of this period may enlighten us as to what the future has in store for our planet."

(1) Da Silva, AC., Kershaw, S. and Boulvain, F. (2011) Sedimentology and stromatoporoid paleoecology of Frasnian (Upper Devonian) mud mounds from southern Belgium. Lethaia, 44, 255-274.
Da Silva, AC., Kershaw, S. and Boulvain, F. (2011) Stromatoporoid palaeoecology in the Frasnian (upper Devonian) Belgian platform, and its applications in interpretation of carbonate platform environments. Palaeontology, 54 (4), 883-905.

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