The extraordinary fossil beds of Anticosti Island have shed light on what would have caused the first mass extinction on Earth, 445 million years ago. Researchers believe that this fundamental discovery will flesh out Anticosti’s candidacy for accession to the UNESCO World Heritage List.
“Anticosti is the best natural laboratory in the world for the study of fossils and sedimentary layers from the geological period going from the end of the Ordovician to the beginning of the Silurian, that is to say from 447 to 437 million years ago, during which place the first mass extinction of life on Earth ”, underlines the geologist, stratigrapher and paleontologist André Desrochers of the University of Ottawa. The latter is the co-author of an article published in Nature Geoscience which relates the results of a study seeking to shed light on the cause of this first extinction.
Recall that at the time of this extinction, life was mainly concentrated in the shallow seas near the continents. And Anticosti was then a shallow tropical sea, located a little south of the equator. “If we had been snorkeling in an Ordovician sea, we would have seen familiar groups like clams, snails, sponges, corals, but also many other groups whose diversity is today very small, and even groups that are completely extinct, such as trilobites, brachiopods and crinoids: so many invertebrates, but not quite the same as today, and few vertebrates as such, so very few fish. And on emerged lands, only very primitive plants, ”describes Mr. Desrochers.
This late Ordovician mass extinction – the first of five mass extinctions of life on Earth – led to the disappearance of about 85% of species in the space of several hundred thousand years, even a million years. “This extinction was longer than the one that caused the sudden disappearance of the dinosaurs at the end of the Cretaceous, 65.5 million years ago, but from a geological point of view, it is quite fast”, comments the geologist.
The cooling trail
According to a hypothesis put forward, it would be a decrease in the oxygen concentration in seawater that would have caused the extinction that occurred at the end of the Ordovician. A multidisciplinary team of French, American, Canadian and Chinese researchers, including André Desrochers, therefore applied to verify this hypothesis in the sedimentary rocks that outcrop on Anticosti Island.
Since it was impossible to directly measure the quantities of oxygen that were present in seawater from today’s rocks, an indirect geochemical indicator, called a proxie, which is the ratio between the iodine and calcium concentration. “This ratio which is measured in limestone rocks varies according to the quantity of oxygen which was present in the sea water”, specifies the geologist.
From the data collected in the rocks, the researchers found that the shallow seas were well oxygenated immediately before the mass extinction, and that they remained so during and after it. “It was a surprise for us, because according to the traditional interpretation, anoxia [c’est-à-dire la diminution d’oxygène] was the cause of the degradation of ecosystems, ”says the researcher.
The study also found that while the shallow seas remained well oxygenated throughout this period, anoxia had greatly increased in the deeper parts of the oceans.
Alexandre Pohl, an expert in climate modeling in ancient geological times, who is the first author of the article, then proceeded to modeling with the geochemical data obtained, adding information on the paleogeography of continents and oceans. 445 million years ago and the characteristics of the climate of the time that was going through an intense Ice Age.
“This modeling came to corroborate our hypotheses developed from geochemical proxies. So, yes, at the end of the Ordovician, a stratified ocean was seen to develop, the upper part where most organisms lived was still oxygenated, while the lower part had become anoxic. And as life was concentrated in the shallow parts, the anoxia did not therefore play the role we thought in extinction, ”summarizes Mr. Desrochers.
Rather, the researchers believe that it is a great cooling of the shallow parts that would have led to the collapse of biodiversity, because we know that there were great glaciations at that time.
Modeling has shown that the cooling of the climate would probably have modified the oceanic circulation, that it would have probably interrupted the flow of cold and oxygen-rich waters from shallow seas to the deeper ocean, hence the appearance of this stratification of the oceans.
To make sure that the geochemical signal they measured at Anticosti was the same globally, the researchers also measured the same geochemical relationships in Nevada. The latter turned out to be similar to those of Anticosti, even though Nevada was located more than 1000 kilometers from Anticosti at that time and that it did not bathe in the same ocean. “This therefore confirms that the signal is indeed global and not simply local”, concludes the researcher.
“There may be other factors that combine with this global cooling, but it is the studies of the next few years that will allow us to know them,” says the geologist while arguing that understanding the environmental conditions that led to such mass extinctions could help us predict and prevent a similar event from happening again.
UNESCO application
One thing is certain, it is surely in Anticosti that researchers will find the answer to these puzzles. Given the immense stratigraphic and palaeontological value of the island’s sedimentary rocks, which “contain 10 million years of the history of life on Earth”, the researchers strongly wish that this site be included on the World Heritage List. so that it is protected in perpetuity and that UNESCO grants funding for a few professionals to ensure the protection, conservation and interpretation of the site.
The municipality of Anticosti and its many partners (including Quebec ministries and representatives of the Innu nation) must submit an application file by the end of the year to UNESCO in the hope that 550 kilometers and the outcrops of the Jupiter and Vauréal rivers are inscribed on the World Heritage List.
“Anticosti is without a doubt the best place in the world to study the first mass extinction that life on Earth experienced. Among the forty or so sites of the same age, it is the richest, the most complete, the best preserved and where the strata are very well exposed, ”notes Mr. Desrochers. He notes that more than 1,450 species have been described in its rocks, while barely a hundred have been listed on the most interesting site after Anticosti.
Renowned paleontologists recognize this unequivocally: the fossiliferous sites of Anticosti “stand out from the others by their abundance, their level of preservation and the great diversity of fossil specimens found there”, continues the professor. “The island’s exceptional paleontological value has been recognized for over a century, and today it continues to attract researchers from around the world. The study that we have just published will surely strengthen the application file, ”argues Mr. Desrochers.
Experts are expected to come next summer to Anticosti to do a field assessment, says the man who, with his long experience of 30 years of research at Anticosti, will guide these assessors to the most representative outcrops of the island.
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