The threat posed to biodiversity today is nothing compared to the violent volcanic events that affected the Earth in the past. These events created vast continents of lava, altered the climate and led to the extinction of numerous living species. Researchers at UNIL have retraced how such exceptional geological events unfolded.
When we talk about extinction, we immediately think about the famous biological crisis in the Cretaceous-Tertiary period 65 million years ago that triggered the disappearance of the dinosaurs and wiped out between 60% and 80% of species living on the planet. And yet well before this period, our planet had experienced other upheavals. On several occasions it was subject to violent volcanic phenomena, which also had a powerful impact on biodiversity. The consequences at this level were sometimes significant and sometimes relatively limited. The question may be asked therefore why the same causes do not always produce the same effects. UNIL researchers have just provided an answer to this precise question.
Gigantic lava-covered surfaces
As senior lecturer and researcher at UNIL Institute of Earth Sciences, Sébastien Pilet explains geological time has witnessed “five or six extreme volcanic events”. Such cataclysmic events were capable of emitting several million cubic kilometres of lava over periods of between 500,000 to 1 million years (an extremely short space of time in geological terms). “Such an event is gigantic” notes the researcher. To give an idea of the scale of this phenomenon, he points out that, if reproduced today, “It would cover the whole of Europe in a layer of lava 100 to 200 metres thick.”
Not cones but fissures
Such events should not be imagined as resulting from the cone-shaped volcanoes with which we are familiar. They arise from “massive fissures which trigger fractures in the continents”, such as the Laki fissure to the south of Iceland, which comprises more than 100 craters over a 27-kilometre stretch. When they release their lava, these fissures give birth to what scientists refer to as volcanic provinces.
The Siberian Traps – vast plateaus comprising thick deposits of basaltic rocks – were formed in this way around 250 million years ago. CAMP, the Central Atlantic Magmatic Province – “traces of which can be found on the fringes of the Atlantic from present-day Canada to Brazil and Portugal to Senegal” –, the province of Karoo-Ferrar – “located in today’s South Africa, Australia and Antarctica” – and the Deccan province in India then followed in succession. Added to these are “events on a lesser scale”, such as the one dating from at least 20 million years ago, which was the origin of the Columbia River gorges in the northwest United States.
Varied impact of eruptions
Although they released enormous quantities of molten rock, surprisingly some of these volcanic events had only a relatively limited effect on ecosystem diversity. This observation aroused the curiosity of researchers at UNIL, who set about updating knowledge of “the mechanisms which were necessary to the disappearance of living species”. To find the answer, they focused on the two volcanic provinces of CAMP and Karoo-Ferrar in particular.
The study published in March 2016 in Nature Scientific Reports was the fruit of “collaboration which brought together very different skill sets” points out Sébastien Pilet. Aside from his specialist team of volcanologists, the work saw contributions from palaeontologists from UNIL and from the National Museum of Natural History in Paris, “who studied the fossil archives of these two major extinction events”. They were joined by geochronologists from the University of Princeton in the United States and the University of Geneva, whose laboratory is “one of the best on the planet for dating”, according to Sébastien Pilet.
Correlating fossil archives and lava emissions
The Geneva team helped date the CAMP formation to precisely within the Triassic/Jurassic boundary interval and the Karoo-Ferrar to within the Pliensbachien/Toarcian boundary interval. The major difficulty with understanding the link between volcanism and its effect on biodiversity is being able to establish a temporal correlation between fossil archives and lava emissions, which are distributed across different locations on the globe. “My palaeontologist and geochronologist colleagues had the bright idea of studying the sedimentary rocks containing volcanic deposits in Peru and in Nevada.”
Accurate dating to within 100,000 to 200,000 years
Both these volcanic provinces feature “among the rare places where zircon (zirconium silicate) is to be found in sediments”, explains the Lausanne researcher. When formed, the structure of this mineral includes uranium and thorium, two radioactive metals which decay over time to produce lead. “By measuring the relationship between the parent and daughter elements, an event can be dated to within almost 100,000 to 200,000 years.” In this way, the researchers were able to establish that the CAMP began to form 201.48 million years ago and Karoo-Ferrar 183.25 million years ago. This accuracy “has made it possible to correlate the date of the sediments and the positioning of the volcanic event.”
Two periods of climatic extremes
When studying marine sediments, Jean Guex, Honorary Professor of Palaeontology at UNIL, had observed that geological episodes from the Triassic-Jurassic and Pliensbachien-Toarcian “were associated with not one but two different extreme climatic periods.” Cooling led firstly to a drop in sea level and then warming raised it again. At this point, Sébastien Pilet and his colleagues became involved to try to understand “the role of volcanism in this series of events.”
Thickening of the lithosphere
“To our great surprise,” Sébastien Pilet recalls, “we realised that in the two cataclysms studied, volcanism had occurred at places where the lithosphere – the rigid part of our planet comprising the crust and upper part of the terrestrial mantle – had thickened.” This had consequences for the release of magma, the ‘paste’ of molten rocks which gives rise to lava at the surface. As Sébastien Pilet explains, the material that rises from the centre of the Earth to the crust “is hot but not liquid”. It is only when it is found at a depth of 150 to 180 km beneath the surface that it becomes molten.
Two successive periods
Researchers have established that, when the terrestrial envelope is thicker than normal, this hinders the formation of magma and thus delays the release of lava. The lithosphere must first warm up and, during this process, “large quantities of sulphur dioxide (SO2), which had been stored at its base, are released into the atmosphere”. This triggers a rapid cooling of the earth’s surface. It is only subsequently, “when the magma succeeds in acquiring real depth” that the basaltic lava flows emit CO2, a gas well known for causing global warming.
Crucial role of gases
This is significant as these gases play a crucial role in species extinction. As the researcher points out, “Laval flows kill only those animals and plants in the vicinity”. In contrast, SO2 and CO2 can wipe out numerous species living thousands of kilometres from the volcano, as not only do they alter the climate but they also cause atmospheric and ocean acidification, which is harmful to life.
The scientists at UNIL are therefore the first to have highlighted the succession of events that has led to the loss of biodiversity when the CAMP and Karoo-Ferrar were formed. But how does this explain why other equally violent cataclysmic events have not had the same impact on the living world? “It is likely that some species are able to adapt to the cooling of the atmosphere and to resist this climate change,” replies Sébastien Pilet. “But when the atmosphere subsequently warms up, they disappear.”
An as yet unconfirmed theory
Part of the mystery has therefore been clarified. The geosciences expert goes on to stress, however, that “it’s simply a theory which we have yet to confirm.” In response to the question of how this will be done, he responds, “By studying sulphur isotopes, for example, to try to identify the origin of this element.” Another avenue to explore would be “to see if the sequence of events that we have uncovered could explain other major extinctions, such as those linked to the Deccan Traps in India”.
No immediate threat
One crucial question remains: could such cataclysms strike the earth’s surface again? “This type of major volcanic event occurs every 10 to 20 million years,” observes Sébastien Pilet. Given that the last violent reactivation of the fissures that gave rise to the immense basaltic plateaus covering the states of Oregon and Washington took place around 14 or 15 million years ago, “it is not inconceivable that such a phenomenon could shortly – in terms of the geological timescale of course – reoccur.” But rest assured; it appears that there is nonetheless no immediate threat, as no geophysical indicator suggests that such a process is underway.
Proof that CO2 raises ocean levels
Although the study conducted by the UNIL researchers and their colleagues deals with the distant past, it nevertheless resonates with the highly topical issue of global warming. While the majority of scientists agree in their predictions that an increase in CO2 emissions in the atmosphere will produce a rise in sea levels, this point is still a subject for debate. “Past records clearly show that this is indeed the case,” stresses Sébastien Pilet. “There is a direct correlation between greenhouse gas emissions and rising ocean levels.” The rise has the potential to be all the greater “as we know that, on a geological timescale, sea levels have sometimes varied by one hundred metres”. The IPCC (Intergovernmental Panel on Climate Change) predictions, which indicate that levels could rise by 50 cm to 1 m between now and the end of the century, “are therefore completely credible”. This is the case even without an extreme geological episode.