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Time matters....

 

Precisely dating the Frasnian–Famennian boundary: implications for the cause of the Late Devonian mass extinction 

2018

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L. M. E. Percival, J. H. F. L. Davies, U. Schaltegger, D. De Vleeschouwer, A.-C. Da Silva
& K. B. Föllmi

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Scientific Reports volume 8, Article number: 9578 (2018)

https://www.nature.com/articles/s41598-018-27847-7

 

In this research, we focus on the Frasnian Famennian boundary (Upper Devonian),  around 370 Myr ago. This extinction was one of the largest in Earth’s history, with coral reefs and marine invertebrates amongst the major to suffer. However, unlike the more famous extinction of the dinosaurs that is widely thought to have occurred when an asteroid collided with the Earth 66 million years ago, the cause of the Devonian extinction remains largely unknown. Although asteroid impacts are known from the Upper Devonian, massive volcanism could also have been responsible for the big ecological overturn. Emission of volcanic gases (for example the greenhouse gas CO2) could have caused extreme climate change, resulting in increased weathering and erosion leading to excess nutrients in the oceans and algal blooms. This altogether lead to de-oxygenation of the seawater, suffocating marine animals.

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Key to this debate, is time, rates and when the extinction did happen. When we study massive geological events like that, we study them in very different locations in the world and we try to gather as much information as possible. Although we have to be sure that we are looking really at the same event. For this we have what is called biostratigraphy, we look at the fossils in the rocks and we try to find the same species. So it gives us what is called a relative dating, we know if one rock is older, younger or the same age of another. But what do we do when we want to compare a massive volcanic accumulation with a sedimentary succession, it becomes tricky, because there is no fossils into the volcanic accumulation. So what we need is to precisely know the age of the rock we are looking at the different places in the world, we need to give a number. This is called absolute dating and it's mostly made with Radiometric techniques (cf : https://en.wikipedia.org/wiki/Radiometric_dating). 

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                      At the hammer, a layer of black rocks corresponding to the development of anoxia in the ocean at the Frasnian-Famennian crisis

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So what happens if we want is to compare the event of the extinction itself, with the ages of Devonian asteroid impacts and volcanic eruptions. Researchers from the Universities of Lausanne and Geneva, together with colleagues from Germany and Belgium collected a volcanic ash bed within the sedimentary rocks that record the extinction, which contained abundant crystals of the mineral zircon. Zircon crystals contain tiny amounts of uranium, a radioactive element that decays to lead at a known rate over time and permits determination of its age.

 

The previous estimate of the Frasnian-Famennian boundary was 372.2 Million years with an undertainty of 1.6 Million years, which is small compared to 372.2 Million years of course...  but when you want to compare events and to see if something happened at the same time, it becomes problematic.

By analyzing some zircon crystals from an ash bed just below the extinction $, the researchers pinpointed the date of the extinction to 371.86 million years ago, with a precision of 0.06 Million years. Interestingly though, it does not match the ages of any known Devonian asteroid impact or huge volcanic eruption!

Whilst evidence for an asteroid impact or a giant volcanic province may still be waiting to be discovered, it is also possible that neither was linked to the extinction, and that the Devonian event was triggered by processes profoundly different to those that led to later extinctions in Earths’ history like the one causing the end of the dinosaur era.

So time matters and as Dr. Lawrence Percival, lead author and a geochemistry researcher at the Université de Lausanne, said: ‘A precise new date for the Devonian extinction gives a platform for us to understand much better the events going on at the same time as this event, and that may have caused this ecological catastrophe.’

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