The earth was hit by major volcanic events 183 million years ago that caused climatic and biological critical perturbations, termed the Toarcian Oceanic Anoxic Event, leading to some of the most devastating extinction events in the planet’s history.
At that time, large-scale volcanism was taking place in southern Africa, Antarctica, and Australia, which is known as the Karoo-Ferrar Large Igneous Province. Also, significant ocean acidification, widespread ocean anoxia and seawater warming took place, leading to major extinctions in the oceans.
Large Igneous Provinces (LIPs) represent an extreme type of volcanism that occurs on earth on average every 20-30 million years.
It was commonly thought that the cause of the Jurassic volcanism is to be linked to the ‘upwelling’ of magma from deep in the earth’s interior, which is called a mantle plume. When this magma reaches the base of the continent, it is assumed that it will relatively quickly erode (or melt) the continent from below to allow for volcanic activity to take place at the surface.
But, a team of researchers led by Micha Ruhl, the Assistant Professor of Sedimentology in Trinity’s School of Natural Sciences challenges this idea suggesting that a slowing of continental plate movement may have been the most significant driver for the devastating volcanic events.
The paper was published on 9 September in Science Advances.
A new model
Findings of the study suggest that there is a direct link between greenhouse gas release from volcanism, and climatic and environmental perturbations that can cause mass extinction. It also provides a new way to explain why some of the largest volcanic events in Earth’s past occurred, and what controlled their duration and termination.
The team collected sediments that formed on the seabed during the Toarcian Oceanic Anoxic Event and show highly elevated levels of Hg, orders of magnitude higher than sediments that formed before and after. They then concluded that there was a direct temporal link between volcanism, and associated greenhouse gas release, and climatic/environmental change at that time.
However, when they compared the age of volcanic activity with models of plate reconstruction (these are models that reconstruct where the continents were in the past), Ruhl and his team observed that the Karoo-Ferrar Large Igneous Province, but also several other Large Igneous Provinces in the earth’s past (e.g. the Siberian Traps, which are linked to the End-Permian mass extinction, the East African Rift volcanism) all coincide with a significant reduction in the velocity of the continents, to less than 2 centimetres per year.
“This allows us to present a new model to explain the timing of these large volcanic events, in which not the arrival of a mantle plume at the base of a continent, determines the onset of volcanism, but rather the slowdown of the continent, which allows for the mantle plume to longer thermally erode the base of the continent and ultimately lead to surface volcanism,” he explains. “We don’t know exactly why continental plates slow down at some times and speed up at others, but it is likely linked to global plate tectonic forces which are in turn linked to large-scale (global) patterns in mantle convection.”
The authors presented a very strong case for increased Hg loading most likely derived from extensive volcanic outpourings during the Toarcian. This in itself makes it a very important study, according to professor of geology at the University of Copenhagen, Denmark, Christian J. Bjerrum.
He said that the team presented an unprecedented thorough analysis of Hg together with a multi-element record. This analysis permits unique insight into the Hg loading and the timing of the volcanic sources.
For Bjerrum, the most surprising thing about these results is the link between plate motion slowing down and huge volcanic outpouring and carbon cycle change. “It makes perfect sense, and is simple and elegant – yet it requires careful analysis and a keen eye to see the dots line up.”
Interesting results, more work is needed
But the professor of geology believes that despite the great progress this study has made, it is still not clear why the Hg loading seems controlled by different mechanisms up through time – so there is some work needed. “Also, it is still a question why this Toarcian event only caused a so-called “second-order mass extinction”, while the plate movement lull at the Permian-Triassic boundary is related to the mother of all extinctions.”
Richard Ernst, the earth scientist in residence at Carleton University, Ottawa, Canada, called the hypothesis of the paper very interesting – of the importance of a drop in plate motion to allow the plume to more efficiently erode and thin the lithosphere to allow a LIP event to occur.
He added that the authors analyse the Karoo-Ferrar and three other cases, Siberian Traps, North Atlantic LIP and East African LIPs, but there is one counter-example that they should have commented on.
“The Deccan LIP was emplaced into the Indian craton during very rapid movement of India to the north— and this Deccan LIP had devastating environmental consequences—along with the Yucatan meteorite impact caused the end-Cretaceous extinction and end of the dinosaurs,” he explains.
Ernst believes that the authors have introduced a new insight about slowing of plate motion being an important control on the timing of plume-generated LIPs—but the Deccan example would indicate that it is not the only control on the timing of plume-generated LIPs. “If the lithosphere is already thin then once a plume arrives the LIP will develop, even for fast moving plates.”
The lead author explained that the work was conducted as part of a large, international research programme, the Early Jurassic Earth System and Timescale (JET) project aims to understand the causes and consequences of changes in the biological, geochemical and physical processes within the Earth system in the Early Jurassic. So, “work in this research programme is ongoing and the current paper is a contribution to that.”