MIT researchers use geochronology to link Earth's most severe extinction to one of its most voluminous volcanic events.
Complex life has inhabited our Earth for roughly 550 million years. Since this time our biosphere has evolved greatly and endured much, the result of which is the Earth we currently inhabit and enjoy. The evolutionary arc of Earth’s biosphere is affected by a multitude of environmental factors, which serve to make survival easier for some organisms and harder for others. The gradual ebb and flow of organism radiation and demise can be tracked throughout the fossil record. However, at five discreet instances in the last 550 million years, this trend was not at all gradual – at these points in Earth history, organism demise far outstripped radiation, such that the majority of life on Earth was rendered extinct. Geologists call these events “Mass Extinctions,” and they represent fundamental inflection points in the evolutionary trajectory of life on Earth. The most severe of these mass extinctions occurred over a maximum duration of 60 thousand years at the end of the Permian period, roughly 252 million years ago. This geologically abrupt event killed over 90% of ocean life and over 75% of life on land – the most catastrophic loss of biodiversity ever recorded. So what was responsible for this environmental cataclysm? According to recent MIT graduate and the lead author of a new study in Science Advances, Dr. Seth Burgess, the answer to that question will help solve “the most compelling murder mystery in Earth history.”
The Earth at the time of the end-Permian extinction was characterized by abnormally high ocean and atmosphere temperature, an acidic ocean, and an abrupt and global perturbation to the carbon cycle, all of which suggest rapid and voluminous addition of greenhouse gasses to the atmosphere. These records indicate that whatever triggered the mass extinction must be rapid, and capable of generating incredible amounts of greenhouse gas, namely carbon dioxide and methane. The lead suspect, and the one on which Burgess and his colleagues focus, is the Siberian Traps large igneous province. Preserved in Siberia, the Traps are the remains of the largest continental volcanic event in Earth history. The combined present-day volume of erupted and intruded (into the shallow crust) rock is conservatively estimated at ~ 4 million cubic kilometers – that’s enough to cover a piece of land the size of the U.S. in nearly a half kilometer of lava. These rocks release greenhouse gasses when erupted, and so do the sediments they contact and cook during ascent through the crust. It’s these gasses that many researchers think are the key link between magmatism and mass extinction. Previous work indicates that the Siberian Traps has the potential to release the quantity of gas necessary to trigger extinction, but one major question needed answering before magmatism could be pinned as the culprit. Is the timing right? That’s where Burgess and Bowring have focused.
Working in the MIT radiogenic isotope lab, they have used U/Pb geochronology to date erupted rocks (lavas) and intruded rocks (sills) from the Siberian Traps. With this dataset, according to Burgess, “we can constrain the timing and tempo of magmatism, and have a representative picture of what was occurring when.” This information is powerful, as the efficacy of magmatism as a trigger for extinction depends on the relative timing of the two events and how quickly gasses were generated. Having previously dated the mass extinction interval in the same lab (Burgess, Bowring, Shen, PNAS, 2014), Burgess and Bowring were able to directly compare their results from the two disparate events, thereby eliminating any lab-based or technique-based bias, thus maximizing the age precision with which the timing of each event is known. In their current paper Burgess and Bowring present a timeline for magmatism that begins with explosive eruption ~ 300 thousand years before the onset of extinction, followed by effusion of lavas and concurrent emplacement of sills. They suggest that ~ 2/3 of the total volume of rock was erupted/emplaced prior to and during the mass extinction, with the other 1/3 following extinction and persisting for another few hundred thousand years. What emerges from their timeline is a relatively protracted (~ 1 million years) period of active magmatism, with short-lived, immensely high-volume pulses possible within this interval.
This new study by Burgess and Bowring resolves the relative timing of magmatism and mass extinction and shows that, as has been suspected for decades, the timing permits a causal connection. Andrew Knoll, Fisher Professor of Natural History at Harvard, who was not involved in this study, comments, “They say that good ideas must pass the test of time. As Burgess and Bowring show, the hypothesis that massive volcanism caused the great end-Permian extinction quite literally passes the test of time as revealed by high-precision geochronology – a beautiful study and important for our understanding of life’s turbulent history.”
The next step, says Burgess, is to “focus on finding which part of the magmatism did the most damage and why.” Indeed, as one question is answered in this mystery, many more will crop up. For now, more attention then ever will be placed on the Siberian Traps, as its presence at the time of the murder has been proven.
Thanks to Seth Burgess PhD '14(XII) for help preparing this article.
A central theme of Sam Bowring’s work is using high-precision U-Pb zircon geochronology to examine rates of geological and biological processes in deep time. Areas of particular interest include the earliest history of the Earth, the origin of continental crust and thermal evolution of orogenic belts, and the age and duration of major extinction events. His work also extends to environmental applications of tracer isotopes using the isotopic composition of Pb, Sr, and U in natural waters to examine mixing and the fate and transport of contaminants.
Bowring Lab alum Seth Burgess Phd '14(XII) is currently a Mendenhall Postdoctoral Fellow at the United States Geological Survey in Menlo Park, CA, where he is developing a refined approach for dating Quaternary tephras by combining in-situ U-Th and U-Pb analyses on the SHRIMP-RG with (U-Th)/He dates using a multicollector noble gas mass spectrometer.
Burgess and Bowring (2015) High-precision geochronology confirms voluminous magmatism before, during, and after Earth’s most severe extinction, Science Advances, Vol. 1, no. 7, e1500470, doi: 10.1126/sciadv.1500470
Mountain composed entirely of Siberian Traps lava flows, one on top of another, being eroded away into a passing river in central Siberia - Image credit: Henrik Svensen
View from the top of an enormous sill, exposed after 250 million years of erosion. In the distance can be seen table-top flat mountains in the distance, all composed of Siberian Traps lava flows and sills - Image credit: Seth Burgess