About 250 million years ago, one of the largest known volcanic events in history flooded the area that is now Siberia with lava, to create the Siberian flood basalts. A superheated mantle plume beneath the continent likely created the event, which erupted continuously for more than 1 million years. It led to the end-Permian mass extinction, wiping out more than 70 percent of all terrestrial species and 96 percent of all marine life. While scientists agree that the massive eruptions caused tremendous and rapid changes to the environment, there is ongoing debate about exactly why they were so destructive.
DCO researchers propose that the erupted Siberian flood basalts released giant quantities of halogens, such as chlorine and bromine, into the atmosphere. These halogens may have eroded the ozone layer, allowing UV radiation from the sun to inundate Earth. Reservoirs and Fluxes Community member Michael Broadley (Centre de Recherches Pétrographiques et Géochimiques, France), and Deep Energy Community members Peter Barry and Chris Ballentine (both at University of Oxford, UK), examined the halogen content of rock samples that surfaced in Siberia before and after the eruption of the flood basalts. Their results, reported in a new paper in the journal Nature Geoscience , suggest that massive halogen release to the atmosphere may have kicked off the largest mass extinction in Earth’s history.
“Given the size of the Siberian flood basalts, the effect the eruptions had on life at that time is much larger than expected,” said Broadley. “So there must be another source of volatiles driving the environmental change and the associated mass extinction.”
Halogens are an unusual group of elements in that they are abundant and highly reactive on Earth’s surface, but are fairly inert in the mantle where they occur in trace amounts. These characteristics make halogens an excellent tracer for understanding how surface material enters into the mantle through subduction and later returns to the surface via volcanism.
Broadley and his colleagues quantified halogen concentrations in xenolith rocks, which get transported to Earth’s surface along with magma during an eruption. These xenoliths came from two Siberian kimberlite pipes, which are a bizarre type of volcanic eruption that starts deep in the mantle and quickly blasts through continental crust, often carrying diamonds and other deep minerals. The Udachnaya kimberlite eruption occurred 360 million years ago, before the Siberian flood basalts, and the Obnazhennaya eruption occurred 160 million years ago, after the end-Permian extinction.
The researchers discovered that the older xenoliths were loaded with halogens and had other characteristics signifying that they came from pieces of halogen-enriched crust that had sunk into the mantle. The younger xenoliths, however, had far lower halogen concentrations. The difference suggests that during the Siberian flood basalts, the mantle material picked up halogens stored in the solid outer part of Earth, called the lithosphere, and deposited them at the surface.
“This may have had a huge effect on the environment and climate at that time,” said Broadley.
Halogens catalyze reactions that split ozone into oxygen and so this rush of halogens likely would have decimated the ozone layer. The effects were much like the ozone hole created by the use of chlorofluorocarbons (CFCs) in the 1980s, but on a much larger scale. These findings align with observations that plant spores preserved from the end of the Permian period appear to be malformed and mutated, as though exposed to UV light.
Terrestrial species disappeared initially, followed by marine ones. The authors suggest that the burst of halogens may have been the first of a one-two punch from the eruption. Later, as the eruption liberated carbon dioxide from carbon-rich sediments, the climate likely warmed, which may explain the marine die-offs.
The researchers don’t yet know whether the vast store of halogens in the lithosphere below Siberia is unusual, or if there are other enriched locations on Earth. One possible explanation is that this section is crust is incredibly old, dating back to the Archean era, and so it had hundreds of millions of years to accumulate subducted halogens before the eruption of the Siberian flood basalts.
The project initially came about when Broadley, then a graduate student in the lab of Ray Burgess and Chris Ballentine at the University of Manchester, UK, met Barry at Goldschmidt 2013 in Florence, Italy. Barry and his coworkers had already made multiple analyses of the samples [2,3,4], which pointed to a subducted origin, and wanted to confirm by testing their halogen content. “I had no idea what kind of halogen signatures we would find within these samples when we started, but based on previous work we thought it would be unusual,” said Broadley.
Now, Broadley has begun looking at diamonds brought up in these kimberlite pipes to see if they have the same halogen-rich signature. The findings will reveal whether the diamonds also formed from the same subducted crust materials as the xenoliths in the current study.
Image: The eruption of the Siberian flood basalts created a giant plateau that is still visible across northern Russia today. Credit: Photo by Olga Chumachenko via Wikimedia Commons