Erupting volatiles may have caused Earth’s deadliest mass extinction

Around 250 million years ago, at the boundary between the Permian and Triassic periods, most species on Earth were decimated. The prevailing hypothesis pins the blame on a massive eruption of molten basalt, known as the Siberian Traps, that flooded an area roughly the size of Greenland. During the million-year incident, carbon dioxide, sulfur, water, and other volatiles spewed into the atmosphere. The resulting sharp fluctuations in global temperature and UV radiation, compounded with protracted ozone depletion, would have substantially stressed plants and animals, possibly to the point of death. To bolster that hypothesis, researchers have sought a source of the volatiles in the flood basalt that could have caused the extinction.

Now Michael Broadley at the Center for Petrographic and Geochemical Research and his colleagues may have found it: A magma plume that was propelled through the mantle incorporated enough volatiles to provoke Earth’s biggest extinction. The researchers measured the concentrations of halogen elements chlorine, bromine, and iodine in two Siberian formations of peridotite, an igneous rock widespread in the upper mantle. One formed 360 million years ago, before the mass extinction. The other, shown in the figure, formed 160 million years ago. The enriched halogen composition of the preextinction peridotite matches that of the metamorphic rock eclogite, which forms when oceanic crust teeming with halogen-rich seawater subducts deep into the mantle. Together with previously published helium isotope data, the new measurements suggest the preextinction peridotite mixed with some eclogite. In contrast, the postextinction peridotite was depleted in halogens.

The discrepant compositions of the two formations suggest that the upwelling magma plume that fueled the Siberian Traps eruption acquired halogens on its way up through the mantle—only to expel them into the atmosphere when it reached the surface. Broadley and colleagues used their data to estimate the volume of Cl that could have degassed during the eruption. By their calculations, the amount of Cl ejected into the atmosphere by the Siberian Traps episode was about 300 000 times that ejected by the Mount Pinatubo eruption of 1991, which reduced global ozone by 15–20%. That’s enough volatiles to cause mass extinction. (M. W. Broadley et al., Nat. Geosci. 11, 682, 2018 .)