The late Permian extinction, which kicked off roughly 250 million years ago, has a rather gruesome nickname: the Great Dying. Over 90 percent of the species in the oceans went extinct in the geological blink of an eye, and similar devastation took place on land. It's about as close as we've come to having multicellular life wiped out. The timing of the event coincides with a volcanic outburst that covered an area the size of Western Europe in volcanic rock. That might be enough to trigger a major catastrophe on its own, but new research indicates that the hot magma ignited coal deposits, sending toxic coal ash into the oceans.

The remains of these eruptions, called the Siberian Traps, now cover about 2 million square kilometers of Russia. The rock formation is what's called a flood basalt, thought to be caused by a plume of hot mantle breaking through to the surface. The Siberian Traps may be the largest event of this sort we know about, and the dimensions are staggering: over 1,000 Gt (Gigatonnes) of magma were released during the eruptions that created them, and they are thought to have put material into a plume that rose over 40 kilometers into the atmosphere.

The effects on life were devastating. Massive ash falls, huge changes in the carbon cycle, ocean acidification, and climate change all accompanied the eruptions, and all of those are capable of pushing species to extinction. But is that really enough to account for wiping out over 90 percent of the life in the oceans?

Some researchers don't think so, and have focused on a secondary effect of the eruptions: burning coal. There is evidence that the hot magma intruded into large deposits of coal found in Siberia and set it alight. Some estimates suggest that over 3 trillion tons of carbon could have been placed into the atmosphere through the burning of coal alone (that's in addition to the carbon dioxide released by the volcanism proper). That release would come in the form of methane, a potent greenhouse gas. Methane is rapidly oxidized into carbon dioxide, which could then contribute to ocean acidification.

As if all of that weren't enough, the new paper, published in Nature Geoescience, indicates that the Siberian Traps eruptions might have added another insult to the oceans: toxic coal ash. The authors examined deep ocean sediments from a site that was off the west coast of the supercontinent at the time. To get there with the prevailing winds, material from the eruption would have to travel around the globe, a distance the authors estimate as more than 20,000 kilometers. And yet the sediments contain organic material that, under the microscope, looks remarkably similar to coal ash obtained from a modern power plant.

The authors were able to detect three pulses of this material derived from coal burning in the half-million years before the onset of the Great Dying, with the third and most significant ending just as marine life collapsed. Each of them were associated with changes in the carbon cycle, either resulting from the large release of the eruptions themselves, or the burning of organic materials that continued in their wake.

This combustion material is known to stress aquatic ecosystems in two ways. To begin with, the coal ash will block enough sunlight to inhibit photosynthesis, a major source of the ocean's dissolved oxygen. That creates anoxic conditions, and isotope ratios confirm that the oceans probably experienced an anoxia event that coincided with the last major eruption. In addition, coal ash carries toxic metals with it; levels of chromium increased with each of the eruptions, and peaked with the third and final one. Thus, the coal ash itself probably contributed directly to the conditions that were so harmful to life in the oceans.

It's clear from this data that the formation of the Siberian Traps created conditions that would severely stress life in the oceans through a variety of mechanisms. Although problems would obviously have started before the Great Dying itself, the final, critical event seems to have been the most severe. The preponderance of evidence clearly links the extinction to the eruptions that made Traps themselves.

The biggest open question is whether all the factors were global in nature. Diffusion of gasses is rapid, so issues like ocean acidification would clearly have a global impact. But the spread of fly ash is very dependent on the prevailing winds, and the site examined by the authors is at roughly the same latitude as the eruptions themselves. Sampling of sites further removed from the Siberian Traps may indicate whether what the authors see there (which they refer to as "Catastrophic dispersion of coal fly ash" in the title) was a truly global catastrophe.

Nature Geoscience, 2011. DOI: 10.1038/NGEO1069 (About DOIs).

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