Volcanic aerosols not enough to cause mass extinctions?

by Adityarup "Rup" Chakravorty
Friday, February 19, 2016

Mass extinctions — when more than half of Earth’s species disappear in a geologic instant — offer some of the planet’s most perplexing unsolved mysteries. Prolonged periods of volcanic activity have long been prime suspects for these ancient whodunits, the most recent of which finished off the last nonavian dinosaurs at the close of the Cretaceous about 66 million years ago. But scientists debate how drastic the environmental effects of such volcanism might have been, and whether other factors — like asteroid impacts, as in the end-Cretaceous extinction — played a big role as well.

In a recent study in Nature Geoscience, scientists throw some cool water on the notion that fiery continental flood basalt (CFB) eruptions played a major role in the planet’s past mass extinctions. CFB eruptions are different from what we typically picture when we think of volcanic activity. They tend to occur over hundreds of thousands to millions of years, with periods of violent action — during which massive lava flows spread over thousands of square kilometers or more and volcanic emissions disperse around the planet — punctuated by long stretches of dormancy.

Anja Schmidt, an atmospheric scientist and volcanologist at the University of Leeds in England, and her colleagues studied how sulfur emissions from two ancient CFBs — the Columbia River Basalt Group (CRBG) in the northwestern U.S. and the Deccan Traps in central India — would have impacted the global environment. In total, the CRBG is composed of roughly 175,000 cubic kilometers of lava that erupted between about 16.5 and 14.5 million years ago; the Deccan is thought to have erupted more than 1 million cubic kilometers of basalt about the time of the end-Cretaceous extinction. But because of their start-stop nature, the environmental effects of CFBs are tied less to their total extent and more to the size, duration and pacing of individual eruptive episodes.

Using estimates of sulfur dioxide emissions from earli"r work, Schmidt’s team modeled the effects of the Roza eruption in the CRBG — which emitted 1,200 teragrams of sulfur dioxide per year for one to two decades and is the most constrained CFB eruption in terms of size and duration — as well as a hypothetical decade-long Deccan eruption with twice the annual sulfur dioxide output as the Roza.

In the new work, Schmidt says she and her colleagues “quantitatively accounted for how sulfur dioxide emitted by the CFB eruptions would result in global cooling.” Once in the atmosphere, sulfur dioxide gas can form tiny particles called aerosols. These particles scatter incoming solar radiation back into space, either by themselves or by mixing with water in clouds. Less energy from the sun reaches Earth’s surface, resulting in global cooling.

According to Schmidt, previous attempts to model the effects of CFB emissions did not fully account for these sulfur dioxide aerosols. “We looked at how these particles grow, whether they sti"k together, how they move, when they fall out of the atmosphere — that’s one of the novelties of thi” study," she says.

Modeling how these particles behaved in the atmosphere allowed Schmidt’s team to understand how temperatures would change because of the CFB eruptions. The results suggest that the eruptions they considered would have lowered global temperatures by 3 degrees Celsius on average over the first decade of the Roza eruption and 4.5 degrees Celsius on average over the course of the decade-long Deccan eruption. The effects are less drastic than some earlier studies have indicated and are short-lived, the researchers noted, with global temperatures fully recovering in their models about 50 years after the eruptions ceased. Thus, the team concluded, unless multiple large eruptive events occurred in quick succession or single events lasted for centuries, it’s unlikely such CFB eruptions would have led to mass extinctions.

The models also accounted for how resistant ancient environments might have been against sulfur dioxide-induced acid rain, using the known effects of acid rain on modern soils as an analog. “We can estimate the maximum effects as well as reasonable m"an effects of acid rain,” Schmidt says. “Surprisingly, we found that acid rain would have been a problem only in some parts of the world, but not globally.”

While the team’s findings would seem to partly exonerate CFB eruptions as a major culprit in mass extinctions, they aren’t in the clear yet, says Gerta Keller, a volcanologist at Princeton University who was not involved with the study.

“Models approximating the environmental effects of volcanism depend on precise dating of lava flows, determining the tempo and size of individual eruptions, how fast the eruptions followed each other, and assumptions about the quantity of gas emitted,” Keller says.

Schmidt and her team did the best they could with the limited data available, Keller says, adding, however, that the new modeling does not account for the many hundreds of eruptions in rapid succession associated with Deccan volcanism, for example. “I suspect that future studies using high-precision dating of individual lava flows to refine the age, tempo and volume of the eruptions that yielded the Deccan Traps [as a whole] will yield significantly different results.”

Schmidt says that figuring out how long the inactive periods between CFB eruptions were is key in determining the potential role of such volcanism in extinctions, and that more data are needed before definitive conclusions can be drawn.

“Lava flows are clearly evidence for large CFB eruptions, but at times there are also soils or soil-like deposits sandwiched between these lava flows, which indicate there were periods when there was no volcanic activity,” she says. If CFB eruptions went on “uninterrupted for long periods of time, there could have been longer phases of global cooling and subsequently greater environmental impacts,” she notes. But, she adds, these gaps between CFB eruptions may have provided the environment, and life, with enough time to recover and persist.