No, ‘volcanic winter’ didn’t decimate humans in E. Africa

(Credit: Luke Mackin/Flickr)

The massive Toba volcanic eruption on the island of Sumatra about 74,000 years ago didn’t bring about a six-year-long “volcanic winter” in East Africa that caused the human population in the region to plummet, a new study reports.

The findings run counter to the Toba catastrophe hypothesis, which says the eruption and its aftermath caused drastic, multiyear cooling, and severe ecological disruption in East Africa.

“This is the first research that provides direct evidence for the effects of the Toba eruption on vegetation just before and just after the eruption,” says Chad L. Yost, a doctoral candidate in the University of Arizona geosciences department and lead author of the study in the Journal of Human Evolution. “The Toba eruption had no significant negative impact on vegetation growing in East Africa.”

“That a singular event in Earth history 75,000 years ago caused human populations in the cradle of humankind to drop is not a tenable idea.”

Researchers can use ancient plant parts that wash into and accumulate on the bottoms of lakes to reconstruct a region’s past ecosystem. Yost and colleagues studied microscopic bits of plants preserved in two sediment cores from Lake Malawi, which is approximately 570 kilometers (354 miles) long and is the southernmost of the East African Rift lakes.

Previous investigators found material from the Toba eruption in the Lake Malawi cores that pinpoints the time of the eruption and allowed Yost and colleagues to peer back in time 100 years before to 200 years after the Toba eruption. The team analyzed samples that represented, on average, every 8.5 years within that 300-year interval.

“It is surprising,” Yost says. “You would have expected severe cooling based on the size of the Toba eruption—yet that’s not what we see.”

The researchers didn’t find marked changes in lower-elevation vegetation post-eruption—but did find some die-off of mountain plants. Cooling from the eruption might have injured frost-intolerant plants.

Had the region experienced the drastic, multiyear cooling post-Toba suggested by the catastrophe theory, the cores would have evidence of a massive die-off of the region’s vegetation at all elevations, Yost says.

Part of the hypothesis suggests the eruption caused human populations to shrink.

“We know anatomically modern humans were living within 50 kilometers of Lake Malawi,” Yost says. “People would have been able to travel to habitats and lower elevations that had little to no cooling effect from the Toba eruption.”

Most of the region’s known archaeological sites are from low elevations, not the mountains.

“That a singular event in Earth history 75,000 years ago caused human populations in the cradle of humankind to drop is not a tenable idea,” says coauthor Andrew S. Cohen, professor of geosciences.

The Lake Malawi Drilling Project took the cores from the lake bottom in 2005. The lake is one of the deepest in the world–material archived in the cores goes back more than 1 million years.

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Plant and animal material washes into lakes and is deposited on the bottom in annual layers, so a sediment core contains a record of the past environments of a lake and of the surrounding land.

Yost studied two cores taken from the lake: one from the north end of the lake, which is closer to the mountains, and the other from the central part of the lake. Other researchers had pinpointed what layer in those cores had glass and crystals from the Toba eruption, Cohen says.

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Yost took samples from the cores that straddled the eruption and analyzed the samples for charcoal and for silica-containing plant parts called phytoliths.

If the Toba catastrophe hypothesis is true, the massive die-off of vegetation would have resulted in more wildfires and therefore more charcoal washing into the lake. However, researchers didn’t find an increase in charcoal outside the range of normal variability in the sediments deposited after the eruption.

“We determined that the Toba eruption had no significant negative impact on vegetation growing in East Africa,” Yost says. “We hope this will put the final nail in the coffin of the Toba catastrophe hypothesis.”

Lily J. Jackson of the University of Texas, Austin, and Jeffery R. Stone of Indiana State University are coauthors of the study. The National Science Foundation and the International Continental Scientific Drilling Program funded the research.

Source: University of Arizona