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Not Toba's Fault

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malawibarge.jpgBY CHERYL REITAN

Tom Johnson, a University of Minnesota Duluth Regents professor and Institute on the Environment resident fellow, knew his work on Lake Malawi in 2005 would yield significant scientific discoveries. Now, eight years later, he and his colleagues have announced research that impacts our knowledge of the near extinction of the human race. They have determined that 75,000 years ago, the Toba volcanic eruption in Sumatra did not cause a volcanic winter or the dramatic drop in human population in Africa, as some anthropologists had proposed.
Johnson and colleagues from UMD's Large Lakes Observatory and an international team of scientists extracted ancient sediment in 2005 from two sites under Lake Malawi, the second largest lake in the East African Rift Valley. The lake sediment in the drilling samples accumulated over 1.2 million years, providing the longest undisturbed history of the environment and climate yet obtained for the African continent.

In October 2010, Johnson arranged for UMD graduate student Ben Chorn to take a two-weektomandrillers.jpg course with Christine Lane, a geographer at the University of Oxford, to examine Lake Malawi sediment samples from 75,000 years ago, looking for the Toba ash. Following this training, Chorn returned to UMD to carry on the search, with frequent communication with Lane.  

After more than a year of analyzing a number of volcanic ash layers derived from volcanoes in nearby Tanzania, they found a specific volcanic ash from the Toba explosions on the Indonesian island of Sumatra.  This was the largest volcanic eruption of the past 2.5 million years, thousands of times greater than Krakatoa (1883) or Pinatubo (1991). Evidence of Toba's giant plume of ash was previously found from the South China Sea to the Arabian Sea, but never before in Africa.

About the same time as the Toba eruption took place, genetic evidence points to a drop in the number of our species, Homo sapiens, in Africa. Lake Malawi is located in the East African Rift Valley, where our ancestors emerged about 200,000 years ago. "Something created a population 'bottleneck' around 75,000 years ago, at the time of the Toba eruption," said Johnson. The human population dropped cataclysmically to near-extinction levels, some estimating the remaining population consisted of only 1,000-10,000 individuals. For decades, many scientists have embraced the theory that most of the people in Africa were killed when Toba's eruption caused a global volcanic winter where sunlight was obstructed, plants died, and the planet cooled.

Lane, Chorn and Johnson have disproved that theory. They published the paper, "Ash from the Toba super eruption in Lake Malawi shows no volcanic winter at 75 ka," in the Proceedings of the National Academy of Sciences April 29, 2013.

llo-shards.jpgThrough examination of the sediment cores taken from Lake Malawi, Chorn found a thin layer of the distinctive glass-like shards of Toba ash. "Ben had to conduct an elaborate search, one centimeter at a time, to find the tiny shards, smaller than the diameter of a human hair," said Lane. The chemical analysis by Lane definitively ties the fragments to the Toba eruption. Now it is proven that the volcanic ash traveled 4,350 miles from its source in Sumatra, about twice the distance as previously thought.
In addition, Johnson examined the algae and other organic matter in the layer that contained the ash from Toba. The organic matter showed no evidence of a significant temperature drop in East Africa, indicating the environment recovered quickly from the atmospheric disturbance from Toba.

"There may have been effects from the Toba eruption for a few seasons, but no prolonged, substantial impact on the climate," said Johnson. If the area had seen dramatic cooling because of all the ash and sulfur spewed into the atmosphere, the lake's ecosystem would have changed measurably, significantly altering the composition of the lake's mud.

The results of this study at UMD's Large Lakes Observatory indicate that the Toba volcanic eruption did not sway the course of human history as much as some previously thought. "We can now rule out the widely touted theory that humans nearly became extinct 75,000 ago as a result of the Toba super-volcano eruption," said Johnson. "The data don't support it." Johnson said it may have been another catastrophe, such as a disease, that caused the drop in human population at about that time.

Cheryl Reitan is interim director of public relations and marketing for the University of Minnesota Duluth. Photos courtesy of Tom Johnson. A version of this article originally appeared on the UMD website. Reprinted with permission.

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This page contains a single entry by Mary Hoff published on May 7, 2013 10:35 AM.

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