The Dark Ages May Have Really Been Dimmer

Ken Wohletz said an eruption in the Indonesian archipelago could have produced a 150-meter-thick cloud layer over the entire Earth, triggering a chain of climatic, agricultural, political and social changes that ushered in the Dark Ages.

Evidence supporting the catastrophe includes tree-ring and ice-core measurements, indications of a huge underwater caldera, and ash and pumice in the same area, said Wohletz, who discusses his work modeling such an eruption today (Dec. 17) at the fall meeting of the American Geophysical Union.

The 6th century was a turbulent, unsettling period in human history. The Roman Empire began to fall; nomads of central Asia migrated to Europe and the Near East; civilizations in Persia, Indonesia and South America collapsed; major religions experienced considerable change as natural events were viewed as omens.

Many of these social transformations resulted from widespread crop failures and the explosion of plague around the globe, which in turn were caused by major climatic changes, Wohletz said. Beginning in about the year 535, according to historical and archeological records, the weather was colder and drier, sunlight diminished, snow fell in summer and regions of persistent drought suffered floods.

Wohletz was a resource for a book postulating that the climate changes resulted from a huge volcanic eruption. The book, "Catastrophe: A Quest for the Origins of the Modern World" by David Keys, was published earlier this year.

Wohletz said he worked with Keys to try to identify a volcano that could produce such dramatic climate change. "We came up with an eruption that would certainly be the largest in recorded history, some four or five times bigger than the (1815) eruption of Tambora, which is usually considered the biggest eruption in the past few millennia," he said.

Such an explosion, he said, would eject some 200 cubic kilometers of material, and one-third to one-half of it would be lofted into the stratosphere, where it would remain suspended for months to years while being carried around the globe.

"It would have produced enough dust and water vapor (in the form of ice crystals) to form a cloud layer 150 meters thick over the entire globe, and that's a conservative estimate," he said, adding that a cloud of particles that thick may have diminished the transmission of sunlight by as much as 50 percent.

Wohletz said tree-ring data collected around the world and ice-core measurements in Greenland and Antarctica support the possibility of a huge eruption in the 6th century. Ocean depth measurements between Sumatra and Java ­ where Krakatoa exploded in a well known 1883 eruption ­ indicate the presence of a caldera up to 50 kilometers in diameter, and a recent survey uncovered evidence of ash and pumice layers formed in the area during the appropriate time frame.

Under a likely scenario, a large volcano, which Wohletz calls proto-Krakatoa, connected the islands of Sumatra and Java. When it erupted and then subsided, it created the Sundra Strait and left a ring of smaller volcanoes, including the present day Krakatoa. The ash, dust and water vapor blown into the stratosphere would disperse across both the Northern and Southern Hemispheres.

"This volcano would have had the potential to be a major player in destabilizing the climate around the world," he said. "An eruption that could produce a caldera 50 kilometers across would have been big enough."

Although definitive evidence for such a catastrophic eruption has not been discovered, the possibility deserves a full-scale field study, Wohletz said, in part because of the potential impact on the world if another such catastrophe happens.

"(Key's book) is the first detailed account of how closely humanity is linked to the natural world," he said. "If the natural world goes through some large upheaval, we'll all be affected."

Los Alamos National Laboratory is operated by the University of California for the U.S. Department of Energy.