Stalagmites hold clues to the Fall of an Empire

A recent study by researchers at Oxford Earth Sciences has revealed a record of changing climatic conditions and heightened dust activity that may be linked to the abandonment of ancient Mesopotamian settlements, 4,200 years ago. These findings were published in December 2018 in Proceedings of the National Academy of Sciences of the United States of America, and summarised in an Eos article by Freelance Journalist Katherine Kornei (@katherinekornei), excerpts of which are included below. For the full article, please click here.

Forty-two centuries ago, highly advanced city-state settlements in Northern Mesopotamia, an area once imperialized by the Akkadian Empire, were suddenly abandoned. Paleoclimatologists and other geoscientists now have one possible explanation for why. Using precisely dated chemical measurements from a stalagmite collected in a cave in Iran, researchers found an abrupt increase in Mesopotamia-sourced dust at that point in history. The associated heightened aridity, which persisted for 300 years, might have made for uncomfortable living conditions and difficulties in rain-fed agriculture, the researchers suggest.

“There’s this specific point in time where everything just disappears [from the archaeological record],” said Stacy Carolin, a paleoclimatologist who led this research while at Oxford University in the United Kingdom and is currently at the University of Innsbruck in Austria. Sediment cores obtained from the Gulf of Oman have suggested that increased dust activity in Mesopotamia might have been the culprit, but the imprecise ages of marine records make it difficult to definitively link the geophysical and archaeological evidence, said Carolin. “Unless you know the exact timing…you can’t say anything about a cause and effect relationship.”

Carolin and her colleagues, including Richard Walker, Chris Day and Gideon Henderson from Oxford Earth Sciences, as well as geochemists and members of the Iranian Cave and Speleology Association and the Geological Survey of Iran—have now analyzed the chemical composition of an 18-centimeter-long, semitranslucent stalagmite from Gol-e-Zard (Yellow Flower) cave near Tehran to look for signatures of dust. Because stalagmites slowly build up over time as dissolved minerals percolate through rock and drip from cave ceilings, they’re like clocks recording local environmental conditions. Using uranium-thorium radiogenic age dating, Carolin and her team found that their stalagmite started growing about 5,200 years ago. The researchers estimated an age uncertainty of 31 years for their measurements, far more precise than the centennial-scale resolution of other studies.

Photograph taken from outside the cave where the stalagmite sample was collected. The cave name is Gol’e Zard, which means Yellow Flower Cave. Photo credit: Morteza Talebian

The researchers found an increase in magnesium, a component of dust, in the stalagmite beginning 4,260 years ago (in a twist of geophysical convention, all dates are referenced as years before 1950). This dust likely derived from the alluvial plains of the Tigris and Euphrates Rivers—that is, the location of the Akkadian Empire—and was transported by prevailing winds, the researchers suggest. High levels of dust might have caused the Akkadian Empire settlements to be abandoned, the team concluded. “Dry, arid conditions could have been detrimental to agricultural settlements,” said Carolin. “The dustiness may have made living conditions unbearable as well.”

The stalagmite records exhibited heightened dust activity for the following 290 years, the researchers showed, remarkably consistent with the 300-year interval of abandoned settlements revealed in the archaeological evidence. These findings were published in December 2018 in Proceedings of the National Academy of Sciences of the United States of America. The data are suggestive of a link between the geophysical and archaeological records, but it’s hard to know for sure, said Carolin. “The idea that there’s a relationship between climate and society is a provocative one.”

Other researchers are supportive of a real cause and effect correlation, however. This work “provides another line of evidence supporting the idea of a link between climate variability and societal change,” said Matthew Lachniet, a paleoclimatologist at the University of Nevada, Las Vegas, not involved in the research.

Carolin is currently analyzing other Iranian stalagmite samples that afford a deeper look into the past. She and her colleagues are looking at climate change over ice age cycles on the timescale of centuries. “We are interested in reconstructing the climate state of this region…when our human ancestors were migrating out of Africa and into Asia through southern Iran.”

Citations:

Kornei, K. (2019), The Akkadian Empire—Felled by dust?, Eos, 100, https://doi.org/10.1029/2019EO115307. Published on 31 January 2019.