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Long-Term Drought Doomed Indus Valley Civilization, Researchers Say
The decline of Bronze-Age civilizations in Egypt, Greece and Mesopotamia has been attributed to a long-term drought that began around 2000 bc.
Now palaeoclimatologists propose that a similar fate was followed by the enigmatic Indus Valley Civilization, at about the same time.
Based on isotope data from the sediment of an ancient lake, the researchers suggest
that the monsoon cycle, which is vital to the livelihood of all of South Asia, essentially stopped there for as long as two centuries.
The Indus Valley, in present Pakistan and northwest India, was home to the civilization known as the Harappan Civilization.
It was characterized by large, well-planned cities
with advanced municipal sanitation systems
and a script that has never been deciphered.
But the Harappans seemed to slowly lose their urban cohesion, and their cities were gradually abandoned.
The link between this gradual decline and climate has been tenuous because of a dearth of climate records from the region.
So Yama Dixit, a palaeoclimatologist at the University of Cambridge, UK, and her colleagues examined sediments from Kotla Dahar, an ancient lake near the northeastern edge of the Indus Valley area in Haryana, India, that still seasonally floods.
The team assigned ages to sediment layers using radiocarbon dating of organic matter.
In various layers, they collected the preserved shells of tiny lake snails (Melanoides tuberculata), which are made of a form of calcium carbonate (CaCO3) called aragonite.
The team also looked at the oxygen in the argonite molecules, counting the ratio of the rare oxygen-18 isotope to the more prevalent oxygen-16.
Two-hundred-year hiatus
Kotla Dahar is a closed basin, filled only by rain and runoff and without outlets. Thus precipitation and evaporation alone determine its water volume. During drought, oxygen-16, which is lighter than oxygen-18, evaporates faster,
so that the remaining water in the lake and, consequently, the snails' shells, become enriched with oxygen-18.
The team's reconstruction showed a spike in the relative amount of oxygen-18 between 4,200 and 4,000 years ago.
This suggests that precipitation dramatically decreased during that time. Moreover, their data suggests that the regular summer monsoons stopped for some 200 years.
The result, reported last week in Geology,
supports the idea that monsoon failure led to the civilization’s decline, although David Hodell, a co-author of the study and a palaeoclimatologist also at the University of Cambridge,
hastens to add that uncertainties in the shell and archaeological records mean that the dates could be off by some 100 years in either direction.
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