In a ground-breaking discovery led by
an Indian-origin
palaeoclimatologist, researchers have found that a 200-year-long monster
drought nearly 4,200 years ago doomed
the Indus Valley civilisation in present Pakistan and northwest India.
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 to wipe out the Indus Valley civilisation — also knows as the
Harappan civilisation.
“The Indus Valley was characterised 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,” explained Yama
Dixit, a palaeoclimatologist at University of Cambridge.
The team examined sediments from Kotla Dahar — an ancient lake near the
northeastern edge of the Indus Valley area in
Haryana — 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 which are made of a form of calcium carbonate 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.
Kotla Dahar is a closed basin, filled only by
rain and runoff and without outlets. 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.