The Indus Valley Civilisation is
one of the earliest known instances of organised
urban human settlements. It flourished in the northwestern parts of the
Indian subcontinent, in the region around north Rajasthan, Haryana, Punjab, and
more prominently in the areas around the Indus river in Pakistan, for about 1,500 years between 3000 and 1500 BC.
Its most glorious phase was the 600-700
years between 2600 and 1900 BC, which saw the emergence of some of the most
modern and mature townships of that
era, like Harappa, Mohenjo-daro, and
Rakhigarhi.
The decline and disintegration of the Indus Valley Civilisation are
attributed to several reasons —
climatic, tectonic, and even social. There are varying degrees of evidence
to support these. Most scientists and archaeologists agree that the availability of abundant water was the most
crucial enabler for the sustenance of the civilisation in this region, just
as it is seen in most other ancient civilisations as well. Presence of
prominent buried channels between Fatehabad and Tohana in Haryana, and the
concentration of archaeological mounds on its bank suggest the dependence of Indus people on a perennial
source of water.
There are plenty of geological and climatic studies that point to good rainfall
patterns in that area in those times. There are a few examples of counter-evidence as well, some that show that the
civilisation flourished mostly in drier times. A group of geologists,
archaeologists and climate scientists from India, United Kingdom and France
have recently published fresh evidence in Nature Scientific Report that shows
that Indus Valley Civilisation was at its peak in the wettest phase.
These scientists, led by Yama
Dixit of Godwin Laboratory for Palaeoclimate Research at the Department of
Earth Sciences in Cambridge University, have managed to establish a high resolution
chronology of the wet and dry phases in the area between 9000 and 2000 BC, and
show that the peak of the civilisation
almost coincided with the wet phase when monsoon intensified for about 600
years between 5,000 and 4,400 years from now (about 3000 to 2400 BC). The dispersal of the civilisation also
coincides with the onset of the dry phase in the region.
The group relied on the studies of gypsum deposits at the site of a
now-dried-up lake near Karsandi village in the Nohar-Bhadra area of northern
Rajasthan, which is on the margins of the Thar desert. Such palaeo-lakes — there are many in this
area —have been the subject of previous investigations as well, but this is the
first time that the scientists have been able to propose a detailed chronology
of rainfall variation in those areas having a bearing on the expansion and
contraction of Indus urbanism. The Karsandi palaeo-lake is about 120 km
northeast of Rakhigarhi, an important Indus settlement that has seen some
exciting excavations very recently, and near Kalibangan and Karanpura, also
important centres of the Indus civilisation.
Gypsum, chemically calcium sulphate, is one of the common evaporites —
the mineral deposits that remain after the evaporation of saline water — found
in the palaeo-lakes in these areas. Its chemical analysis gives scientists
some good indications about the source and composition of water in these lakes
and the environmental condition at the time of precipitation. For example, if the deposit is pure gypsum, it can be an
indication of no or very less rainfall in the region. This is because the
surrounding areas in the region are all very sandy, and if there was rainfall,
the rains would have brought sand to the lake, and the deposits would have a
mixture of gypsum and sand. Similarly, pure sand can be indicative of very good
rainfall.
The scientists collected samples
of different layers of gypsum and studied them in detail. They carried out AMS carbon-dating
of tiny ostracod fossils to assess the ages of different layers, and
measured the isotopic composition of oxygen and deuterium in gypsum from these
layers. Based on their studies, they have been able to propose a specific
timeline for rainfall variation in this region. They have inferred that this
region, northern part of Rajasthan, was largely dry till about 11,200 years
ago, that is until about 9000 BC. But between 9000 and 3000 BC, there was
substantial precipitation in the area, making it conducive for human
settlements. The scientists say a further intensification of monsoon was
witnessed between 3000 and 2400 BC, after which another dry phase is supposed
to have begun similar to the modern condition.
This timeline almost coincides
with the rise and fall of the Indus civilisation. The evidence from Karsandi
lake, therefore, strongly suggests that there were areas that were receiving
favourable rainfall in the period leading up to the development of urban
centres along the northern part of the Thar desert.“Our findings add to the evidence that climatic reasons could have been
one of the main factors behind the sustenance and decline of the great cities
of Indus civilisation. Of course, much more work needs to be done in this
and many other areas of this region to bolster this evidence,” Saini, the
former Director of Geological Survey of India, said. The study has implications
for modern society as well, which is witnessing climate change and perceptible
variations in precipitation and temperature.
Credit: Indian Express Explained
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