From the lab: How adequate rainfall helped Indus Valley Civilisation flourish

The scientists 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.

Written by Amitabh Sinha | New Delhi | Updated: April 10, 2018 1:10:47 am
How adequate rainfall helped Indus Valley Civilisation flourish The decline and disintegration of the Indus Valley Civilisation are attributed to several reasons — climatic, tectonic, and even social.

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.

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