According to recent research, scientists analysed cave deposits and found the ebb and flow in the hydroclimate of the Indian subcontinent corresponded to the changing fortunes of ancient civilisations. To reconstruct patterns of the Indian summer monsoon (ISM), which hits large swathes of the Indian subcontinent from June to September, the researchers stepped back in time by stepping into a cave in Uttarakhand.In addition to the reconstructed ISM data, they took into account records of northern hemisphere temperatures and Himalayan river discharges for the last 5,700 years.
They found that, in the words of Ashish Sinha, a professor at California State University Dominguez Hills, USA, “There appears to be a rhythmic change in monsoon rainfall that appears to correlate with societal changes.”Gayatri Kathayat, a post-doctoral researcher Xi’an Jiaotong University, China, told The Wire that the Sahiya cave is one of the toughest she has explored in her life. “You have to crawl inside.” Entering through a narrow opening, she inched along. Falling through a hole, an underground cave intersection offered many branches.
Kathayat chose one narrower than the entrance. As the roof and the floor almost connected, “My caving helmet was getting stuck on the way,” said Kathayat. Along with limited space, she had to contend with moving streams of groundwater. Two hours of edging along through dark, wet albeit refreshing environs, jumping through another hole in the ground, she arrived at a water pond within the caves. With no interaction with the outside world, the ideal speleothem samples developed right there.
“We knocked it down using a chisel and a hammer,” said Kathayat. When rain falls on land, it trickles through the ground picking up minerals like calcium carbonate. This mineral-rich water drips into caves, falling onto the floor to form stalagmites. neral-rich water. by drip-drip-drip of rainwater trickling. On its journey, the water picks up minerals like calcium. Also known as speleothems, these cave deposits were sampled in this study to measure the rainfall over the past 5700 years.
These speleothems, as they are also known, bear a signature thousands of years old, of both rainwater and trapped minerals. Shiny, elongated lumps, the best speleothem samples form in quiet, undisturbed caves with stable temperatures, relatively high humidity and suffering from little erosion.In 2010, Kathayat made several trips to the Sahiya cave, carrying back with her speleothem samples. In the lab, the scientists cut open a speleothem to reveal a timeline, one that Ashish Sinha compares to growth rings within trees. The oldest part of the speleothem lies towards the bottom with the youngest deposits on its tip.
Kathayat analysed speleothems from the Sahiya cave to extract rainfall data. “This cave is in the path of the way the Indian monsoon progresses even today, so actually, it’s very well located,” Rajiv Sinha, a professor at IIT Kanpur who was not involved in this research, told The Wire.Rainwater contains two isotopes of oxygen – a lighter one, O16, and a heavier one, O18. The calcium carbonate in a speleothem reflects the oxygen isotope composition of the rainwater. “The isotope composition of rain becomes fossilised in speleothems,” Ashish Sinha said. According to Kathayat, they measure the ratio of “the oxygen isotopes captured in these stalagmites” – which varies with air temperature and amount of precipitation.
The added presence of uranium, a radioactive isotope with a predictable decay time, means this data is essentially timestamped. The history of rainfall in that region is trapped within the layers of a speleothem.The researchers mined data put together by other paleoclimatology groups to compare temperatures in the northern hemisphere temperatures plus the discharge of Himalayan rivers (like the Indus) to their data on the ISM. “The stronger monsoon corresponds with warmer temperatures in the northern hemisphere and cooler temperatures mostly link with reduced rainfall,” Ashish Sinha said.The Himalayan glaciers are partly fed by the ISM – so when it’s colder, the Himalayan don’t flow with the same strength as they do when it’s warmer.“When we started to compare this record with the archaeological and the historical accounts, we found a very strong temporal relation – the period when the monsoon was weak, the northern hemisphere was cold, river discharge was low – those were the times when ancient civilisations collapsed,” according to Ashish Sinha. The de-urbanisation period of the Indus valley civilization, for example, coincided with such climate.The colder temperatures, consequently reduced river discharge and poor rainfall might have led to drier conditions, coinciding with the time the Harappan civilisation is thought to have moved towards greener pastures.
Kathayat and her colleagues were also able to associate variations in rainfall with lifestyle. The periods of the Indus valley and Vedic civilisations that were primarily agrarian have been linked to greater precipitation, among other things. On the other hand, nomadic intervals happened during times of drier weather. Therefore, the collapse of the Mauryan empire in 180 BCE and Tibet’s Guge kingdom in 1623 AD coincide with colder temperatures and reduced rainfall. While impressed with the high resolution data collected, Anindya Sarkar, a professor at IIT Kharagpur, has reservations about the cave’s location.
The Sahiya cave is a ways off from where the Indus valley civilisation flourished, so high resolution data may also be vulnerable to “inconsistencies in the monsoonal records”. “The reason for these inconsistencies, I think, are local parameters that affect the monsoons in these kinds of timescales,” Sarkar told The Wire. Of course, finding a speleothem in semi-arid or arid regions where the civilisation once thrived would be impossible and other methods might be required.
Ashish Sinha agrees that the Sahiya cave is a single record but he defends it as being a “strategic” one. In the future, “more local, regional datasets” analysed together with archaeological records could help paint a fuller picture.There were factors besides a gradually changing hydroclimate that altered the spread of ancient Indian civilizations at a local level. However, our understanding of their motivations adds a depth of detail to the story of our forebears and how they learned to cultivate our lands and sustain themselves in the face of new challenges. But at higher resolutions, there are highs and lows in the amount of rainfall.
This ebb and flow is analogous to the changing fortunes of the ancient civilisations of the Indian subcontinent. Despite worrying weather events due to an increasingly changing climate, we are better equipped to deal with these problems than civilisations of the past, according to Rajiv Sinha. However, this doesn’t mean we can be complacent. While the ISM has had highs and lows over the last 6,000, overall, it shows a gradual decline, one that Ashish Sinha believes India must be prepared to deal with. Interview Sources:Ashish SinhaGayatri KathayatRajiv SinhaAnindya Sarkar