Differentiating between summer and winter precipitation in South Asia around a 4.2 ka climatic “event”

Climate change has the potential to have affected ancient civilizations by spurring migrations and shifts in cultivation strategies, and these questions are increasingly relevant as we examine how modern civilization is coping with climate change today. today.

An article published today in a special issue of Climate of the past on “4.2 ka BP climatic event” provides rich information on how rainfall in northwestern South Asia changed during the critical period between 5400 and 3000 years ago.

The marine sediment core known as 63KA was taken near the mouth of the Indus Delta and contains important and intriguing information about past changes in river flow and depth of ocean stratification. The flow of the Indus River is directly related to the amount of summer rainfall. The depth of ocean mixing is related to the strength of winds and evaporation over the Arabian Sea during the winter which, in turn, correlates with winter precipitation over the northwest of the ‘India. Three species of planktonic foraminifera with distinct ecological niches have been used to track relevant changes in salinity and temperature by measuring oxygen isotopes of their calcareous shells, which produce a synchronous record of the strength of the summer monsoon. and winter.

Work on samples of this sea core aroused great interest in 2003 when a summer monsoon mega-drought about 4.2 thousand years ago was linked to the cultural transformations of the Indus Civilization, which had an urban phase extending from about 4.5 to 3.9 thousand years ago.

Professor Michael Staubwasser of the University of Cologne, lead author of the 2003 paper and co-author of the latest research, has stored samples of this core for more than 20 years. “We always thought there could be more valuable information in these samples,” he says, “and now we can see that it’s possible to track summer and winter precipitation from the same core. “

The new results point to a 200-year period of anomalously heavy winter precipitation about 4.5 to 4.3 thousand years ago, and after that both winter and summer precipitation decreased to a minimum about 4 thousand years ago, 1 thousand years.

“Humans are completely dependent on constant access to water. The possibility of a simultaneous decrease in winter precipitation 4.1 thousand years ago completely changes the picture of year-round water availability in this region. A shift of heavy winter rains to the opposite extreme, in combination with the summer monsoon rains which were already diminishing, will have had a dramatic impact on the inhabitants of this region,” says Alena Giesche, lead author of the new publication and PhD candidate in Earth Sciences at the University of Cambridge.

Changes in climate during this period coincided with significant cultural shifts in the Indus Civilization, which saw the culmination of a process of deurbanization by c. 3.9 thousand years ago. “This finding has important implications for our understanding of the Indus Civilization, particularly our interpretation of changes in settlement patterns and cultivation strategies. There was a marked decline in major urbanized centers, but there There has also been an increase in the number of rural settlements in summer monsoon-dominated regions to the east, suggesting that populations have adapted to changing conditions,” says co-author and reader Dr Cameron Petrie. archeology at the University of Cambridge.

The new results are particularly interesting because they track two precipitation patterns in the exact same core. “It is a unique core because the sediments are laminated and undisturbed by the mixing of organisms (bioturbation). It has a detailed radiocarbon chronology and, as the summer and winter precipitation proxies are recorded in the same samples, the relative timing of the two can be determined with confidence,” says Professor David Hodell, co-author and Woodwardian Professor of Geology at the University of Cambridge.

This research was conducted as part of an ongoing collaborative project between the Hindu University of Benares, Varanasi, India, and the University of Cambridge, under the TwoRains project.

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