Shillong

A 6,000-year climate record preserved in a stalagmite in Meghalaya's Mawsmai Cave has provided evidence of long-term changes in monsoon intensity in northeastern India, with researchers identifying links between ancient rainfall patterns, global climate systems and solar variations, officials said on Wednesday.

The study, led by Yachna Verma and Anil K Gupta of IIT Kharagpur, along with Prasanta Sanyal of IISER Kolkata and co-authors Priyantan Gupta, Som Dutt, Pankaj Kumar and Abhijit Mukherjee, examined changes in the Indian Summer Monsoon (ISM) between 11,807 and 5,622 years ago.

Published in the international journal Palaeogeography, Palaeoclimatology, Palaeoecology earlier this year, the research found that the monsoon was significantly stronger during the early Holocene, followed by a gradual weakening during the middle Holocene. Holocene is the current geological period, which began about 11,700 years ago after the last Ice Age.

Titled 'Deciphering Holocene monsoon dynamics in northeastern India', the research is based on the analysis of a stalagmite recovered from Mawsmai Cave, about 60 km south of Shillong in Meghalaya's East Khasi Hills district.

The researchers analysed the stable oxygen isotope composition of the stalagmite, along with its petrographic and mineralogical characteristics, to reconstruct past rainfall and monsoon conditions.

The findings indicate that the ISM was particularly strong between approximately 11,807 and 7,700 years ago, with lower oxygen isotope values pointing to intensified monsoon activity.

However, the period between 7,700 and 5,622 years ago witnessed a gradual weakening of the monsoon, reflected in higher oxygen isotope values.

The study also identified several fluctuations in monsoon intensity over periods spanning hundreds of years, suggesting that rainfall patterns in northeastern India were influenced by climate changes extending well beyond the Indian subcontinent.

These variations corresponded with a series of North Atlantic climate events, known as Bond events 8, 7, 6, 5b, 5a and 4, indicating possible links between North Atlantic climate variability and the Indian monsoon through changes in the Atlantic Meridional Overturning Circulation (AMOC), a major system of ocean currents that transports heat across the Atlantic.

The researchers also found recurring climate patterns at intervals of around 118 and 110 years in the stalagmite record.

These periodicities are consistent with the Gleissberg solar cycles, associated with long-term variations in solar activity.

According to the study, solar variability, together with interactions involving the Pacific Decadal Oscillation (PDO), ocean-atmosphere systems and the temperature gradient between the Tibetan Plateau and the Indian Ocean, played an important role in modulating monsoon intensity during the period.

The findings underline the influence of both regional and global climate processes on the Indian Summer Monsoon, which is critical to water availability, agriculture and livelihoods across South Asia.

Northeastern India, recognised for its climatic sensitivity and distinctive geographical setting, is considered an important region for understanding monsoon variability, according to the researchers.

They noted that although several previous investigations have used cave deposits and other geological records to reconstruct past monsoon patterns, integrating petrographic evidence with conventional geochemical proxies provides an additional dimension to understanding the region's prehistoric climate.

The stalagmite sample, measuring 11 cm in length, was collected from a chamber approximately 300 metres inside Mawsmai Cave, where relative humidity was above 95 per cent.

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The researchers acknowledged the support of the Meghalayan Adventure Association, led by B D Kharpan Daly, along with Ksan Kupar Mawlong and Kyrmen Skhembha Rani, for their assistance during fieldwork.

Kolkata-based IISER provided facilities for isotope analysis, while IIT Kharagpur provided access to petrographic analysis facilities, the researchers said.