Ancient pollen hidden below Raja Rani Lake tells the story of a stronger monsoon

India may have experienced a much stronger Indian Summer Monsoon (ISM) in the past than previously believed. Scientists have uncovered evidence of intense rainfall, dense forests, and significant climate variability between 1,060 and 1,725 CE in central India, preserved within sediments of Raja Rani Lake in Korba district, Chhattisgarh.

Discovery from the Core Monsoon Zone (CMZ)

The findings come from India’s Core Monsoon Zone (CMZ)—a climatically critical region where rainfall is predominantly controlled by the ISM and which contributes nearly 89–90% of India’s annual rainfall. Due to its high sensitivity to monsoon fluctuations, the CMZ offers crucial insights into monsoonal variability during the Late Holocene (Meghalayan Age).

Research by Birbal Sahni Institute of Palaeosciences

Scientists from the Birbal Sahni Institute of Palaeosciences (BSIP), Lucknow, conducted the study by extracting a 40-centimetre-long sediment core from Raja Rani Lake.
The sediment layers record environmental changes spanning approximately 2,500 years.

The 4.2 ka Mega-Drought (Meghalayan Age)

Around 4,200 years ago, Earth experienced a mega-drought event that devastated many regions across the globe. Known as the 4.2 ka (kilo-annum) event, it led to the collapse of several agriculture-based societies and widespread socio-economic disruption. Owing to its global or near-global extent, this event is regarded as a time-stratigraphic marker in Earth’s climate history.

Evidence from Mawmluh Cave, Meghalaya

Critical evidence of the 4.2 ka event comes from stalagmite samples collected from Mawmluh Cave in Meghalaya, one of the officially designated Meghalayan Age caves. Scientific analysis shows that these stalagmites:

• Were not affected by diagenesis or erosion
• Are well-preserved archives of past climate
• Retain chemical signatures of the mega-drought

High-Resolution Oxygen Isotope Records

Through intensive oxygen isotope (δ¹⁸O) analysis, scientists found that the stalagmites preserved a very high-resolution record of climatic changes associated with the 4.2 ka event.
Such isotopic data help reconstruct:

• Changes in monsoon intensity
• Shifts in precipitation patterns
• Long-term hydro-climatic variability

Mawmluh Cave is among the few globally recognized sites that preserve such a clear and continuous climatic signature from the Holocene.

The Holocene Epoch

We currently live in the Holocene Epoch, which began about 11,700 years ago following a rapid warming that ended the last Ice Age. The Holocene encompasses the entire period of human civilization, including the rise of agriculture and complex societies.

Subdivision of the Holocene: Meghalayan Age

According to the International Commission on Stratigraphy (ICS), the Holocene is subdivided into stages.

• The Meghalayan Age is the youngest stage
• It began 4,200 years ago, marked by the 4.2 ka mega-drought
• It continues to the present day

The climatic signal preserved in Mawmluh Cave serves as the Global Boundary Stratotype Section and Point (GSSP) for the Meghalayan Age.

Source: PIB