Pollens help trace why the Harappan Civilization shrank

Why in News?
Researchers from the Birbal Sahni Institute of Palaeosciences (BSIP) have reconstructed past vegetation and hydroclimatic changes in the Garhwal Himalaya using sediment records from Deoria Tal. The study provides evidence linking an abrupt dry phase around 4,200 years ago (Onset of the Meghalayan Age; 4.2 ka event) with changes in the Indian Summer Monsoon (ISM), which may have contributed to the eastward migration of populations associated with the Harappan Civilization.

Key Findings

• The study analysed a well-dated sediment core from Deoria Tal, a lake in the Garhwal Himalaya.
• Researchers used 10 Accelerator Mass Spectrometry (AMS) radiocarbon dates obtained from Trapa seed cases to establish the chronology.
• Pollen and spores were analysed to reconstruct past vegetation and climatic conditions.
• An abrupt increase in the Oak/Pine ratio around 4,250 cal yr BP was identified.
• This coincided with evidence of an abrupt, short-lived dry climatic phase around 4,200 cal yr BP.
• This period corresponds to the globally recognised “4.2 ka event”, a major climatic disturbance during the Holocene.

Link with Harappan Civilization

• The study suggests that a weakening of the Indian Summer Monsoon (ISM) affected the hydro-climatic conditions of northwestern India.
• Reduced and less reliable river flows and seasonal flooding may have affected populations dependent on the Indus and Ghaggar-Hakra river systems.
• The disappearance of reliable seasonal flooding around the margins of the Thar Desert may have encouraged populations to migrate towards the eastern Ganga plains, where conditions were more favourable for agriculture and settlement.
• Researchers therefore associate the 4.2 ka abrupt climate change with the geographical contraction and transformation of the Harappan Civilization.

What is the 4.2 ka Event?

• The 4.2 ka event refers to an abrupt climatic shift that occurred around 4,200 years ago which is also the onset of the Meghalayan Age. It marks the most recent subdivision of the Holocene Epoch, which began approximately 11,700 years ago, following the last Ice Age.
• The Meghalayan Age represents the last 4,200 years of Earth’s history, a period of increased aridity and climatic instability in several parts of the world. These clues were found in Mawmluh Cave, near Cherrapunji (Meghalaya), one of the wettest places on Earth.
• In the Indian subcontinent, changes in monsoon intensity and hydroclimatic conditions could have significantly affected agriculture, river systems and human settlements.

Source: PIB

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