Rhythmic Indian Summer Monsoon variability inferred from stalagmite δ¹⁸O and δ¹³C record during Middle to Late Holocene transition

The Middle to Late Holocene transition was associated with a notable episode of aridity throughout many parts of the world known as the 4.2 ka event. Indian summer monsoon (ISM) records of this period are of great importance as this region witnessed the collapse of the Indus Valley civilization. However, the temporal and spatial extents of ISM records during this interval are still scarce. The focus of the present study is (1) to generate a new high resolution climate proxy record to understand the causative factors of various dry (weak) and wet (strong) ISM phases during the Middle to Late Holocene transition; (2) to infer the temporal extent of the dry phase recorded around 4.2 ka; and (3) to investigate any potential link of abrupt climate change with the collapse of the Indus Valley civilization. We present high-resolution stalagmite δ¹⁸O and δ¹³C proxy records from Borra Cave, eastern India. Isotopic results provide insight into ISM over a time span of ∼925 years between ∼4.8 ka and ∼ 3.9 ka and uncover four major dry phases. The time series indicate that dry Phase I (4.06–3.98 ka) and II (4.24–4.08 ka) were associated with ∼25 years of periodicity. Phase III (4.2–4.1 ka) was the major dry phase which is also known as 4.2 ka event. Phase IV (4.50–4.45 ka) was observed to be associated with ∼10 years of periodocity. δ¹⁸O and δ¹³C time series reveal quasi-millenial to centennial and decadal periodicities and suggest that the ISM variability has been modulated by long range solar insoltion and coupled ocean-atmospheric processes. The potential link between abrupt climate change and collapse and/or migration of the Indus Valley civilization is also discussed in this work.