Geochemical insights into the 5.4 ka event in the eastern Arabian Shelf

Abstract

This study explores the historical presence of the El Niño-Southern Oscillation (ENSO) phenomenon during the Holocene and its impact on the Indian Summer Monsoon (ISM) and the East Asian Summer Monsoon (EAM). This investigation sheds light on an area with limited prior understanding. The primary objective is to analyse ISM variations from ~ 6000 to 1700 calibrated years before the Present (cal yr BP) and decipher their connection with the EAM. Sediment samples were obtained from core SK-291/GC-15, collected off the coast of Goa, and underwent comprehensive analysis, including examination of major, trace, and rare earth elements (REEs). The findings from geochemical proxies reveal that variations in sample compositions are primarily attributed to shifts in chemical weathering intensity rather than alterations in the source rock composition, and the sediments were deposited under consistent anoxic conditions. A noteworthy shift in the chemical weathering pattern was identified, particularly during the ~6000–4400 cal yr BP period, coinciding with the onset of intensified ISM around ~5400 cal yr BP. This intensified monsoon phase, recognised as the 5.4 ka event, coincides with the development of the Harappan civilisation, highlighting its historical significance. Notably, an inverse relationship between the ISM and EAM was observed during this 5.4 ka event – a phenomenon explained by the influence of ENSO on the Asian monsoon system.