Palaeoclimatic shifts in the Central Ganga Basin during the Middle- to Late Holocene: Exploring the 4.2 ka arid event and its implications in northern India

The Central Ganga Basin is one of the most densely populated regions of India. It is agriculturally diverse and contributes much to the Indian economy. The region has housed numerous ancient and mediaeval empires. This study presents a continuous record of the paleomonsoon from the Chandrika Devi lake, Lucknow district of Uttar Pradesh, India which is linked with paleo vegetational shifts over the last ~6000 years (5871–75 cal yr BP). The chronology of the lake core is based on three accelerated mass spectrometry (AMS) radiocarbon and two Optically Stimulated Luminescence (OSL) dates. The multiproxy data (grain size, major and trace element ratio, total organic carbon (TOC wt%), carbon isotopes (δ13Corg‰) and pollen), suggest that the lake was initially a part of the Gomti river that began to transform into a lake at ~5000 cal yr BP with weakening of the Indian summer monsoon (ISM) in the Central Ganga Basin. The lake formation was completed at ~4100 cal yr BP under the influence of the 4.2 ka arid event. This phase marks the beginning of human presence as well as agricultural activities in the lake region with the appearance of Cerealia pollen and other agricultural taxa. The agricultural activity surrounding the lake catchment peaked at ~3000 cal yr BP. The lake gradually shrank and became a marshy lowland at ~75 cal yr BP. Our study is significant because it is the first comprehensive multiproxy study from the Lucknow region in the Central Ganga Basin on paleomonsoonal variability and its relationship to human activity, agricultural practices during the Late-Holocene with a focus on the 4.2 Ka arid event. Also, pollen record suggests that the changes in agriculture and human activity began just after 4.2 ka arid event in the study area.