Last 10 millennial history of Indian summer monsoon in the Bengal region – a multi-proxy reconstruction from a lacustrine archive

Abstract

We combine phytoliths, non-pollen palynomorphs, stable carbon isotopes of sediment organic matter, environmental magnetic parameters and sediment texture data of a lacustrine sedimentary archive from the Bengal region to understand the ecosystem response to past hydroclimatic changes. The region experienced a high Indian summer monsoon (ISM) rainfall during c.10.2–5.6 ka corresponding to the Holocene Climatic Optimum (HCO) when the lake level was high. Our study reveals that ISM weakened at c.4.3 ka with a decline in lake level, became strong between c.3.7 and 2.1 ka and then shifted towards a drier mode. However, this weakened phase was punctuated by a stable phase between c.1–0.8 ka, and a comparatively stronger phase between c.0.2–0.1 ka corresponding to the Medieval Warm Period (MWP) and Little Ice Age (LIA), respectively. An eutrophication of the lake began at c.3.7 ka which was completely in-filled during c.0.2–0.1 ka. A comparison of the present results with published records suggests millennial-scale coherence in the Holocene ISM variability across the Bay of Bengal (BoB) moisture source regions while, the finer-scale incongruities are attributable to differential proxy response and dating uncertainty. Our proxy-based observations are also compared with a few palaeo-model outputs for selected time-spans [such as, 1550–1850 CE (0.4–0.1 ka), 900–1300 CE (1–0.6 ka), 2100–2000 BCE (4.1 − 4 ka), and 4050–3350 BCE (6–5.3 ka)] from the Palaeo Modeling Intercomparison Project Phase 3–4 (PMIP3/PMIP4) experiments which support the assumption that changes in lake ecosystem may be strongly influenced by ISM rainfall, and the region surrounding northern BoB received higher precipitation than the other parts of India since the past 10.2 ka. The millennial-scale variations in the ISM rainfall may largely be attributed to gradual decline in orbital insolation and dynamics of Inter tropical Convergence Zone (ITCZ). However, centennial scale variations may be collectively triggered by forcings like NAO, ENSO and IOD.