The role of active channels in sediment transport to the Bengal Fan and their implications for climate and sediment source changes since 16 ka

Abstract

The Bay of Bengal (BoB) is crucial in understanding sediment dynamics in the Indian Ocean. Analysis of sediment cores from the western BoB, focusing on the Middle and Lower Bengal Fan since 16,000 years ago, reveals rapid deposition within the submarine canyon. This deposition was driven by the Ganges-Brahmaputra (G-B) river system during lower sea levels, leading to the formation of turbidity currents. Increased glacial activity during cold phases enhanced physical weathering in the Himalayan highlands, affecting sediment transport to the BoB until the early Holocene. Following the early Holocene, the lower fan displayed a mixture of sources, while the deeper fan predominantly reflected Indian sources throughout the Holocene. Sediment contributions from the Indian subcontinental commenced after 9,100 years ago, becoming dominant since 7,000 years ago, particularly in arid-semiarid conditions in the middle to lower fan, primarily driven by monsoonal water currents. Cores located near active channels exhibited consistent sourcing patterns influenced by regional factors such as river systems and active channels since the mid-Holocene. Sediments with coarser-grain, higher smectite content, and primarily sourced from Indian rivers indicated an intensification of monsoonal currents since 9,100 years ago. Fluctuations in smectite/(illite + chlorite) ratios were correlated with changes in summer monsoon rainfall, impacting erosion and sediment dynamics. Active channels played a crucial role in transporting Himalayan sediments via the G-B river system to the deep ocean during periods of low sea levels, but their influence has diminished with rising sea levels. Therefore, the proximity of sediments to active channels denotes the dominance of Himalayan sources; however, sediment from the lower fan transitioned to Indian sources since 9,100 years ago, influenced by the combined effects of monsoonal currents, active channels, and underwater currents shaping sediment transport dynamics. During the Holocene Climatic Optimum (HCO), intensified summer monsoons led to increased sediment contributions from Indian sources, driven by water currents during the winter monsoon, with notable impacts after 7,000 years ago. This study highlights the complexity of sedimentary processes and provides insights into past environmental conditions, contributing to our comprehension of sediment deposition patterns in the northeastern Indian Ocean.