Changing the paleo-depositional environment in the last 2300 years: a study through sedimentology and geochemistry of a sediment core, western Bay of Bengal

Abstract

The sediment core collected at a water depth of 260.2 m off Mahanadi, western Bay of Bengal was analyzed for grain size fractions, clay minerals, and major and trace elements to understand changing depositional environments, source and processes with time, from ~ 2300 cal yr before present (BP) to the year of the collection (2014). Age was estimated using accelerator mass spectrometer (AMS) 14C dates of two planktonic foraminifera samples. The core was divided into six distinct zones based on the significant changes in the sediment components and clay minerals. The presence of higher than average clay and smectite and lower silt and illite, in Zone II (2000 years BP–1070 cal yr BP) and Zone V (300–130 cal yr BP) revealed their deposition in low energy conditions, while, Zone III (1070 to 530 cal yr BP) and Zone VI (132 cal yr BP to present) exhibited higher silt and illite and lower clay and smectite than average suggesting their deposition in a high energy environment. Similar depositional environments observed between Zones II and V and also between Zones III and VI indicated the reoccurrence of similar hydrodynamic conditions with time. Zone II represents the Roman Warm Period and part of the early Medieval Warm Period, while, zone V corresponds to the retreating phase of the Little Ice Age. High Al in zone III and fluctuating trend in zone VI indicates higher Al release along with silt and illite from physical weathering in a cold and arid climate, while, smectite showed a positive correlation with Ti indicating the source of Ti and smectite as basic igneous rocks released during the enhanced southwest monsoon and warm climate. Mn and Zn distribution revealed fluctuations in the Oxygen Minimum Zone (OMZ) intensity to changing hydrodynamic conditions and depositional environments. The present study revealed changing depositional environments in the western Bay of Bengal with time with respect to changing material influx, reflecting fluctuating monsoons and climate.