Reconstruction of Holocene Paleoenvironment and Evidence of Sea-level Changes in the Bengal Lowland

Abstract

This paper provides a detailed description of sedimentary facies including pollen and diatom analyses, and 14C ages obtained from a hand-excavated outcrop in the central part of the Bengal Lowland. Based on this examination, seven sedimentary facies were recognized in relation to relative sea-level (RSL) changes since the mid Holocene. With the help of a standard reference datum, the required Mean Sea Level (MSL) has been calculated at the surface of the outcrop. The top of the outcrop was about 1.9 m above MSL and the base is 4.1 m below the MSL. The lowermost bioturbated sand flat facies, 3.75 m below the present MSL, contains mangrove pollen and represents an intertidal coastal-plain and estuarine-channel deposits that were dominated by tidal current. A 14C age of 7570-7430 cal BP was obtained for this unit. The evidence supports a transgressive mode in the Bay of Bengal during the middle-Holocene epoch. A subsequent regression interval was found in a salt marsh facies before 6670-6410 cal BP. The salt marsh facies has a sharp contact with an underlying (lower) mud flat facies, shaped by downcutting of the upper bed, indicating a regression. In response to this regression, the environment changed gradually from a mud flat to a salt marsh. The salt-marsh deposits contain abundant mangrove pollens and marine- and brackish-water diatoms, providing support for inflow of shallow-marine water into a supratidal, brackish-water mangal environment. The bioturbated mud flat facies indicates that the succession was dominated by tidal current, and the relative sea-level started to rise again after a small rebound. Hence, the transgressive conditions prevailed during the deposition of this mud flat facies in an intertidal coastal-plain environment. The underlying bluish-black medium-bedded peat layer contains grass pollen. This indicates that after the mid-Holocene the environment around the site changed gradually from mangrove to fresh-water swamp vegetation, in response to regression of the bay between 4080-4030 cal BP. The above evidence suggests that the central part of the Bengal Lowland represented as an intertidal to supratidal coastal-plain estuarine environment that experienced with mid-Holocene relative sea-level changes between 7570-7430 and 4080-4030 cal BP.