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Systematics of Modern Benthic Foraminiferal Assemblages from the Deltaic Mangrove Ecosystem of Sundarbans

TUMPA SAHA
Beach Sand Mineral Exploitation Center, Bangladesh Atomic Energy Commission, Kalatoli, Cox’s Bazar-4700, Bangladesh

SUBROTA KUMAR SAHA*
Department of Geology, University of Dhaka, Dhaka-1000, Bangladesh

Abstract/ Executive Summary:

The contemporary intertidal foraminifera and estuarine environment information were collected from the four sites adjoining the deltaic mangrove environment in the Sundarbans. The marsh zones of the south-western coastal region of Bangladesh were examined for modern benthic foraminifera and to expound on the relationship of the foraminiferal assemblages with the environment. Due to high inaccessibility and remoteness, the taxonomic study of foraminifera and its depositional environment remains largely overlooked in the Sundarbans of Bangladesh. This study includes a detailed survey of depositional environment of these fluvio-deltaic to shallow marine sediments. The seventeen species belonging to fourteen genera representing nine families were recorded from surficial sediment of supratidal, intertidal, and dune environment of Kotka, Jamtala, Kochikhali, and Dimer char area. In the present study, these foraminiferal assemblages are characterized calcareous and agglutinated foraminifera. The Kotka beach is recognized as Nonionina assemblage zone, Jamtala beach designated as Ammonia assemblage zone, Kochikhali as Nonionina assemblage zone and Dimer char as Rosalina-Nonionina-Nonionella assemblage zone. The deposition of foraminifera is restricted to Sundarbans’ low to high marsh zone.

Keywords: Mangroves, Intertidal foraminifera, Depositional environment, Sediment texture.

Location: The deltaic mangrove environment in the Sundarbans

Start Date:

End Date:

Program Area: Comparative study

Commodity: Evaluation, Sundarban

Objectives:

The present research’s main objective was to systematically describe the recorded foraminiferal species and determine the bio-assemblage constituents and prepare assemblage zone for the overall study area, characterize modern benthic foraminifera, their distribution, assemblage patterns from four sampling sites and investigate the southwestern coastal margin of Bangladesh.

Methodology:

Regionally, the study area covers the South-western coastal margin of Bangladesh. The study area had been chosen for its uniqueness and then to get something new because the micropaleontology of that area was completely unexplored and unknown.

The sampling sites cover an area of almost 16-17 km extending west to east and represent the coastal environments of the Sundarbans forest. The study area belongs to four sampling sites, Kotka (210 51′24″N, 890 46′16″E), Jamtala (210 50′49″N, 890 49′01″E),

Kochikhali (210 51′55″N, 890 50′12″E) and Dimer char (210 51′04″N, 890 51′01″E) of the Sundarbans of Bangladesh.

This area selection is important because there is no such micro-paleontological research in the south-western coastal area of Bangladesh. All samples were carried out during low tide as the water regressed; thereby, the intertidal zone was exposed. From these sections, samples were collected at depth about 0.5-1m using a hand auger and scrapper. The collected samples were labeled and preserved in sample bags.

The collected samples were washed carefully to remove the unwanted fraction of materials like clay and silt, dried in an oven at 100^oC temperature for 30-40 minutes, and 100gm of samples were weighted from oven dry samples for each and every location. Then, the samples were sieved for 15 to 20 minutes, and the samples were stored in individual packets after labeling the mesh number and retained weight.

After picking under a binocular microscope, foraminifera was placed on a gridded tray, observed under Leica EZ4E microscope (Leica Corporation) at 35X magnification, and bright-field images were captured using cellSens software (Leica). In order to accurately identify foraminifera species based on morphological features, some representative samples sent to Jadavpur University, India for Scanning Electron Microscopy (SEM).

Source of Information: J. Asiat. Soc. Bangladesh, Sci. 47(2): 121-136, December 2021

Article Link (Online Journal): DOI: https://doi.org/10.3329/jasbs.v47i2.57276

Funding Source:

Budget:

Total Budget (Tk.) :

Achievement/Findings:

The marsh areas of Sundarban are influenced by a multitude of factors comprising monsoon, meso-tidal range, and biotic effects. These environmental factors have a marked effect on foraminiferal assemblage and mangrove vegetation. The present integrated study of foraminifera and vegetation distinguishes the high and low marsh zones: a) High Marsh: Foraminifera- Trochammina inflata, Miliammina fusca, and Haplophragmoides canarienses; b) Low marsh: Foraminifera-Ammonia beccarii, Haynesina germanica, Rosalina bradyi and Elphidium sp. Saltmarsh foraminifera differentiates the high and low marsh zone, and paleo-environment of the study area has been identified. Numerous environmental factors influence and these factors take prominent attributes on foraminiferal assemblages. The dominance of high marsh foraminifera like Haplophragmoides canariensis, Jadammina macrescens, Trochammina inflata and Miliammina fusca is a distinguished feature, and in fact, Jadammina and Trochammina are confined to mangrove habitats. This research aimed to create a thorough study of benthic foraminifera from remote, inaccessible, and unexplored Sundarbans of Bangladesh. This foraminiferal dataset may provide a better understanding of sea level rise in the south-western coastal region of Bangladesh.

Knowledge Management: Journal