PALEOENVIRONMENTAL INVESTIGATIONS AND CORAL REEF MONITORING OF ST. MARTIN’S ISLAND, BANGLADESH USING FORAMINIFERAL ASSEMBLAGES

Benthic foraminifera occur in almost all marine environment with higher abundance than other micro fauna. They have been used for paleoenvironmental investigation for long ranges because of their specific depth preferences, salinity tolerance, and high preservation potential in sediment. The present research is concerned with the investigations of paleoenvironment in association with old coral reefs of the St. Martin’s Island based on foraminiferal assemblages. For conducting this study, the island has been divided into three broad regions; namely Uttar Para, Madhya para and Dakkhin Para and samples were collected from the three sections at different depths. Based on the abundance of micro fauna in sediments three sections were identified from these three regions. About 300 specimens of foraminifera are collected; among them 44 species, 39 genera and 25 families are identified which are mostly benthic in origin. Astrorhiza, Elphidium, Cyclammina, Robulus, and Eponoides are the top five most abundant genera all over the island based on their relative abundance and frequency of occurrence. Among the three sections, Uttar Para can be designated as Cyclammina-Astrorhiza assemblage zone, Golachipa as Astrorhiza-Carterina assemblage zone and Dakkhin Para as Triloculina-Astrorhiza assemblage zone. Relative abundance chart of foraminifera and its correlation of Saraswati (2016) Model Diagram and Shell Type Ratios on Murray’s Ternary diagram indicates inner shelf to middle shelf region as their paleoenvironmental position. Van der Zwaan’s Empirical Equation suggests the average paleodepth of water to be 36.15m at which the foraminifera lived in. Gibson’s Index of Oceanity also indicates shelf environment where the diversity of benthic foraminifera is high. Murray’s Index of Diversity for the island is on category VII which indicates normal salinity of the ocean prevails at that time. ¹⁴C dating indicates two cycles of coral growth in the island. The first cycle was in 2058-1866 BP and the second cycle continued from 1290-797 BP. The Hallock’s FORAM Index was calculated for these two cycles of corals to have an idea of water and sediment quality of the coral habitat during that time. The FORAM Index value for the first cycle is 2.74 which indicates marginal condition for reef growth and suggests that coral cover will not follow by recovery after one mortality event.