Benthic Foraminifera from Shelf Sediments of the Bay of Bengal, Central East Coast, India

Abstract

Benthic foraminiferal study, carried out in 23 surface sediment samples from different water depth, between 15 and 250 m in the Bay of Bengal, has led to the identification of 187 foraminiferal species. Principal Component and Cluster Analysis performed on 23 abundant species (> 5%) resulted in four different assemblages that correlate well with bathymetry, nature of sediment and other physicochemical parameters of the bottom water. The shallowest inner shelf assemblage (I) dominated by Amphistegina radiata, Asterorotalia pulchella, Elphidium crispum, Rotalidium annectens and Rotalinoides gaimardii, is characteristically found up to 30 m water depth on sandy sediment formed under high energy condition. The second shallower inner shelf assemblage (II) composed mainly of Ammonia beccarii, Cibicidoides wuellerstorfi, Hanzawaia concentrica, Quinqueloculina seminula, Rotalinoides compressiuscula and Textularia agglutinans is restricted to 30-80 m of water depth and sandy to slightly muddy sand sediments. The third middle shelf assemblage (III) which is confined to the 80–150 m of water depth and slightly muddy sand to muddy sand, includes Asterorotalia dentata, Cancris communis, Cassidulina laevigata, Lenticulina thalmani, Triloculina oblonga, Nonion costiferum, Quinqueloculina venusta and Uvigerina proboscidea. The fourth outer shelf assemblage (IV) consisting of Brizalina subspathulata, Globulimina notovata, and Uvigerina mediterranea exists from 150 to 250 m depth in the muddy sand and sandy mud with low dissolved oxygen in the bottom water. The high abundance of Amphistegina radiata at 0-30 m water depth and Ammonia beccarii at 30-80 m water depth represents low salinity and high energy conditions prevailing inner shelf region of the study area. The presence of Cancris, Cassidulina, and Lenticulina, which are found to be more associated with moderate dissolved oxygen condition, reflects the occurrence of suboxic environment in the middle shelf area. The deeper water sediment dominated by Brizalina, Globulimina, and Uvigerina indicates the oxygen depleted environments with low energy condition. The presence of relict forms in the sediments at 50, 80 and 100 m water depth in the study area denotes different sea stands suggesting past sea level changes. The principal component analysis of the abundant species with grain size and other physicochemical parameters of the bottom water reveal that the distribution of benthic foraminifera in this shelf region is chiefly controlled by the nature of sediments, bathymetry coupled with salinity, dissolved oxygen and hydrodynamic condition.