Biodiversity of radiolarians in surface sediments from the East Indian Ocean and their implication for water masses

Abstract This study focused on radiolarian diversity in the deep-sea sediments of the East Indian Ocean (EIO) to demonstrate their potential as a paleoceanogrpahic proxy. Radiolarians are in general abundant in deep-sea sediment of the EIO. However, around Sri Lanka, their abundances decrease potentially because of terrigenous matter inputs. The significantly high abundance of radiolarians in the northern Ninety-East Ridge (NER) area may indicate high levels of productivity in the overlying waters. Further, redundancy analysis (RDA) demonstrated that, in the EIO, Tetrapyle group and Phorticium group mainly inhabited the water depth between 25 m and 100 m; Acanthodesmia vinculata, Botryostrobus scutum and Dictyocoryne group can potentially be a nutrient indicator; the distribution of Didymocryrtis tetrathalamus tetrathalamus may possibly be related to the dissolved oxygen concentration; the distributions of Cornutella profunda, Cycladophora sp., and Cycladophora cornuta may be mainly influenced by flows of AAIW. Our analysis documented that the distribution of an Equatorial Undercurrent (EUC) assemblage, including A. vinculata, B. scutum, Dictyocryphalus hispidus, D. tetrathalamus tetrathalamus, Tetrapyle group, Phorticium group and Dictyocoryne group, may be controlled by the eastward-flowing EUC with the salinity-high and dissolved oxygen-rich waters. The Antarctic Intermediate Water may also affect the distribution pattern of Intermediate-Deep assemblage, including Cycladophora sp., C. profunda, Clathrocanium diadema and C. cornuta, with a possible northward spread along the NER into the southern Bay of Bengal.