The distribution of deep-sea benthic foraminifera in core tops from the eastern Indian Ocean

Abstract

Relative abundances of benthic foraminifera in 57 core tops collected within a depth-range between 700 and 4335 m below sea level [b.s.l.] from the eastern Indian Ocean (mostly between Australia and Indonesia) were investigated quantitatively using Detrended Correspondence Analysis (DCA) to analyse species spatial-distribution. Canonical Correspondence Analysis (CCA) and correlation matrices were used to evaluate the relationships between the species distribution and environmental variables (temperature, salinity, dissolved oxygen, nitrate and phosphate concentrations, carbon-flux rate). Seven key-species proved useful for distinguishing environmental parameters. Two groups of species are identified by means of the first DCA ordination axis. The first group increases in relative abundances with depth and includes three taxa: Oridorsalis tener umbonatus, Epistominella exigua and Pyrgo murrhina. These three taxa prefer a cold (< 3°C) and well-oxygenated (> 3.5 ml/l) environment, with low carbon flux to the sea floor (< 3 g C m-2 year-1). O. tener umbonatus and P. murrhina tend to indicate reduced food availability, whereas E. exigua may indicate periodic delivery (seasonal) of organic matter to the sea floor. The second group includes Nummoloculina irregularis and Cibicidoides pseudoungerianus, typical of upper-bathyal depths. C. pseudoungerianus is correlated with a warm (> 2.5°C) environment characterised by high carbon-flux rate (> 2.5 g C m-2 year -1). N. irregularis is associated with high dissolved-oxygen concentrations (>3 ml/l) and its distribution is limited to south of 20°S. In this area, the contemporary presence of the low salinity and well oxygenated Antarctic Intermediate Water and low primary productivity at the sea-surface (which causes low oxygen consumption at the sea floor) create the ideal conditions for this species. The second ordination-axis scores identify another taxon, Uvigerina proboscidea. The distribution of this species is mainly limited to low latitudes (north of 25°S), where carbon flux rate is high (> 3.5 g C m-2 year-1), due to higher primary productivity levels at the sea surface, and oxygen levels are low (<3 ml/l) due to the organic matter oxidation and the presence of oxygen-depleted Indonesian Intermediate Water and North Indian Intermediate Water.