Abstract: The northeastern Indian Ocean Ridge area along the 90°E meridian exhibits complex and unique topographic features. To gain deeper insights into the benthic ecological characteristics, regional biological productivity, and terrigenous input influences across different parts of the ridge, a comprehensive identification and statistical analysis of benthic foraminifera (>150 μm) was conducted on 13 surface sediment samples from this region. The study revealed that benthic foraminifera in the 150-250 μm size fraction dominated the area with relative abundances of 60-84%, while the fraction larger than 250 μm exhibited lower relative abundances of 16-40%. The dominant benthic foraminiferal species were primarily epifaunal and shallow infaunal types. Among the three major benthic foraminiferal shell types, hyaline shells predominated, followed by agglutinated shells, with porcellaneous shells being least abundant. Although no significant differences in dominant species were observed among stations, distinct regional variations emerged in shell-type proportions and epifaunal or infaunal distributions: the abundance of benthic foraminifera is relatively high on both sides of the northern ridge and relatively low on the ridge; The relative content of agglutinated benthic foraminifera is significantly higher in the northeast side of the study area compared to other areas, and the content of agglutinated benthic foraminifera increases with increasing water depth in the central part of the Indian Ocean Ridge. The particles selected for the construction of the shell walls of agglutinated benthic foraminifera are mainly composed of detrital minerals in the northeast, while multi-species planktic foraminifera have been used to build their test in the central part of the Indian Ocean Ridge. Analysis suggests that benthic foraminifera in the northeast Indian Ocean ridge area are mainly influenced by the transport of terrestrial materials, surface productivity distribution, and dissolution caused by changes in water depth. In addition, we have noticed for the first time that the content of surface species is much higher than that of epifaunal species in the components larger than 250 μm. We believe this is the result of the adaptation and evolution of benthic foraminifera to high oxygen ecological environments.

Key words: Surface sediment, Benthic foraminifera, 90°E ridge, the Indian Ocean