Abstract: Enhanced the observation and research on the vertical settling gradient variation of siliceous radiolarian remains, an important component of marine settling particles, is conducive to understanding the deep-sea silicon cycle process and the paleoceanographic environmental indication significance of radiolarians. This study selected two deep-sea regions in the tropical southeastern Indian Ocean. Utilizing a large multi-net system to conduct continuous stratified sampling of the 0~3000 m water column across nine layers, and based on the Rose Bengal staining method combined with traditional morphological analysis, it systematically reveals for the first time the settling patterns of siliceous radiolarian remains at different water depths in this region: Vertically, the layer with the highest abundance of radiolarian remains occurs either within the same layer or directly below the layer exhibiting the highest abundance of living radiolarians. The entire settling process consistently displays a three-layer differentiation pattern: the shallow layer group serves as the accumulation layer for remains, the middle layer group constitutes the stable settling layer, and the deep layer group functions as the dissolution loss layer. Spumellarian dominant species exhibit high stability and strong dissolution resistance during vertical settling, whereas nassellarian content decreases significantly with increasing water depth, especially in deep layers. Regionally, variations in radiolarian remains abundance are not only related to their productivity but also influenced by regional hydrodynamic environments. For instance, at southern stations, the density, species number, and diversity of remains within the 100~2000 m depth interval are significantly higher than at equatorial stations, closely corresponding to higher living radiolarian productivity. However, within the 2000–3000 m deep water layer, radiolarian density at southern stations is lower than at equatorial stations, potentially related to lateral transport by deep-water dynamics. Furthermore, the quantitative radiolarian remains data collected in this study will provide crucial observational data and scientific support for estimating radiolarian silicon export flux and its biological pump effect in the tropical eastern Indian Ocean.

Key words: Tropical Southeast Indian Ocean, Radiolaria, Deep Sea, Different depths, Settling changes