Siliceous microfossil assemblages in the southern Emperor Seamount Chain sediments and their biogeographical and paleoceanographical implications

The taxonomic composition and abundance of siliceous microfossils in sediments is a valuable source of information about environmental changes at the sea surface today and in the geological past. This paper presents the results of a study of siliceous microplankton (radiolaria, diatoms, and silicoflagellates) in the surface calcareous sediments (silty foraminiferal oozes and sands) of the Nintoku, Jingu, Ojin, Koko, and Yuryaku guyots at the southern end of the Emperor Seamount Chain (ESC). Bottom sediments were collected using an underwater remote-operated vehicle (ROV) Comanche-18. The study revealed the taxonomic diversity and relatively high abundance of siliceous microplankton. Radiolarians are represented by 237 taxa (104 taxa from 68 genera of Spumellaria, 120 taxa from 61 genera of Nassellaria, and 13 taxa from 7 genera of Collodaria). Diatoms are represented by 60 species from 29 genera, and silicoflagellates by 5 species from 3 genera. Differences in siliceous microplankton reflect the northern boundary of warm water in the ESC, passing at approximately 38°N, between the Jingu and Ojin guyots, which coincides with the boundary of the Kuroshio Bifurcation Current, confirmed by the corals and the bathyal fauna of brittle stars (Ophiura). The absence of remains of siliceous microplankton in some sediments is probably due to dissolution, and the presence of extinct taxa from older sedimentary rocks and neritic species is due to process of reworking caused by active hydrodynamics on ESC guyots and transport by currents.