Planktonic Foraminiferal Assemblage from a Deep-Sea Sediment Core in the Western North Pacific Ocean

Planktonic foraminiferal assemblages preserved in deep-sea sediments provide valuable information on assemblage composition, diversity, and upper-ocean habitat structure. However, baseline assemblage data from remote high-seas regions of the western North Pacific remain comparatively limited. This study documents the planktonic foraminiferal assemblage recovered from a deep-sea sediment core in the western North Pacific Ocean and evaluates its assemblage composition, relative abundance, diversity, morphological characteristics, and taxonomic similarity with previously reported Pacific assemblages. The analyzed sediment was obtained from the 44–46 cm interval of piston core WPPC190208 (18°17′40.3″ N, 149°50′24.0″ E; water depth 5315 m; core length 643 cm), collected during the 2019 R/V Onnuri expedition. A total of 299 specimens representing 25 planktonic foraminiferal taxa belonging to 14 genera were identified using stereomicroscopy and scanning electron microscopy. The assemblage was dominated by Globigerinoides ruber (19.73%), Neogloboquadrina dutertrei (12.71%), and Trilobatus sacculifer (11.04%), together with other mixed-layer and thermocline-associated taxa. The Shannon–Wiener diversity index (H′) was 2.779, Pielou’s evenness (J′) was 0.863, and Simpson’s diversity index (1 − D) was 0.912, indicating a relatively even assemblage structure. Presence/absence-based Sørensen similarity analysis using 34 taxa showed substantial overlap with Pacific reference assemblages, with similarity values ranging from 0.596 to 0.818 and the highest similarity observed with Core MC931 from the central equatorial Pacific. UPGMA clustering further indicated that WPPC190208 shares major assemblage components with assemblages from multiple Pacific sectors rather than forming an exclusively regional western Pacific group. Although the analyzed material represents a single subsurface interval without independent age control, the assemblage provides a sediment-integrated record of a broadly shared tropical to subtropical Pacific taxonomic assemblage. These results expand baseline information on planktonic foraminiferal assemblages preserved in deep-sea sediments from a comparatively understudied sector of the western North Pacific and provide a reference for future ecological, biogeographic, and sedimentary studies in subtropical open-ocean environments.