Radiolarian Microfossils as a Tool for Reconstructing Sea Surface Temperature of the past in the Northwest Pacific

We investigate the suitability of radiolarian species as palaeoceanographic proxies in the Northwest Pacific Ocean using 33 new core-top samples collected since 2021 during Joint Usage/Research Center for Atmosphere and Ocean Science (JURCAOS, Japan), combined with existing datasets published by authors of this study. The main target of this study is to review the suitability of radiolarian species as a paleoceanographic proxy and to develop a robust methodology to estimate past Sea Surface Temperature based on radiolarian species abundances. For this purpose, we compiled our new data from the East China Sea and Central Northwest Pacific with previous datasets obtained in the same area, Japanese coast, and Japan Sea. Our analysis revealed considerable differences between Sea of Japan and Northwest Pacific radiolarian assemblages, suggesting different responses of biota to environmental changes in this marginal sea; thus, we excluded Sea of Japan data from Northwest Pacific Sea Surface Temperature (SST) reconstructions. Factor Analysis identified four radiolarian assemblages in the Northwest Pacific and East China Sea, each associated with specific water masses and SST ranges: Subtropical, Sea of Okhotsk-related subarctic, Oyashio Current to transitional zone-related, and coastal water assemblages. Warm-water species (e.g., Tetrapyle circularis/fruticosa, Dictyocoryne tetrathalamus) showed strong correlation with temperatures above 24°C, while cold-water species (e.g., Lithomelissa setosa, Ceratospyris borealis) were linked to temperatures below 14°C. Literature review suggests these radiolarian-based SST reconstructions primarily reflect summer conditions. Using weighted averaging partial least squares analysis, we reconstructed past summer SSTs at IODP Site U1429 in the northern East China Sea with high precision (R²=0.97, ±1.4°C). These reconstructions align well with Globigerinoides ruber Mg/Ca-based summer SSTs, despite minor glacial period discrepancies, while showing consistent offsets from alkenone-based estimates, likely due to seasonal biases.