Review of radiolarian microfossils as a tool for reconstructing sea surface temperature of the past in the Northwest Pacific

Abstract In this review we re-evaluated the potential of radiolarian species as palaeoceanographic proxies in the Northwest Pacific Ocean relying on 33 new samples collected since 2021, combined with already published datasets. Our re-evaluation revealed significant differences between Sea of Japan and Northwest Pacific radiolarian assemblages, leading to the exclusion of the Sea of Japan dataset for further sea surface temperature (SST) reconstructions in the Northwest Pacific. By employing factor analyses, we were able to identify four distinct radiolarian assemblages characterizing the Northwest Pacific and East China Sea as follows: the Subtropical, the Sea of Okhotsk-related subarctic, the Oyashio Current to transitional zone-related, and the coastal water assemblages. Each assemblage showed specific species associations with different water masses and SST ranges. Species like Tetrapyle circularis / fruticosa and Dictyocoryne tetrathalamus were associated with waters above 24 °C, while Lithomelissa setosa and Ceratospyris borealis were associated temperatures below 14 °C. Based on a review of the literature about modern radiolarian species blooming conditions in the Northwest Pacific, we suggest that radiolarian species-based reconstructed SSTs more likely infer summer SSTs. Applying weighted averaging partial least squares transfer function on selected radiolarian species showing strong affinities with changes in summer SST, we re-evaluated past summer SSTs at IODP Site U1429 in the northern East China Sea with high accuracy (R 2 = 0.97, error margin ± 1.4 °C). Our radiolarian-based summer SST reconstruction is similar to Globigerinoides ruber Mg/Ca-based summer SSTs, though showing minor disparities during glacial periods, while constant disparities were observed with alkenone-based SST estimates at the same site, which is likely due to seasonal biases. Notably, we identified L. setosa as a potential proxy for East Asian Winter Monsoon intensity.