The Japan Sea was a semi-closed marginal sea mainly connected to the subarctic northwestern Pacific via shallow seaways during the late Miocene. We use a multiple regression analysis with common extant radiolarian species groups to estimate the sea-surface temperature (SST) for the period between 9.1 and 5.3 Ma. Our results show a cooling of 8 °C between 7.9 and 6.6 Ma, when the SST dropped from 24 °C to 16 °C. We infer that this cooling dominantly reflects wintertime cooling related to an intensified East Asian winter monsoon. On the other hand, cooling of the summertime SST occurred from 6.6 to 5.8 Ma, suggesting that the late Miocene global cooling is composed of a wintertime cooling phase from 7.9 to 6.6 Ma and summertime cooling phase from 6.6 to 5.8 Ma.
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