Shallow-marine ostracode assemblages from late Pliocene sediments of the upper part of the Sagavanirktok Formation and tower part of the Gubik Formation record the last series of warm periods that occurred before the onset of significant cooling of the Arctic Ocean, which ultimately led to Northern Hemisphere continental glaciation. The informally named Colvillian and Bigbendian transgressions represent the oldest deposits of the Gubik Formation and are dated, based on various lines of

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... between 2.48 and 3 Ma. Ostracode faunas from the lower part of the Gubik Formation indicate a cold temperate to subfrigid marine climate with summer bottom temperatures 3-5°C warmer than today. Deposits of the upper part of the Sagavanirktok Formation at Barter Island and Manning Point are older than Colvillian sediments, but are believed to be late Pliocene in age and contain an ostracode fauna with many species in common with the lower Gubik assemblages. The Sagavanirktok ostracode faunas indicate a cold temperate to subfrigid marine climate, with summer bottom temperatures 3-5°C warmer than today. The opening of Bering Strait between 3 and 4 Ma altered Arctic Ocean assemblage composition as Pacific species migrated into the Arctic and North Atlantic Oceans. The admixture of evolutionary distinct faunas from the two oceans identifies Colvillian (and younger) faunas and the first appearance of these mixed faunas provides a convenient reference horizon in the Alaskan fossil record. The marine climatic deterioration that followed the Bigbendian appears to have been generally abrupt and is documented by biotic turnover, with large numbers of species extinctions and first appearances of new species. The change in species composition can be correlated with the cooling of the Arctic Ocean during 1 the Pliocene.