Abstract: The paleoceanographic history of the central Arctic Ocean since Marine Isotope Stage (MIS) 13 was reconstructed based on ostracode assemblages in a gravity core from the Alpha Ridge (modern water depth 2280 m). Over 7000 ostracode valves, including 8 genera and 11 species, were obtained from the core. The biological productivity, as represented by ostracode abundance, is low during MIS 13-10 but is markedly high throughout MIS 9-1. The distribution of Acetabulastoma arcticum, a sea ice-dwelling species, indicates that perennial sea ice was probably absent during MIS 13-9. The ostracode assemblages are predominated by Polycope spp. (an indicator of Arctic Intermediate Water, AIW) and Cytheropteron sedovi (an indicator of Arctic Ocean Deep Water, AODW), and are accompanied by Microcythere medistriatum, Pseudocythere caudata, Pedicythere spp., Cytheropteron scoresbyi, Cytheropteron higashikawai, and Henryhowella asperrima. The relative frequencies (%) of Polycope spp. and Cytheropteron sedovi show inverse correlation throughout the core. The reconstructed paleo-watermass history is as follows: initially, the core site was occupied by the upper part of AODW (MIS 13-12) and lower part of AODW (MIS 11-10); after then, the overlying AIW shifted downward and took the place of AODW (MIS 9-early MIS 5); later, the lower part of AODW shifted up rapidly (middle to late MIS 5) and finally the upper part of AODW came to settle down at the site (MIS 4-1).