The migration of thermophilic marine Ostracoda into the Arctic Ocean during the Pliocene indicates that winter and summer ocean temperatures around Arctic margins were > 0 øC and > 3 øC, respectively, and that ice-free conditions existed for most or all of the Arctic. By at least 3.5-3.0 Ma, probably earlier, the opening of the Bering Strait allowed marine organisms to migrate through the Arctic Ocean, mostly from the Pacific Ocean. Migrant taxa such as Cythere, Hemicythere, and Neomonoceratina are known from Pliocene deposits of Alaska and Canada as well as Neogene deposits of the North Pacific and Atlantic oceans. On the basis of ecological and zoogeographic information on ostracode species from more than 800 modern "core top" samples for the North Atlantic, North Pacific, and Arctic Oceans, we determined winter and summer temperature tolerances for migrant taxa to be at or above about 0 øC and 3 øC. This suggests ice-free summers, and probably, a perennially ice-free Arctic Ocean in some regions. Elevated water temperatures in the Arctic Ocean between 3.5 and 2.0 Ma is supported by evidence for late Pliocene increased meridional heat transport in the North Ariantic Ocean.Paper number 93PA00060. 0883-8305/93/93PA-00060510.00 INTRODUCTIONThe Arctic Ocean, in general, and Arctic sea ice, in particular, play an important but poorly understood role in oceanic circulation and global climate. Raymo et al. [1990] conducted an experiment using the GISS II atmospheric general circulation model (GCM) to examine the influence of reduced Arctic sea ice, a situation postulated for the Pliocene, on global climate. Among their conclusions, they suggested that late Pliocene cooling, well documented in the North Atlantic deep-sea record and many other paleoclimate proxy records, may have been linked in part to a shift in the Arctic Ocean from perennially ice-free to ice-covered conditions. However, they conclude that paleoclimate evidence from the Arctic is still too fragmentary to identify the forcing mechanisms that changed late Pliocene climate.Perhaps equally important as its influence on atmospheric parameters, Arctic Ocean sea ice contributes in a significant way to deepwater formation and the overall thermohaline circulation of the world's oceans [Aagaard, 1981; Aagaard et al., 1991; Rudels et al., 1991]. Freezing of sea ice and brine rejection in Arctic shelf seas leads to the formation of cold, dense water [Aagaard et al., 1985] and the maintenance of the modem Arctic Ocean halocline [Aagaard, 1981], which serves a key role as a heat sink and which buffers Arctic sea ice from underlying warm Arctic Intermediate Water (AIW) = (the Ariantic layer). A catastrophic breakdown of the halocline may have occurred during Termination I when rapid runoff lowered salinity so much that convection and deep ventilation stopped [Rooth, 1982; Broecker et al., 1985], Smaller-scale oscillations in fresh water from sea ice over historical times can similarly alter convection and deepwater formation [Aagaard and Carmack, 1989] because th...

Abstract. The Omma-Manganji fauna of Japan signifies a time during the late Pliocene and Pleistocene when arctic-subarctic species migrated far south of their present geographic range in response to oceanographic changes. Omma-Manganji deposits exposed on Hokkaido, northern Honshu, and Sado Islands yielded about 224 species of marine Ostracoda. At least 26 are circumpolar species known previously from Arctic seas off the British Isles, eastern North America, Scandinavia and Europe, comprising between 14 and 47% of the ostracod assemblage in eight of ten formations studied. The 26 circumpolar species and 21 other western Pacific cryophilic species are illustrated and their distribution in Japanese deposits is discussed.

The genus Semicytherura belongs to the family Cytheruridae, and was distinguished from Cytherura on the basis of carapace features. Species of Semicytherura from Japan and adjacent seas can be divided into two groups. One is represented by Semicytherura miurensis Hanai, 1957, characterized by a thin, oval carapace covered with fine reticulation. The other is represented by Semicytherura henryhowei Hanai & Ikeya, 1977, characterized by a thick sub-rectangular carapace in lateral view. Semicytherura henryhowei, which is distributed from Hokkaido to Okinawa in Japan, has been regarded as having several morphotypes distinguishable on outline and reticulation of carapace. However, as a result of detailed observations on the copulatory organ, carapace outline and distributional pattern of pore systems, remarkable differences are shown to exist between the two most frequently occurring morphotypes. In order to recognize S. henryhowei sensu stricto, the carapace of the holotype was re-examined. Consequently, neither of the two morphotypes are considered to belong to S. henryhowei due to differences of carapace outline and distribution of pore systems. The two morphotypes are here regarded as independent taxa, described as new: S. kazahana n. sp. and S. sasameyuki n. sp. The geographical distributions of the two new species overlap, but their micro-habitats differ from each other; the former lives on calcareous algae on rocky shores, the latter lives on silty sand bottom within the inner bay. A third new species, S. slipperi sp. nov., is also described. In view of their present geographical distributions and fossil records, the origin of this group of species would appear to be the Japanese islands or adjacent areas in and after the Miocene. This group then migrated to the Arctic Ocean and East Pacific Ocean during or before the middle Pliocene.

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