Late Miocene ostracods from the Fujikotogawa Formation, northern Japan – with reference to cold water species involved with trans-Arctic interchange

Seventy-eight ostracod species belonging to 38 genera are recognized from the late Miocene Fujikotogawa Formation (c. 7–8 Ma), 40 km NE of Akita City, northern Japan. Some 30–40% of the ostracod species belong to the cold water groups (circumpolar and cryophilic species) reported from Plio-Pleistocene formations yielding the Omma-Manganji Fauna, the name given by Otuka (1939) to the Pliocene Japanese cold water molluscan fauna. This study demonstrates that most ostracod species distinguished in deposits yielding the Omma-Manganji Fauna had already appeared in the late Miocene. At least 13 of the ostracod species have been reported from both the Arctic and northern Atlantic Oceans, implying migration from the Pacific to the northern Atlantic through the Arctic after the Bering Strait had been breached. The 13 circumpolar, nine cryophilic and four endemic cold water species are illustrated, with brief taxonomic notes.


INTRODUCTION
One of the most significant geological events in the late Neogene of high northern latitudes is the Pliocene opening of the Bering Strait. It is thought to have brought about a drastic launal change in northern oceans. In order to investigate the dynamics of ostracod species in relation to the opening of the Bering Strait, it is important to compare Miocene to Recent ostracod assemblages from Arctic, northern Atlantic and northern Pacific Provinces. Ostracod species inhabiting such frigid to cold water environments as the Arctic and northern Atlantic have been reported long since by many workers. In recent years, Hazel (1967Hazel ( , 1970 and Hazel &: Valentine (1969) investigated detailed distributions of many ostracod species from off northeastern North America; Neale & Howe (1973) studied ostracod assemblages from high latitude seas such as Novaya Zemlia; Siddiqui & Grigg (1975) investigated Recent ostracods from Nova Scotia, North America and Canada; Penney 11989) investigated Recent ostracods of the Ikerssuak (Bredefjord) District in southwestern Greenland; Hartmann (1992) reported living and subfossil ostracods of the Liefdefjorden (North Spitsbergen). Late Neogene fossil ostracods in eastern North America, northern Europe, and the Arctic have also been studied by a large number of workers (Swain, 1963;Hazel, 1968;Cronin, 1981Cronin, , 1988Cronin, , 1991aCronin & Dowsett, 1990;Marincovich et a/., 1990;Brouwers i:t ul., 1991).
On the other hand, in northern Japan many cold water ostracod species have been reported by many workers from Plio-Pleistocene formations yielding the Omma-Manganji Fauna, which was assigned to cold water fossil molluscan assemblages in the Pliocene of the Japan Sea side by Otuka (1939). Hanai (1957aHanai ( , b, c, 1959aHanai ( , b, 1970 conducted the taxonomic work of typical high latitude genera such as Palmenella, Cythere and Semicytherura from the Plio-Pleistocene Setana and Sawane Formations (a and e in Fig.   1). Okada (1979) and Ishizaki Rr Matoba (1985) showed some cold water ostracods from F'lio-Pleistocene formations in Akita Prefecture. Tabuki (1986) recognized some genera and species, reported from the north Atlantic to Arctic Provinces, in the Plio-Pleistocene Daishaka Formation (c in Fig. 1) . Hanai & Yamaguchi (1987) also found those species in the early Pleistocene Hamada Formation (b in Fig. 1) . Cronin & Ikeya (1987) presented a preliminary report of ostracod assemblages from the representative formations yielding the Omma-Manganji Fauna and distinguished three different species groups: 26 circumpolar and 21 cryophilic species in the northwestern Pacific, and the remaining temperate endemic species. Hayashi (1988) discussed a palaeoenvironment during deposition of the Plio-Pleistocene Setana Formation by means of fossil ostracod assemblages. Irizuki (1989b) investigated ostracods from the late Pliocene Sasaoka Formation (d in Fig. 1) and suggested that 20-40% of the assemblages were species inhabiting cold waters. Ishizaki et al. (1993) reported ostracod assemblages from the Omma Formation distributed in the type locality ( f i n Fig. 1) and delineated vertical environmental changes during early to middle Pleistocene time. As a result, some of these ostracod species in the Plio-Pleistocene are common to those in the Arctic and north Atlantic, which suggests that species in the Arctic and/or those in the northwestern Pacific migrated through the Bering Strait.
In Japan, only a few attempts have so far been made at the study of ostracod assemblages of pre-opening of the Bering Strait. The Fujikotogawa Formation is almost the only deposit yielding abundant late Miocene shallow water calcareous fossils on the Sea of Japan side of northern Japan. The purpose of the present study is to report some ostracod species which inhabit the Arctic and northern Atlantic and had already appeared during late Miocene time (pre-opening of the Bering Strait) in the northwestern Pacific and to discuss the origin of such cold water ostracods and their migration during the late Neogene. geology was investigated in detail by Sumi & Moritani (1973) and Ogasawara et al. (1986). It overlies conformably the Iwaya Formation. The Lower Fujikotogawa Formation can be divided into four parts on the basis of lithology: a lower part consisting of fine to medium calcareous black  sandstone yielding many calcareous fossils (FC in Fig. 2); this is succeeded by fine sandstone (FS); followed by alternations of sandstone and black mudstone (FA); the upper part consists of black mudstone rich in siliceous fossils (FM). The lower Fujikotogawa Formation is assigned to the late Miocene and correlated with the Funakawa Formation distributed around Akita City . Saito & Izawa (1988) reported such planktonic foraminifers as Glohorotalia praemargaritae Catalan0 & Sprovieri and Glohorotaloides falcotzarae Giannelli & Salvatorini, which indicate zone N. 17 of Blow (1969) from the Fujikotogawa Formation. Izawa (pers. comm.) reported a nannofossil species, Discoaster quinquerarnas Gartner, showing zone CN 9 of Okada & Bukry (1980) from sandstones of the lower part (near sample FC-11). The age of the lower part of this formation is thus suggested to be 7-8 Ma.

MATERIALS AND METHODS
Fifteen rock samples were collected from the lowest part of the Fujikotogawa Formation (FC in Fig. 2). However, only five samples yielded sufficient numbers of ostracod specimens for study   (Fig. 2). Eighty-gram sediments were disaggregated by using a saturated sodium sulfate solution and naphtha, washed through a 200 mesh sieve screen, and dried. A fraction coarser than 125 p m ( 1 15 mesh) was sieved and divided by a sample splitter into aliquot parts, from which 200-odd specimens of ostracods were picked and placed on an assemblage slide.

DISCUSSION
Extensive studies on the origin of the molluscan Omma-Manganji Fauna and its migration have been made by many workers (e.g. Ogasawara, 1986). Durham & MacNt:il (1967) cited more than 12.5 megainvertebrates of Pacific origin which entered the Arctic-Atlantic region, while only 16 Arctic--Atlantic taxa occur widely in the North Pacific late Neogene strata. Recently, Vermeij (1991) identified 295 molluscan species that either took part in the trans-Arctic interchange or are derived from stocks that so did, and suggested that out of these species, 261 are of Pacific origin, whereas only 34 are of Arctic-Atlantic origin.
The first appearance of Pacific molluscs in the Atlantic region has been reported from a late Pliocene formation (Serripes groenlandicus zone) in Iceland, which is correlated to the Red Crag of England (MacNeil, 1973;Gladenkov, 1981). MacNeil (1973) etc.) and Hemicythere uillosa (Sars), are representative species ranging in age from late Pliocene (e.g. Red Crag of England, Tjornes in Iceland) to Recent in the Arctic and northern Atlantic Provinces, but they have never been reported from northeastern Japan. Several different congeneric species are found in the late Miocene Fujikotogawa Formation, which suggests that some species migrated from the northwestern Pacific to the Arctic-Atlantic to evolve into new species after 3.0 Ma.
Whether water temperature around northeastern Japan at the time when the Fujikotogawa Formation was deposited (about 7-8 Ma) was warmer or colder than today is yet to be determined. This question will be dispelled by further investigations in combination with isotope measurements.
Fossil assemblages from the Fujikotogawa Formation are correlated with those from Plio-Pleistocene formations yielding the Omma-Manganji Fauna based on Cronin & Ikeya (1987), Hayashi (1988), Ishizaki & Matoba (198.5), Irizuki (1989b), Okada (1979), Tabuki (1986) and unpublished data. About 90% of ostracod species from the Fujikotogawa Formation are common to those of Plio-Pleistocene formations. This indicates that the species composition of the Fujikotogawa Formation is very similar to that of Plio-Pleistocene sediments yielding the Omma-lrizuki Manganji Fauna, but generally the ratio of circumpolar and cryophilic species is higher in Plio-Pleistocene deposits than that in the Fujikotogawa Formation (see Cronin & Ikeya, 1987).

CONCLUSIONS
The late Miocene Fujikotogawa Formation yields important ostracod species for discussing the origin of cold water ostracod faunas and their migration. Major conclusions from the present study are as follows: 1. At least 13 sublittoral species from the Fujikotogawa Formation are found in the Arctic and northern Atlantic, suggesting migration from the Pacific through the Bering Strait (Table 1). 2. Atlantic species belonging to three hemicytherin genera, Bajinicythere, Finmarchinella and Hemicythere, may be descended from Pacific ancestors. 3. Ostracods such as Hemicythere and Sernicytherura were already diversified during the late Miocene and most ostracod species from the Fujikotogawa Formation are found common to the Plio-Pleistocene of northern Japan.
(PI. 1, fig. 2) Family Leptocytheridae Hanai, 1957 Genus Cluthia Neale, 1973 * Cluthia cfuthae Neale, 1973 Remarks. A specimen from the Fujikotogawa Formation resembles Cluthia juponicu (Tabuki, 1986) reported from the Pliocene Daishaka Formation, northern Japan, but differs from the latter in having two distinct swellings in the anterocentral area. This species is distributed in Recent sediments of the northwestern Atlantic to Arctic and its fossils were reported around Great Britain (Neale, 1973). Brouwers (1990) recognized it from the middle sublittoral zone of the northeast Gulf of Alaska.
+ Hemicythere ochotensis Schornikov, 1974 Remarks. This species resembles Hemicythere quad- rinodosa Schornikov, 1974 but the former has a posterior rim as opposed to a tubercle in the latter.

Remarks.
Poorly-preserved specimens from the Fujikotogawa Formation seem to be assigned to Bafinicythere sp.

Remarks.
The type specimen of Elofsonella concinna is a pitted form with a conspicuous anterior marginal rim, but specimens from the Fujikotogawa Formation are notably reticulate, with a less conspicuous anterior marginal rim. Bassiouni (1965) assigned pitted and reticulate forms respectively to E. concinna and E. neoconcinna, but Hazel (1967) thought that both types are conspecific because of the existence of intermediate forms.
Only juvenile specimens were found in the Fujikotogawa Formation.
* Finmarchinella japonicu (Ishizaki, 1966) Remarks. This differs from F. angulata (Sars) in having an anterior tubercle and more prominent posterior nodes. It (PI. 1, fig. 15) is a matter of argument, however, whether these differences of characters represent intraspecific variation or geographically separate subspecies, which is now being studied by the author.
Genus Urocythereis Ruggieri, 1950 -Urocythereis? gorokuensis Ishizaki, 1966 Remarks. This species was first described from the Pliocene Tatsunokuchi Formation in northern Japan (g in Fig. l), where the Tatsunokuchi Fauna, applied to cold shallow water molluscan assemblages in the Japanese Pliocene on the Pacific side, was reported (Nomura, 1938;Otuka, 1941). It was also found in abundance in Plio-Pleistocene formations yielding the Omma-Manganji Fauna.
(Pl. 2, fig. 3) Genus Robertsonites Swain, 1963 + Robertsonites hanaii Tabuki, 1986 Remarks. Specimens from the Fujikotogawa Formation are all juveniles. This species differs from R. reticuliformu Ishizaki in having a subcentral tubercle and ridges, and its valve is less protrudent toward the posterior than the latter.
(PI. 2, figs 10, 11) Genus Semicytherura Wagner, 1957 * Semicytherura mainensis Hazel & Valentine, 1969 Remarks. Specimens from the Fujikotogawa Formation have less conspicuous reticulation and subcentral tubercle than the type specimens but their ridge patterns are very similar. They also resemble Howeina higashimeyaensis Ishizaki, 1971, of which type specimens have prominent vertical ridges in the posterior portion, but have curved ridges. According to Hazel & Valentine (1969) and Siddiqui & Grigg (1975), this species ranges from the Pleistocene to Recent around Nova Scotia, eastern Canada and USA.

ACKNOWLEDGEMENTS
The author expresses his deep gratitude to Professor Kunihiro Ishizaki of the Department of Geoenvironmental Science, Faculty of Science, Tohoku University, for advice and continuous encouragement throughout the course of the present study and critical reading of the manuscript. Deep appreciations are also expressed to Emeritus Professor Yokichi Takayanagi and Professor Tsunemasa Saito of the same university for their assistance, suggestions and discussions concerning various aspects of the study, and reading of the manuscript. The author is indebted to Ilr Thomas M. Cronin of the US Geological Survey for constructive discussions and suggestions about North Atlantic ostracod assemblages. Sincere thanks are also due to Professor Kenshiro Ogasawara of Tsukuba University for constructive suggestions concerning the Omma-Manganji Fauna. Acknowledgements are also due to Shohei Otomo and Jun Nemoto of the Department of Geoenvironmental Science. Faculty of Science, Tohoku University, for their photographic assistance. A part of this study is supported financially by the Katayama Memorial Scholarship from Tohoku University.