This study estimates paleoenvironmental changes in water depths and temperatures from approximately 0.6 Ma during the marine oxygen isotope stage (MIS) 15 of the Middle Pleistocene along coastal Japan. We used the modern analog technique (MAT), which is based on ostracod assemblages from the Naganuma Formation in the Sagami Group of Kanagawa Prefecture in central Japan. The analysis revealed that the Naganuma Formation was deposited in the shallow-marine environment at paleo-water depths of 25–41 m, similar to current depths of southwestern Japan coastal areas such as the Setonaikai Sea. The estimated summer paleo-water temperature (ca. 28℃) was approximately 2℃ higher than current Sagami Bay water temperatures at depths of shallower than 30 m. The estimated temperatures were also similar to or slightly lower (~2℃) than current summer temperatures in Sagami Bay at depths of 40 m. The estimated winter paleo-water temperatures (6–11℃) were slightly lower (~2℃) than current Sagami Bay temperatures at depths of shallower than 40 m. This study reports for the first time fluctuations in paleo-water temperature estimates quantitatively from 0.6 Ma during the MIS 15 along coastal Japan.
The middle to upper Miocene Shida Group is widely distributed in the northeastern Sendai area, Miyagi Prefecture, Northeast Japan. This group is divided into the Irisugaya, Bangamoriyama, Aoso and Nanakita Formations in ascending order. Among them, the Aoso Formation is mainly composed of ocean current-dominated sedimentary facies and yields abundant molluscan and marine vertebrate fossils. Therefore, this formation has been focused for paleoenvironmental, paleogeographic and paleontological studies of Northeast Japan. In this study, we established integrated biostratigraphy using planktonic foraminifera and diatom for the Shida Group in the northeastern Sendai Plain, which includes the type locality of the Aoso Formation. We also report fission track (FT) ages of two pyroclastic layers of Aoso and Nanakita Formations. Consequently, the Irisugaya Formation is correlated with the diatom zone NPD5B. The Aoso Formation is assigned to the diatom zone NPD5D and planktonic foraminiferal zone N.16–N.17A. The Nanakita Formation is correlative with zones NPD6A and NPD6B. FT ages of the uppermost part of the Aoso Formation and the lower part of the Nanakita Formation were determined at 9.3 ± 0.4 Ma and 7.6 ± 0.7 Ma, respectively. These results are concordant with each other and support previous chronostratigraphic data of adjacent areas.
The age of pelagic Panthalassic deep-sea bedded chert has been assigned based on radiolarian biostratigraphy. However, Triassic radiolarian biostratigraphy is in many cases not precisely correlated to the conodont zones and the standard geological timescale. In this study, we investigated the conodont biostratigraphy of two radiolarian-controlled bedded chert sections of Anisian age: the Ajiro Island section in Oita Prefecture and the Kurusu section in Aichi Prefecture. We recognised six conodont biozones in the studied sections: the upper Olenekian Novispathodus brevissimus-Icriospathodus collinsoni and Triassospathodus homeri Zones, the lower Anisian Chiosella timorensis Zone, the middle Anisian Paragondolella bulgarica Zone, the upper Anisian Paragondolella excelsa Zone and the uppermost Anisian to lowermost Ladinian Paragondolella trammeri Zone. These conodont zones were successfully correlated to the standard Triassic radiolarian zonation proposed by Sugiyama (1997, Bull. Mizunami Foss. Mus., vol. 24, p. 79–193). Sugiyama’s radiolarian TR 1 Zone, previously considered to be of Olenekian age, extends to the middle Anisian. The TR 2A Zone, the TR 2B Zone and the lower part of the TR 2C Zone are correlated to the middle Anisian, while the upper part of the TR 2C Zone and the lower part of the TR 3A Zone are correlated to the upper Anisian. The upper part of the TR 3A Zone and the lower part of the TR 3B Zone are probably correlative to the uppermost Anisian, but the possibility that they are correlative to the lowermost Ladinian cannot be ruled out.
The Middle Miocene Tsurushi Formation distributed in Sado Island is composed of the lower basalt and upper mudstone. This formation is important to elucidate the geohistory of Sado Island and the expansion process of Japan Sea.
We tried to clarify the geologic age of the Tsurushi Formation by using biostratigraphical data on microfossils from this formation. We identified radiolarian fossils from three sites (Sites 1 to 3) of the Tsurushi Formation and correlated the radiolarian assemblages with the previously proposed radiolarian zones. At the Site 1, we discriminated Eucyrtidium inflatum. E. inflatum indicate the E. inflatum Zone, which corresponds to 15.3–11.7 Ma. At the Site 2, E. inflatum, Lychnocanoma kamtschatica and Lychnocanoma magnacornuta were identified. On the basis of the occurrences of these species, the formation of the Site 2 can be correlated with the E. inflatum Zone (15.3–11.7 Ma) to the L. magnacornuta Zone (11.7–9.1 Ma). L. kamtschatica has been reported only from the high latitudes of the North Pacific.The occurrence of this species from the Tsurushi Formation is its first occurrence from the Japan Sea region. At the Site 3, relatively a lot of specimens of Cyrtocapsella tetrapera and Cyrtocapsella japonica are identified, which indicate the Subzone a of the E. inflatum Zone or older zones.
In late August 2014, a living polycystine radiolarian assemblage was collected from the surface waters off the shore of Sakata, Yamagata Prefecture, in the eastern margin of the Japan Sea. The assemblage was dominated by the spumellarian Spongosphaera streptacantha, and included a few individuals of Pseudocubus obeliscus and Tetrapyle octacantha. The intraspecific variation in S. streptacantha observed in the main spine and spongiose layer was thought to reflect skeletal growth. Morphometric analysis of the main spine and spongiose layer indicated that most individuals of S. streptacantha were growing. On the sampling day, the surface water attained the annual maximum water temperature and was strongly influenced by the Tsushima Warm Current. S. streptacantha inhabited the summer surface water of this ocean area almost exclusively.
The latest Middle Devonian to early Carboniferous (Mississippian) radiolarian fossils (e.g. Trilonche cf. vetusta) has been obtained from the muddy matrix or mud-chips in the conglomerate in the early Carboniferous Nedamo Complex of the Nedamo Terrane, Northeast Japan. This conglomerate bed lies conformably on the mudstone bed including early Carboniferous radiolarian fossils reported by the previous study. The conglomerate is also almost composed of terrigenous rock clasts (e.g. felsic tuff, tuffaceous clastic rocks, mudstone), and strongly deformed. The radiolarian fossils in the conglomerate support adequacy of the early Carboniferous accretion age of the Nedamo Complex, which was constrained by the Late Devonian conodont age from the chert in accordance with the oceanic plate stratigraphy. The discovery of the radiolarian fossils in this study is a second report from the terrigenous rocks in the Nedamo Complex. These reports are important to discuss the tectonics of an early Carboniferous island-arc-trench system, because the Nedamo Complex is the only Carboniferous accretionary complex recognizable in the Japanese Islands.
The Cretaceous Shimanto accretionary complex, which is composed of the Taniyama, Hiwasa, and Mugi units, is distributed in the Umaji district, Kochi Prefecture, Southwest Japan. The Taniyama and Mugi units are characterized by mélange units including blocks of sandstone, chert and basalt. We report radiolarian fossils from cherts of the Taniyama and Mugi units, indicating a latest Barremian to early Aptian age and a late Santonian to early Campanian age, respectively. These ages obtained in the Umaji district are coincident with the chert radiolarian ages reported from the both units distributed in surrounding area of eastern Shikoku. A clear gap of chert ages clarified from both units supports that subducted oceanic slabs between the Taniyama and Mugi units were changed from the old Izanagi Plate to the young oceanic plates with ridge.
Progress in Neogene radiolarian biostratigraphy in Japan and the North Pacific region during the last two decades is characterized by (1) the direct correlation between the radiolarian biostratigraphy and the magnetostratigraphy, which has now been extended back to the Middle Miocene using deepsea cores, (2) advances in regional Neogene radiolarian biostratigraphy from the Northwest Pacific, the California margin, and the Japan Sea, and (3) application of the revised zonations to correlate onshore Neogene sections. These results have been supported by the reinvestigation of type-species described by earlier studies in the 19th to earliest 20th century, revision of taxonomic systems from lower to higher taxa, and researches for new biostratigraphic index fossils. Great advances have also been made in Quaternary radiolarian biostratigraphy in the nearshore seas surrounding the Japanese islands, proposing new radiolarian zones for the Kuroshio and Oyashio regions and the Japan Sea.
Totally 183 species or species group of radiolarians including 78 spumellarians and 105 nassellarians were identified from the lower to middle Miocene at IODP Site U1335 in the eastern equatorial Pacific. Photographs have been illustrated in 18 plates. These taxa include 70 unidentified forms, particularly within the families Actinommidae, Collosphaeridae, Hexalonchidae, Litheliidae, Pyloniidae, Spongodiscidae and Stylodictyidae. Some of these taxa might be new species.
The purpose of this study is to present microphotgraphs of all the encountered taxa of the middle to late Miocene age from Ocean Drilling Program (ODP) Site 1021 in the eastern North Pacific in order to analyze the species diversity of radiolarians. Totally 149 species or species groups of radiolarians were identified from Site 1021 in the eastern North Pacific. Micro-photographs have been illustrated in the 24 plates, and a new species, Lychnocanoma californica, is described.
In this study, we have illustrated the 84 species/species group of polycystine radiolarians, which were commonly encountered in the upper Miocene sediments at the Site U1425 of the IntegratedOcean Drilling Program (IODP) in the Japan Sea. The micro-photographs of these species/species group are illustrated in 9 plates.
Recently, taking long-slab samples from marine/lake sediment cores and preserving them for research and outreach purposes has gained attention by researchers and educators. Epoxy resin is one of the ways to preserve the sediment materials permanently. Here, we report a procedure to make epoxyresined samples with long and thin sediment samples, which were subsampled with aluminum sampling tools (long-slab samples). We used marine sediment cores retrieved from the Yamato Rise, Japan Sea during the Integrated Ocean Drilling Program (IODP) Expedition 346. The long-slab samples were dehydrated with acetone and impregnated with the Spurr resin, epoxy resin. The state of the curing of the resined samples was examined in the cross-section of one of the test samples after hardened by heating.The surface of the resined samples was hardened enough as expected. Although the inside of the long-slab samples was not enough hardened, the resined samples were good enough in quality for outreach purpose.
The Ashio terrane, which is a Jurassic accretionary complex, is exposed in the Hachioji Hills, eastern Gunma Prefecture, central Japan. The Ashio terrane in the Hachioji Hills mainly comprises chert and muddy mixed rock with siliceous mudstone and sandstone. Early Permian (Sakmarian, isuralian), Late Triassic (Carnian–Norian), and Jurassic radiolarians occurred in the chert, whereas Middle Jurassic (Bajocian–Bathonian) radiolarians occurred in the siliceous mudstone.
Paleozoic-Mesozoic radiolarian research developed rapidly in the latter half of the 20th century in Japan. Based on passage of the author’s research, the development process of radiolarian research in Japan is divided into 4 periods: (1) before 1968: Paleozoic-Mesozoic radiolarians were not admitted the biostratigraphical validity, (2) 1969–1978: Mesozoic-type radiolarian fossils have begun to be found from non-limestone facies “Paleozoic”, (3) 1979–1988: Paleozoic-Mesozoic radiolarian biostratigraphy developed rapidly, and the reality as sedimentary complex of “Paleozoic-Mesozoic strata” was elucidated, and (4) after 1989: based on the analysis of radiolarian fossil assemblage, approach to paleoenvironmental reconstruction has started.
Next factors existed as background of rapid development of radiolarian research: (1) radiolarian characteristics (high diversity and variability of species, high population, comparatively stable shell component, stratigraphically continuous occurrence), (2) spread of modernization of research method such as hydrofluoric acid (HF) treatment, scanning electron microscope (SEM) and computer, (3) organization of research system (Grants-in-Aid Co-operative Research (A), International Cooperation Research Project, International Collaborative Research etc.) and (4) organization and internationalization of information exchange (Radiolarian Symposium, International Conference etc.)
I have started to study the recent radiolarian biostratigraphy, when I have met Prof. E.F.Pessagno of UTD (University of Texas at Dallas), who presented an excellent view of a radiolarian body extracted from a Mesozoic siliceous sedimentary rock, June 4, 1976, at International Meeting of IGCP 115 (Siliceous deposits of the Circum-Pacific region) held at USGS, Menlo Park, California. During my stay in Canada and US, I informed to Prof. Pessgno, where I was, and asked to teach me how to separate the fossils from siliceous rocks, and how to examine them. I moved from Canada, to Harvard and finally to UT at Austin. February 16, 1977, I flied from Austin to Dallas-Fortworth, Prof. Pessagno kindly sent a guide who can understand Japanese in order to bring me from the airport to the campus of his lab. Prof. Pessagno demonstrated how to treat the rock-specimen and to separate the fossils and how to observe under SEM (Scanning Electoron Microscope), and suggested me what kind of literature is good for us to study the radiolarian biostratigraphy. After my sabbatical leave, I return to Nagoya; fortunately, I have got a budget for the scientific study in the fall of 1978 in order to purchase an inexpensive SEM (JM-T20) in our lab. The first discovery of a Jurassic radiolaria had been made by a student (Sakai, 1979). Thereafter, we have worked for the evidence of wide distribution of Jurassic formations in the Mino area, central Japan. The results have been published as an authorized paper in Proceedings of Japan Academy (Mizutani et al., 1981). This paper, entitled “Personal history on my study of radiolarian biostratigraphy”, as noted above, has outlined the memories of my own study on the radiolarian biostratigraphy particularly in relation to the Pessagno’s school of UTD.
More than ten volumes of Proceedings of the Japanese radiolarian symposiums had been published for 25 years as Special Volumes of News of Osaka Micropaleontologists (NOM). The term, NOM is an abbreviation of these publications and had also been used as a name of the meetings of micropaleontologists around the Kansai area, including Osaka, Kyoto, Kobe and Nara. The histories of NOM and both Japanese and international radiolarian symposiums are briefly reviewed in this paper.