Kaoru Ogane

A Cenozoic record of the equatorial Pacific carbonate compensation depth

Atmospheric carbon dioxide concentrations and climate are regulated on geological timescales by the balance between carbon input from volcanic and metamorphic outgassing and its removal by weathering feedbacks; these feedbacks involve the erosion of silicate rocks and organic-carbon-bearing rocks. The integrated effect of these processes is reflected in the calcium carbonate compensation depth, which is the oceanic depth at which calcium carbonate is dissolved. Here we present a carbonate accumulation record that covers the past 53 million years from a depth transect in the equatorial Pacific Ocean. The carbonate compensation depth tracks long-term ocean cooling, deepening from 3.0–3.5 kilometres during the early Cenozoic (approximately 55 million years ago) to 4.6 kilometres at present, consistent with an overall Cenozoic increase in weathering. We find large superimposed fluctuations in carbonate compensation depth during the middle and late Eocene. Using Earth system models, we identify changes in weathering and the mode of organic-carbon delivery as two key processes to explain these large-scale Eocene fluctuations of the carbonate compensation depth.

Stratigraphic relationships between the last occurrence of Neogloboquadrina inglei and marine isotope stages in the northwest Pacific, D/V Chikyu Expedition 902, Hole C9001C

The stratigraphic relationships between the last occurrence (LO) of the planktic foraminiferan Neogloboquadrina inglei in the middle Pleistocene and established marine isotope stages (MIS) was investigated using a 365-m-long sediment core from a continental slope in the northwest Pacific near the Shimokita Peninsula, Japan. Two tephra layers (Shikotsu-Daiichi and Aso-4 tephras) and two nannofossil datum planes (first occurrence of Emiliania huxleyi and LO of Pseudoemiliania lacunosa) were used as age- control points, and the oxygen isotope stratigraphy of Hole C9001C was established by correlating the oxygen isotope values of the benthic foraminiferan Uvigerina akitaensis with the standard oxygen isotope curve LR04. Hole C9001C provides the first continuous, high-sedimentation-rate (20-90 cm/kyr) record from MIS 18 to present in the northwestern Pacific near Japan. The stratigraphic position of the LO of N. inglei is in late MIS 16 or near the MIS 16/15 boundary.

The Ehrenberg type species of flat-shaped radiolarian genera (Spongodiscidae and Stylodictyidae, Spumellaria, Polycystina)

Synopsis Original material for almost all of the polycystine radiolarian species described by Christian Gottfried Ehrenberg (∼8o genera, ∼530 species) are housed in the Museum für Naturkunde, Berlin. Eleven type specimens of 10 spongodiscid and stylodictyid radiolarian genera were found in Ehrenbergs original mica strips. These micas are from six different sample locations and range in geological age from (approximately) late Eocene to Holocene. Ten specimens are confirmed as those sketched by Ehrenberg and are designated as lectotypes. Consequently, Dictyocoryne profunda is the senior valid synonym of Euchitonia für cata and Rhopalodictyum abyssorum. The Ehrenberg collection contains several specimens identified as F. concentrica, the type species of the genus Flustrella. After careful examination, the best preserved specimen was defined as the lectotype and, as a result, Flustrella (type species: F. concentrica) is confirmed as a valid name, while Porodiscus (Trematodiscus orbiculatus) and Trematodiscus (T. orbiculatus) are junior synonyms of Flustrella. The type species of Spongodiscus (S. resurgens), Spongaster (S. tetras) and Dictyocoryne (D. profunda) possess concentric internal structures, suggesting close phylogenetic relationships with Flustrella. Lectotypes of Stylodictya (S. gracile), Perichlamydium (Flustrella praetexta), and Stephanastrum (S. rhombus) are designated.

Paleoceanographic affinities of radiolarian faunas in late Aalenian time (Middle Jurassic) recorded in the Jurassic accretionary complex of Japan

Abstract A total of 140 polycystine radiolarians (84 nassellarians and 56 spumellarians; called Mn-03 assemblage herein) were identified from a manganese nodule in mudstone of the chert–clastic sequence distributed in the Jurassic accretionary complex of the Kuzumaki–Kamaishi Belt, Northern Kitakami Mountains, Northeast Japan. The horizon yielding this well-preserved assemblage was correlated to the uppermost part of the radiolarian JR4 Zone (the Upper Aalenian, Middle Jurassic) on the basis of the presence of Tricolocapsa tegiminis , the probable decendant of Laxtorum (?) jurassicum , and the absence of L. (?) jurassicum and Tricolocapsa plicarum . The Mn-03A assemblage is of high diversity but shows nearly half of the diversity of the contemporaneous assemblage which was reported from manganese nodules in another Jurassic accretionary complex of the Mino Belt in Japan. Both assemblages are considered to have been derived from the same province due to faunal similarity. The difference between both assemblages is explained by lower productivity in the depositional region of the Mn-03A assemblage. Comparison of the Mn-03A assemblage to the contemporaneous assemblage indicates that the fauna lived in the equatorial to lower latitudinal zone of the Pacific Superocean in late Aalenian time.

Pseudopodial silica absorption hypothesis (PSA hypothesis): a new function of pseudopodia in living radiolarian polycystine cells

The secretion process of the siliceous skeleton in polycystine radiolarians has drawn a great deal of interest during the last century; however, little is known about the actual physiological process of silica deposition. Recently, the PDMPO (2-(4-pyridyl)-5-[(4-(2-dimethylaminoethylaminocarbamoyl) methoxy)-phenyl] oxazole) method for staining silica deposition sites in polycystines was developed. In the present study we examined over 30 polycystine cells with PDMPO and found that both the skeletons and pseudopodia of three species (Lithelius sp., Rhizosphaera trigonacantha and Arachnosphaera hexasphaera) were stained and emitted green fluorescent light. Staining of the skeleton was probably the result of skeletal thickening growth, whereas staining of the pseudopodia may indicate that siliceous matter is assimilated within pseudopodia. We refer to this hypothesis as the ‘pseudopodial silica absorption hypothesis’ (PSA hypothesis). If this hypothesis is correct, PSA is an intermittent process, and the absorbed silica within pseudopodia is quickly transferred to the cytokalymma where it is deposited on the skeleton. To date, the PSA process has been observed in only the three species cited above; therefore we are unable to evaluate whether the PSA process is unique to these species or a common process that occurs in all polycystines; further investigation is necessary.

A SECTIONING METHOD USING NAIL POLISH FOR OBSERVATION OF THE INTERNAL STRUCTURE OF DISK-SHAPED POLYCYSTINE RADIOLARIANS

Examinations of the internal structures of microfossils provide important information about phylogenetic relationships and growth patterns. The internal structure of microfossils was first observed by sectioning during the late nineteenth century (e.g., Schlumberger, 1893, 1896), and further sectioning methods were developed in the twentieth century (e.g., Hemleben et al., 1977; Kennedy, 1978; Coleman, 1979; Nomura, 1983; Suzuki, 1998). The internal structure of polycystine radiolarians provided important information on phylogenetic relationships and classification due to the varying internal structures that are characteristic within each family or subfamily (Dumitrica, 1970, 1983, 1985, 1989; Takemura, 1986; Takemura and Nakaseko, 1986; De Wever et al., 2001). An array of disk-shaped polycystine forms that appears to be uniform may in fact contain many different internal structures (Kozlova, 1967a, 1967b; Dumitrica, 1973, 1983, 1985, 1988, 1989, 1991; De Wever et al., 2001; Dumitrica and Zugel, 2002). Kozlova (1967a, 1967b) studied the internal structure of the family Porodiscidae, the initial skeleton of which is similar to that of the family Larcoidea. She described three new genera on the basis of internal structure, according to the method detailed by Petrushevskaya and Kozlova (1972). Dumitrica (1973, 1983, 1985, 1988, 1989, 1991) and Dumitrica and Zugel (2002) also observed the internal structure of many spumellarian or entactinarian polycystines, including discoidal types. In these previous studies, disk-shaped polycystine individuals were observed using traditional sectioning methods. Kozlova (1967a, 1967b) examined the internal structure of disk-shaped polycystines by grinding the specimens while they were embedded in Canada balsam. Dumitrica (1970, 1973, 1983, 1985, 1988, 1989, 1991), De Wever et al. (2001), and Dumitrica and Zugel (2002) sectioned …