Harumasa Kano

Geochemistry and geochronology of the Troodos ophiolite: An SSZ ophiolite generated by subduction initiation and an extended episode of ridge subduction?

New trace-element, radiogenic isotopic, and geochronologic data from the Troodos ophiolite, considered in concert with the large body of previously published data, give new insight into the tectonic history of this storied ophiolite, as well as demonstrating the variability of suprasubduction-zone ophiolites, and differences between them and commonly used modern analogs. Similar to earlier studies, we find that island-arc tholeiite of the lower pillow lava sequence erupted first, followed by boninite. We further divide boninitic rocks into boninite making up the upper pillow lava sequence, and depleted boninites that we consider late infill lavas. We obtained an Ar-Ar age from arc tholeiite of 90.6 ± 1.2 Ma, comparable to U-Pb ages from ophiolite plagiogranites. New biostratigraphic data indicate that most of the basal pelagic sedimentary rocks that conformably overlie the boninitic rocks are ca. 75 Ma. This suggests that voluminous eruption of boninitic rocks persisted until ca. 75 Ma. Limited eruption of boninitic lavas may have continued until 55.5 ± 0.9 Ma, based on the Ar-Ar age we obtained. The duration of arc magmatism at Troodos (at least 16 m.y., with some activity perhaps extending 35 m.y.) without the development of a mature arc edifice greatly exceeds that of other well-studied suprasubduction-zone ophiolites. We propose that Troodos was formed over a newly formed subduction zone, similar to many proposed models, but that the extended period of magmatism (boninitic) resulted from a prolonged period of ridge subduction.

PERMIAN RADIOLARIAN FAUNAS FROM CHERT IN THE KHABAROVSK COMPLEX, FAR EAST RUSSIA, AND THE AGE OF EACH LITHOLOGIC UNIT OF THE KHABAROVSK COMPLEX

Abstract The Khabarovsk Complex, a Jurassic accretionary complex distributed in and around the Khabarovsk city area, Far East Russia, comprises mélange and schist facies. From the review of previous studies including Russian papers, the lithology and age of the constituent rocks of the mélange facies can be summarized as follows: Upper Paleozoic basic volcanic rocks, mainly pillow lava, and altered gabbro, Upper Carboniferous to Upper Permian fusulinoidean-bearing limestone associated with tuff, Lower and Middle Jurassic siliceous mudstone, Upper Jurassic tuffaceous mudstone, uppermost Jurassic carbonate concretions embedded in mudstone, and age-unknown sandstone. Newly found sequences of limestone-chert and of basalt-chert in the mélange facies crop out along the Amur River in the Khabarovsk city area. A chert sample of the limestone-chert sequence contains Albaillella aff. asymmetrica and Pseudoalbaillella aff. lomentaria, and a chert sample of the basalt-chert sequence includes Follicucullus monacanthus, Follicucullus porrectus, and Pseudoalbaillella cf. yanaharaensis. The radiolarian assemblages from the limestone-chert and basalt-chert sequences have a maximum age of middle Early Permian and late Middle Permian, respectively, overlapping the time of deposition of the fusulinoidean-bearing limestone. The co-occurrence of chert and limestone indicates that the fusulinoidean-bearing limestone was formed on a basaltic topographic high in a pelagic ocean whereas the radiolarians accumulated in a deeper part. Limestone debris occasionally flowed into the depositional site of the radiolarian chert. Although the Khabarovsk Complex was simply considered as a northern extension of the Mino–Tamba Belt of the Inner Zone of southwest Japan, we propose a new correlation based on the lithologic associations. The mélange facies of the Khabarovsk Complex is correlative with one of the Kasugano, Funabuseyama, Nabi, and Yabuhara Formations in the Mino–Tamba Belt, whereas the schist facies is correlative with the Hikami Formation of the Ultra–Tamba Belt.

Observation of the dissolution process of Globigerina bulloides tests (planktic foraminifera) by X‐ray microcomputed tomography

We performed a 9 day dissolution experiment with tests of the planktic foraminifer Globigerina bulloides at pH 6.7 ± 0.1 in water undersaturated with respect to calcite. The initial stage of the dissolution process, which is not recognizable from the surface structure of the tests, was quantitatively evaluated by X-ray microcomputed tomography (XMCT). XMCT revealed three distinct test structures: early-developed calcite formed during the juvenile stage of G. bulloides, an inner calcite layer, and an outer calcite layer. The test ultrastructure was observed by scanning electron microscopy, and CT number evaluated the density distribution in the test. The early-developed calcite and inner calcite layer had low CT numbers (500–1300; low density, porous) and were sensitive to dissolution, whereas the outer calcite layer had high CT numbers (<1300; high density) and resisted dissolution. Both the modes and the frequencies of the CT numbers decreased with progress of dissolution. Changes in the CT number histogram with progress of dissolution were quantified in terms of the percentage of calcite volume accounted for by low-density calcite (% Low-CT-number calcite). A clear linear relationship (R2 = 0.87) between % Low-CT-number calcite and % Test weight loss was found. This relationship indicates that the amount of test dissolution can be estimated from the distribution of CT numbers. We propose that XMCT measurements will be useful for quantitatively estimating the amount of carbonate loss from foraminiferal tests by dissolution.

The role of symbiotic algae in the formation of the coral polyp skeleton: 3‐D morphological study based on X‐ray microcomputed tomography

Symbiotic algae of primary polyps play an important role in calcification of coral skeletons. However, the function of the symbiotic algae, including the way they influence the physical features of their host skeleton under various conditions, is not well understood. We used X-ray micro-computed tomography to observe skeletal shape characteristics in symbiotic and aposymbiotic primary polyps of Acropora digitifera that were cultured at various temperature and pCO2 levels (temperature 27, 29, 33 °C; pCO2 400, 800, 1000 µatm). Symbiotic polyps had a basal plate with a well-developed folding margin supporting the branched skeleton, whereas aposymbiotic ones did not. The features of the folding margin suggest that it might be the initial growth stage of the epitheca. In addition, three-dimensional (3-D) morphological measurements made by X-ray micro-computed tomography show that the branched skeletons of symbiotic primary polyps were taller than those of aposymbiotic ones, suggesting that zooxanthellae in coral primary polyps play a critical role in the height growth of skeletal branches. Furthermore, results of the temperature- and pCO2-controlled experiments suggest that global warming might greatly affect the activity of zooxanthellae, whereas ocean acidification might reduce calcification by damaging the coral host itself. Our findings provide new knowledge about the role of zooxanthellae in coral calcification. This article is protected by copyright. All rights reserved.

Thylacocephala (Arthropoda) from the Lower Triassic of the South Kitakami Belt, Northeast Japan

Abstract. A thylacocephalan fauna from the upper Olenekian (Lower Triassic) Osawa Formation in the South Kitakami Belt, Northeast Japan, is described. The fauna comprises three species belonging to three genera: Ankitokazocaris bandoi Ehiro and Kato sp. nov., Kitakamicaris utatsuensis Ehiro and Kato gen. et sp. nov., and Ostenocaris sp. This is the first report of fossils belonging to the class Thylacocephala from Japan, and it expands their paleogeographic distribution. As almost all genera of Thylacocephala inhabited low-latitude areas in each relevant geological time, their occurrence from the Osawa Formation supports previous opinions that the South Kitakami Belt was located in the equatorial region during the Triassic.

The effects of surface roughness of sediment particles on the respiration of case-bearing caddisfly larvae

For aquatic tube dwellers, the ventilation system is an essential trait because the external structure impedes gas exchange. Their gas-exchange efficiency is determined by various environmental factors. We assessed the effect of microscale surface roughness (texture) of the inner wall of the case on the respiration of caddisfly larvae Psilotreta kisoensis Iwata (Odontoceridae:Trichoptera). We compared the O2 consumption of larvae in 2 types of cases constructed from smooth and rough artificial particles at 2 water temperatures (15 and 23°C). Larvae in smooth cases consumed more O2 than those in rough cases at 23 but not at 15°C. Larvae in rough cases exhibited no significant respiration response to water temperature. In contrast, larvae in smooth cases increased their O2 consumption as the temperature increased. In addition, the increasing degree of O2 consumption tended to be higher in lightweight immature larvae. These results indicate that a rougher surface depresses larval respiration activity, probably because of friction between the larval body and case wall, which may lead to high mortality and low growth. The roughness of sediment particles is determined by local geology and causes plasticity or intraspecific local variation in case-making behavior. Therefore, the roughness of sediment particles is not only a limiting factor for animal distribution but also exerts a locally different evolutionary pressure on animal respiration systems.

Ammonoid Fauna of the Upper Olenekian Osawa Formation in the Utatsu Area, South Kitakami Belt, Northeast Japan

Abstract. A late Olenekian (Early Triassic) ammonoid fauna is described from the Osawa Formation in the Utatsu area, South Kitakami Belt, Northeast Japan. The fauna comprises 14 species belonging to 13 genera: Hemilecanites discus (Arthaber), Albanites sheldoni (Kummel), Pseudosageceras? sp., Pseudokymatites tabulatus sp. nov., Columbites parisianus Hyatt and Smith, Tardicolumbites aff. tardicolumbus Guex et al., Yvesgalleticeras sp., Subcolumbites sp., Hellenites elegans Guex et al., Metadagnoceras sp., Procarnites sp., Leiophyllites wakoi sp. nov., Leiophyllites? sp., and Nordophiceratoides bartolinae Guex et al. Among them, genera Hemilecanites, Albanites, Pseudokymatites, Tardicolumbites, Yvesgalleticeras, Hellenites, and Nordophiceratoides are described for the first time from the Japanese Triassic. The generic composition of the ammonoid fauna of the Osawa Formation has thus increased to 25 genera by adding these to the 18 existing genera. This high taxonomic diversity indicates that the nektonic faunas had already recovered from the end-Permian mass extinction by the beginning of the late Olenekian in the South Kitakami Belt. The ammonoid fauna of the Osawa Formation is similar to that of the late Olenekian of Idaho, North America, and in addition, it has some faunal similarity to those in the western Tethys province. These faunal characteristics are likely to reflect the paleogeographic position of the South Kitakami Belt, which was located at the low-latitude area of the Tethys—Panthalassa border during the Triassic.

Effect of seawater turbulence on formation of coral primary polyp skeletons

Though seawater movement is among the important factors that maintain healthy conditions on coral reefs, little is known about the effects of seawater turbulence on the growth of coral primary polyps. We cultured aposymbiotic primary polyps of Acropora digitifera with and without seawater turbulence and investigated how seawater turbulence affected the physical characteristics of the polyp skeleton. Results showed that seawater turbulence increased the amount of skeletal carbonate and directly influenced calcification of coral hosts. Three-dimensional morphological measurements by X-ray micro-computed tomography revealed that seawater turbulence increased the thickness of the skeletal branches of polyps. Greater thickness of the skeletal branches and increased horizontal growth may significantly enhance survival of primary coral polyps during early life stages before the establishment of algal symbioses.