Reishi Takashima

New perspective on Aptian carbon isotope stratigraphy: Data from δ13C records of terrestrial organic matter

Carbon isotope analyses were performed on detrital woody materials in Aptian (mid- Cretaceous) marine sediments of central Hokkaido, northern Japan. A positive δ13Cwood excursion (from −25.4‰ to −21.8‰) following a remarkable negative isotope shift is recognized in the early Aptian, and a small positive anomaly is also found in the latest Aptian. This δ13Cwood profile is exactly in phase with the δ13Ccarbonate curve from a Pacific guyot when the age of the guyot carbonates is revised using published Sr isotope stratigraphies. The highly conformable δ13C profiles of Pacific and Hokkaido sections suggest that δ13C compositions of Aptian marine and terrestrial carbon reservoirs changed simultaneously by the same amplitude within the ocean-atmosphere-biosphere system. Fluctuation patterns of Tethyan δ13Ccarbonate curves are slightly different from those of Pacific and Hokkaido sections. Such differences in δ13C profiles may be attributed to the local paleoceanographic setting of the Aptian Mediterranean Tethys.

Planktonic foraminiferal zonation in the Cretaceous Yezo Group, Central Hokkaido, Japan

Abstract The mudstone of the Yezo Group exposed in Central Hokkaido yields abundant microfossils of calcareous nannofossils, foraminifers, radiolarians and dinoflagellates. Benthic foraminifers consisting of both agglutinated and calcareous species occur abundantly and consistently throughout the sequence, while specimens of planktonic foraminifers are generally fewer than benthics in all samples. We recognized the following 13 planktonic foraminiferal zones assigned to the late Aptian to early Campanian in the Oyubari and Haboro–Kotanbetsu areas; (1) Globigerinelloides spp., (2) Ticinella primula , (3) Biticinella breggiensis , (4) Rotalipora subicinensis – Rotalipora ticinensis , (5) Rotalipora appenninica , (6) Rotalipora globotruncanoides , (7) Rotalipora cushmani (8), Whiteinella archaeocretacea (9) Helvetoglobotruncana helvetica , (10) Marginotruncana pseudolinneiana , (11) Marginotruncana sinuosa , (12) Contusotruncana fornicata , (13) Globotruncana arca . The Globigerinelloides spp. to H. helvetica Zones (late Aptian to early Turonian) can be correlated with standard zones in the Tethyan regions, whereas the assemblages from the M. pseudolinneiana to G. arca zones lack tropical zonal markers of Dicarinella concavata , D. asymetrica and Globotruncanita elevata in many studied sections. The scarcity or lack of tropical zonal species during the late Turonian to early Campanian suggests that the Oyubari and Haboro–Kotanbetsu regions in Hokkaido were located in the Transitional to Boreal biogeographical provinces.

Prevailing oxic environments in the Pacific Ocean during the mid-Cretaceous Oceanic Anoxic Event 2

The occurrence of Oceanic Anoxic Event 2 (OAE2) 94 million years ago is considered to be one of the largest carbon cycle perturbations in the Earths history. The marked increase in the spatial extent of the anoxic conditions in the worlds oceans associated with OAE2 resulted in the mass accumulation of organic-rich sediments. Although extensive oceanographic studies of OAE2 have been undertaken in the Atlantic Ocean, the Tethys Sea, and the epicontinental seas of Europe and America, little is known about OAE2 in the Pacific Ocean. Here, we present high-resolution carbon-isotope and degree of pyritization (DOP) data from marine sequences that formed along the continental margins of North America and Asia below the northeastern and northwestern Pacific Ocean. The predominance of low DOP values in these areas revealed that the continental margins of the Pacific Ocean were oxic for most of the OAE2 interval.

Stratigraphic carbon isotope fluctuations of detrital woody materials during the Aptian Stage in Hokkaido, Japan: Comprehensive δ13C data from four sections of the Ashibetsu area

Abstract Geological and organic geochemical studies were performed on an Aptian (mid-Cretaceous) sedimentary succession of the Sorachi and Yezo groups in the Ashibetsu area, central Hokkaido, northern Japan. Microscopic observations of kerogen samples indicate that the constituents of sedimentary organic matter (SOM) are predominantly detrital woody materials (opaque and translucent phytoclasts). Hydrogen/carbon atomic ratios of the kerogen samples indicate that the SOM have not been affected by thermal alteration significantly and thus preserved the original δ13C compositions. It is found that δ13Cwood profiles of four sections from the Ashibetsu area fluctuate systematically with stratigraphy. The δ13Cwood profiles show a remarkable negative shift followed by a positive excursion of ∼3.6‰ (from −25.4 to −21.8‰) exhibiting a bifurcate shape in the early Aptian. δ13Cwood values become nearly constant between −23.0 and −23.5‰ and then approximate to ∼−24.0‰ in the late Aptian. A relatively small positive anomaly is also identified in the latest Aptian. The similarity in δ13Cwood profiles among different sections suggests that SOM with homogenized δ13C values of C3 plants were deposited in sediments. Transportation for a long distance from the sufficiently large provenance is postulated to mix C3 plant materials, resulting in the homogenization of δ13Cwood values. The δ13Cwood profile is essentially conformable with that of Pacific marine carbonates, with their isotopic difference almost constant at all times. Such conformable marine and terrestrial δ13C variations support the previous hypothesis that the temporal δ13C change of the Aptian ocean–atmosphere–biosphere system is precisely traced by bulk isotopic analyses of detrital woody materials, and the supplemental paleoenvironmental effects onto δ13Cwood composition, such as pCO2 changes, are probably negligible.

Geology, petrology and tectonic setting of the Late Jurassic ophiolite in Hokkaido, Japan

Abstract The Gokurakudaira Formation, which has a N–S zonal distribution within a latest Jurassic greenstone belt in Hokkaido Island, Japan, constitutes the uppermost ultramafic–mafic unit of the Horokanai Ophiolite. The following three hypotheses for the origin of the ophiolite have been proposed: (1) a mid-oceanic ridge; (2) an oceanic plateau; and (3) an island arc. The Gokurakudaira Formation can be subdivided into four zones extending NNW to SSE, from east (Zone I) to west (Zone IV), based on lithofacies and areal distribution. Zones I and III consist of aphyric tholeiite resembling back-arc basin basalt (BABB), while Zone II is characterized by the coexistence of BABB-like tholeiite along with high-Mg andesite. Zone IV has a different lithology from the other zones, and is composed mainly of picrite and thick sedimentary sequences of island arc tholeiite (IAT) type andesitic subaqueous pyroclastic deposits and terrigenous sediments. These stratigraphic and petrological characteristics of the Gokurakudaira Formation cannot be explained by the oceanic plateau or mid-oceanic ridge models, but they could correspond to the marginal sea model, as in the Lau Basin. Therefore, we conclude that the Horokanai Ophiolite was formed in the Late Jurassic in a marginal basin above a supra-subduction zone on the margin of the Asian continent.

The first Pacific record of the Late Aptian warming event

Carbonate platforms consisting of hermatypic coral, rudists and larger foraminifers formed along the margin of the NE Japanese islands during the mid-Late Aptian. The geographical northern limit of the carbonate platforms extended to c. 36°N at that time, which is the highest latitude for coral and/or rudist formation in the North Pacific region. This geographical extension of the carbonate platform in the NW Pacific indicates extreme climatic warmth in the mid-latitudes and strong poleward heat transport systems. Contemporaneous warming has been detected in European and Australian regions, as represented by the migration of Tethyan fauna toward the boreal realm and the positive excursion of δ13C and negative excursion of δ18O in deep-sea carbonates. Our new constraint on the northern limit of ‘reef’ growth reinforces the view that an interval of greenhouse-like warmth punctuated the Late Aptian climate.

Late Jurassic–Early Cretaceous intra-arc sedimentation and volcanism linked to plate motion change in northern Japan

The Sorachi Group, composed of Upper Jurassic ophiolite and Lower Cretaceous island-arc volcano-sedimentary cover, provides a record of Late Jurassic–Early Cretaceous sedimentation and volcanism in an island-arc setting off the eastern margin of the Asian continent. Stratigraphic changes in the nature and volume of the Sorachi Group volcanic and volcaniclastic rocks reveal four tectonic stages. These stages resulted from changes in the subduction direction of the Pacific oceanic plate. Stage I in the Late Jurassic was characterized by extensive submarine eruptions of tholeiitic basalt from the back-arc basin. Slab roll-back caused rifting and sea-floor spreading in the supra-subduction zone along the active Asian continental margin. Stage II corresponded to the Berriasian and featured localized trachyandesitic volcanism that formed volcanic islands with typical island-arc chemical compositions. At the beginning of this stage, movement of the Pacific oceanic plate shifted from northeastward to northwestward. During Stage III, in the Valanginian, submarine basaltic volcanism was followed by subsidence. The Pacific oceanic plate motion turned clockwise, and the plate boundary between the Asian continent and the Pacific oceanic plate changed from convergent to transform. During Stage IV in the Hauterivian–Barremian, in situ volcanism ceased in the Sorachi–Yezo basin, and the volcanic front migrated west of the Sorachi–Yezo basin.

A mid-Cretaceous (Albian–Cenomanian) shell-rubble bryozoan fauna from the Goshoura Group, Kyushu, Japan

Early Cretaceous bryozoans are key to understanding the evolutionary radiation of order Cheilostomata, the dominant modern group. Prior to the current study, there were few records of any Cretaceous cheilostomes from eastern Asia, and no pre-Cenomanian records. We found bryozoan fossils to be common in brackish-water to marine deposits representing estuary–tidal-flat and shoreface–inner-shelf habitats in the late Albian–early Cenomanian Goshoura Group, Goshoura Island, Kyushu, Japan. Bryozoans from the Goshoura Group were poorly preserved; for identification, we made silicone casts of colony moulds and studied them by scanning electron microscopy (SEM). We detected six cheilostome and one cyclostome species (Berenicea sp. 1), all forming multiserial encrusting colonies. We erect the new genus Haplostoechios for two new Conopeum-like malacostegans (H. hayamiae and H. clusum), and describe another new malacostegan as Charixa goshouraensis. Additionally, we briefly describe three species detected from a single specimen each: the malacostegan Incertae sedis sp. 1 and neocheilostomes Incertae sedis sp. 2 and sp. 3. The Goshoura fauna is dominated by cheilostomes rather than cyclostomes, with malacostegans marginally outnumbering neocheilostomes. Bryozoans were a common component of the shell-rubble community, encrusting shells of one brachiopod and a broad variety of molluscan species, including 15 bivalves, two gastropods, an ammonite, a nautiloid and wood fragments containing Teredolites. Bryozoans were less diverse in estuary–tidal-flat deposits (three cheilostomes) than in shoreface–inner-shelf deposits (four cheilostomes and Berenicea sp. 1. Cheilostome average zooid length differed between the estuary–tidal-flat deposits (≤0.38 mm) and the shoreface–inner-shelf deposits (most specimens ≥0.49 mm). This study demonstrates that bryozoan assemblages worldwide were not of similar composition at the onset of cheilostome radiation, and underscores the need for further work in less studied regions. http://www.zoobank.org/urn:lsid:zoobank.org:pub:F1B0D7FC-5E0B-4E9E-8098-CD1E6DBE47B8

Tectonic Synthesis: A Plate Reconstruction Model of the NW Pacific Region Since 100 Ma

Based on the results of interdisciplinary study from Chapters 1–4, a plate tectonic model of the northwestern Pacific region since 100 Ma is presented in this chapter. The evolution of the Pacific margin is viewed as a longstanding history of migration/amalgamation of allochthonous blocks onto the subduction zone. Such a process inevitably provoked diverse tectonic events, spatiotemporal positions of which have been discussed in this book. In order to reconcile paradoxical discrepancies in the docking process of arc fragments, the authors introduce a marginal sea plate with a spreading center that was alive in the Cretaceous. Oblique subduction of the ridge caused specific migratory igneous activity along the rim of the overriding plates, together with flips of shearing direction. Arc-trench systems on the eastern and western sides of the marginal sea plate developed following different timelines and were eventually mixed up during the plate’s closure that prompted formation of a coincident Oligocene clinounconformity widespread on the Eurasian margin. Since the demise of the hypothetical plate, the tectonic regime of the northwestern Pacific margin has been controlled by the growth, namely, the rotational history and modes of convergence of the Philippine Sea Plate.

Distribution of Recent Benthic Foraminifera off Western Costa Rica in the Eastern Equatorial Pacific Ocean

Abstract. Benthic foraminifera provide essential information for paleobathymetric reconstructions. However, the modern distribution of benthic foraminifera, especially at depths below 1000 meters below sea level (mbsl), is still obscure in the offshore regions near Central and South America. To characterize the bathymetric scale in the eastern equatorial Pacific Ocean, we examined the depth distribution of benthic foraminifera using piston core samples taken off the coast of Costa Rica. Foraminiferal assemblages vary according to water depth: 1) U1 (mainly composed of Ammonia beccarii, Cancris sagra, Elphidium tumidum, Hanzawaia concentrica, Pseudononion basispinata, and Planulina exorna) represent inner shelf faunas (shallower than 50 mbsl). 2) U2 (mainly composed of Ammobaculites foliaceus, Bolivina striatula, Cassidulina minuta, Hanzawaia concentrica, Uvigerina incilis, Bulimina denudata, and Cancris sagra) is correlated with mid-shelf depth assemblages, from 50 to 100 mbsl. 3) U3 (mainly composed of Uvigerina incilis, Hanzawaia concentrica, Angulogerina semitrigona, Bolivina acuminata, Bolivina bicostata, and Cibicorbis inflatus) is assigned to outer shelf assemblages from 100 to 200 mbsl. 4) U4 (mainly composed of Bolivina humilis, Bolivina seminuda, Bolivina subadvena, Cassidulina tumida, Epistominella obesa, Angulogerina carinata, and Cibicorbis inflatus) is the upper bathyal faunas (200–600 mbsl). 5) U5 (mainly composed of Brizalina argentea, Uvigerina peregrina, Uvigerina auberiana, Brizalina seminuda, Bulimina striata, Epistominella smithi and Globocassidulina subglobosa) is the mid-bathyal faunas (600–1000 mbsl). 6) U6 (mainly composed of Uvigerina auberiana, Uvigerina peregrina, Brizalina argentea, Bulimina mexicana, Cassidulina carinata, Epistominella smithi, and Lenticulina cushmani) represents the lower bathyal assemblage (1000–2000 mbsl). 7) U7 (mainly composed of Uvigerina auberiana, Brizalina argentea, and Eubuliminella tenuata) represent upper abyssal faunas (2000–3000 mbsl). 8) U8 (mainly composed of Glomospira sp. A, Lagenammina arenulata, Chilostomella oolina, Hoeglundina elegans, Melonis barleeanum, Nonion affine, Oridorsalis umbonatus, Pullenia bulloides, and Uvigerina proboscidea) is characterized by deep-water cosmopolitan faunas (deeper than 3000 mbsl). On the basis of a comparison with several environmental parameters, dissolved oxygen concentrations are likely to be the most effective factor controlling foraminiferal depth distributions in the eastern equatorial Pacific especially below the oxygen minimum zone (OMZ). Around OMZ, nitrate concentration also might be related with the benthic assemblage due to the nitrate respiration.