First record of a Cretaceous cheilostome bryozoan from Hokkaido, Japan

A small collection of recrystallised, encrusting colonies of a single species from the Mikasa Formation (lower Middle Cenomanian), represents the first record of cheilostome (malacostegan or anascan) bryozoans from Hokkaido, Japan.     

The Kellwasser events in the Upper Devonian Frasnian to Famennian transition in the Toc Tat Formation,northern Vietnam

Upper Devonian carbonates of the Toc Tat Formation in the Si Phai Pass area of Dong Van District, northern Vietnam were deposited in carbonate platform, slope, and basin environments. These carbonates yield abundant conodonts indicative of the Palmatolepis nasuta, Pa. linguiformis and Pa. triangularis zones, the Frasnian–Famennian stage boundary being identified by the first occurrence of Pa. triangularis. Two positive carbon isotope excursions are recognized, the lower excursion peaking in the interval of the lower to middle Pa. nasuta Zone, whilst the upper excursion peaks just above the local Frasnian–Famennian boundary. Based on the biostratigraphy, these excursions equate to the Lower and Upper Kellwasser events. Locally, tentaculitoid taxa (Nowakia, Styliolina, Homoctenus, and Metastyliolina?) are abundant in the interval of the Pa. nasuta Zone, but show a drastic decline in abundance before the Lower Kellwasser Event, and only two taxa survived into the Famennian.     

Evolution of animal multicellularity stimulated by dissolved organic carbon in early Ediacaran ocean: DOXAM hypothesis

Oxygenation of the ocean is presumed to be an important factor stimulating the evolution of multicellular animals. The appearance of the Ediacaran‐type biota (ca 575 Ma) was assigned to the aftermath of the Gaskiers glaciation (ca 580 Ma), when substantial oceanic oxygenation is believed to have started. However, several lines of evidence reveal that at least sponges evolved before this oxygenation. For understanding the first stage of animal evolution, we propose the hypothesis that Dissolved Organic Carbon (DOC) Stimulated the evolution for Animal Multicellularity (DOXAM). Recent geochemical studies of the Ediacaran sedimentary sequences have indicated that a substantial DOC mass was developed in the stratified ocean after the Marinoan glaciation (655–635 Ma), and this was supported by the inorganic and organic carbon isotope profiles of the Doushantuo Formation in South China. The DOC mass was an oxygen consumer in the water column; however, it could have provided a food source for filter‐feeding animals such as sponges and cnidarians, and established a primitive food‐web. Such an ecological structure is recognized in modern deep‐sea coral mounds. Results from the integrated ocean drilling program (IODP) Expedition 307 for a mound in northeastern Atlantic suggested that organic carbon suspended around the density boundary in the water column is the key feature to feed the heterotrophic deep‐sea coral community. Our hypothesis is consistent with the fact that the two most primitive animal phyla (Porifera and Cnidaria) are filter feeders. The evolution of filter feeding ecosystems removed the DOC mass and may have contributed to ocean oxygenation in the terminal Neoproterozoic when animal evolution passed into the second stage, with the appearance of bilaterians.     

Variations in trace elements,isotopes, and organic geochemistry during the Hangenberg Crisis,Devonian–Carboniferous transition,northeastern Vietnam

The Hangenberg Crisis at the Devonian–Carboniferous boundary is known as a polyphase extinction event that affected more than 45 % of marine and terrestrial genera. As the cause of this event is still debated, analyses were carried out on sedimentary samples from the Devonian–Carboniferous Pho Han Formation in northeastern Vietnam to reconstruct the paleoenvironment around the time of this event using stable carbon isotopes; total sulfur; manganese; vanadium; molybdenum; and sedimentary organic matter, such as dibenzothiophenes, cadalene, and regular steranes. These geochemical signatures provide a high‐resolution redox history for this section and show that transgression‐driven high primary productivity, possibly enhanced by terrestrial input, caused severe oxygen depletion along the continental margin of the South China block during the Hangenberg Crisis.     

Depositional environments and maturity evaluated by biomarker analyses of sediments deposited across the Cenomanian–Turonian boundary in the Yezo Group,Tomamae area,Hokkaido, Japan

Biomarker analyses for evaluating maturity of organic matter and depositional environments such as redox conditions, were performed in sediments across the Cenomanian–Turonian boundary (CTB) in the Saku Formation of the Yezo Group distributed along the Shumarinai‐gawa River and the Omagari‐zawa River, both in the Tomamae area, Hokkaido, Japan. Maturity indicators using steranes and hopanes, show that organic matter in sediments from the Shumarinai‐gawa and Omagari‐zawa sections are of lower maturity than those from the Hakkin‐gawa section (Oyubari area). Moreover, the ββ hopane ratios clearly show that the maturity of the Shumarinai‐gawa samples is lower than that of the Omagari‐zawa samples. These variations in the maturity of organic matter presumably reflect the difference in their burial histories. The results for the pristane/phytane (Pr/Ph) ratios suggest that the Shumarinai‐gawa samples were deposited under dysoxic to anoxic environments across the CTB, while the depositional environments of the Omagari‐zawa samples were relatively oxic. By another paleoredox indicator using C₃₅ homohopanoids including a homohopene index (HHenI), higher values are observed in the Shumarinai‐gawa section, particularly in the horizons of the preceding period and an early stage of the first negative shift phase and the latest oceanic anoxic event 2 (OAE2) interval. These results suggest that the Shumarinai‐gawa samples record dysoxic to anoxic environments across the CTB. In contrast, the signals for the C₃₅ homohopanoid index values show a relatively oxic condition in the Omagari‐zawa section. The trends of stratigraphic variations in redox conditions are different from those in the OAE2 interval in the proto‐Atlantic and Tethys regions as reported previously. Hence, the redox variations in the Tomamae area were basically related to a local environmental setting rather than global anoxia. However, the prominent anoxic emphasis observed in the HHenI profile of the Shumarinai‐gawa section can be a distinctive, and possibly global, event in the North‐West Pacific just before the OAE2.     

High-resolution terrestrial carbon isotope and planktic foraminiferal records of the Upper Cenomanian to the Lower Campanian in the Northwest Pacific

Making Upper Cretaceous biostratigraphic correlations between the Northwest Pacific and Tethyan–Atlantic sections have been difficult because of rare frequencies of age-diagnostic macro- and microfossils in the sequences in the Northwest Pacific region. In order to correlate these sections precisely, an integrated planktic foraminiferal and bulk wood carbon-isotope stratigraphy from the upper Cenomanian to the lower Campanian succession (the middle–upper part of the Yezo Group) of Hokkaido, northern Japan is established with an average resolution of 50 k.y. The δ¹³C curves from bulk wood of the Yezo Group and from bulk carbonate of English Chalk show remarkably similar patterns of isotopic fluctuation, allowing the correlation of 22 carbon isotopic events between these sections. This high-resolution correlation greatly improves the previous micro- and macrofossil biostratigraphic schemes in the Northwest Pacific region, and reveals that global events, such as the oxygen depletion at the OAE 2 horizon, the constant decrease in pCO₂ during the Late Cretaceous, and the eustatic sea-level falls in the late middle Turonian, Santonian/Campanian Boundary and early Campanian, are recorded in the Upper Cretaceous sequence of the Northwest Pacific.     

越南东北部Cat Ba岛泥盆系-石炭系过渡层的初探

The Pho Han Formation is exposed on southern Cat Ba Island, Hai Phong Province in northeastern Vietnam, and intercalates the Devonian and Carbonif-erous (D-C) boundary (Ta and Doan, 2007; Komatsu et al., 2012). The D-C boundary section consists mainly of limestone beds, numbered from 1 to 167, interca-lated with alternating black organic-rich shales. The limestone yields abundant brachiopods, crinoid-stems and conodonts. Preliminary investigations on strati-graphy (conodont biostratigraphy and δ13C) and sedi-mentology of beds 113-133 were undertaken in this study.     

Tectonic setting of volcanic rocks in the Akara Au mine, north-central Thailand using petrochemical and stratigraphic investigations

Akara gold mine in north-central Thailand is situated within the Loei-Phetchabun-Nakhon Nayok volcanic belt. The roughly north-south trending, Permo-Triassic volcanic rocks earlier mapped by Thailand Department of Mineral Resources were re-mapped and samples were collected from the main active open pit. Forty-four samples were petrographi-cally classified and geochemically analyzed to docu-ment their stratigraphy. Two types of volcanic rocks are recognized, namely coherent and non-coherent units, in which the former is older on the basis of stratigraphic succession. Several lines of evidence suggest that the studied rocks occurred nearby the volcanic edifices and were dominated by debris flows of submarine environment.     

Identification of the source caldera for the Middle Miocene ash-flow tuffs in the Kii Peninsula based on apatite trace-element composition

A large caldera cluster consisting of at least four calderas (Omine, Odai, Kumano-North and Kumano calderas) existed in the central–southern part of the Kii Peninsula approximately 14–15 Ma. On the other hand, thick Middle Miocene ash-flow tuffs, referred to as the Muro Ash-flow Tuff and the Sekibutsu Tuff Member, are distributed in the northern part of the Kii Peninsula. Although these tuffs are considered to have erupted from the caldera cluster in the central-southern Kii Peninsula, identifying the source caldera in the cluster has been controversial because of similarities in the petrological characteristics and identical radiometric ages of the volcaniclastic rocks of these calderas. We successfully discriminated the characteristics of the eruptive products of each caldera in the caldera cluster based on the apatite trace-element compositions of the pyroclastic dikes and ash-flow tuffs of the calderas. We also demonstrated that the source caldera of at least the lower main part of the Muro Ash-flow Tuff and the Sekibutsu Tuff Member was the Odai Caldera, which is located in the central Kii Peninsula. Our findings show possible correlations among the pyroclastic conduits and ash-flow tuffs of the caldera-fill and/or outflow deposits, even in cases where they have been densely welded and diagenetically altered. This method is useful for the study of deeply eroded ancient calderas.     

Bacterial symbiosis forming laminated iron‐rich deposits in Okuoku‐hachikurou hot spring,Akita Prefecture,Japan

Banded iron formations are the most characteristic of Archean–Paleoproterozoic sediment records. Laminated textures resembling banded iron formations can be observed in modern hot‐spring environments. Using sedimentological and microbiological techniques, we investigated the processes of laminar formation and considered the origin of lamination textures. An iron‐rich deposit at the Okuoku‐hachikurou hot spring in Japan exhibits sub‐millimeter laminations consisting of bacteria‐induced ferrihydrite and aragonite. The ferrihydrite particles are spherical and exhibit fine lamination, up to 100 µm thick in ferrihydrite‐rich parts. In aragonite‐rich parts, ferrihydrite particles form filamentous textures with diameters of 10–30 µm, but not laminations. Textural analysis using scanning electron microscopy and phylotype analysis using 16S rRNA indicated the bacterial contribution to ferrihydrite precipitation. A sheath‐like fabric showing a meshwork of nanometer‐order organic filaments, and sheath‐forming bacteria were observed in the deposit specimen etched by citric acid. Phylotype analysis detected in the iron‐rich deposits some bacterial types related to cyanobacteria, purple bacteria, and iron‐oxidizing bacteria. Iron‐oxidizing bacteria probably were responsible for precipitation of the ferrihydrite. Chemolithoautotrophic iron‐oxidizing bacteria are microaerophilic and thrive on Fe(II) in a redox gradient, but dissolved oxygen was not detected in the Okuoku‐hachikurou hot spring. Thus, a certain supply of oxygen is needed for metabolism of the microaerophilic iron‐oxidizing bacteria. The distribution of photosynthetic pigments in the iron‐rich parts indicates that the most likely source of oxygen is photosynthesis by cyanobacteria. This symbiotic relationship between cyanobacteria and iron‐oxidizing bacteria can explain the laminated texture of iron‐rich deposits in the Okuoku‐hachikurou hot spring. These laminations may reflect changes in photosynthetic intensity. There is presently some debate about the bacterial groups that may have played roles in precipitation of banded iron formations. This study presents a new bacterial model for iron precipitation and may provide a mechanism for sub‐millimeter laminations in banded iron formations deposited in shallow water.