Shun Chiyonobu

Timing of Shell Size Increase and Decrease of the Planktic Foraminifer Neogloboquadrina pachyderma (Sinistral) During the Pleistocene, IODP Exp. 303 Site U1304, the North Atlantic Ocean

We made shell size measurements of the planktic foraminifer Neogloboquadrina pachyderma (sinistral) using samples from IODP Site U1304, the North Atlantic. We found that mean and maximum shell sizes began to increase around 1.1 Ma, and that several episodic changes in test size occurred during 0.6 - 0.35 Ma. Test size reached a maximum during the late Quaternary. Based on correlation with previous investigations, we have confirmed that these changes in fo- raminiferal shell size occurred on an inter-ocean scale.

Systematic Development of Submarine Slope Failures at Subduction Margins: Fossil Record of Accretion-Related Slope Failure in the Miocene Hota Accretionary Complex, Central Japan

A comparative study of fossil sliding masses and the results of analog experiments indicates the systematic accumulation of submarine sliding masses at subduction margins. An analysis of sliding masses in the Middle Miocene Hota accretionary complex, Central Japan, combined with the results of analog experiments that simulated an accretionary wedge, revealed two types of slope failures at the toe of the accretionary wedge or at the head of an out-of-sequence thrust: relatively small but frequently developed failures on the lower parts of the slope (Type I failures), and relatively large but less frequent failures that affect the entire slope (Type II failures). The Type I slides are precursors of the larger Type II failures. These successive failure processes are recorded in the Hota accretionary complex as the following depositional sequence: three thin conglomerate layers containing small clasts (type I failure deposits) are overlain by a thick slope-failure deposit containing larger clasts (type II). In such an environment, carbonate-cemented and brecciated sandstones, which contain web structures, generally form at depth and behave as competent layers during failure. Clasts of such sandstones are indicative of large slope failures and are found only in type II failure deposits. Type II failures affect the sediment to a depth of ∼2–10 m, whereas type I failures are surficial (<1 m depth), as inferred from the results of analog experiments and field observations. This systematic pattern of slope failure is important in terms of understanding the formation mechanism of submarine slope failures and in predicting such failures.

Temporal changes in biotic and abiotic composition of shallow-water carbonates on submerged seamounts in the northwestern Pacific Ocean and their controlling factors

ABSTRACT The lithology of Cretaceous to Pleistocene shallow-water carbonates, which were collected from 29 sites on 24 submerged seamounts in the northwestern Pacific Ocean using the Deep-sea Boring Machine System, are described. The shallow-water carbonate deposits examined in the present study can be roughly divided into three types based on their composition: Cretaceous, Eocene (to lowest Oligocene?), and Oligocene to Pleistocene. The Cretaceous type is characterized by an abundance of molluscs (including rudists), smaller foraminifers, microencrusters, non-skeletal grains (e.g., peloids, cortoids, and intraclasts), and microbial sediments. Most components have been micritized and possess thick micrite envelopes. The Eocene type is characterized by the dominance of larger foraminifers, Halimeda spp., nongeniculate and geniculate coralline algae, bryozoans, and dasycladacean algae. Scleractinian corals are very minor components. The Oligocene to Pleistocene type is similar in composition to the Eocene type, but it differs from the latter by the abundant occurrence of scleractinian corals and nongeniculate coralline algae. Corals, nongeniculate coralline algae, and Halimeda spp., which precipitate carbonates within closed to semi-closed spaces in and around their bodies (intra-tissue), are major components of the Eocene and Oligocene to Pleistocene types. In contrast, the Cretaceous-type sediments contain relatively more carbonates of extra-tissue origin (i.e. carbonates deposited in relatively open spaces around the bodies of organisms, such as rudists, as well as microbialite and ooids) than the Eocene and Oligocene to Pleistocene types. The changes in the major constituents of the carbonate factory depend on local environments, such as nutrient availability, as well as a global factor: seawater chemistry in the surface waters. Temporal variations in the abundance of the shallow-water carbonates on the examined seamounts suggest that carbonate accumulation was not necessarily controlled by climatic conditions; instead, it was related to the volcanism and tectonics that served as the foundations for reef/carbonate-platform formation.