Mahito Watanabe

The Cenozoic palaeoenvironment of the Arctic Ocean

The history of the Arctic Ocean during the Cenozoic era (0–65 million years ago) is largely unknown from direct evidence. Here we present a Cenozoic palaeoceanographic record constructed from >400 m of sediment core from a recent drilling expedition to the Lomonosov ridge in the Arctic Ocean. Our record shows a palaeoenvironmental transition from a warm ‘greenhouse’ world, during the late Palaeocene and early Eocene epochs, to a colder ‘icehouse’ world influenced by sea ice and icebergs from the middle Eocene epoch to the present. For the most recent ∼14 Myr, we find sedimentation rates of 1–2 cm per thousand years, in stark contrast to the substantially lower rates proposed in earlier studies; this record of the Neogene reveals cooling of the Arctic that was synchronous with the expansion of Greenland ice (∼3.2 Myr ago) and East Antarctic ice (∼14 Myr ago). We find evidence for the first occurrence of ice-rafted debris in the middle Eocene epoch (∼45 Myr ago), some 35 Myr earlier than previously thought; fresh surface waters were present at ∼49 Myr ago, before the onset of ice-rafted debris. Also, the temperatures of surface waters during the Palaeocene/Eocene thermal maximum (∼55 Myr ago) appear to have been substantially warmer than previously estimated. The revised timing of the earliest Arctic cooling events coincides with those from Antarctica, supporting arguments for bipolar symmetry in climate change.

Late Pliocene climate change 3.4-2.3 Ma: paleoceanographic record from the Yabuta Formation, Sea of Japan

Abstract Late Pliocene paleoceanographic changes in the Sea of Japan between 3.4 and2.3 Ma were investigated through study of molluscs, diatoms, and ostracodes from the Yabuta Formation in Toyama Prefecture. The period 3.4−2.7 Ma was characterized by relatively high sea level and cool water benthic faunas. A progressive paleoceanographic shift towards colder oceanic conditions and lower sea level occurred beginning near 2.7 Ma, intenifying about 2.5 Ma, when important changes in ostracode and molluscan faunas occurred. Between 2.7 and 2.3 Ma, eight glacial events can be inferred based on drops in sea level of 50–60 m and increasing proportions of cold, shallow water ostracode species whose modern ecology and zoogeography indicate colder winter water temperatures (3–4°C). The glacial events between 2.5 and 2.3 Ma were the most intense. Preliminary interpretation of the faunal and oceanographic events of the Yabuta Formation suggests that they correspond to Northern Hemispheric cooling also known from North Atlantic deep-sea oxygen isotope, IRD, and planktic foraminiferal records, North Pacific diatom and radiolarian record, and the Chinese loess sequences. The eight glacial events may record a 41,000-yr obliquity cycle which characterized other late Pliocene climate proxy records. Inferred sea level drops near 2.5−2.3 Ma of about 50–60 m provide direct evidence from an ocean margin setting that supports deep sea oxygen isotopic evidence indicating major changes in global ice volume changes.

TAXONOMY OF THE FOSSIL MARINE DIATOM RESTING SPORE GENUS GONIOTHECIUM EHRENBERG AND ITS ALLIED SPECIES

Taxonomic notes are provided on the genus Goniothecium and its allied groups from Arctic core materials, in addition to a synonymy list, microscope observations and several key references for each species. Moreover, the type material of Goniothecium from the Ehrenberg Collection (no. 1651a and 1651b) in the Museum für Naturkunde, Humboldt Universität, Berlin, was obtained in order to examine the structure of G. rogersii. The extinct genus is characterized by a subrectangular frustule and forming paired frustules connected by two epivalves, which possess interlocking linking spines. The genus presently includes 3 species: G. rogersii Ehrenberg, G. danicum Grunow and G. decoratum Bran. A fourth species, Goniothecium loricatum Fenner is here transferred to Hemiaulus loricatus (Fenner in Schrader et Fenner) Suto comb, nov., based on the valve similarity of genus Hemiaulus. Goniothecium has a long stratigraphic history from the Cretaceous to the Pliocene. Goniothecium rogersii and G. decoratum appeared in the Southern Hemisphere in the late Cretaceous and in the middle Eocene, respectively. From the Oligocene, they spread to the Northern Hemisphere. On the other hand, G. danicum appeared in the North Atlantic and/or Arctic Ocean and spread to the Southern Hemisphere in the Oligocene. The spread (or migration) of Goniothecium species from North to South and vice versa might have been influenced by changes in the thermohaline circulation and ocean current structure across the Eocene/Oligocene boundary. Since diatoms live in the euphotic zone, transportation of living cells over long distances most likely occurs by surface (or intermediate) water currents, however, more resistant resting spores may be laterally advected by bottom water currents.

Taxonomy of the fossil marine diatom resting spore genus Odontotropis

This article describes the taxonomy of Odontotropis Grunow including nine taxa with two new species (O. circularis Wise, O. danicus Debes ex Hustedt, O. carinata Grunow, O. arctica Suto sp. nov., O. reticulata Suto sp. nov., O. cristata Grunow, O. galeonis Hanna, O. hyalina Witt and O. birostrata Pantocsek) mostly from Arctic core materials (Integrated Ocean Drilling Program, IODP Expedition 302), in addition to a synonymy list, light and scanning electron microscopy observations and several key references for each taxon. This genus is characterized by having a strongly vaulted frustule, a boat-shaped or circular outline with a sharp carina around the margin and a long history from the Cretaceous to the middle Eocene. The resting spores of Odontotropis may have had a similar ecological strategy to that of dinoflagellate cysts rather than Chaetoceros resting spores. The Odontotropis resting spore extinction occurred before the Eocene/Oligocene Event of Suto which was characterized by a Chaetoceros resting spore explosive diversification, an increase in abundance, and a decrease in valve size. These changes indicate that coastal conditions became unstable, whereby there was a regular seasonal environmental change involving the depletion and sporadic supply of nutrients, conditions more favorable to Chaetoceros resting spores than to dinoflagellate cysts.

The Origin and Development of Geoparks in Japan: Reflections from a Personal Perspective

This chapter provides a historical overview of geopark development in Japan, especially focusing on the initial stage. The author was a key stakeholder in these efforts, and the chapter is based on his personal experience. In the initial stage, excepting a few geoscientists, the geopark concept was virtually unknown. The International Year of Planet Earth (IYPE) was a major catalyst for introducing geopark activities into Japan. The first key stakeholders were academic societies and scientists, and it was realized at that stage that local administrative participation was vital for the growth of this concept in Japan. The Japan Geopark Committee (JGC) was established to evaluate the scientific merit of applications, and the first designated geoparks voluntarily set up the Japanese Geoparks Network (JGN). The first global geoparks from Japan were recognized in 2009, and this helped generate media attention and popular interest. Today, Japan has 35 national geoparks and 8 UNESCO global geoparks. The activities have become much more diverse compared to the initial years, the stakeholder base have become broader, and educational programs in geoparks have gained praise from evaluators; however, several issues such as conservation of geological heritage and long-term plan for sustainable development needs more attention.

Geologically old and ontogenetically young Herpetocetus sp. from the late Miocene of Hokkaido, Japan

ABSTRACT An ontogenetically young fossil baleen whale from the lower part of the Horokaoshirarika Formation of Hokkaido, Japan, includes a partial skull, periotics, bullae, mandible, vertebrae, and a scapula. It is identified as Herpetocetus sp. because it exhibits a postglenoid process of the squamosal more transversely compressed than in Nannocetus, a deep and anteroposteriorly long fossa on the dorsal surface of the squamosal between the zygomatic process and the lateral wall of the brain case, and a prominent squamosal flange of the periotic. The Hokkaido specimen differs from currently described Herpetocetus species in having a large hiatus fallopii, and an angle at the anteromedial edge of the pars cochlearis, just medial to the hiatus fallopii. Because the lower part of the Horokaoshirarika Formation is late Miocene in age (approximately 7.7 to 6.8Ma), the Hokkaido Herpetocetus specimen is the only record of Miocene Herpetocetinae from the western Pacific. Previously, Miocene Herpetocetinae were reported from the eastern North Pacific and western Atlantic. Accordingly, this new record of Herpetocetus from the upper Miocene of the western North Pacific suggests an earlier origin for both the genus and the subfamily.