On the so-called "Mid-Neogene climatic optimum" in Japan

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  • Sato Seiji
    北海道大学理学部地学質鉱物学教室

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Other Title
  • 日本における "Mid-Neogene climatic optimum" について
  • ニホン ニ オケル Mid Neogene climatic optimum

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Abstract

The "Mid-Neogene climatic optimum at 16Ma", widely used among the geologists in Japan, was proposed by invertebrate paleontologists based on the fossils in the marine sediments after 16Ma. However, because the marine fossils are scarce in the sediments before 16Ma, the students have not referred to the climatic condition before 16Ma. On the other hand, paleobotanists also use the same or similar terms on the basis of plant fossils, the Daijima flora from the non-marine sediments underlying the above noted marine sediments. Namely, the former usage is slightly differnt from the latter in age. Fossil pollens are commonly found both in marine and non-marine sediments. The pollen analysis of the above noted marine and non-marine sediments leads to the following conclusions : The age of the warm climate represented by the Daijima type flora is assigned to ca. 26-13Ma based on the published radiometric and biostratigraphic data for the formations including the flora. During the time, warm climate culminated at the middle Daijima stage, which is supposed to be older than 16Ma, represented by the Daijima flora, and after then the climate became cooler towards 13Ma with some fluctuations in temperature (at least two periods of cool temperate climate were intercalated) in Northeast Honshu and Southwest Hokkaido. The above noted climatic optimum at 16Ma is supposed to correspond to the minor climatic culmination in the above noted fluctuations after the Daijima stage. Some floras reported as the Aniai and Daijima types may correspond to the floras at the times of the minor culminations of cool and warm climates in the climatic change after the Daijima stage. In Hokkaido except the southwestern part, the evidence of warm climate is found in the sediments older than 16Ma and has not found at and after 16Ma.

Journal

  • Fossils

    Fossils 53 (0), 7-19, 1992

    Palaeontological Society of Japan

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