モンスーン気候はいつ始まったのか? : その地質学的証拠

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This paper reviews the recent geological-research results on climatic changes and initiation of monsoon climate caused by uplifting of the Himalayas and Tibetan Plateau. All lines of evidence obtained from the Siwalik Group of foreland basin as well as deep sea drilling cores from the Indian Ocean indicate that monsoon was set up in the Late Miocene about 10 to 8Ma. Abrupt increase of sedimentation rate at 10Ma and appearance of flood splay deposits at 9 Ma suggest seasonal flood has begun around 10 to 9Ma. The first appearance of detrital grains of kyanite and gneiss at 9.2 Ma implies metamorphic rocks in the core of the Himalayas exposed on earth surface and rapid denudation has started by that time. Faunal and floral changes at 9.5 Ma and 7.4 Ma demonstrate that vegetation and climate have gradually changed from wet subtropical evergreen forest to dry grassland. Synchronous changes of σ13C and σ18O value of pedigenic carbonate also support the interpretation. The first appearance of diatomaceous mudstone in the core collected from the upwelling area off Oman suggests upwelling triggered by southwest summer monsoon was set up at about 10Ma. Abrupt increase of Globigerina bulloides in 8.5-7.4 Ma and radioralia Colloshaera in 8 Ma indicates southwest monsoon was strengthened and upwelling was active around 8 Ma. Intraplate deformation recorded in deep sea fan sediments has started in between 7.5 and 8 Ma. This fact also indirectly supports rapid uplifting of the Himalayas and Tibetan Plateau. Initiation of north-south normal faulting in the High Himalayas and Tibetan Plateau at 10-5 Ma is another evidence that the Himalayas and Tibetan Plateau may have attained at least 4, 000 m by the end of Late Miocene. In opposition to those opinions, Chinese paleobotanists insist that elevation of the Tibetan Plateau and the Himalayas were lower than 3, 000 m at the end of Late Miocene. They estimate very rapid uplifting after 2 Ma on the basis of fossil leaves and pollen of evergreen oaks from the Late Pliocene beds in the north slope of the Himalayas.

モンスーン気候はいつ始まったのか? : その地質学的証拠

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