Rock Magnetic Study of Holocene Marine Sediments Recovered from the Okhotsk Sea: Study of Two Core Sediments Near Kitamiyamato Bank off Abashiri, Hokkaido

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  • Kawamura Noriko
    Department of Biosphere-Geosphere System Science, Okayama University of Science Present Address: Graduate School of Science, Kochi University
  • Ikehara Ken
    Geological Survey of Japan
  • Oda Hirokuni
    Geological Survey of Japan Present Address: Paleomagnetic Laboratory ‘Fort Hoofddijk’, Utrecht University
  • Torii Masayuki
    Department of Biosphere-Geosphere System Science, Okayama University of Science

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Other Title
  • オホーツク海完新世堆積物の岩石磁気学的研究―北海道網走沖北見大和堆周辺で採取された２本のコア試料―
  • オホーツクカイ カンシンセイ タイセキブツ ノ ガンセキ ジキガクテキ ケンキュウ ホッカイドウ アバシリオキ キタミ ヤマトタイシュウヘン デ サイシュ サレタ 2ホン ノ コア シリョウ
  • Study of Two Core Sediments Near Kitamiyamato Bank off Abashiri, Hokkaido
  • 北海道網走沖北見大和堆周辺で採取された2本のコア試料

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Marine sediment formed at a high sedimentation rate is distributed in the Okhotsk Sea, off Abashiri, Hokkaido. In order to make a detailed study of environmental changes in the area, we applied rock magnetic methods to two gravity cores covering 2, 700yrs (sediment rate about 150cm/yrs), GH00-1001 (water depth 461m) and GH00-1006 (water depth 1, 348m), recovered from the northern and southern basins near Kitamiyamato Bank. The dominant magnetic mineral in both cores is magnetite. A variety of rock magnetic parameters show three types of downcore variation, which are spiky anomalies suggesting the presence of volcanic ash and basaltic granules. Stepping down of the parameters indicates chemical change in a sedimentary environment, and the rather broad peaks suggest changes in the origin of detrital grains. Based on magnetic parameters, we also magnetically illustrated that increase in grain-size below -300cm (GH00-1001) and -150cm (GH00-1006) is a result of the dissolution of the fine part of magnetic grains in a reducing environment. Magnetic parameters are sensitive enough to pinpoint an episodic input of detrital material of different grain size and mineralogy at 275cm and 420cm (GH00-1001), which corresponds to the Kofun cold age (AD246-732).

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