日本地球惑星科学連合2018年大会

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[EE] Eveningポスター発表

セッション記号 S (固体地球科学) » S-IT 地球内部科学・地球惑星テクトニクス

[S-IT22] 核-マントルの相互作用と共進化

2018年5月22日(火) 17:15 〜 18:30 ポスター会場 (幕張メッセ国際展示場 7ホール)

コンビーナ:飯塚 毅(東京大学)、渋谷 秀敏(熊本大学大学院先端科学研究部基礎科学部門地球環境科学分野)、土屋 卓久(愛媛大学地球深部ダイナミクス研究センター、共同)、太田 健二(東京工業大学大学院理工学研究科地球惑星科学専攻)

[SIT22-P34] Paleomagnetic and rock magnetic results from 17-22 ka sediment of Jeju Island, Korea: Excursional behavior of the geomagnetic field or rock magnetic anomalies?

*安 ヒョンソン1Sohn Young Kwan1Lee Jin-Young2Kim Jin Cheul2 (1.Department of Geology and Research Institute of Natural Science, Gyeongsang National University、2.Geologic Environment Division, Geo-Environmental Hazards and Quaternary Geology Research Center, Korea Institute of Geoscience and Mineral Resources)

キーワード:paleomagnetism、rock magnetism、geomagnetic excursion、17-22 ka、Jeju Island、nonmarine unconsolidated sediment

Paleomagnetic and rock magnetic investigations were performed on a 64-cm-thick section of nonmarine unconsolidated muddy sediment from the so-called Gosan Formation in Jeju Island, Korea. This sediment was recently dated to have been deposited between 22 and 17 kyr BP calibrated, with a sedimentation rate of 13-25 cm/kyr, based on many radiocarbon ages. From 30 out of 33 horizons characteristic directions of natural remanent magnetization are successfully determined by the stepwise alternating field demagnetization method: Of these, six horizons (5, 11, 29, 43, 65, and 69 cm depth from the top) shows marked deviations in virtual geomagnetic pole (VGP) position, more than 45 ° away from the North Pole. A suite of rock magnetic analyses suggest that the sediment mainly could be characterized by the dominance of low-coercivity magnetic particles of a range of sizes up to pseudo-single domains, which are magnetite plus (occasionally maghemitized?) titanomagnetite with an unblocking temperature of ~300 °C. These also seem to exclude the possibility that the unusual directions might be caused by self-reversal or gyroremanent magnetization. On one hand, it is found that there are some peaks, for instance, in saturated isothermal remanence for 7-9, 17, 29, 39, and 65 cm depths. This kind of the rock magnetic anomalies may allow us to suspect an adverse influence on the directions. Nevertheless, the anomalous VGP deviations, at least, for 5, 11, 43, and 65 cm depths seem not to be associated with the rock magnetic anomalies, thereby giving a possibility of recording the geomagnetic excursional behavior. Excursion-like or high-amplitude secular variation records in the direction and/or the intensity have been observed around the world including Tianchi of NE China, Lake Baikal of Siberia, Bay of Bengal of India, Hawaii, western USA, western North Atlantic Ocean, Arctic Ocean, and some in mid- to high-latitudes of the South Hemisphere, but the occurrence of geomagnetic excursions around the 17-22 ka period is not yet well accepted by the paleomagnetic community. Our current results seem to advocate a possibility of the geomagnetic instability at the time, which might be manifested by multiple short-lived excursions.