[S-CG52] 海洋底地球科学

[SCG52-20] 琉球海溝南西端海底堆積物の地域的輸送: 海底地形と堆積学特徴

*熊 衎昕¹、金松 敏也¹、池原 研²、村山 雅史³、山本 裕二⁴ (1.国立研究開発法人海洋研究開発機構、2.国立研究開発法人産業技術総合研究所、3.高知大学農林海洋科学部海洋資源科学科、4.高知大学 海洋コア総合研究センター)

キーワード：タービダイト、磁硫鉄鉱、帯磁率異方性、ITRAX、琉球海溝、台湾

The southwestern Ryukyu Trench is an ideal place to study the sediment transport along trench axis. Taiwan-sourced sediments transported by turbidity currents via submarine canyons/channels can be investigated in the southwestern Ryukyu Trench floor (Hsiung et al. 2021). Piston cores YK1501PL14/PC14 and KR1518PL04/PC04 located in the hadal Ryukyu Trench (deeper than 6,000 m water depth) were selected in this study. Based on regional bathymetry between the sites YK1501PL14/PC14 and KR1518PL04/PC04, a slightly sinuous deep-sea channel shows clear braided patterns, indicating active transport morphology along the trench axis. The lithology shows homogeneous gray mud (hemipelagites) intercalated with thin-bedded very fine-sand to silt (turbidites). The lithological characteristics of high occurrence and frequency of turbidites may help to estimate the reoccurrence intervals and spatial distribution of events (e.g., paleoseismology records). Total 60 turbidites of YK1501PL14/PC14 and 36 turbidites of KR1518PL04/PC04 were depicted on basis of visual core description, X-ray images, and element Ca intensity of Itrax. Each turbidites is 0.3–4.0 cm thick and average around 1.0 cm. Detrital pyrrhotite episodically eroded from the Taiwan orogenic belt during the Plio-Pleistocene may be preserved in adjacent marine sedimentary basins (Horng et al. 2012). Magnetic Properties Measurement System (MPMS) not only provide rapid identification of dominate carrier of magnetic minerals but also provide identification of pyrrhotite in dry bulk samples on the basis of its low-temperature magnetic transition at 34 K. In this study area, magnetite plays a dominate carrier of magnetic minerals. Pyrrhotite plus magnetite signatures are detected in the pilot cores of YK1501PL14 and KR1518PL04. For anisotropy of magnetic susceptibility (AMS) measurements, a total of 123 cubes from YK1501PL14/PC14 and 178 cubes from KR1518PL04/PC04 were conducted. No obvious mean magnetic susceptibility (K_m) trend was observed in YK1501PL14. Several K_m peaks are observed in separate sections in silt to v.f. sand beds which related to each specific turbidites whole downhole. Profiles of AMS parameters including degree of anisotropy (P_j), degree of foliation (F), and degree of lineation (L) were without obvious downhole changes. The AMS results are characterized by a high F value (oblate type). Degree of lineation (L) shows generally low value throughout the two cores, showing the magnetic mineral grains with a strong preferred orientation such like parallel laminae in the core sediments. Equal area projections of the AMS show arbitrary and cannot provide a flow direction. It may due to the hemipelagites intercalated with thin-bedded turbidites of cube measurements.