[H-CG23] Earth surface processes related to deposition, erosion and sediment transport

[HCG23-P03] Transportation process of turbidites in marine cores collected off Cape Muroto – Organic matter and anisotropy of magnetic susceptibility analysis –

Sari Akutsu⁵, *Masafumi MURAYAMA^1,2, Juichiro Ashi⁴, Naomi Harada^4,3, Chisato Anai², Yuhji Yamamoto² (1.Faculty of Agriculture and Marine Science, Kochi University, 2.Marine Core Research Institute, Kochi University, 3.Research Institute for Global Change, Japan Agency for Marine-Earth Science and Technology, 4.Atmosphere and Ocean Research Institute, The University of Tokyo, 5.Graduate school of Integrated Arts and Science, Kochi University)

Keywords:turbidite, transportation process, organic matter, paleocurrent, off Mutoto Cape, marine core

This study aimed to analyze turbidites intercalated between two marine cores collected off the coast of Muroto Cape and to elucidate their transport processes. The two marine cores, PC03 core taken from the base of the continental slope of the Muroto submarine basin (33°20.50'N, 134°25.80'E, W.D. 1,210m) and PC02 core taken approximately 4 km east from the foot of the continental slope (33°20.30'N, 134°29.40'E, W.D. 1,280m), were collected during the KS-22-3 research cruise. Using these marine cores, continuous elemental analysis by XRF core scanner (ITRAX), organic matter analysis, and paleocurrent analysis based on magnetic susceptibility anisotropy were conducted to estimate the origin and transport process of turbidites. The sediment in both cores consisted mainly of hemipelagic mud containing turbidites, with K-Ah tephra (7,300 yr.) in the bottom. A total of 21 turbidite layers were identified in the PC02 core and 28 turbidite layers in the PC03 core through visual and X-CT observations (Ashi et al., 2023, Abstract of Geo. Soc. Japan fall meeting).
Elements detected with peaks in the turbidites were K, Ca, Mn, Fe, and Br in both PC02 and PC03 cores, although not necessarily showing the same trends in all turbidites. This is due to differences in the origin and scale of the turbidites. In organic matter analysis, the carbon isotope ratio (δ¹³C) of organic matter was compared between the turbidites and the hemipelagic mud before turbidite deposition. Turbidites showed a significantly lighter trend compared to the hemipelagic mud before turbidite deposition, indicating the inclusion of terrestrial organic matter. PC03 core showed a maximum difference of 0.38‰ and an average difference of 0.17‰, while PC02 core showed a maximum difference of 0.14‰ and an average difference of 0.07‰, indicating different trends between the base of continental slope (PC03 core) and away from the base of continental slope (PC02 core).
Paleocurrent estimation was conducted using natural remanent magnetization measurements. The paleocurrent direction showed a predominantly north-south direction from magnetic north, with magnetic susceptibility anisotropy intensity plotting turbidites overlapping with hemipelagic mud in PC03 core, while turbidites and hemipelagic mud plotted in different positions in PC02 core. This suggests that turbidites in PC03 core were directly deposited while being transported upward from the continental slope, whereas turbidites in PC02 core were deposited while reworking occurred. Both cores indicate a flow direction from the continental shelf (north) to the south, revealing the transportation of terrestrial organic matter from continental shelf to the base of continental slope by turbidites.

Reference
Ash et al., 2023, Abstract of Geo. Soc. Japan fall meeting.