[M-IS18] 古気候・古海洋変動

[MIS18-P13] Reconstruction of paleo-ocean environment by geochemical analysis of ferromanganese nodule from the tabletop of western Pacific Magellan Seamounts

*Jinsub Park¹、Jaewoo Jung²、Kiho Yang¹ (1.Department of Oceanography, Pusan National University, Republic of Korea、2.Korea Institute of Ocean Science and Technology, Republic of Korea)

キーワード：Ferromanganese nodule, paleo-ocean environment, Magellan Seamount, micro-XRF

Ferromanganese nodules(Fe-Mn nodules) are being widely explored because of their significant economic potential. Due to the recent increase in demand for secondary batteries, Fe-Mn nodules that contain high contents of manganese and rare earth elements are in the spotlight. The economic value of Fe-Mn nodules is related to the elemental contents. Various environmental parameters such as primary production, Oxygen minimum zone(OMZ) and redox condition have been reported as factors affecting the geochemical properties of manganese nodules. Therefore, paleo-ocean environment can be reconstructed by geochemical analysis of Fe-Mn noduels. Three Fe-Mn nodules were collected using an epibenthic sledge from the Gordin guyot(149°34’E, 17°06’N, 1616mbsl) belonging the Magellan Seamount in the western Pacific during the R/V Isabu Expedition Hl-21-06(May 14-June 11, 2021) by the Korea Institute of Ocean Science and Technology(KIOST). Magellan Seamounts was formed in the south of equator during the Cretaceous(about 120Ma), after that moved to the northwest. To reconstruct paleo-ocean environment, 2D elemental distribution mapping was carried out on the cross-section of the Fe-Mn nodules by micro X-ray fluorescence(μ-XRF). The cross-sections of the three nodules are determined by optical observation, and they showed a different shape. The μ-XRF element distribution reveal a distinct alternation between Mn-rich and Fe-rich layers. Mn-rich layer surrounds the nuclei, and next Fe-rich layers appeared as a concentric circle. Finally, Mn-rich layer surrounds the outermost part of the Fe-Mn nodules again. And element distribution maps of Cu, Ni, Co reveal that Ni and Cu follow the distribution of Mn, while Co follows the distribution of Fe. Contrary to optical observation, the elemental variations were similar in the three nodules. Massive structure and Mn-rich layer of the outermost part are thought to be related to the reinforcement of bottom current since 5Ma while the bottom current is strengthened, upwelling becomes stronger together. As a result, primary production increased and diagenesis process was strengthened. Additionally, strong currents prevail sedimentation that interrupt growth of Fe-Mn nodules. In terms of productivity fluctuation, productivity declines as seamounts moved away from the equatorial high productivity zone. Therefore, the formation of hydrogenetic Fe-Mn nodules became dominant. After that, diagenesis was strengthened again due to the reinforcement of the bottom current and increased primary productivity.