[M-ZZ44] Marine Manganese Minerals: Depositional environments, exploration, and development

[MZZ44-06] Distribution of Fe and Mn in red clay on the deep seafloor of the northwestern Pacific

Taisei Hatano¹, *Masafumi MURAYAMA^2,3, Keiji Horikawa⁴, Takuya Sagawa⁵, Hideki Minami⁶, Naomi Harada⁷, Hajime Obata⁷ (1.Graduate school of Integrated Arts and Science, Kochi University, 2.Faculty of Agriculture and Marine Science, Kochi University, 3.Marine Core Research Institute, Kochi University, 4.Faculty of Science, Academic Assembly, University of Toyama, 5.Institute of Sciences and Engineering, Kanazawa University, 6.School of Biological Sciences, Tokai University, 7.Atmosphere and Ocean Research Institute, The University of Tokyo)

Keywords:red clay, element distribution, manganese nodule, northwest Pacific

Ferro-manganese oxides (nodules and crusts), recognized as an important seabed resource, are widely distributed in the Pacific region (Usui, 2010). In particular, Mn nodules are found on the deep-sea floor in red clay with a low sedimentation rate (2-3 mm/kyr.) at depths of 3,500–6,000 m. Although numerous studies have focused on Mn nodules, few have examined the sediments beneath them. Therefore, this study aims to clarify the characteristics of elements such as Fe and Mn in red clay where Mn nodules are located, as well as in other regions. The red clay samples were collected from depths greater than 5,000 meters along a north-south transect at 10° intervals between 10°N and 50°N along 155°E in the northwestern Pacific. A comparison of the elemental compositions was made between the red clay site where Mn nodules were located (OP-7; 20°N) and other sites. Samples were collected using a multiple corer during the research cruises KH-22-7 and KH-23-2 of the R/V Hakuho Maru. A total of 16 red clay samples (from sites OP-5 to OP-22) were analyzed using X-ray CT, organic content analysis, and elemental analysis using XRF and XRF core scanner (ITRAX). Additionally, dissolved oxygen (DO) in bottom water (-15 m) was measured onboard.
The results showed that OP-7, where Mn nodules were located in a low-latitude region, exhibited low organic content (0.3 %) and high dissolved oxygen concentrations (176.9 micro-mol/kg) in the bottom seawater. This suggests that the site is near the pathway of the Lower Circumpolar Deep Water (LCDW) derived from the Antarctic Circumpolar Current, as reported by Kawabe and Fujio (2010). Elemental analysis of surface samples (0-1cm) from each site revealed that OP-7 had a high Fe/Al ratio (0.49–0.51), compared to the crust average (0.39, Wedepohl, 1995). Furthermore, Fe₂O₃, MnO, Al₂O₃, TiO₂ and P₂O₅ also exhibited high values. The content of these elements tended to decrease with increasing latitude.