[M-ZZ42] Process, Environment, and Exploration of Marine Manganese Deposits

[MZZ42-P01] Ferromanganese Encrustation Driven by Active Oxidation and Deposition of Manganese Across Ocean Profiles

*li WENSHUAI¹, Yoshio Takahashi¹, Akira Usui², Ryoichi Nakada³, Teruhiko Kashiwabara⁴, Hajime Obata⁵, Naoya Kanna⁵, Yoshihiro Asahara⁶, Hirofumi Tazoe⁷, Masato Tanaka¹ (1.Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo, Tokyo, Japan. , 2.Center for Advanced Marine Core Research, Kochi University, Kochi, Japan. , 3.Kochi Institute for Core Sample Research, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Kochi, Japan. , 4.Submarine Resources Research Center (SRRC), Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Yokosuka, Japan. , 5.Atmosphere and Ocean Research Institute, The University of Tokyo, Japan. , 6.Department of Earth and Environmental Sciences, Graduate School of Environmental Studies, Nagoya University, Nagoya, Japan. , 7. Institute of Radiation Emergency Medicine, Hirosaki University, Honcho, Hirosaki, Aomori, Japan)

Keywords:ferromanganese crusts , stable cerium isotopes, redox, seafloor

The interaction between a significant dissolved Mn pool in the oxygen minimum zone and the deeper oxygenated water facilitates the formation of Mn oxides at their interface. Yet, whether such interaction is the direct cause of ferromanganese (Fe-Mn) encrustation remains elusive. Marine Fe-Mn crusts enrich metals including those with high affinity for Mn oxides (e.g., Ce), which provide insights into the origin of Mn oxides. We determine the close association of Ce (~80%) with Mn oxides in marine Fe-Mn deposits and present the Ce isotope data (δ¹⁴²Ce) in Fe-Mn crusts and seawater from the Pacific Ocean (up to 6,000 m depth) to bridge Mn and Ce cycles. We identify (i) a heterogeneous seawater δ¹⁴²Ce profile linked to the Mn cycles, and (ii) the crust δ¹⁴²Ce as an integrated signal of Ce scavenged along the path from the precipitation to the deposition locations of Mn oxides. We find little evidence of sinking Mn oxides in Fe-Mn crusts. Instead, consistent isotopic fractionation between Fe-Mn crusts and the surrounding seawater reflects Ce scavenging by locally formed Mn oxides. Our finding reveals that marine Fe-Mn encrustation is driven by active oxidation and deposition of Mn across ocean profiles, which support the seafloor microbial biosphere.