5:15 PM - 6:45 PM
[MGI31-P05] Determination of manganese redox state of microparticles in deep-sea oxic sediment
Keywords:Mn-microparticle, Redox state, STXM
Manganese (Mn) is the third most abundant metallic element in Earth's crust and significant in understanding the global cycling of metallic elements due to its redox-sensitive dynamics. Mn, widely distributed in the ocean's abyssal seafloor as ferromanganese oxides known as Mn nodules or crusts, poses a unique interest for its role beneath the deep subseafloor. Recent investigations have revealed Mn in various mineral forms within subseafloor sediments, notably as submillimeter-scale Mn micronodules and micrometer-scale Mn microparticles, the latter found exclusively in oxygen-rich deep-sea clay environments. These findings highlight significant contribution of these Mn minerals to the global Mn budget, raising intriguing questions about their formation and preservation processes.
Of these, in the course of the studies of Mn-microparticles, techniques were employed using high-energy beam irradiation including flow cytometry, scanning electron microscopy, focused-ion-beam milling, and scanning transmission X-ray microscopy (STXM). These methods have provided insights into the mineralogical and geochemical features of Mn-microparticles. However, these techniques have encountered challenges in accurately determining the redox state of Mn within these particles: despite the Mn-microparticles were derived from an oxygen-rich environment, STXM analyses displayed a reduced Mn (II) state. This contradiction underscores the complexity of Mn's redox dynamics and the need for further investigation.
Addressing the limitations of previous methods, this study proposes a beam-free sample preparation technique to enrich Mn microparticles from geological samples. Analyzing Mn-microparticles from an oxic pelagic clay core sample collected during the Integrated Ocean Drilling Program (IODP) Expedition 329 in the South Pacific Gyre, this study outlined a microparticle enrichment procedure. The procedure enabled the successful isolation of Mn-microparticles, facilitating an accurate determination of their redox state using STXM without the damage of high-energy beam irradiation.
The findings reveal that the Mn-microparticles predominantly exhibit an oxidative Mn (IV) state, contrary to previous observations of reduced Mn (II). This discovery not only clarifies the redox state of Mn in these deep-sea oxic sediments but also supports the hypothesis that hydrogenetic precipitation from oxic deep-sea water is a primary formation process for these particles. Moreover, the presence of Mn microparticles serves as a potential indicator of the oxic conditions prevailing during their preservation, offering new perspectives on the environmental implications of Mn minerals in marine geochemical processes.
Of these, in the course of the studies of Mn-microparticles, techniques were employed using high-energy beam irradiation including flow cytometry, scanning electron microscopy, focused-ion-beam milling, and scanning transmission X-ray microscopy (STXM). These methods have provided insights into the mineralogical and geochemical features of Mn-microparticles. However, these techniques have encountered challenges in accurately determining the redox state of Mn within these particles: despite the Mn-microparticles were derived from an oxygen-rich environment, STXM analyses displayed a reduced Mn (II) state. This contradiction underscores the complexity of Mn's redox dynamics and the need for further investigation.
Addressing the limitations of previous methods, this study proposes a beam-free sample preparation technique to enrich Mn microparticles from geological samples. Analyzing Mn-microparticles from an oxic pelagic clay core sample collected during the Integrated Ocean Drilling Program (IODP) Expedition 329 in the South Pacific Gyre, this study outlined a microparticle enrichment procedure. The procedure enabled the successful isolation of Mn-microparticles, facilitating an accurate determination of their redox state using STXM without the damage of high-energy beam irradiation.
The findings reveal that the Mn-microparticles predominantly exhibit an oxidative Mn (IV) state, contrary to previous observations of reduced Mn (II). This discovery not only clarifies the redox state of Mn in these deep-sea oxic sediments but also supports the hypothesis that hydrogenetic precipitation from oxic deep-sea water is a primary formation process for these particles. Moreover, the presence of Mn microparticles serves as a potential indicator of the oxic conditions prevailing during their preservation, offering new perspectives on the environmental implications of Mn minerals in marine geochemical processes.