The characterization of ferromanganese crust and its redox change, Western Pacific Magellan seamounts

Abstract:

Biotic/abiotic redox reaction is a ubiquitous process in mineral formation and growth, and changes in elemental redox states, particularly Fe/Mn may reflect the redox conditions in the sediment/ocean when the mineral forms. Samples were dredged from the seamounts in the western Pacific, OSM11 in order to investigate the formation, growth and its implications to geological history. The crust consist of five well-defined layers (here after called “layer 1” (rim) through “layer 5” (core)). Quartz, feldspar, and hematite are detected only in the layer 1 in addition to the poorly crystallined Fe-rich vernadite, which is likely to be associated with slower growth rate compared to the layers 2-5. CFA were identified in layers 4 and 5 under XRD measurement. Visible size of white colored well crystallined CFA were only observed in layer 4; whereas nano-sized CFA in layer 5 were identified by TEM. Clay minerals such as smectite were observed by TEM with SAED pattern and EDX in layers 1 and 3. The oxidation states of Fe and Mn in Fe-rich vernadite in entire layers were determined by EELS analysis. All the layers of Mn oxide minerals was consisted with dominantly Mn⁴⁺, which is consistent with appearance of vernadite in Fe-Mn crust. Fe-rich vernadite in layers 1 and 4 were consisted with 26-52 % of Fe³⁺/Fe^tot, dominant reduced form of Fe compared to layers 2, 3, and 5. The observed alternative patterns of Fe oxidation state in five distinct layers of Fe-Mn crust is likely to be associated with the various redox conditions in seawater, changes in growth rate of crust resulting in the various oxygen exposure time, and uplift-subsidence of sea mounts. A non-cultivation-based molecular approach with T-RFLP indicated the presence of functional gene (CumA) association with Mn oxidizing bacteria in Fe-Mn crust layers. The presence of Mn oxidizing gene may suggest that the biotic Mn oxides precipitation may persist locally in the Fe-Mn crust; whereas functional gene of Fe-oxidizing bacteria have not been identified from current T-RFLP experiment suggesting an abiotic Fe-oxidation. The alternative patterns of Fe-oxidation states in layers of Fe-Mn crust might be induced by abiotic processes reflecting the various redox conditions and thus can used as a proxy for explaining the environmental changes in geological history of seamounts.