Spatial distribution of manganese oxide minerals in the natural ferromanganese nodule of the Arctic Sea: A view from Raman spectroscopy

Abstract

Knowledge of the spatial distribution of manganese oxide minerals in the natural ferromanganese nodules provides improved prospects for understanding the geochemical processes involved in nodule formation. In this study, we report a series of micro-laser Raman spectra along with a line profile from the center to the rim of a spherical ferromanganese nodule from the East Siberian Sea in the Arctic region. For the Raman spectra obtained at the center of the nodule, the characteristic Raman bands for todorokite at ∼ 733, ∼ 508, and ∼ 240 cm‐1 were clearly observed, suggesting that the todorokite is predominant in the central part of the nodule. As the data acquisition point moved away from the center to the rim (D), the Raman bands corresponding to the todorokite decreased, while the intensity of the 588 cm‐1 band corresponding to the layered manganate minerals (i.e., birnessite) increased, suggesting that the proportion of manganate phases with a tunnel structure decreases with nodule growth. In addition, the distinct Raman bands at ∼ 280 and 403 cm‐1 indicated that the triclinic birnessite is prevalent in the outer regions of the nodule. Semi-quantitative measurement of the intensity of diagnostic Raman bands for todorokite (I733) allows us to effectively observe the spatial distribution of manganese oxide minerals in the ferromanganese nodule. At the center of the nodule, I733 was ∼ 0.7. Despite a few fluctuations, I733 clearly decreased from ∼ 0.7 to ∼ 0.2 with increasing D from 0 to 7500 μm, indicating that the relative fraction of todorokite in nodules significantly decreases with nodule growth. This study highlights that a highly resolved spatial distribution of manganate mineral phases in ferromanganese nodules can provide a historical record of the formation process and geochemical conditions.