Abstract
Ferromanganese nodules grow by precipitation of metals from seawater and/or sediment pore water. The formation of different genetic types depends on the composition and redox conditions of the water and upper sediment layers, water depth, and primary productivity in surface waters. Many characteristics of nodules have been used to investigate their genesis. In this paper, we compare nodules from different environments using Computed Tomography, Scanning Electron Microscopy-Energy Dispersive X-ray Spectroscopy, and Micro X-ray Fluorescence data to better understand how geochemical differences are linked to different morphological features. We use representative samples of purely hydrogenetic nodules and mixed-type nodules with various proportions of hydrogenetic and diagenetic growth laminae. Our results show a micrometric alternation between high-absorbance massive Mn-enriched (Mn/Fe up to 40) laminae and low-absorbance dendritic Mn-depleted (Mn/Fe about 1) laminae in mixed-type nodules, suggesting the rhythmic alternation of hydrogenetic oxic conditions and suboxic diagenetic input. This micro-rhythmic alternation is absent in purely hydrogenetic nodules, which are homogenous both chemically and morphologically. A conceptual model is proposed to account for these geochemical and morphological differences in terms of the vertical migration of the oxic-suboxic front relative to the base of the nodules.
Highlights
Marine ferromanganese nodules are Fe and Mn oxides precipitated concentrically around a nucleus [1,2,3]
This work is based on four sets of Fe–Mn nodule samples representative of different genetic conditions (Table 1); (1) Four mixed-type Fe–Mn nodules from the Clarion-Clipperton Zone (CCZ) in the Pacific Ocean were collected during cruise JC120 aboard the RRV James Cook (Table 1); (2) Five mixed-type nodules from the Central Indian Basin (CIB) were collected during cruises of RV Akademic Alexander Sidorenko, R.V
Skandi Surveyor; (3) Three hydrogenetic nodules from the Mascarene Basin (MB) in the Indian Ocean were collected during cruise SK35 on the ORV Sagar Kanya (Ministry of Earth Sciences (MoES)), Delhi, India); and (4) one hydrogenetic Fe–Mn coated pebble from Rio Grande Rise (RGR) in the southwest Atlantic was collected during cruise Iatá-Piuna aboard the RV Yokosuka
Summary
Marine ferromanganese (hereafter Fe–Mn) nodules are Fe and Mn oxides precipitated concentrically around a nucleus [1,2,3]. Iron and manganese oxides precipitated from oxic seawater are called hydrogenetic, and those from sediment pore water are called diagenetic. Both are accompanied by the sorption of trace and rare earth elements (REE) on the Fe and Mn oxides [5,8,9,10]. The precipitation occurs concentrically around a nucleus, which is any hard material available in the environment, e.g., rock fragments, pumice, consolidated sediments, pieces of biological material like bone fragments and shark’s teeth, and old nodule fragments [11]. The precipitation rate of Fe–Mn oxides is of the order of a few mm per Myr
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