Iron-Manganese Nodules in Udepts: The Dependence of the Accumulation of Trace Elements on Nodule Size

摘要

Despite extensive studies, little is known about how the properties of Fe-Mn nodules vary according to their size. In this work, we collected nodules from pollution-free Udepts on the west coast of the Pacific Ocean (south Russian Far-East) and studied them using energy dispersive X-ray fluorescence spectroscopy, X-ray diffraction analysis, atomic absorption spectrometry, electron probe microanalysis, and field-emission scanning electron microscopy. The nodules consisted of a complex Mn-Fe-oxide matrix, soil mineral grains, and C-rich areas. The levels of trace element accumulation, morphological features, and chemical and mineralogical compositions of the nodules varied significantly with the size of the nodules. In the small 1- to 2-mm nodules, Co was associated with Mn whereas Zn and Pb were associated with Fe. In the 2- to 3-mm nodules, manganese was the most important determinant for the accumulation of Co, Ni, and Pb. In the large 3- to 5-mm nodules, Co, Ni, Cu, Zn, and Pb were predominantly associated with Fe. Additionally, in the large nodules, the association of Mn with trace elements was less prominent compared to Fe. The main peculiarities of the medium and large nodules were (i) the presence of primary Mn-Fe-containing minerals in the medium (jacobsite and iwakiite) and large nodules (tephroite and bixbyite), (ii) the appearance of a Mn-rich internal core in the medium and large nodules, and (iii) the crystallinity of the core in the large nodules (3-5 mm). These observations led us to suggest that the primary Mn-Fe-containing minerals are the key factors (at least for Udepts) that affect not only the growth of the nodules but also the formation of their reactive matter and enrichment by trace elements