Geochemistry of the authigenic ferromanganese ore formation in sediments of the Northeast Pacific Basin

Abstract

Authigenic ferromanganese formations in sediments from two horizons (0–10 and 240–250 cm) located in the low/high bioproductive transitional zone of the Pacific Ocean were studied. In addition to the compositionally different two types of micronodules, crusts and ferromanganese nodules were detected in the surface horizon (0–1 cm). Three size fractions (50–100, 100–250, and 250–500 μm) of manganese micronodules were investigated. In terms of surface morphology, color, and shape, the micronodules are divided into the dull round (MN1) and angular lustrous (MN2) varieties with different mineral and chemical compositions. The dull MN1 are enriched in Mn and depleted in Fe as compared with the lustrous MN2. The Mn/Fe value in the dull MN1 varies from 13 to 14. Asbolane-buserite and birnessite are the major manganese minerals in them. The lustrous MN2 is mainly composed of vernadite with Mn/Fe = 4.3–4.8. Relative to the dull MN1, fraction 50–100 μm of the lustrous MN2 is enriched in Fe (2.6 times), W (1.8), Mo (3.2), Th (2.3), Ce (5.8), and REE (1.2–1.8). Relative to counterparts from the dull MN1, separate fractions of the lustrous MN2 are characterized by a greater compositional difference. For example, increase in the size of micronodules leads to decrease in contents of the following elements: Fe (by 10 rel %), Ce (2 times), W (2.1 times), Mo (2.2 times), and Co (1.5 times). At the same time, one can see increase in contents of other elements: Th and Cu (2.1 times), Ni (1.9 times), and REE (1.2–1.6 times). Differences in the chemical and mineral compositions of MN1 and MN2 fractions can be related to alternation of oxidative and suboxidative conditions in the sediments owing to the input of a labile organic matter, which serves as the major reducer, and the allochthonous genesis of MN2.