Iron and manganese in bottom sediments and interstitial waters sampled in the White Sea on August 21-30, 2003

Iron and manganese in bottom sediments studied along the sublatitudinal transect from Kandalaksha to Arkhangelsk are characterized by various contents and speciations depending on sedimentation environment, grain size of sediments, and diagenetic processes. The latter include redistribution of reactive forms leading to enrichment in Fe and Mn of surface sediments, formation of films, incrustations, and ferromanganese nodules. Variations in total Fe content (2-8%) are accompanied by changes in concentration of its reactive forms (acid extraction) and concentration of dissolved Fe in interstitial waters (1-14 µM). Variations in Mn content in bottom sediments (0.03-3.7%) and interstitial waters (up to 500 µM) correspond to high diagenetic mobility of this element. Changes in oxidation degree of chemical elements result in redox stratification of sediment strata with maximum concentrations of Fe, Mn, and sulfides. Organic matter of bottom sediments with considerable terrestrial constituent is oxidized by bottom water oxygen mainly at the sediment surface or in anaerobic conditions within the sediment strata. The role of inorganic components in organic matter oxidation changes from surface layer bottom sediments (where manganese oxyhydroxide dominates among oxidants) to deeper layers (where sulfate of interstitial water serves as the main oxidant). Differences in river runoff and hydrodynamics are responsible for geochemical asymmetry of the transect. The deep Kandalaksha Bay serves as a sediment trap for manganese (Mn content in sediments varies within 0.5-0.7%), whereas the sedimentary environment in the Dvina Bay promotes its removal from bottom sediments (Mn 0.05%).

Contents of rare earth and other chemical elements in Fe-Mn micronodules, nodules, and bottom sediments from the Clarion-Clipperton ore province in the Pacific Ocean

Processes governing the formation of rare earth element (REE) composition are under consideration for ferromanganese deposits (nodules, separate parts of nodules, and micronodules of different size fractions) within the Clarion-Clipperton ore province in the Pacific Ocean. It is shown that ferromanganese oxyhydroxide deposits with different chemical compositions can be produced in sediments under similar sedimentation conditions. In areas with high bioproductivity size of micronodules has positive correlation with Mn content and Mn/Fe and P/Fe ratios and negative correlation with Fe, P, REE, and Ce anomaly. Behavior of REE in micronodules from sediments within bioproductive zones is related to increase of influence of diagenetic processes in sediments as a response to the growth of size of micronodules. Distinctions in chemical composition of micronodules and nodules are related to their interaction with associated sediments. Micronodules grow in sediments using hydrogenous ferromanganese oxyhydroxides. As they grow, micronodules are enriched in labile fraction of sediments reworked during diagenesis. Sources of material of ferromanganese nodules are governed by their formation at the water bottom interface. Their upper part is formed by direct settling of iron oxyhydroxides from bottom water, whereas the lower part is accumulated due to diagenetic processes in sediments. Differences of REE compositions in ferromanganese deposits are caused by the reduction of manganese during diagenesis and its separation from iron. Iron oxyhydroxides form a sorption complex due to sorption of phosphate-ion from bottom and pore waters. Sorption of phosphate-ion results in additional sorption of REE.

Pebbles and carbonate nodules Legs 56-57 Holes

At all DSDP Leg 56 drilling sites, exotic pebbles occur commonly, throughout the cores. Chips of carbonate nodules occur only at Site 434 on the lower inner trench wall. Both exotic pebbles and carbonate nodule chips sometimes tend to be concentrated at particular levels of cores. Exotic pebbles are generally well rounded and consist of various rock types, such as dacite, andesite, basalt, tuff, gabbro, granodiorite, metaquartzite, biotite hornfels, lithic wacke, mudstone, etc., of which dacite occurs commonly at all the sites. Almost all pebbles at Site 436 and most at Sites 434 and 435 may have been rafted by ice. Some at the latter sites may have been derived by down-slope slumping. Carbonate nodules consist of microcrystalline dolomite, manganoan calcite, and siderite; CaCO3 content ranges from 22 to 65 per cent. They are also generally characterized by a high content of P2O5. The nodules are commonly rich in diatom remains, some of which indicate that the nodules are autochthonous. Some nodules contain abundant glass shards, with a modal refractive index of 1.499, almost identical to shards in the surrounding mud and ooze. These facts suggest that the carbonate nodules may have been formed diagenetically, in situ. This may throw light on problems of the formation of carbonate nodules in ancient "geosynclinal" sediments. It is also very important to point out that these carbonate nodules were formed within sediment deposited well below the CCD.

Geochemical analyses of chert nodules and beds from deep-sea sediments (Table 1)

Concentrations of Fe, Mg, Ca, Sr, Mn, Zn, and other heavy metals were analyzed by atomic absorption spectrometry in 27 chert samples from the Pacific deep sea, 17 chert samples from land, and 4 associated sediments from the Pacific Ocean. Among the elements, Fe and Mg concentrations are highly correlatable as are the relationships between Ca and Sr, or between Ca and CO2. The correlation between Fe and Mg is particularly high for Pacific deep-sea flints and cherts, and for cherts of deep-sea origin from outcrops on land. Enrichments in heavy metals were recognized in some deep-sea cherts; volcanogenic cherts are enriched in Fe, a chert nodule containing basaltic fragments is enriched in Zn and Cr, and biogenically enclosed carbonates in flint nodules are enriched in Mn. The correlation of Fe and Mg and their constant ratio [Mg(%)/Fe(%)] of around 0.33 might be characteristic features in the pelagic clays contained in deep-sea flints and cherts, and the concentrations of heavy metals in them would be controlled by the concentrations of Fe-Mg correlated clays. Although the mineralogical nature of the Fe-Mg clay in deep-sea cherts was not clarified by dissolution experiments on opaline minerals in chert, the high concentrations of Fe-montmorillonite and fine-grained olivine or other ferromagnesian silicate minerals in the clay may result in the high correlations between Fe and Mg.

Composition of Fe-Mn micronodules, nodules and host bottom sediments from the Northeast Pacific Basin

Authigenic ferromanganese manifestations in bottom 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 two compositionally different 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 dull round (MN1) and angular lustrous (MN2) varieties with different mineral and chemical compositions. MN1 are enriched in Mn and depleted in Fe as compared with MN2. Mn/Fe ratio in MN1 varies from 13 to 14. Asbolane-buserite and birnessite are the major manganese minerals in them. MN2 is mainly composed of vernadite with Mn/Fe ratio from 4.3 to 4.8. Relative to MN1, fraction 50-100 µm of MN2 is enriched in Fe (2.6 times), W (1.8), Mo (3.2), Th (2.3), Ce (5.8), and REE (from 1.2 to 1.8). Relative to counterparts from MN1, separate fractions of MN2 are characterized by greater compositional difference. For example, increase in size of micronodules leads to decrease in contents of 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 (from 1.2 to 1.6 times). Differences in chemical and mineral compositions of MN1 and MN2 fractions can be related to alternation of oxidative and suboxidative conditions in the sediments owing to input of labile organic matter, which acts as the major reducer, and allochthonous genesis of MN2.