Concretions nodules

Niere,/, kidney nodule, concretion (of ore) bunch, pocket. 

Formation of the shallow-water concretions is associated with anoxic conditions that range in duration from seasonal to nearly continuous. For example, in the Baltic Sea, nodules and crusts are mostly found around the margins of the deep anoxic basins. They form from Mn and Fe that accumulates from the reduction of Mn and Fe oxides in the anoxic deep waters. When the basin is periodically flushed with oxic water from the North Sea, about once a decade, the concretions undergo growth as the fresh supply of metals is oxidized. 

It is apparent from our pXRF analyses that at the scale of the analytical window, there are regular, reproducible, variations in element enrichments between sulfide accumulations (nodules, concretions, crusts) and shale matrix. Some of the largest base metal and associated element contents measured in the field (e.g. Zn = 1.4 wt% Cu = 600 ppm Mn = 1 wt% Cd = 90 ppm Hg = 50 ppm) are in apparently (to the naked eye) sulfide-free shale. The majority of such enrichments appear to be due to the incorporation of disseminated, very fine-grained, Fe-rich sphalerite. In many instances this is not evident even when using a hand lens, and care must be taken not to overlook these cryptic enrichments. 

Phosphorites (manne apatites) are dense, hght-brown-to-black concretions, ranging in size from sands to nodules and irregular masses. Phosphorites have been found off Argentina, Chile, Japan, Mexico, Peru, South Africa, and Spain, and several islands in the Indian Ocean. Some also have been found off the west coast of North America and on the eastern North American continental shelf These deposits occur where water upwelling transports phosphorus and where the rate of sedimentation is slow. The... 

Concretions or nodules of iron, aluminum, manganese, or titanium Noncemented, root restricting natural or human-made (plow layers, etc.) root restrictive layers... 

Ferromanganese nodules are primarily centimeter-sized concretions of iron and manganese oxide. They occur on the deep seafloor where pelagic sediment accumulation rates are slow... 

These examples are discussed here in order to indicate the lack of deep burial of sediments in which these carbonate apatites occur. Further discussion of concretions and nodules will appear later. 

Ehrlich (1975) has recently reviewed aspects of microbial participation in the formation of ferromanganese concretions in the deep sea. Other aspects of the biogeochemistry of ferromanganese nodules are given by Lundgren and Dean in Chapter 4. 

Nodular Term synonymous with glaebular and refers to soft to highly indurated concretions of carbonate, or carbonate cemented host material. The margins may be gradational to sharp, and internally the nodules may be uniform, showing concentric laminae or septarian cracks or veins. The nodules can range in shape from spherical to elongate. The nodules typically consist of micrite or less commonly microsparite. Nodular calcrete develops primarily in siliciclastic host material... 

CaC03 (eqn. 6.6). This allows CaC03 to precipitate as nodules (concretions) in the sediment. These are not quantified in the global budget as they are volu-metrically small sinks of CaC03. 

Manganese nodules often exhibit onionskin layering of Feflll) oxides (Arrhenius, 1963). The surfaces of many soil Mn concretions, composed predominantly of birnessite, are coated with ferruginous and siliceous material (Taylor et al., 1964). The intermixing or coating of various chemical species with/on Mn oxides could substantially alter their redox and sorption/desorption behavior and profoundly affect the mobility and fate of many nutrients and pollutants in natural environments. However, there is only limited information in the literature on the subject. Oscarson et al. (1981b) hypothesized that a coating of carbonates on Mn oxides was responsible for a retardation in the rate of oxidation of As(III) by some freshwater lake sediments. 

Unlike calcite and dolomite, siderite rarely forms as an extensive pore-filling cement, but rather as discrete fine crystals, spherules and nodules scattered in the host sediments. Nevertheless, Baker et al. (1996) found that early diagenetic siderite concretions (0.5-2 mm) form up to 30% of Triassic sandstones and mudstones from eastern Australia. Laterally continuous siderite-cemented offshore shelf sandstone sheets (15 cm thick) occur in Upper Cretaceous sequences from Canada (McKay et ai, 1995). 

OF Overbank fines Massive, crudely bedded silts and muds (Fm) finely laminated to rippled silts and muds (FI) laminated silt, sand, and clay (Fsc) silts and clays w/rhizocretions (Fr) Tabular to thin and lobate 0.1-3 m thick 0.5 m to 0.2 km in lateral extent Muds and silts Poorly cemented, isolated nodules, platy, and rod concretions... 

Scattered nodules and rod concretions in a mudstone "matrix, interbedded with thin type-3 (vadose and phreatic) tabular lenses. 

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