Authigenic iron

Fe and Mn were also included in the study because of their reputed involvement in the conservation of trace elements (Jenne, 1968) and because of their sensitivity to redox conditions. Authigenic iron sulfide... 

Table 8.4 Authigenic iron sulfides formed in marine sediments. Modified from Schinzel (1993).

The precipitation of (authigenic) iron sulfides resulting from the reaction between H S and Fe phases exerts an important control on the distribution of HjS in marine pore waters (Goldhaber and Kaplan 1974 Canfield 1989 Canfield et al. 

In coastal environment, detrital and authigenic Fe and Mn oxides, which accumulate in oxic surface sediments, play a pivotal role in determining the geochemical behaviour of arsenic (Mucci et al., 2000) and selenium (Belzile et al., 2000). Arsenic and selenium differ in their affinities for metal oxide surfaces. Although both adsorb onto iron oxides, arsenate (As(V)) adsorbs more strongly than arsenite (As(lll)), and selenite (Se(IV)) adsorbs more strongly than selenate (Se(VI)) (Belzile et al., 2000). 

Over longer time scales, clay minerals can undergo more extensive reactions. For example, fossilization of fecal pellets in contact with a mixture of clay minerals and iron oxides produces an iron- and potassium-rich, mixed-layer clay called glauconite. This mineral is a common component of continental shelf sediments. Another example of an authigenic reaction is called reverse weathering. In this process, clay minerals react with seawater or porewater via the following general scheme ... 

Nontronite apparently forms under the same general environmental conditions as the low-iron montmorillonites. It is formed by hydrothermal alteration and as vein fillings. It is commonly formed by both the hydrothermal alteration and surface weathering of basalt. Nontronite is the dominant clay in some soils (Ross and Hendricks, 1945). Arrhenius (1963) found that much of the authigenic montmoril-lonite in the pelagic muds of the Pacific Ocean has a relatively high iron content. 

Sudo (1954) authigenic clay matrix in tertiary iron sand bed, Moniwa, Japan. 

Most of the saponites described have a hydrothermal origin and have usually been formed by the action of either hydrothermal or surface waters on basic rocks. The iron-rich saponite described by Sudo (1954) occurs as a clay matrix in a Tertiary iron sand bed and is presumably authigenic. A metamorphic origin is proposed for saponite from metalimestones (Wilson et al., 1968). 

This bacterial production occurs in the pore fluids of sediments and in stagnant basins (seas, lakes, rivers and fiords). At the interface between anoxic and oxic waters the H2S can be oxidized. This oxidation is frequently coupled to changes in the redox state of metals (1.2) and non-metals (2). Another major interest in the H-jS system comes from an attempt to understand the authigenic production of sulfide minerals as a result of biological or submarine hydrothermal activity and the transformation and disappearance of these minerals due to oxidation (4). For example, hydrothermally produced H2S can react with iron to form pyrite, the overall reaction given by... 

Slomp, C.P., Epping, E.H., Helden, W., and Raaphorst, W.V. (1996) A key role for iron-bound phosphorus in authigenic apatite formation in North Atlantic continental platform sediments. J. Mar. Res. 54, 1179-1205. 

Figure 7 Characteristic reactions involving phosphorus in the sediments of Toolik Lake, Alaska. The primary processes controlling porewater phosphorus concentrations are adsorption to and desorption from iron oxyhydroxides and the precipitation of authigenic vivianite...

As soon as it was known what to look for, sub-fossilized fungi also began to be found in the Flolocene oceanic basement from the Kolbeinsey Ridge (north of Iceland) from a water depth of nearly 1500 m. This basalt was also riddled with pores, but most were open, some filled with water and some partly filled with sediment or authigenic minerals (organomin-erals or microbialites). Most of these microbialites are iron hydroxide... 

Another surhcial deposit type includes the manganese-rich crusts and nodules that occur on basalt near mid-ocean ridges, and near the sediment-water interface. Whereas the ultimate source of manganese and iron (as well as the associated nickel, copper, and cobalt) is the hydrothermal alteration of MOR basalt, the fields of manganese-rich encrustations and nodules that cover large areas of sediment-starved ocean floor are the result of surficial authigenic upward remobilization of metals and the fixing of those metals at the sediment-water interface. 

Alternative constructions of this reaction are conservative with respect to aluminum (Boles and Eranks, 1979b Land et ai, 1987). Such reactions require less input of potassium from minerals other than smectite, but result in a diminution of the total amount of clay by —25% and the generation of excess silicon, iron, and magnesium in amounts (near 15% of the total shale volume) that are not readily sequestered in known authigenic phases in either shales or sandstones (Awwiller, 1993). 

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