Iron sulfide sediments

Redox Reactions. Aquatic organisms may alter the particular oxidation state of some elements in natural waters during activity. One of the most significant reactions of this type is sulfate reduction to sulfide in anoxic waters. The sulfide formed from this reaction can initiate several chemical reactions that can radically change the types and amounts of elements in solution. The classical example of this reaction is the reduction of ferric iron by sulfide. The resultant ferrous iron and other transition metals may precipitate with additional sulfide formed from further biochemically reduced sulfate. Iron reduction is often accompanied by a release of precipitated or sorbed phosphate. Gardner and Lee (21) and Lee (36) have shown that Lake Mendota surface sediments contain up to 20,000 p.p.m. of ferrous iron and a few thousand p.p.m. of sulfide. The biochemical formation of sulfide is undoubtedly important in determining the oxidation state and amounts of several elements in natural waters. 

The sulfur budget for the Black Sea has been considered in several papers [23, 24,74-77]. Sulfide sources are sulfide production in sediments, sulfide flux at the sediment/water interface, and sulfide production in the water column. Sulfide sinks are sulfide oxidation at the oxic/anoxic interface and in the basin interior by dissolved oxygen of the modified Mediterranean water and iron sulfide formation in the water column. 

Sulfide iron sediments can be formed both by direct chemogenic deposition from solution and in the course of diagenesis of the original sediments (Curtis and Spears, 1968 Skripchenko, 1969). 

Deposition of primary iron sulfides depends on many natural factors—pH, Eh, concentration of iron and sulfur, presence of reactive forms of carbon dioxide and silicic acid, etc. To ascertain the conditions of chemogenic formation of iron sulfides, a special examination of the effect of all these factors is needed. We will limit ourselves to determining the conditions of formation of sulfide iron sediments from waters with a constant sulfur content, and then we will examine the effect of a change in its concentration. 

Oxide-carbonate-silicate-sulfide iron sediments can be formed under conditions of joint interaction of iron compounds with reactive forms of silicic... 

On the basis of thermodynamic constants obtained for hydroxide compounds of iron with different aging time and also of experimental data, the physicochemical character of the diagenetic transformations of iron sediments of various compositions (oxide, silicate, carbonate, sulfide) can be traced. The results obtained are represented graphically in the form of stability diagrams of iron compounds as a function of variations in the main parameters governing the physicochemical character of the environment of diagenesis—pH, Eh, activity of iron and dissolved forms of sulfur and carbon dioxide. 

Diagenesis of oxide-silicate-carbonate-sulfide sediments. If reactive sulfur, carbonic acid, and silicic acid are present at the same time in original iron sediments of any composition, the mineral associations formed are determined by the concentrations of these active forms. Figure 59a gives a diagram of the relationships between the crystalline iron compounds in the system Fe-Si02-C02-S-H,0 for a — 10 and = 10 g- ion/1. 

Dissolved sulfide in this zone builds up because of its bacterial production. Its maximum concentration is less than the total sulfate reduced because a portion of the H2S reacts with iron and organic matter to form insoluble products. At any depth, the concentration gradient of H2S is kinetically controlled and reflects the balance between the rate of these removal processes, the rate of gain or loss by diffusion, and the rate of its formation by reduction. One of these processes, the production of H2S, must cease when all S04 has been consumed. The net result is a concentration maximum that falls in a range from 1 pM to >10 mM. The depth of maximum pore-water H2S commonly correlates closely with the depth of total S04 depletion. In most environments, H2S persists at measurable concentrations (i.e., greater than a few micromolar) in pore waters to depths of a few centimeters to several meters below the point at which S04 is removed. The essentially total removal of pore-water H2S is a reflection of the availability of excess iron over sulfide sulfur in most sediments (see below). Pyrite content may increase gradually within zone III, but the rate of this increase is most rapid at the top of this zone. Frequently, increases in pyrite cannot confidently be distinguished from scatter in the data within this zone. 

The presence of iron minerals and their respective reactivity towards sulfide is of greatest importance for the pore water chemistry and the limitation for pyrite formation. In case of reactive iron rich sediments dissolved iron may build-up in pore water and dissolved sulfide is hardly present although sulfate reduction occurs. In contrast, in sediments characterized by a low content of reactive iron dissolved sulfide can build-up instead of dissolved iron (Canfield 1989). The degree of pyritisation (DOP) was originally defined by Berner (1970) and was later modified by Leventhal and Taylor (1990) and Raiswell et al. (1994). DOP is now defined as ... 

The deep water may well have remained anoxic after the shallow water became oxic [5,36,45]. In their formulations, the bottom water is ferrous iron rich before 2.5 billion years ago and becomes sulfide rich after that UpwelUng of these waters would have triggered high productivity based on iron and sulfide photosynthesis, respectively. That is, the organic sediments produced beneath the anoxic upwellings would show molecular evidence of anoxygenic photosynthesis even though most of the shallow ocean was oxic. 

Dissimilatory sulfate reducers such as Desul-fovibrio derive their energy from the anaerobic oxidation of organic compounds such as lactic acid and acetic acid. Sulfate is reduced and large amounts of hydrogen sulfide are generated in this process. The black sediments of aquatic habitats that smell of sulfide are due to the activities of these bacteria. The black coloration is caused by the formation of metal sulfides, primarily iron sulfide. These bacteria are especially important in marine habitats because of the high concentrations of sulfate that exists there. 

As is the case with assessments of the toxicity of dissolved trace metals, the development of sediment quality criteria (SQC) must be based on the fraction of sediment-associated metal that is bioavailable. Bulk sediments consist of a variety of phases including sediment solids in the silt and clay size fractions, and sediment pore water. Swartz et al. (1985) demonstrated that the bioavailable fraction of cadmium in sediments is correlated with interstitial water cadmium concentrations. More recent work (e.g., Di Toro et al, 1990 Allen et al., 1993 Hansen et al, 1996 Ankley et ai, 1996, and references therein) has demonstrated that the interstitial water concentrations of a suite of trace metals is regulated by an extractable fraction of iron sulfides. 

The role of iron sulfides in regulating sediment interstitial water concentrations is shown by the following reactions (Di Toro et ai, 1990) ... 

Nelson, M. B., Davis, J. A., Benjamin, M. M. and Leckie, J. O. (1977). The Role of Iron Sulfides in Controlling Trace Heavy Metals in Anaerobic Sediments Oxidative Dissolution of Ferrous Monosulfides and the Behavior of Associated Trace Metals." Air Force Weapons Laboratory, Technical Report 425. 

By far the most important ores of iron come from Precambrian banded iron formations (BIF), which are essentially chemical sediments of alternating siliceous and iron-rich bands. The most notable occurrences are those at Hamersley in Australia, Lake Superior in USA and Canada, Transvaal in South Africa, and Bihar and Karnataka in India. The important manganese deposits of the world are associated with sedimentary deposits the manganese nodules on the ocean floor are also chemically precipitated from solutions. Phosphorites, the main source of phosphates, are special types of sedimentary deposits formed under marine conditions. Bedded iron sulfide deposits are formed by sulfate reducing bacteria in sedimentary environments. Similarly uranium-vanadium in sandstone-type uranium deposits and stratiform lead and zinc concentrations associated with carbonate rocks owe their origin to syngenetic chemical precipitation. 

Shannon R.D., White J.R. The selectivity of a sequential extraction procedure for the determination of iron oxyhydroxides and iron sulfides in lake sediments. Biogeochem 1991 14 193-208. 

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