SCOPE 48 - Sulphur Cycling on the Continents

6.1 INTRODUCTION

In this chapter, we show the complexity of and gaps in our knowledge of the sulphur cycle in soils and sediments. Much of our discussion focuses on salt marshes and other marine systems, but we also draw comparisons with other systems. There is a wealth of detailed information on the components of the sulphur cycle in marine environments as compared to freshwater environments, perhaps because marine sediments are dominated by inorganic sulphur chemistry while freshwater sediments are dominated by organic sulphur chemistry. Organic sulphur chemistry is more complex because of the wide variety of compounds which can be formed under natural conditions, and our knowledge and understanding of the chemistry of the organic sulphur cycle in both marine and freshwater environments is still in its formative stages. Clearly, new techniques to determine species and functional groups are needed to advance our knowledge of organic sulphur biogeochemistry.

6.2 AN OVERVIEW OF THE SULPHUR CYCLE

Figure 6.1 shows a comprehensive schematic representation of the sulphur cycle which includes: (1) sulphate reduction to sulphide and its subsequent oxidation; (2) the incorporation of sulphide into reduced inorganic and organic sulphur; and (3) the possible links between the inorganic and organic pools via sulphur compounds of intermediate oxidation states. The sulphur species may be formed via abiotic and/or biotic reactions. Tables 6.1 and 6.2 summarize some of the known laboratory organic sulphur reactions which have been documented to be, or may be, of environmental importance. A glossary of significant inorganic and organic sulphur compounds and ions is given in the appendix to this chapter .

Salt marshes provide excellent systems to understand the cycling of sulphur (and other elements) in the marine environment because of high sulphate concentrations derived from oceanic sources and because of high sulphate reduction rates (Howarth, 1984; Giblin and Wieder, this volume). These conditions lead to the dissimilatory production of hydrogen sulphide which in turn reacts with a variety of other inorganic and organic compounds in the sediments and porewaters. Sulphur compounds with intermediate oxidation states are quickly formed because the surface of the sediments is exposed to the atmosphere and because the rhizosphere of marsh grasses can oxidize sediments at depth. Thus, diffusive pathways for gaseous exchange (e.g. O₂ into and H₂S out of the sediments) are direct. Besides oxygen, other abun dant oxidizing agents (e.g. FeOOH, MnO₂, NO₃-) are available to oxidize reduced sulphur in marine sediments (Aller and Rude, 1988; Lord and Church, 1983; Howarth, 1984; Luther et al.,1986; Luther, 1987) and in forest soils (Fuller et al., 1986; Strickland et al., 1987; Wieder and Lang, 1988). For this reason, abundant oxidized and highly reactive sulphur products coexist, chief in importance being polysulphides (S_x^2-) since they are a common reactant for both pyrite formation (Rickard, 1975; Howarth, 1979; Luther, 1991) and organic sulphur formation (LaLonde, Ferrara and Hayes, 1987; Oae, 1985; Vairavamurthy and Mopper, 1988; Kohnen et al., 1989).

 Figure 6.1. A schematic figure of the sulphur cycle. The upper half of the figure gives the major inorganic species whereas the lower half of the figure gives the principal organic functional groups. Oxidized species are on the right and more reduced species are to the left of the figure. The principal reduced sulphur compounds other than H₂S are considered to be FeS₂ (pyrite), organic sulphides (RSR), thiols (RSH) and organic disulphides (RSSR). Only the well documented stable and metastable sulphur compounds are provided in the figure. Organic pathways marked I and II are described in the text. Pathways marked with boldface lines and reactants with question marks are those that have not been documented in the marine environment

Table 6.1. Known organic sulphur reactions: formation of organic sulphur from inorganic sulphur

Table 6.2. Known organic sulphur reactions: oxidation of reduced organic sulphur compounds

However, other reactants (e.g. S₂O₃^2-, SO₃^2-, FeS₂) may be equally important, participating in reactions indicated with boldfaced lines in Figure 6.1.

6.3 INORGANIC SULPHUR GEOCHEMISTRY

Under anoxic conditions in salt marshes and other marine sediments, dissimilatory sulphate reduction is often the primary means of organic carbon mineralization (Jørgensen, 1982; Howarth, 1984; Ivanov et al., 1989; Giblin and Wieder, this volume). The reduction of sulphate ion to bisulphide involves the transfer of eight electrons per S atom, and the process is dependent on microbial catalysis. However, under varying subsequent redox conditions, re-oxidation reactions serve to cycle sulphide back to sulphate (Howarth and Teal, 1979; Lord and Church, 1983; Giblin and Howarth, 1984). Besides dissimilatory sulphate reduction, assimilatory sulphate reduction can also produce H₂S which can be emitted by higher plants directly to the atmosphere (Rennenberg, 1989; Andreae and Jaeschke, this volume).

As detailed in Figure 6.1, a variety of partially oxidized species can be formed from bisulphide ion oxidation. Thiosulphate, elemental sulphur and polysulphides (Chen and Morris, 1982; Hoffman, 1977; Boulegue, Lord and Church, 1982; Howarth et al., 1983; Luther, Varsolona and Giblin, 1985) are among the most important and are formed by abiotically or biotically catalysed oxidation reactions. Some of these partially oxidized sulphur species can result from microbial metabolism (Goldhaber and Kaplan, 1975) and/or from the reductive dissolution of metal oxide phases (Pyzik and Sommer, 1981).

Elemental sulphur is formed during the oxidation of bisulphide ion by a variety of oxidants. Hoffman (1977) showed elemental sulphur to be an important end-product in the reaction of bisulphide ion with peroxide. Pyzik and Sommer (1981) observed elemental sulphur in the reaction of iron(III) minerals with bisulphide ion, and Burdige and Nealson (1986) observed it in the reaction of MnO₂ with bisulphide ion. Elemental sulphur apparently results from the oxidation of metal monosulphides (Moses et al., 1987) but does not result from the oxidation of pyrite (McKibben and Barnes, 1986; Moses et al., 1987).

Polysulphides (S_x^2-) result from the same oxidation reactions which produce elemental sulphur. They are formed first from the reaction of bisulphide ion with zerovalent sulphur formed from bisulphide oxidation (Hoffman, 1977). As the chain length (x) increases, the polysulphides tend to decompose to elemental sulphur. Polysulphides can also be formed by the reaction of elemental sulphur and bisulphide ion (Giggenbach, 1972). Polysulphides are potentially important in the cycling of metals (Boulegue, Lord and Church, 1982). Polysulphides have been detected as an important component of dissolved sulphur in the interstitial waters of tidal wetland sediments (Boulegue, Lord and Church, 1982; Luther, Varsolona and Giblin; Luther et al., 1986) and of subtidal sediments (Luther, Varsolona and Giblin, 1985). Polysulphides are also important intermediates for the formation of pyrite (Giblin, 1988; Howarth, 1979; Lord and Church, 1983; Luther et al., 1982; Rickard, 1975). Recent work shows that polysulphides react with abundant lignin components from buried marsh grass material to produce organic sulphur (Francois, 1987a), and with activated olefins to produce thiols (Vairavamurthy and Mopper, 1988) and thiophenes (LaLonde, Ferrara and Hayes, 1987).

 Both dissolved organic and inorganic sulphur compounds can provide powerful ligands for complexing and cycling trace metals. For example Cu(I) should be strongly complexed by species like CU(S₄)₂^3- and Cu(II) by species like Cu(HS)₃^-. Both complexes have stability constants (pK) of 18-19. Thiols such as cysteine can strongly complex Cu(I) with pK values of > 15 (Boulegue, Lord and Church, 1982). Recently, Shea and MacCrehan (1988b) have modelled Cu solubility in the anoxic sediments of the Chesapeake Bay and have shown that the major Cu complex is that of cysteine with a small contribution from S_x^2- complexes.

Thiosulphate (S₂O₃^2-) is the first product resulting from the decomposition of the solid sulphide precipitate, FeS₂ (Goldhaber, 1983; Moses et al., 1987; Luther, 1987) and from the decomposition of polysulphides (Chen and Morris, 1972). Thiosulphate has been detected as a significant component of dissolved sulphur at all depths in salt marsh porewaters (Boulegue, Lord and Church, 1982; Howarth et al., 1983; Luther, Varsolona and Giblin, 1985) and in subtidal porewaters (Luther, Varsolona and Giblin, 1985). Thiosulphate is an important nucleophile which can displace the halides and the hydroxide ion in organic compounds to produce thiols, acidity and sulphate (Roy and Trudinger, 1970; Oae, 1985). Luther et al. (1986) hypothesized that thiosulphate from pyrite oxidation can form organic sulphur (carbon-bonded sulphur) in salt marsh sediments.

Sulphite (SO₃^2-) and tetrathionate (S₄O₆^2-) have rarely been observed in the natural environment. Boulegue, Lord and Church (1982) and Luther , Varsolona and Giblin (1985) have reported sulphite as a minor component in a few salt marsh porewater samples, but sulphite was not detected by Howarth et al. (1983) in any samples over a year of sampling. Luther et al. (1986) reported tetrathionate in the porewaters from one salt marsh core.

Metal monosulphides, in particular FeS, are generally minor contributors to the total reduced inorganic sulphur pool in salt marsh sediments, and the major metal sulphide is pyrite, FeS₂ (Howarth and Teal, 1979; Cutter and Velinsky, 1988; Gardner, Wolaver and Mitchell, 1988; King, 1988; King, Howes and Dacey, 1985; Lord and Church, 1983; Swider and Mackin, 1989; Giblin and Wieder, this volume). Howarth and colleagues (Howarth, 1979; Howarth and Teal, 1979; Howarth and Giblin, 1983; Howarth and Merkel, 1984) using ³⁵S-sulphate incubations have shown that pyrite, not FeS, is the major short term end-product of sulphate reduction in many salt marsh sediments. Pyrite is commonly the most abundant metal sulphide in subtidal marine sediments as well (Berner and Westrich, 1985; Chanton, Martens and Goldhaber, 1987; Goldhaber and Kaplan, 1980; Sweeney and Kaplan, 1973). However, in contrast to marsh sediments, FeS is often the initial major end- product of sulphate reduction in subtidal coastal sediments (Howarth and Jørgensen, 1984; Westrich and Berner, 1988; Thode-Andersen and Jørgensen, 1989), although even in some subtidal sediments, pyrite rather than FeS dominates as the end-product in ³⁵S-sulphate incubations (Ivanov et al., 1989; Christensen, 1989).

Pyrite provides an important sink for sulphur and trace metals in salt marsh sediments (Lord and Church, 1983) and in marine sediments in general (Berner, 1984; Berner and Raiswell, 1983; Morse and Cornwell, 1987; Raiswell and Berner, 1986). It is also an important energy component in the metabolism of tidal wetland ecosystems (Howarth and Teal, 1980; Howarth et al., 1983; Howarth, 1984). Pyritization proceeds via the reaction of H₂S with detrital iron minerals also termed 'reactive iron' (Canfield, 1989). Pyritization can be modelled in sediments by the reaction between dissolved sulphide and iron oxide as goethite (Equation (6.1); Lord and Church, 1983 and references therein) or magnetite (Canfield and Berner, 1987):

This equation shows the need to have a source of oxidant to produce FeS₂, which is more oxidized than FeS.

Two general mechanisms have been proposed for the formation of FeS₂. Pyrite may form directly by reaction between polysulphide and Fe(II) (Rickard, 1975; Luther, 1991) or by the reaction of iron monosulphides with S₈ (e.g. Berner, 1970; Sweeney and Kaplan, 1973). The polysulphide reaction with Fe(II) is considered to be rapid and to result in single crystal morphology for FeS₂ whereas the S₈ reaction is thought to be slow and to result in framboidal morphology. Both morphologies have been observed in marine systems (Sweeney and Kaplan, 1973; Luther et al., 1982; Giblin, 1988). The rapid formation from polysulphide reaction would seem to dominate in many saltmarsh sediments, where FeS is often rare (Howarth and Teal, 1979).

Although the rate of pyrite formation may be rapid in the surface zones of marsh sediments, on a net basis much of the pyrite is not preserved due to its oxidation to thiosulphate and sulphate (Boulegue, Lord and Church, 1982). Thus, a major portion of the pyrite which forms is quickly destroyed and recycled (Howarth and Teal, 1979; Lord and Church, 1983; Luther et al., 1986; Luther and Church, 1988). Luther and Church (1988) have observed pyrite oxidation in salt marsh sediments on the time scale of a week or less. They have observed in the upper 10 cm of the sediment, large increased concentrations of porewater iron and thiol-like compounds concurrent with decreased pyrite concentrations before and after the spring tides. These time scales are shorter than recognized by previous studies in salt marsh sediments (Howarth and Teal, 1979; King et al., 1982). Recently, Wieder and Lang (1988) have noted that rapid FeS₂ turnover also occurs in peat from a freshwater wetland. There is also a significant reoxidation of pyrite and other reduced iron compounds within subtidal marine sediments (Jørgensen, 1982; Howarth, 1984; Berner and Westrich, 1985). Recent laboratory studies by Kornicker and Morse (1991) and field studies (Huerta-Diaz and Morse, 1990) show extensive adsorption and co-precipitation of trace elements in pyrite. Thus, rapid FeS₂ turnover will likely affect the geochemical cycles of other trace elements.

At present, the oxidant(s) responsible for rapid pyrite oxidation in salt marsh sediments appears related to Fe(III) species. The oxidation of pyrite by oxygen has been documented as a slow process (Singer and Stumm, 1970; Moses et al., 1987). In contrast, anaerobic oxidation by Fe(III) compounds is quite rapid (Moses et al., 1987). Fe(III) may also be important in the oxidation of pyrite in anoxic, subtidal sediments (Howarth, 1984). Recently a molecular orbital theory (MOT) approach (Luther, 1987) has been used to explain the molecular basis for the reactivity of pyrite. Fe(III) can form a covalent bond to the surface of the pyrite through S, but molecular oxygen cannot. Thus, electron transfer (and oxidation of pyrite) is rapid with Fe (III) . In pyrite oxidation, oxygen in the thiosulphate and sulphate formed comes mainly from water, not molecular oxygen (Taylor, Wheeler and Nordstrom, 1984a,b). Equations (6.2), 6.3 and  (6.4) describe the overall complete oxidation of FeS₂ by Fe(III). Equation (6.4) is the sum of Equations (6.2) and (6.3)

               
The MOT approach predicts that other oxide phases may also oxidize pyrite. Recently, Aller and Rude (1988) have shown that MnO₂ and other manganate solids oxidize sulphidic material in subtidal sediments, These oxidants are supplied by bioturbation (Aller and Rude, 1988; Berner and Westrich, 1985). The nature of this sulphidic material is unknown, but is assumed to be iron monosulphides or acid volatile sulphur (A VS), This oxidation can be mediated by chemolithotrophic microorganisms, Figure 6.2 describes the major processes that can occur in an estuarine ecosystem.

Figure 6.2. This schematic diagram depicts the major processes that can occur in an estuarine ecosystem

6.4 ORGANIC SULPHUR GEOCHEMISTRY

The role of organic sulphur in sulphur and trace element cycling is not well understood because of the diversity of organic sulphur compounds. Organic sulphur compounds in salt marsh environments are not well characterized, other than the volatile compounds dimethylsulphide (DMS), methanethiol (CH₃SH), and dimethyldisulphide (DMDS) (Adams et al., 1981; Cooper et al., 1989; Steudler and Peterson, 1984; Dacey, King and Wakeman, 1987; Andreae and Jaeschke, this volume). In particular, DMS results from the decomposition of dimethylsulphoniumproprionate (DMSP) in Spartina alterniflora (Dacey, King and Wakeman, 1987) and also phytoplankton (e.g. Charlson et al., 1987; Nguyen et al., 1988; Turner, Malin and Liss, 1989; Andreae and Jaeschke, this volume). Cooper et al. (1989) have found DMS, DMDS, and CS₂ to be produced in a freshwater Cladium jamaicense swamp in Florida, and Haines, Black and Bayer (1989) have found emission of CS₂, DMS, and ethanethiol (C₂H₅SH) from the forest litter, soil, and roots of Stryphnodendron excelsum in a central American rainforest. Biogenic sulphur emissions of these volatile compounds as well as COS and H₂S have been reviewed by Aneja and Cooper (1989), Rennenberg (1989), Andreae and Jaeschke (this volume), and Giblin and Wieder (this volume).

Non-volatile reduced organic sulphur materials are expected of the types R-S-H (thiols), R-S-R (organic sulphides), R-S-S-R (organic disulphides) with R being alkyl groups, or sulphur containing humic or fulvic compounds (Francois, 1987a; LaLonde, Ferrara and Hayes, 1987). Generally, such organic sulphur compounds could be diagenetic products of reactions under reducing conditions between high molecular weight lignin type materials and inorganic reduced sulphur (SH^-, FeS, FeS₂ and S_x^-2). These high molecular weight compounds can decompose to intermediate sulphur organic by-products which are soluble and perhaps volatile (Balzer, 1982).

In salt marshes and other sediments, partially to fully oxidized organic sulphur compounds are probably formed as well. These compounds should result from aerobic oxidation (by O₂) and from anoxic oxidation (by the metal oxides in reactions analogous to those described for inorganic reduced sulphur compounds) of reduced organic sulphur (Table 6.2). Soluble inorganic S compounds (e.g. S₂O₃^2- and SO₃^2-) with sulphur in oxidation states ranging from -2 to +6 may react with organic compounds to form oxidized organic sulphur compounds. This range of oxidation states makes these compounds likely reactants for cycling between the inorganic and organic sulphur pools (Figure 6.1 and Table 6.1).

The organic sulphur compounds may be represented by two pathways: (1) sulphoxides and sulphones which are formed from R-S-R; and (2) sulphonates which can be produced by the oxidation of thiols and their disulphides (Oae, 1985). Within each pathway there is the possibility of reversibility which includes the reduction of RSO₃H to RSSR as shown in laboratory studies (Oae, 1985). Pathway 1 appears to be a process by which stable solid phase organic sulphur compounds such as thiopenes (cyclic R-S-R) can be buried (Brassell et al., 1986; LaLonde, Ferrara and Hayes, 1987), whereas pathway 2 may be a process that serves to recycle sulphur through carbon compounds.

Organic sulphur pathways 1 and 2 are known to be linked at the RSH to RSR conversion, and recent work has shown that the link is reversible for methylated compounds via microbial processes (Kiene and Visscher, 1987). It is not clear that there is any other link between these two pathways. This work established that both pathways are occurring in the marine environment in the low molecular weight organic material.

Organic sulphur compounds that play important roles in the biochemical metabolism of plants, animals, and microbes include amino acid and peptide types (e.g. cysteine, cysteic acid, glutathione, taurine, methionine). These biochemically important sulphur-containing compounds may be transformed during microbially mediated decomposition processes (Balzer, 1982; Freney, 1986; Guerin and Braman, 1985; Kiene et al.,1986; Kiene and Taylor, 1988), which follow reactions similar to those in pathways 1 and 2.

There has not been a detailed or comprehensive characterization of organic sulphur compounds in any waters and sediments, much less information on their means of formation or their fates. Much of the organic sulphur research has centred on freshwater wetlands, lake sediments, and forested mineral soils. Radiolabelled ³⁵S-sulphate incubations of soil and litter samples from a hardwood forest (Strickland et al., 1987), from peats (Wieder and Lang, 1988), from cedar swamps (Spratt, Morgan and Good, 1987), and from lake sediments (Rudd, Kelly and Furutani, 1986) show that sulphate is incorporated into carbon-bonded S (C-S) and ester sulphate (C-OSO₃), with carbon-bonded sulphur being the major component or end-product. Using chemical extraction methods, several other groups have also noted the importance of carbon-bonded sulphur in freshwater peats (Bustin and Lowe, 1987; Lowe, 1986; Wieder and Lang, 1986; Wieder, Lang and Granus, 1987; Giblin and Wieder, this volume), in forest soils (Fitzgerald, Strickland and Ash, 1985; Fuller et al., 1986; Mitchell and Fuller, 1988; Watwood et al., 1988; Mitchell, David and Harrison, this volume), and in lake sediments (Nriagu and Soon, 1985; Rudd, Kelly and Furutani, 1986; Cook and Kelly, this volume). However, Tabatabai (1984) shows that organic sulphate esters predominate in some agricultural soils.

Lowe and Bustin (1985) have suggested that these two organic components (C-S and C-OSO₃) can be further subdivided into five other pools: hydrolysable C-S, hydrolysable HI-S forms, non-hydrolysable C-S forms, non-hydrolysable HI-S forms, and volatile forms lost on hydrolysis. Unfortunately, each of these pools may contain more than one functional group. For example, the non-hydrolysable C-S forms which they describe include sulphonates and sulphoxides. These compounds represent both of the organic sulphur pathways in Figure 6.1.

Although the long-term incorporation of sulphur is into the organic fraction, ³⁵S-sulphate studies show that inorganic sulphur species are the short-term end-products in peats (Wieder and Lang, 1988), in a bog (Urban, Eisenreich and Grigal, 1989), and in lake sediments (Rudd, Kelly and Furutani, 1986). These studies show that dissimilatory sulphate reduction is of importance in freshwater systems. These studies also support the earlier stable isotope data of Nriagu and Soon (1985) who showed that inorganic reduced sulphur transforms to organic sulphur in eutrophic lakes. In the Everglades, Altschuler et al. (1983) proposed that organic oxysulphur compounds are reduced in dissimilatory sulphate reduction.

Thiols (C-S-H) have been reported in salt marsh porewaters by Boulegue, Lord and Church (1982) and by Luther et al. (1986) using independent electrochemical techniques. Thiols complexed with iron (iron thiolates) have been reported using polarography (Luther and Church, 1988). In marine porewaters, Mopper and Delmas (1984) and Mopper and Taylor (1986) have reported measuring discrete thiols by derivatizing the thiols with o- phthalaldehyde followed by HPLC separation and fluorescent detection of the derivative. Using HPLC followed by direct electrochemical detection, Shea and MacCrehan (1988a) reported discrete thiols in Chesapeake Bay porewaters. Matrai and Vetter (1988) and Vetter et al., (1989) have measured thiols in sediment porewaters and coastal waters by derivatizing the thiols with monobromobimane followed by HPLC separation and fluorescent detection of the derivative.

Most recent work in the marine environment has centred on specific organic sulphur compounds or functional groups in subtidal sediments. Guerin and Braman (1985) have observed the production of 1-14 µmol CH₃SH g^-l dry weight in sediments from Tampa Bay during the analysis of sediments by the Cr(II) reduction method after the AVS had been removed. Their laboratory results show that Cr(II) solutions do not react with a variety of organic sulphur compounds. Thus, CH₃SH production comes from as yet unidentified solid inorganic or organometallic phases. Mopper and co-workers (1986, 1987, 1988) have reported on thiol compounds in shallow subtidal sediments and porewaters. On the addition of tributyl-phosphine (a S-S bond cleaving agent) to sediment slurries, up to nine times the porewater thiol concentrations were released from the sediments, demonstrating the importance of pathway II (Figure 6.1) in the marine environment.

Francois (1987a,b) observed sulphur enrichment of humic acids in the sediments of Jervis Inlet. Sulphur-stable isotope results and a peak of organic polysulphide (RS_x^-) sulphur at the oxic-anoxic interface suggests that active sulphidation of humic compounds occurs at this interface. Carbon to sulphur mole ratios below the redoxcline reached a low of 53 : 1. Francois (1987a) concluded that the depth of the redoxcline and iron limitation may be the important factors contributing to organic sulphur enrichment in the humic fraction. Canfield (1989) has noted the importance of iron speciation and limitation in determining whether organic or inorganic sulphur will be deposited in marine sediments. Francois's data on organic sulphur formation show similar characteristics to those observed in the Great Marsh, Delaware (Ferdelman, et al., 1991) where the major zone of organic sulphur formation is in the upper 4-6 cm, which is the zone characterized by high rates of both sulphate reduction and pyrite oxidation.

It is unclear how all organic sulphur compounds or functional groups are formed in situ. However, there are several recent reports which shed some information on mechanisms. Activated olefins react with H₂S (Vairavamurthy and Mopper, 1987; Sinninghe Damste et al., 1989) and inorganic polysulphides (Vairavamurthy and Mopper, 1988) to form thiols in marine porewaters (Table 6.1, reaction (1a)); polysulphides are more reactive. LaLonde, Ferrara and Hayes (1987) showed the basis for this reactivity using a molecular orbital approach and also showed that polysulphides react with conjugated olefin-carbonyls to form solid phase sulphur heterocyclic compounds (e .g. thiophenes). Isoprenoid thiophenes (cyclic C-S-C) are considered diagenetic products of chlorophyll-derived phytol or archaebacterial phytenes in marine sediments (Brassell et al., 1986). Recently, Kohnen et al. (1989) have provided evidence for the direct incorporation of polysulphides into solid phase organic compounds to form six membered cyclic disulphides (C-C-S-S-C-C) and seven membered cyclic trisulphides (C-C-S-S-S-C-C) .

Thayer, Olson and Brinckman (1984) reported on the direct reaction of FeS with organic halides to produce alkyl sulphides (R-S-R) and disulphides (RSSR) via a R-S-H intermediate. This reaction is analogous to the nucleophilic displacement of halides from organic halogen compounds by bisulphide ion which has been observed in polluted groundwaters (Schwarzenbach et al., 1985). Thiosulphate formed from FeS₂ oxidation can oxidize further to sulphate or react with organic compounds (e.g. organic halides) to form Bunte salts (RS₂O₃^-) which lead to reduced organic sulphur formation (Oae, 1985). This reaction has not been documented in the marine environment, but has been hypothesized by Francois (1987c) and Luther et al. (1986). Recently, Barbash and Reinhard (1989) have reviewed the reactivity of various nucleophiles, including S₂O₃^2- , towards halogenated organic compounds and concluded that such reactions are feasible.

6.5 CONCLUSIONS

Although there have been numerous studies concerning the forms of sulphur in the environment, we are still far from a complete understanding of sulphur chemistry in natural ecosystems. This is due to the wide range of oxidation states of sulphur (-2 to +6), to sulphur's tendency to form a variety of compounds with carbon, oxygen and itself, and to methodological problems with measuring sulphur compounds. More data are needed on which specific organic sulphur compounds or functional groups are prevalent in the environment under varying redox conditions. In addition, there is a great need to identify and understand the mechanisms by which sulphur is incorporated into these organic compounds. Are the mechanisms abiotic, biotic or both, and are these mechanisms dependent upon the conditions characteristic of particular types of ecosystems? Clearly, more research is needed to understand and to predict accurately how ecosystems will respond to changes in sulphur inputs, to transformations, and to interactions with other element cycles.

REFERENCES

Adams, D. F., Farwell, S. O., Pack, M. R. and Robinson, E. (1981). Biogenic sulphur gas emissions from soils in eastern and southeastern United States. APCA J., 31, 1083-9.

Aller, R. C. and Rude, P. D. (1988). Complete oxidation of solid phase sulphides by manganese and bacteria in anoxic marine sediments. Geochim. Cosmochim. Acta, 52, 751-65.

Altschuler, Z. S., Schneppe, M. M., Silber, C. C. and Simon, F. O. (1983). Sulphur diagenesis in Everglades peat and origin of pyrite in coal. Science, 221, 221-7.

Andreae, M. O. and Jaeschke, W. A. (This volume).

Aneja, V. P. and Cooper, W. J. (1989). Biogenic sulphur emissions: A review. In: Saltzman, E. S. and Cooper, W. J, (Eds). Biogenic Sulphur in the Environment. American Chemical Society, Washington DC, pp. 2-14.

Balzer, W. (1982). Organic sulphur in the marine environment, Chapter 13 In: Duursma, E. K. and Dawson, R. (Eds). Marine Organic Chemistry, Elsevier, pp. 395-414.

Barbash, J. E. and Reinhard, M. (1989).'Reactivity of sulphur nucleophiles toward halogenated organic compounds in natural waters. In: Saltzman, E. S. and Cooper , W. J. (Eds). Biogenic Sulphur in the Environment. American Chemical Society, Washington DC, pp. 101-38.

Berner, R. A. (1970) , Sedimentary pyrite formation. Am. J. Sci, , 268, 1-23.

Berner, R, A. (1984). Sedimentary pyrite formation: An update. Geochim. Cosmochim. Acta, 48, 605-15.

Berner, R. A. and Raiswell, R. (1983). Burial of organic carbon and pyrite sulphur in sediments over Phanerozoic time: a new theory .Geochim. Cosmochim. Acta, 47, 855-62.

Berner, R. A. and Westrich, J. T. (1985). Bioturbation and the early diagenesis of carbon and sulphur. Am. J. Sci., 285, 193-206.

Boulegue, J., Lord, C. J. and Church, T. M. (1982). Sulphur speciation and associated trace metals (Fe, Cu) in the porewaters of Great Marsh, DE. Geochim. Cosmochim. Acta, 46, 453-64.

Brassell, S. C., Lewis, C. A., deLeeuw, J. W., deLange, F. and Sinninghe Damste, J. S. (1986). Isoprenoid thiophenes: novel products of sediment diagenesis. Nature, 320, 160-2.

Burdige, D. J. and Nealson, K. H. (1986). Chemical and microbiological studies of sulphide mediated manganese reduction. Geomicrobiol, J., 4, 361-87.

Bustin, R. M. and Lowe, L. E. (1987). Sulphur, low temperature ash and minor elements in humid-temperate peat of the Fraser River Delta, British Columbia. J. Geol. Soc. (London), 144, 435-50.

Canfield, E. (1989). Reactive iron in marine sediments. Geochim. Cosmochim. Acta, 53, 619-32.

Canfield, D. E. and Berner, R. A. (1987). Dissolution and pyritization of magnetite in anoxic marine sediments. Geochim. Cosmochim. Acta, 51, 645-59.

Chanton, J. P., Martens, C. S. and Goldhaber, M. B. (1987). Biogeochemical cycling in an organic-rich coastal marine basin. 7. Sulphur mass balance, oxygen uptake and sulphide retention. Geochim. Cosmochim. Acta, 51, 1187-99.

Charlson, R. J., Lovelock, J. E., Andreae, M. O. and Warren, S. G. (1987). Oceanic phytoplankton, atmospheric sulphur, cloud albedo and climate. Nature, 326, 655- 61.

Chen, K. Y. and Morris, J. C. (1972). Kinetics of oxidation of aqueous sulphide by oxygen. Environ. Sci. Technol., 6, 529-37.

Christensen, J. P. (1989). Sulphate reduction and carbon oxidation rates in continental shelf sediments, an examination of offshelf carbon transport. Cont. Shelf Res., 9, 223-46.

Cooper, D. J., Cooper, W. J., de Mello, W. Z., Saltzman, E. S. and Zika, R. G. (1989). Variability in biogenic sulphur emissions from Florida wetlands. In: Saltz man, E. S. and Cooper, W. J. (Eds). Biogenic Sulphur in the Environment. American Chemical Society, Washington DC, pp. 31-43.

Cutter, G. A. and Velinsky, D. J. (1988). Sulphur diagenesis in a coastal salt marsh. Mar. Chem., 23, 311-27.

Dacey, J. W. H., King, G. M. and, Wakeman, S. G. (1987). Factors controlling emission of dimethylsulphide from salt marshes. Nature, 330, 643-5.

Ferdelman, T. G., Church, T. M. and Luther, G. W. III. (1991). Sulfur enrichment of humic substances in a Delaware salt marsh. Geochim. Cosmochim. Acta, 55, 979-88.

Fitzgerald, J. W., Strickland, T. C. and Ash, J. T. (1985). Isolation and partial characterization of forest floor and soil organic sulphur. Biogeochemistry, 1, 155- 67.

Francois, R. (1987a). A study of sulphur enrichment in the humic fraction of marine sediments during early diagenesis. Geochim. Cosmochim. Acta, 51, 17-27.

Francois, R. (1987b). A study of the extraction conditions of sedimentary humic acids to estimate their true in situ sulphur content. Limnol. Oceanogr., 32, 964-72.

Francois, R. (1987c). The influence of humic substances on the geochemistry of iodine in nearshore and hemipelagic marine sediments. Geochim. Cosmochim. Acta, 51, 2417-27.

Freney, J. R. (1986). Forms and reactions of organic sulphur compounds in soils. In: Tabatabai, M. A. (Ed.). Sulphur in Agriculture, Agronomy Monograph No.27. American Society of Agronomy, Madison, Wisconsin, pp. 207-32.

Fuller, R. D., Driscoll, C. T., Schindler, S. C. and Mitchell, M. J. (1986). A simulation model of sulphur transformations in forested Spodosols. Biogeochemistry, 2,313-28.

Gardner, L. R., Wolaver, T. G. and Mitchell, M. (1988). Spatial variations in the sulphur chemistry of salt marsh sediments at North Inlet, South Carolina. J. Mar. Res., 46, 815-36.

Giblin, A. E. (1988). pyrite formation in marshes during early diagenesis. Geomicrobiol. J., 6, 77-99.

Giblin, A. E. and Howarth, R. W. (1984). Porewater evidence for a dynamic sedimentary iron cycle in salt marshes. Limnol. Oceanogr., 29, 47-63.

Giblin, A. E. and R. K. Wieder. This volume.

Giggenbach, W. (1972). Optical spectra and equilibrium distribution of polysulphide ions in aqueous solution at 20°C. Inorg. Chem., 11, 1201-7.

Goldhaber, M. B. (1983). Experimental study of metastable sulphur oxyanion forma- tion during pyrite oxidation at pH 6-9 and 30° C. Am. J. Sci., 283, 193-217.

Goldhaber, M. B. and Kaplan, I. R. (1975). The sulphur cycle. In: Goldberg, E. D. (Ed.). The Sea. Wiley, New York, pp. 569-655.

Goldhaber, M. B. and Kaplan, I. R. (1980). Mechanisms of sulphur incorporation and isotope fractionation during early diagenesis in sediments of the Gulf of California. Mar. Chem., 9, 95-143.

Guerin, W. F. and Braman, R. S. (1985). Patterns of organic and inorganic sulphur transformations in sediments. Org. Geochem., 8, 259-68.

Haines, B. , Black, M. and Bayer, C. (1989). Sulphur emissions from roots of the rain forest tree Stryphnodendron excelsum. In: Saltzman, E. S. and Cooper, W. J. (Eds). Biogenic Sulphur in the Environment. American Chemical Society, Washington DC, pp. 58-69.

Hoffman, M. R. (1977). Kinetics and mechanisms of oxidation of hydrogen sulphide by hydrogen peroxide in acidic solution. Environ. Sci. Technol., 11, 61-6.

Howarth, R. W. (1979). pyrite: Its rapid formation in a salt marsh and its importance to ecosystem metabolism. Science, 203, 49-51.

Howarth, R. W. (1984). The ecological significance of sulfur in the energy dynamics of salt marsh and coastal marine sediments. Biogeochemistry, 1, 5-27.

Howarth, R. W. and Giblin, A. (1983). Sulfate reduction in the salt marshes at Sapelo Island, Georgia. Limnol. Oceanogr., 28, 70-82.

Howarth, R. W. and Jorgensen, B. B. (1984). Formation of ³⁵S-labelled elemental sulfur and pyrite in coastal marine sediments (Limfjorden and Kysing Fjord, Denmark) during short-term ³⁵S₄^2- reduction measurements. Geochim. Cosmochim. Acta, 48, 1807-18.

Howarth, R. W. and Merkel, S. (1984). Pyrite formation and the measurement of sulfate reduction in salt marsh sediments. Limnol. Oceanogr., 29, 598-608.

Howarth, R. W. and Teal, J. M. (1979). Sulfate reduction in a New England salt marsh. Limnol. Oceanogr., 24, 999-1013.

Howarth, R. W. and Teal, J. M. (1980). Energy flow in a salt marsh ecosystem; the role of reduced inorganic sulphur compounds. Am. Nat., 116, 862-72.

Howarth, R. W., Giblin, A., Gale, J., Peterson, J. and Luther, G. W. (1983). Reduced sulfur compounds in the pore waters of a New England salt marsh. Environ. Biogeochem. Ecol. Bull., 35, 135-52.

Huerta-Diaz, M. A. and Morse, J. W. (1990). A quantitative method for determination of trace metal concentrations in sedimentary pyrite. Mar. Chem., 29, 119-44.

Ivanov, M. V., Lein, A. Yu, Reeburgh, W. S. and Skyring. (1989). Interaction of sulphur and carbon cycles in marine sediments. In: Brimblecombe, P. and Lein, A.

Yu. (Eds), Evolution of the Global Biogeochemical Sulphur Cycle, Wiley, Chichester, pp. 125-179.

Jørgensen, B. B. (1982). Mineralization of organic matter in the sea bed: the role of sulfate reduction. Nature, 296, 643-5.

Kiene, R. P. and Taylor, B. F. (1988). Biotransformations of organosulphur compounds in sediments via 3-mercaptopropionate. Nature, 332, 148-50.

Kiene, R. P. and Visscher, P. T. (1987). Production and fate of methylated sulphur compounds in sediment via 3-mercaptopropionate. Nature, 332, 148-50.

Kiene, R. R. and Visscher, P. P. (1987). Production and fate of methylated sulphur compounds from methione and dimethylsulphoniopropionate in anoxic salt marsh sediments. Appl. Environ. Microbiol., 53, 2426-34.

Kiene, R. P., Oremland, R. S., Catena, A., Miller, L. G. and Capone, D. G. (1986). Metabolism of reduced methylated sulphur compounds in anaerobic sediments and by a pure culture of an estuarine methanogen. Appl. Environ. Microbial., 52, 1037- 45.

King, G. M. (1988). Patterns of sulphate reduction and the sulphur cycle in a South Carolina salt marsh. Limnol. Oceanogr., 33, 376-90.

King, G. M., Howes, B. L. and Dacey, J. W. J. (1985). Short term end products of sulphate reduction in a salt marsh: formation of acid volatile sulphides, elemental sulphur, and pyrite. Geochim. Cosmochim. Acta, 49, 1561-6.

King, G. M., Klug, M. J., Wiegert, R. G. and Chalmers, A. G. (1982). Relation of soil water movement and sulfide concentration to Spartina alterniflora production in a Georgia salt marsh. Science, 218, 61-3.

Kohnen, M. E. L., Sinninghe Damste, J. S., ten Haven, H. L. and de Leeuw, J. W. (1989). Early incorporation of polysulphides in sedimentary organic matter. Nature, 341, 640-1.

Kornicker, W. A. and Morse, J. W. (1991). Interactions of divalent cations with the surface of pyrite. Geochim. Cosmochim. Acta, 55, 2159-71.

LaLonde, R. T., Ferrara, L. M. and Hayes, M. P. 1987). Low temperature polysulphide reactions of conjugated ene carbonyls: A reaction model for the geologic origin of S-heterocycles. Org. Geochim., 11,563-71.

Lord, C. J. and Church, T. M. (1983). The geochemistry of salt marshes: sedimentary ion diffusion, sulphate reduction, and pyritization. Geochim. Cosmochim. Acta, 47, 1381-91.

Lowe, L. E. (1986). Application of a sequential extraction procedure to the determination of the distribution of sulphur forms in selected peat materials. Can. J. Soil Sci., 66, 337-45.

Lowe, L. E. and Bustin, R. M. (1985). Distribution of sulphur forms in six facies of peats of the Fraser River Delta. Can. J. Soil Sci., 65, 531-41.

Luther, G. W., III (1987). Pyrite oxidation and reduction: molecular orbital theory considerations. Geochim. Cosmochim. Acta, 51, 3193-9.

Luther, G. W., III (1991). Pyrite synthesis via polysulfide compounds. Geochim. Cosmochim. Acta, 55, 2839-49.

Luther, G. W., III and Church, T. M. (1988). Seasonal cycling of sulphur and iron in porewaters of a Delaware salt marsh. Mar. Chem., 23, 295-309.

Luther, G. W., III, Varsolona, R. and Giblin, A. E. (1985). Polarographic analysis of sulphur species in marine porewaters. Limnol. Oceanogr., 30, 727-36.

Luther, G. W., III, Giblin, A., Howarth, R. W. and Ryans, R. A. (1982). Pyrite and oxidized iron mineral phases formed from pyrite oxidation in salt marsh and estuarine sediments. Geochim. Cosmochim. Acta, 46, 2665-9.

Luther, G. W., III, Church, T. M., Scudlark, J. R. and Cosman, M. (1986). Inorganic and organic sulphur cycling in salt marsh porewaters. Science, 232, 746-9.

McKibben, M. A. and Barnes, H. L. (1986). Oxidation of pyrite in low temperature of light and nutrients on their planktonic production. Limnol. Oceanogr., 33, 624- 31.

Matrai, P. A. and Vetter, R. D. (1988). Particulate thiols in coastal waters: the effect acidic solutions: rate laws and surface textures. Geochim. Cosmochim. Acta, 50, 1509-20.  

Mitchell, M. J. and Fuller, R. D. (1988). Models of sulphur dynamics in forest and grassland ecosystems with emphasis on soil processes. Biogeochemistry, 5, 133-63.

Mopper, K. and Delmas, D. (1984). Trace determination of biological thiols by liquid chromatography and precolumn labeling with ophthaldehyde. Anal. Chem., 56, 2557-60.

Mopper, K. and Taylor, B. (1986). Hiogeochemical cycling of sulphur: Thiols in coastal marine sediments. In: Sohn, M. (Ed.). Organic Marine Chemistry. Amer. Chem. Soc., Washington DC, pp. 324-39.

Moses, C. O., Nordstrom, D. K., Herman, J. S. and Mills, A. L. (1987). Aqueous pyrite oxidation by dissolved oxygen and by ferric iron. Geochim. Cosmochim. Acta, 51, 1561-71.

Nguyen, B. C., Belvino, S., Mihalopoulos, N., Gostan, J. and Nival, P. (1988). Dimethylsulphide production during natural phytoplanktonic blooms. Mar. Chem., 24, 133-41.

Nriagu, J. O. and Soon, Y. K. (1985). Distribution and isotopic composition of sulphur in lake sediments of northern Ontario. Geochim. Cosmochim. Acta, 49, 823-34.

Oae, S. (1985). Historical development of sulphur bonding: A view of an experimental organosulphur chemist. In: Bernardi, F., Csizmalia, I. F. and Mangini, A. (Eds). Organic Sulphur Chemistry, Elsevier, New York, pp. 1-67.

Pyzik, A. J. and Sommer, S. E. (1981). Sedimentary iron monosulphides: Kinetics and mechanism of formation. Geochim. Cosmochim. Acta, 45, 687-98.

Raiswell, R. and Berner, R. A. (1986). Pyrite and organic matter in Phanerozoic normal marine shales. Geochim. Cosmochim. Acta, 50, 1967-76.

Rennenberg, H. (1989). Synthesis and emission of hydrogen sulphide by higher plants. In: Saltzman, E. S. and Cooper, W. J. (Eds). Biogenic Sulphur in the Environment. American Chemical Society, Washington DC, pp. 44-57.

Rickard, D. T. (1975). Kinetics and mechanisms of pyrite formation at low temperatures. Am. J. Sci., 275, 636-52.

Roy, A. B. and Trudinger, P. A. (1970). The Biochemistry of Inorganic Compounds of Sulphur. Cambridge University Press, London.

Rudd, J. W., Kelly, C. A. and Furutani, A. (1986). The role of sulphate reduction in long term accumulation of organic and inorganic sulphur in lake sediments. Limnol. Oceanogr., 31,1281-91.

Schwarzenbach, R. P., Giger, W., Schaffner, C. and Wanner, O. (1985). Groundwater contamination by volatile halogenated alkanes: abiotic formation of volatile sulphur compounds under anaerobic conditions. Environ. Sci. Technol., 19, 322-7.

Shea, D. and MacCrehan, W. A. (1988a). Determination of hydrophilic thiols in sediment porewater using ion-pair liquid chromatography coupled to electrochemical detection. Anal. Chem., 60, 1449-1454.

Shea, D. and MacCrehan, W. A. (1988b). Role of biogenic thiols in the solubility of sulphide minerals. Sci. Total Environ.,73, 135-41.

Singer, P. C. and Stumm, W. (1970). Acid mine drainage-the rate limiting step. Science, 167,1121-3.

Sinninghe Damste, J. S., Rijpstra, I. C., Kock-van Dalen, A. C., de Leeuw, J. W. and Schenck, P. A. (1989). Quenching of labile functionalized lipids by inorganic sulphur species: Evidence for the formation of sedimentary organic sulphur compounds at the early stages of diagenesis. Geochim. Cosmochim. Acta, 53, 1343-55.

Spratt, H. G., Morgan, M. D. and Good, R. E. (1987). Sulphate reduction in peat from a New Jersey Pinelands cedar swamp. Appl. Environ. Microbiol., 53, 1406-11.

Steudler, P. A. and Peterson, B. J. (1984). Contribution of gaseous sulphur from salt marshes to the global sulphur cycle. Nature, 311, 455-7.

Strickland, T. C., Fitzgerald, J. W., Ash, J. T. and Swank, W. T. (1987). Organic sulphur transformations and sulphur pool sizes in soil and litter from a southern Appalachian hardwood forest. Soil Sci., 143, 453-8.

Sweeney, R. E. and Kaplan, I. R. (1973). Pyrite framboid formation: laboratory synthesis and marine sediments. Econ. Geol., 68, 618-34.

Swider, K. T. and Mackin, J. E. (1989). Transformations of sulphur compounds in marsh-flat sediments. Geochim. Cosmochim. Acta, 53, 2311-23.

Tabatabai, M. A. (1984). Importance of sulphur in crop production. Biogeochemistry, 1, 45-62.

Taylor, B. E., Wheeler, M. C. and Nordstrom, D. K. (1984a). Oxygen and sulphur compositions of sulphate in acid mine drainage: evidence for oxidation mechanisms. Nature, 308, 538-41.

Taylor, B. E., Wheeler, M. C. and Nordstrom, D. K. (1984b). Stable isotope geochemistry of acid mine drainage: experimental oxidation of pyrite. Geochim. Cosmochim. Acta, 48, 2669-78.

Thayer, J. S., Olson, G. J. and Brinckman, F. E. (1984). Iodomethane as a potential metal mobilizing agent in nature. Environ. Sci. Technol., 18, 726-9.

Thode-Andersen, S. and Jorgensen, B. B. (1989). Sulphate reduction and the formation of ³⁵S-labeled FeS, FeS₂, and S^o in coastal marine sediments. Limnol. Oceanogr., 34, 793-806.

Turner, S. M. , Malin, G. and Liss, P. S. (1989). Dimethyl sulphide and (dimethylsulphonio) propionate in European coastal and shelf waters. In: Saltzman, E. S. and Cooper, W. J. (Eds). Biogenic Sulphur in the Environment. American Chemical Society, Washington DC, pp. 183-200.

Urban, N. R., Eisenreich, S. J. and Grigal, D. F. (1989). Sulphur cycling in a forested Sphagnum bog in northern Minnesota. Biogeochemistry, 7, 81-109.

Vairavamurthy, A. and Mopper, K. (1987). Geochemical formation of organosulphur compounds (thiols) by addition of H₂S to sedimentary organic matter. Nature, 329, 623-5.

Vairavamurthy, A. and Mopper, K. (1988). Organosulphur formation in marine sediments: laboratory studies on the reactivity of sulphur nucleophiles with organic Michael acceptors. In: Saltzman, E. S. and Cooper, W. J. (Eds). Biogenic Sulphur in the Environment. American Chemical Society, Washington DC, pp. 231-42.

Vetter, R. D., Matrai, P. A., Javor, B. and O'Brien, J. (1989). Reduced sulphur compounds in the marine environment. In: Saltzman, E. S. and Cooper, W. J. (Eds). Biogenic Sulphur in the Environment. American Chemical Society, Washing- ton DC, pp. 243-61.

Watwood, M. E., Fitzgerald, J. W., Swank, W. T. and Blood, E. R. (1988). Factors involved in potential sulphur accumulation in litter and soil form a coastal pine forest. Biogeochemistry, 6, 3-19.

Westrich, J. T. and Berner, R. A. (1988). The effect of temperature on rates of sulphate reduction in marine sediments. Geomicrobiol. J., 6, 99-117.

Wieder, R. K. and Lang, G. E. (1986). Fe, Al, Mn, S chemistry of sphagnum peat in four peatlands with different metal and sulphur input. Water Air Soil Pollut., 29, 309-20.

Wieder, R. K. and Lang, G. E. (1988). Cycling of inorganic and organic sulphur in peat from Bog Run, West Virginia. Biogeochemistry, 5, 221-42.

Wieder, R. K., Lang, G. E. and Granus, V. A. (1987). Sulphur transformations in sphagnum-derived peat during incubation. Soil BioI. Biochem., 19, 101-6.

APPENDIX TO CHAPTER 6: GLOSSARY OF ENVIRONMENTALLY IMPORTANT INORGANIC AND
ORGANIC SULPHUR COMPOUNDS AND IONS (WITH SULPHUR OXIDATION STATES)

INORGANIC SULPHUR COMPOUNDS

MAJOR ORGANIC FUNCTIONAL GROUPS
 
R = organic group, for R = methyl the name is in parentheses.

Accurate oxidation states are not as easily obtained or calculated for organic compounds. Thus, formal charges [fc] are given. These are calculated by the equation below which requires a correct Lewis electron dot structure:

[fc] = maximum valence electrons of the atom in its standard state -1/2 (the number of electrons in bonds) -the number of electrons not in bonds

e.g. S in R-:S:-R

      [fc] = 6 -1/2 (4) -4 = 0

'Reduced'

Special Cyclic Structures