RMZ - Materials and Geoenvironment, Vol. 52, No. 1, 103-105, 2005
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Sulfurization of organic matter in recent estuarine sediments
(Authie Bay, N France)
Sonja Lojen1, Cedric Gabelle2, Branko Cermelj3, Michel Wartel2
'Department of Environmental Sciences, J. Stefan Institute, Ljubljana, Slovenia; sonja.lojen@ijs.si laboratory of Analytical and Marine Chemistry, Faculty of Sciences and Technology of Lille I,
France; cedric.gabelle@ed.univ-lillel.fr QMarine Biology Station, National Institute of Biology, Piran, Slovenia; cermelj@mbss.org
Abstract: Biogeochemical cycling of sulfur in estuarine sediments was studied in Authie Bay, N France, based on Distribution and stable isotope composition of sedimentary S species. The accumulation of biosynthetic and diagenetic organic S was not related to the fluxes of organic matter, but critically depended on the competitive processes of pyrite formation and sulfurization of organic substrates.
Key words: sediment, sulfur, isotope, humic acid, fulvic aid
Introduction
The enrichment of organic matter with sulfur (S) during earliest stages of diagenesis has been amply documented in marine sediments, while the importance of such reactions in freshwater environments is not very much appreciated (Bruchert, 1998, Urban et al., 1999). However, recent studies of lacustrine and soil-derived humic and fulvic acids showed, that their S-containing functional groups play an important role in binding and immobilization of metals (Hutchinson et al., 2001). In estuarine hu-mic and fulvic acids, S is present predominantly in oxidized forms as sulfate or sulfoxide rather than in reduced forms (e.g. in oxidation states close to 0) (Billon et al., 2002). Laboratory experiments showed, however, that polysulfides are principal sul-furizing agents for organic matter in environments with pH close to neutral (Amrani and Aisenstrat, 2004).
The aim of the present study was to investigate the timing of sulfurization of humic and fulvic acids in estuarine environment along the salinity gradient in a relatively pristine environment of the Authie Bay (N France). Distribution and stable isotope composition (d34S) of dissolved and sedimentary S species were analyzed in three sediment cores collected along the last 5 km of the Authie River flow, with salinity ranging from 0 to 23. Dissolved sulfide and sulfate, as well as sedimentary S forms: acid volatile sulfide (AVS, consisting predominantly of amorphous FeS, greigite and mackinvite), CRS (pyrite and elemental S), elemental S, and S bound to humic (HAS) and fulvic acids (FAS) were extracted using sequential extraction procedures described in the literature (Bruchert, 1998, Billon et al., 2002, Henneke et al., 1997). Stable isotope composition of extracted species was determined
Short scientific paper
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Lojen, S. et al.
using a continuous flow IRMS (Europa 20-20). Concentrations of organic carbon and total nitrogen, as well as reactive Fe (as degree of pyritization) were also analyzed.
Results and discussion
The total S concentration generally decreased with depth and was the highest in the marine environment. The distribution of individual S species in the sediment varied as well. With increasing marine influence, inorganic S forms (AVS, CRS and elemental S) became prevailing over organosulfur compounds. No correlation between sedimentary organic C and organic S was observed, indicating that the accumulation of biosynthetic and diage-netic organic S was not related to the fluxes of organic matter, but critically depended on the competitive processes of pyrite formation and sulfurization of organic substrates. Fe2+ is supposed to be an effective sink for
H2S, however, in the Authie Bay the formation of FAS and HAS was generally independent on the availability of reactive Fe. At the same time, the formation of FAS and HAS above the AVS and CRS in the freshwater or mixed environments indicates that the predominant reactants for organosulfurs are partially reduced S ionic species, such as polysulfides, rather than sulfide.
The stable isotope composition of sedimentary S species is shown in Figure I. Dissolved sulfate was the most 34S-enriched species and its 534S value increased with depth, however, the enrichment varied from site to site. Among the sedimentary S species, CRS generally exhibited the most negative 534S values. 534S values of HAS and FAS were generally not correlated with their concentration and lied between those of dissolved sulfate and sulfides, confirming the assumption that dissolved sulfide was not directly bond to the organic substrate. Using a simple mix-
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Figure I. Stable isotope compositions of dissolved and sedimentary S species in the sediments of Authie Bay; AI - Authie Aval (marine environment); A2 - Port de la Madelon (marine with strong frehswater influence); A3 - Authie Amont (freshwater).
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Sulfurization of organic matter in recent estuarine sediments (Authie Bay, N France)
93
ing equation for determination of sources of S in the organic matter, it was estimated that around 70 % of organosulfUrs are of biosyn-thetic origin, while the rest is diagenetic. The 534S of fluxes of added diagenetic S to the organic substrates were estimated to be between -12 and + 8 %o, i.e. values that are lower than those of dissolved sulfate, showing that the sulfur incorporated into the organic matter was subject to sulfate reduction and most probably also to recycling of reduced S in the sediment.
Conclusions
The occurrence of organosulfur compounds
in the uppermost sediment layer and their
References
Amrani, A., Aisenstrat, Z. (2004): Mechanisms of sulfur introduction chemically controlled: 834S print. Org. Geochem., in press (available on Internet via ScienceDirect).
Billon, G., Ouddane, B., Recourt, P., Boughriet, A. (2002): Depth Variability and some Geochemi-cal Characteristics of Fe, Mn, Ca, Mg, Sr, S, P, Cd and Zn in Anoxic Sediments from Authie Bay (Northern France) Estuar. Coast. Shelf Sci. 55, 167-181.
Bruchert, V. (1998): Early diagenesis of sulfur in estuarine sediments: The role of sedimentary hu-mic and fulvic acids. Geochim. Cosmochim. Acta 62, pp.1567-1586.
enrichment in 34S relative to AVS and CRS suggest their biosynthetic origin, but also a considerable contribution of diagenetic sulfurization of humic and fUlvic acids in the earliest stages of diagenesis. Based on S isotope composition of HAS and FAS, the main sulfurizing agents are supposed to be partially reduced and/or recycled dissolved S species rather than sulfide or sulfate.
Acknowledgements
This study was financed by the INTERREG III STARDUST project, CNRS research fellowship SPRH 41/2003, and the University of Sciences and Technologies of Lille 1.
Henneke, E., Luther III, G.W., de Lange, G.J., Hoefs. J. (1997): Sulphur speciation in anoxic hypersaline sediments from the eastern Mediterranean Sea. Geochim. Cosmochim. Acta 61, 307-321. Hutchinson, K.J., Hesterberg, D., Chou, J.W.( 2001): Stability of reduced organic sulfur in humic acids as affected by aeration and pH. Soil Sci. Soc. Am. J .65, 704-709. Urban, N.R., Ernst, K., Bernasconi, S. (1999): Addition of sulfur to organic matter during early diagenesis of lake sediments. Geochim. Cosmochim. Acta 63, 837-853.
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