Sulfur flux sediments

Measurements of S cycling in Little Rock Lake, Wisconsin, and Lake Sempach, Switzerland, are used together with literature data to show the major factors regulating S retention and speciation in sediments. Retention of S in sediments is controlled by rates of seston (planktonic S) deposition, sulfate diffusion, and S recycling. Data from 80 lakes suggest that seston deposition is the major source of sedimentary S for approximately 50% of the lakes sulfate diffusion and subsequent reduction dominate in the remainder. Concentrations of sulfate in lake water and carbon deposition rates are important controls on diffusive fluxes. Diffusive fluxes are much lower than rates of sulfate reduction, however. Rates of sulfate reduction in many lakes appear to be limited by rates of sulfide oxidation. Much sulfide oxidation occurs anaerobically, but the pathways and electron acceptors remain unknown. The intrasediment cycle of sulfate reduction and sulfide oxidation is rapid relative to rates of S accumulation in sediments. Concentrations and speciation of sulfur in sediments are shown to be sensitive indicators of paleolimnological conditions of salinity, aeration, and eutrophication. 

The insolubility of iron sulfides offers the potential for the burial and mineralization of large amounts of sulfur in sediments. However, sulfur shares with nitrogen a complex organic chemistry, but organosulfur compounds have not always been widely studied. Nevertheless the discovery of large fluxes of DMS to the atmosphere has revealed just how important organosulfur compounds can be. 

Various workers have estimated the rate of pyrite formation. Berner (1972) summed the sulfur accumulation rates of various sediment types in proportion to their areal coverage and found a flux of about 10% of the river flux. Li (1981) carried out a similar calculation and finds 30% of the river flux, probably indicative of the uncertainty of the approach. Toth and Lerman (1977) established that the decrease of sulfate with depth in sediment pore waters is a function of sedimentation rate. This information was used to estimate the diffusive flux of sulfur into sediments driven by pyrite formation, again a value about 10% of the river flux. Apparently, pyrite... 

Table 5.39. Diffusion fluxes of sulfur across sediment-seawater interface in the Nansha coral reef lagoon waters, South China Sea (Song and Li, 1996b)...

Song JM, Li PC (1996b) -2 valance sulfur in sediment interstitial waters of Nansha Islands waters. Oceanol Limnol Sin 27(6) 642 (in Chinese with English abstract) Song JM, Li PC (1996c) Studies on characteristics of nutrient diffusion fluxes across sediment-water interface in the district of Nansha islands, South China Sea. Mar Sci 20(5) 43-50 (in Chinese with English abstract)... 

Arsenic(As) in ocean is mainly removed by formation of pyrite in marine sediments. The production rate of sulfur in pyrite is 3.3 X 10 mol my (2.5 X 10 ° g my ) (Holland 1978). As/S ratio of pyrite in sediments previously reported is (8.7 3) x 10" (Huerta-Diaz and Morse 1992). Thus, As sink by pyrite is (1.7-3.9) x 10 mol my . This flux seems to be not different from As input to ocean ((1.6-8.1) x lO mol my (Table 5.3). As concentration of ocean is considered to be controlled by hydrothermal input, riverine input and pyrite output. Fluxes by volcanic gas from atmosphere and by weathering of ocean-floor basalt are small, compared with hydrothermal, riverine and pyrite As fluxes. Residence time of As in seawater is estimated as the amount of As in seawater (4.2 x 10 g) divided by As input to seawater (1.6-8.1) X 10 mol my which is equal to (1.7-3.8) x 10" year. This is shorter than previously estimated one (10 year by Holland 1978). Subducting sulfur flux is estimated to be 6.1 x 10 g my from S contents of altered basalt and sediments ( 0.1 wt%) (Kawahata and Shikazono 1988) and subducting rates of... 

Interest in the possible persistence of aliphatic sulfides has arisen since they are produced in marine anaerobic sediments, and dimethylsulfide may be implicated in climate alteration (Charlson et al. 1987). Dimethylsnlfoniopropionate is produced by marine algae as an osmolyte, and has aronsed attention for several reasons. It can be the source of climatically active dimethylsulfide (Yoch 2002), so the role of specific bacteria has been considered in limiting its flux from the ocean and deflecting the prodncts of its transformation into the microbial sulfur cycle (Howard et al. 2006). 

Sulfur in the sediments and oceanic crust which is derived from seawater subducts to deeper parts. This subduction flux is estimated to be ca. 4 x lO mol/m.y. (Shikazono, 1997). Therefore, degassing S flux from back-arc and island arc ((2.3-8.2) x lO mol/m.y.) seems to be not different from the subduction flux, although uncertainty of estimated degassing and subduction flux is large. 

Figure 7. Sulfur transformations of seston-S and microbially reduced S in sediments of Little Rock Lake. This diagram is based on information collected from the lake (cores and sediment traps) and from laboratory sediment-water microcosms to which 35S was added as labeled algae or as 35S042 Sizes of circles and arrows are roughly proportional to magnitudes of pools and fluxes, respectively. ROSO3 = ester-S MeS = metal sulfide, C-S = carbon-bonded S.

Figure 12.5 Sulfur isotopic budget based on mass isotopic fluxes (bar graphs) and their isotopic composition on a monthly basis in coastal marine sediments of cape Lookout Bight, NC (USA). Units are in mol S m-2 m-1. (Modified from Chanton and Martens, 1987b.)...

The flux of DS across the sediment-water interface can in some cases be strongly influenced by the presence of chemoautotrophic bacterial mats in estuaries. These sulfur oxidizing bacteria occur at the oxic-anoxic interface and can occur as colorless or as pigmented forms (GSB and PSB). 

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. 

Figure 1 Conceptual model for the origin of mixed detrital-biogenic facies relating the three major inputs to the processes that control them. The major inputs are shown in boxes with bold-type labels. ControlUng factors are shown in italics. Large and medium scale arrows represent fluxes of key components involved in sedimentation and the biogeochemical cycles of carbon, sulfur, and oxygen. Thin arrows illustrate relationships between major controlling factors and depositional processes and/or feedback. Dashed thin arrows apply to major nutrient fluxes only. Dotted thin arrows apply to major authigenic fluxes only. See text for further explanation.

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