Sulfur isotopes models

There is another explanation for the variations in values of sulfide sulfur. It was cited that oxidation state (/02) od pH of ore fluids are important factor controlling values of ore fluids (e.g., Kajiwara, 1971). According to the sulfur isotopic equilibrium model (Kajiwara, 1971 Ohmoto, 1972), of sulfides in predominance... 

Janecky, D.R. and Shanks, W.C. Ill (1988) Computational modeling of chemical and sulfur isotopic reaction processes in seafloor hydrothermal systems, chimney, massive sulfides, and subjacent alteration zones. Can. Mineral, 26, 805—826. 

Another factor that is of great importance for the observed sulfur isotope variations of natural sulfides is whether sulfate reduction takes place in an open or closed system. An open system has an infinite reservoir of sulfate in which continuous removal from the source produces no detectable loss of material. Typical examples are the Black Sea and local oceanic deeps. In such cases, H2S is extremely depleted in " S while consumption and change in " S remain negligible for the sulfate. In a closed system, the preferential loss of the lighter isotope from the reservoir has a feedback on the isotopic composition of the unreacted source material. The changes in the " S-content of residual sulfate and of the H2S are modeled in Fig. 2.21, which shows that 5 S-values of the residual sulfate steadily increase with sulfate consumption (a linear relationship on the log-normal plot). The curve for the derivative H2S is parallel to the sulfate curve at a distance which depends on the magnitude of... 

Naturally occurring stable isotopes of C, N, and S have been used extensively for over a decade as direct tracers of element cycling in marine and terrestrial food webs (34-39). Carbon and sulfur isotopes fractionate very little between food and consumer thus their measurement indicates which primary producers or detrital pools are sources of C and S for consumers. For example, a study of plants and animals in Texas sand dunes showed that insect species had 813C values either like those of C3 plants or like those of C4 plants (-27 and -13%o, respectively). Rodent species had intermediate values near -20%o that indicated mixed diets of both C3 and C4 plants (40). The 13C measurements, used in simple linear mixing models, proved to be quick and reliable indicators of which plant sources provided the carbon assimilated by higher trophic levels. 

Summary of Biogenic Sulfur Emissions from the Ocean. To summarize the source fractionation patterns for biogenic sulfur emissions from the oceam a simple isotopic model has been constructed (Figure 1). From this, the S S value of sulfate from a marine biogenic source of atmospheric sulfur can be estimated. 

Rees C.E. (1970) The sulfur isotope balance of the ocean An improved model. Earth. Planet. Sci. Lett. 7, 366-370. 

Jprgensen, B.B. (1979) A theoretical model of the stable sulfur isotope distribution in marine sediments. Geochim. Cosmochim. Acta 43, 363-374. 

Figure 29 Model sulfur isotopic evolution in the course of geologic history. The lower trend (labeled 6 Sred) represents the for sulfides. The upper curves (labeled 6 Sox) are the of marine sulfates. TML and VJ simulations assume a logistic type of continental growth as proposed by Taylor and McLennan (1985) and Veizer and Jansen (1979), respectively. BB simulation assumes an instantaneous generation of continental crust, BB-evol simulation assumes instantaneous continental generation, but with delayed invention of oxygen generating photosynthesis. The Phanerozoic trend as in Figure 30. Dots represent measurements of Precambrian sulfates (Claypool et al, 1980) and the hatched field represents sulfates from Holser et al. (1988) (after Godderis...

Petsch and Berner (1998) expanded the model of Van Cappellen and Ingall (1996) to include the sulfur system, as well as carbon and sulfur isotope effects. This study examined the response of the C-S-O-Fe-P system, and in particular carbon and sulfur isotope ratios, to perturbation in global ocean overturn rates, changes in continental... 

Applying these newly recognized modifications of carbon and sulfur isotope discrimination in response to changing O2 availability has allowed development of new numerical models of the evolution of the coupled C-S-O systems and variability of Phanerozoic atmospheric O2 concentration (Figure 17). 

The reduction is typically limited by the availability of organic carbon and often occurs in shallow waters at continental margins. Thus, global sulfide production would be dependent on the availability of biological productive areas over geological time. Sulfur-isotope data can be used to constrain simple models of the sulfur cycle over geological time and establish the size of the reservoirs as shown in Figure 5(b). 

Modeling Dynamic Hydrothermal Processes by Coupling Sulfur Isotope Distributions with Chemical Mass Transfer Approach... 

Chemical reaction models of seafloor hydrothermal vent processes and accompanying sulfur isotopic distribution pathways illustrate the capabilities of coupling EQPS S with EQ6 calculations, including the extent of differences that can exist due to the isotopic bounding condition assumptions described above. 

Chemical reaction pathways for input into the isotopic model have been computed using the EQ3/6 reaction pathway modeling codes (6). Distribution of sulfur isotopes between aqueous species and minerals are calculated using a new computer code (EQPS.S). Isotopic fractionation factors (I) are used by the code to determine the distribution among components as described below. Thus, this approach does not make or apply any assumptions about the chemical mechanism by which isotopic exchange or transfer occurs. The descriptive , rather than mechanistic approach, is due in part to the lack of understanding of such mechanisms and inability of chemical reaction codes to handle kinetics of homogeneous solution reactions. 

Mid-ocean ridge hydrothermal processes provide an ideal application for geochemical reaction path modeling, involving temperature dependent reactions, fluid mixing, reaction with sulfide products, and reaction with seafloor basalts (5). The solution and solid compositions are well characterized, including that of sulfur isotopes (10.11). However, measured fractionations between solution and solid samples can not be the result of simple equilibrium processes f 10-121. The ability to track reactions involving seawater sulfate = 21 per mil) and hydrother-... 

Adiabatic-mixing pathways, where seawater (2°C) mixes into hydrothermal fluid (350°C), have been successfully used to model formation of sulfide minerals associated with venting hydrothermal solutions at mid-ocean ridges (9). Sulfate reduction can be quantitatively and isotopically important in such reactions (. Combinations of three types of isotopic path constraints discussed above have been examined, using the mixing reaction pathways calculated by Janecky and Seyfried ( ) for chemical equilibrium and initial sulfur isotopic compositions of 1 per mil for the hydrothermal solution and 21 per mil for seawater (Figure 1). 

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