Sulfur input components

Total moles of all sulfur-bearing components present in the system are required by EQPS from data on tabular files output by EQ6 or input from the user. The input modules allow expansion to integrate with other codes as necessary. The choice of a post-processor mode of operation for EQPS, rather than integration into the EQ6 code as for the oxygen and hydrogen isotopic calculations of Bowers and Taylor (4), was made primarily because of the complexities of sulfur chemistry discussed above. By using the post-processor approach the computationally intensive EQ6 code is only run once for each chemical reaction path, while the simpler and much faster EQPS code can be run multiple times to examine the effects of various isotopic reaction constraints. 

To show mass balance, we add the molalities of each species containing a component (but not species concentrations in mg kg-1, since the mole weight of each species differs) to arrive at the input constraint. Taking component SO4 as an example, we find the total mole number (A/,) from the molalities (m, and m.j) of the sulfur-bearing species... 

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.

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. 

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