Application of Gibbs Energy Minimization to Model Early-Diagenetic Solid-Solution Aqueous-Solution Equilibria Involving Authigenic Rhodochrosites in Anoxic Baltic Sea Sediments

摘要

The natural early-diagenetic environment "anoxic porewater-authigenic mineral phases" has been characterized in sediment of the Gotland Deep. Baltic Sea, by a closed-system model. Occurrence of carbonate precipitates as thin almost pure white laminae was considered as a natural experiment for long-term equilibration between these phases and porewater. Plots of distribution coefficients indicate that metastable equilibrium exists between porewater and the authigenic Ca-rich rhodochrosite phases below 7 cm depth. A thermodynamic model of porewater geochemistry at in situ P = 25 bar and T = 5 ℃ was developed using the Gibbs energy minimization (GEM) approach. The values of isobaric-isothermal potentials of Mn, Ca, Fe, Mg, Sr, Ba, C, and O, calculated from the porewater composition, were used in a new "dual thermodynamic" calculation approach to estimate solid activity coefficients of the end-members in the non-ideal solid solution (Mn, Ca, Mg, Sr, Ba, Fe)CO_3, i.e., at full major and minor multi-component complexity. The regular Margules interaction parameters for the composing binaries estimated by this model were #alpha#_(Mn-Ca) = 1.9 +- 0.5, #alpha#_(Mn-Mg) = 0.6, #alpha#_(Ca-Mg) = 3.7, #alpha#_(Mn-Fe) = 0.2, #alpha#_(Ca-Fe) = 2.8, #alpha#_(Mn-Sr) = 9.7, #alpha#_(Ca-Sr) = 2.15, #alpha#_(Mn-Ba)= 4.0, #alpha#_(Ca-Ba) = 1.4, validating the theoretical predictions given by Lippmann in his pioneering 1980's paper. The strictly thermodynamic equilibrium model is not only able to match both the measured porewater and carbonate solid-solution composition, but also to predict that the porewater pH, pe, alkalinity, and dissolved Mn, Fe, and S concentrations are controlled by the authigenic mineral buffering assemblage mackinawite-greigite-rhodochrosite. Our model is only compatible with the idea of ACR formation with typical composition (X_(Mn) between 70-75%) in the topmost sediment layer which, however, needs a major source of Mn_(aq)~(II). This is provided by reduction of particulate Mn oxides precipitated in significant amounts in the water column upon major inflow events in the Baltic Sea. The model enables also to set up scenarios of changing environmental conditions, e.g., to predict the non-linear response of the carbonate solid-solution composition to changes in Mn loading, alkalinity and salinity of the sediment-water system. The results suggest that the major and especially minor element contents (Sr, Mg, Ba) in authigenic carbonates can be applied as an environmental paleoproxy.