Application of Gibbs energy minimization to model early-diagenetic solid-solution aqueous-solution equilibria involving authigenic rhodochrosites in anoxic Baltic Sea sediments

The natural early-diagenetic environment "anoxic porewater - authigenic min eral phases'' has been characterized in sediment of the Gotland Deep, Balti c Sea, by a closed-system model. Occurrence of carbonate precipitates as th in almost pure white laminae was considered as a natural experiment for lon g-term equilibration between these phases and porewater. Plots of distribut ion coefficients indicate that metastable equilibrium exists between porewa ter and the authigenic Ca-rich rhodochrosite phases below 7 cm depth. A the rmodynamic model of porewater geochemistry at in situ P = 25 bar and T = 5 degrees C 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 coefficien ts of the end-members in the non-ideal solid solution (Mn, Ca, Mg, Sr, Ba, Fe)CO3, i.e., at full major and minor multi-component complexity. The regul ar 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 theor etical predictions given by Lippmann in his pioneering 1980's paper. The st rictly thermodynamic equilibrium model is not only able to match both the m easured porewater and carbonate solid-solution composition, but also to pre dict that the porewater pH, pe, alkalinity, and dissolved Mn, Fe, and S con centrations are controlled by the authigenic mineral buffering assemblage m ackinawite-greigite-rhodochrosite. Our model is only compatible with the id ea 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). T his is provided by reduction of particulate Mn oxides precipitated in signi ficant amounts in the water column upon major inflow events in the Baltic S ea. The model enables also to set up scenarios of changing environmental co nditions, e.g., to predict the non-linear response of the carbonate solid-s olution composition to changes in Mn loading, alkalinity and salinity of th e 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.