ISSN:
1573-1421
Source:
Springer Online Journal Archives 1860-2000
Topics:
Chemistry and Pharmacology
,
Geosciences
Notes:
Abstract The natural early-diagenetic environment ``anoxicporewater – authigenic mineral phases'' has beencharacterized in sediment of the Gotland Deep,Baltic Sea, by a closed-system model. Occurrence ofcarbonate precipitates as thin almost pure whitelaminae was considered as a natural experiment forlong-term equilibration between these phases andporewater. Plots of distribution coefficientsindicate that metastable equilibrium exists betweenporewater and the authigenic Ca-rich rhodochrositephases below 7 cm depth. A thermodynamic model ofporewater geochemistry at in situ P = 25 barand T = 5 °C was developed using the Gibbsenergy minimization (GEM) approach. The values of isobaric-isothermal potentials of Mn, Ca, Fe, Mg,Sr, Ba, C, and O, calculated from the porewatercomposition, were used in a new ``dual thermodynamic''calculation approach to estimate solid activitycoefficients of the end-members in the non-idealsolid solution (Mn, Ca, Mg, Sr, Ba, Fe)CO3,i.e., at full major and minor multi-componentcomplexity. The regular Margules interactionparameters for the composing binaries estimated bythis model were αMn-Ca = 1.9 ± 0.5,αMn-Mg = 0.6,αCa-Mg = 3.7,αMn-Fe = 0.2,αCa-Fe = 2.8,αMn-Sr = 9.7,αCa-Sr = 2.15,αMn-Ba = 4.0,αCa-Ba = 1.4,validating the theoretical predictions given byLippmann in his pioneering 1980's paper. Thestrictly thermodynamic equilibrium model is not onlyable to match both the measured porewater andcarbonate solid-solution composition, but also topredict that the porewater pH, pe, alkalinity, anddissolved Mn, Fe, and S concentrations arecontrolled by the authigenic mineral bufferingassemblage mackinawite-greigite-rhodochrosite. Ourmodel is only compatible with the idea of ACRformation with typical composition (XMnbetween 70–75%) in the topmost sediment layerwhich, however, needs a major source ofMnaq II. This is provided by reduction ofparticulate Mn oxides precipitated in significantamounts in the water column upon major inflow eventsin the Baltic Sea. The model enables also to set upscenarios of changing environmental conditions, e.g.,to predict the non-linear response of the carbonatesolid-solution composition to changes in Mn loading,alkalinity and salinity of the sediment-watersystem. The results suggest that the major andespecially minor element contents (Sr, Mg, Ba) inauthigenic carbonates can be applied as anenvironmental paleoproxy.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1023/A:1009694703207
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