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Aquatic Geochemistry (v.8, #1)
Pyritization of Iron in Sediments from the Continental Slope of the Northern Gulf of Mexico by John W. Morse; Dwight K. Gledhill; Karen S. Sell; Rolf S. Arvidson (pp. 3-13).
In sediments from the continental slope of the Northern Gulf of Mexico, generally,the degree of iron pyritization (DOP) is low (<0.1) and dissolved sulfide is belowdetection limits (∼5 μM), whereas dissolved Fe is typically about 50 to100 μM. Therefore, the dissolution of kinetically reactive iron minerals generallydominates over the rate of sulfide production in sediments throughout this region.However, in sediments where hydrocarbons have been added via seepage from thesubsurface, dissolved-Fe is undetectable, DOP can approach 1, and high concentrationsof dissolved sulfide (up to ∼11 mM) are commonly present. Even though thesesediments have high total reduced sulfide (TRS) concentrations (typically 150 to370 μmol gdw-1), their average C/S ratio is about 4 times that of “normal” marine sediments reflecting the major input of hydrocarbons. DOP is significantly (∼20%) higher when calculated using reactive-Fe extracted by citrate dithionite than by cold 1N HCl. This difference is primarily due to the greater extraction efficiency of the cold HCl method for silicate-Fe. TRS tends to rise to a maximum, and remains close to constant even at high (mM) dissolved sulfide concentrations. These TRS concentrations, therefore, represent the size of the ``kinetically'' reactive-Fe pool during early diagenesis.
Keywords: iron; sulfides; sediments; kinetics; Gulf of Mexico
Kinetics of the Abiotic Reduction of Polymeric Manganese Dioxide by Nitrite: An Anaerobic Nitrification Reaction by George W. Luther III; Jeannette I. Popp (pp. 15-36).
Manganese oxides are strong environmental oxidants recently found to be involvedin the nitrogen cycle. Of the several possible reactions with reduced nitrogen species,the reduction of MnO2 by nitrite has only received marginal attention. Yet, this reaction might explain why nitrification can occur in the absence of O2, observed in both sediments and water columns. We have determined the stoichiometry of this reaction, as well as the chemical kinetics and the activation parameters, using a soluble polymeric form of MnO2. The reaction rate decreases with increasing pH and decreasing temperature. The reaction is first order in each reactant with a second order rate constant (k) = 493 M-1 min-1 at 21.5 °C and pH = 5.00. The energy of activation (Ea = 9.370 kJ/mole) and the entropy of activation (Δ S‡ = -169.5 J/mole) show the reaction to be associative and diffusion controlled, occurring via an inner-sphere mechanism, likely with O atom transfer from MnO2 to HNO2. The reaction is proton assisted and slowsdown at pH ≥ 5.5 where NO2 - and MnO2 (unprotonated and negatively charged) become the dominant species. In natural waters and sediments where anaerobic nitrification has been observed the pH is higher than this. Thus, the thermodynamically favorable reaction will likely proceed by microbial mediation.
Keywords: Manganese dioxide; nitrite; nitrification; kinetics
Determining Biogenic Silica in Marine Samples by Tracking Silicate and Aluminium Concentrations in Alkaline Leaching Solutions by Erica Koning; Eric Epping; Wim Van Raaphorst (pp. 37-67).
This study introduces an alkaline leaching technique for the simultaneous analysis of biogenic silica and aluminium in sediments. Measuring aluminium facilitates the discrimination between silica from the biogenic (BSiO2) and the non-biogenic fraction, because it originates almost solely from the lithogenic phase. The method was tested using fine-grained silicagel, standard clay minerals, artificial sediments, and natural samples ranging from fresh diatoms to aged sediment from different depositional settings. To determine the BSiO2 content, four different models each describing the dissolution curves, but of increasing complexity, were applied and for each different type of sample the optimum model was selected on the basis of F-test statistics. For mixtures of silicagel and clay minerals, the contribution of Si from the dissolution of clay was negligible compared to Si originating from silicagel. For natural samples with high clay content, complex dissolution curves were observed and single-phase first order dissolution was the exception. This deviation from `ideal' behavior could only be recognized because of high-resolution sampling, especially in the first 20 minutes of the experiment. For most of the samples, the distinction between the biogenic silica fraction and the silica originating from dissolution of clays could be made on the basis of the Si/Al ratios and reactivity constants of the dissolving phases calculated with the models. Clay minerals typically dissolve slowly at a Si/Al ratio close to 1–2, depending on the type of clay mineral. In contrast, biogenic silica displays a wide range of reactivities and Si/Al ratios. Fresh biogenic silica from the water column usually has a high reactivity and a low Al content. Aged biogenic silica from the sediments has a lower reactivity, but Si/Al ratios as low as 5 were found. The method as described here therefore presents an accurate method to analyze biogenic silica in marine sediments with a relatively high clay mineral content.
Keywords: biogenic silica; aluminium; alkaline extraction; analytical method