MANGANESE DIAGENESIS IN DIFFERENT GEOCHEMICAL ENVIRONMENTS OF THE RIA DE VIGO (GALICIA, NW IBERIAN PENINSULA)
Description
Two geochemical scenarios with sharp differences have been identified in the Ria de Vigo (NW Spain). One of them corresponds to San Simon Bay, located in the innermost section of the Ria de Vigo, where the high organic matter content present in the sediment promoted high production of methane, which diffuses throughout the sediment column until it reaches the sediment-water interface. This diffusion process promotes anaerobic oxidation of sulfate with subsequent generation of H2S and intense Mn pyritization throughout the sediment column, a process similar to those observed in euxinic environments. The second scenario corresponds to the rest of the Ria de Vigo (middle and outermost sections), where the dominant Mn fraction was Mn-carbonate, even at depth in the sediment and in the presence of methane, where the concentration of this fraction is comparable to that of Mn-pyrite.
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Mn geochemical forms extraction The extraction of Mn was carried out on bulk wet sediment samples by combining the sequential extraction procedures of Tessier et al. (1979), with small modifications to allow the separation of easily reducible Mn forms into those associated with crystalline Fe oxyhydroxides, according to the method of Fortin et al. (1993), and to separate the pyrite fraction according to the method of Huerta-Díaz and Morse (1990, 1992). This combination of methods, similar to those used by other authors (e. g. Yu et al., 2015; Otero et al., 2009), allows for the differentiation of five different fractions, according to the following classification: Fraction F1: exchangeable Mn. 1 g of wet sediment was extracted with 30 mL of 1 M MgCl2 solution at pH 7 (adjusted with concentrated acetic acid) at 4 ºC with continuous stirring for 30 min, centrifuged at 10,000 rpm at 4 ºC for 20 min, and filtered through Albet filter paper. The extract was stored at 3 °C until analysis. The residue was washed twice with 20 mL of deoxygenated Milli-Q water (18 Ω) before starting the next extraction step. The same centrifugation, filtration, and washing procedures were repeated at the end of each of the following extraction steps. Fraction F2: Mn associated with carbonates. Extracted with 30 mL of 1 M NaOAc at pH 5; samples were shaken for 5 h at room temperature. Fraction F3: Mn associated with amorphous iron oxides (easily reducible Mn (e.g. Ferrihydrite or Lepidocrocite). Extracted with 30 mL of 0.04 M hydroxylamine hydrochloride? + acetic acid (25% v/v) solution and shaking the samples for 6 h at 96 °C. Fraction F4: Mn associated with crystalline iron oxides (e.g. goethite or hematite). Extracted using 20 mL of 0.25 M sodium citrate + 0.11 M sodium bicarbonate with 3 g sodium dithionite solution and shaken for 30 minutes at 75 °C. Fraction F5: Mn associated with pyrite. Extracted using 10 mL of concentrated HNO3; samples were shaken for 2 h at room temperature. Before extracting this fraction, samples were pre-treated with 30 mL of 10 M HF with constant shaking at room temperature for 16 h to eliminate the silicate-associated Mn, followed by a 2 h digestion with 15 mL concentrated H2SO4 to eliminate the Mn bound to organic matter (Huerta-Díaz and Morse, 1990; Huerta-Díaz and Morse, 1992). The degree of Mn pyritization (DTMP-Mn), as proposed by Huerta-Diaz and Morse (1990), provides an estimate of the content of a particular metal incorporated into the pyritic phase. DTMP-Mn was calculated according to equation 1, which defines reactive Mn as the sum of the fractions F1 to F4 (∑F1Mn→F4 Mn).