Manganese accumulation and solid-phase speciation in a 3.5 m thick mud sequence from the estuary of an acidic and Mn-rich creek, northern Baltic Sea

• Published in: Chemical geology Vol. 437; pp. 56 - 66
• Main Authors: Yu, Changxun, Virtasalo, Joonas J., Österholm, Peter, Burton, Edward D., Peltola, Pasi, Ojala, Antti E.K., Hogmalm, Johan K., Åström, Mats E.
• Format: Journal Article
• Language: English
• Published: 25-10-2016
• Subjects: Acid sulfate soil; anoxic; black-shale basins; Earth and Related Environmental Sciences; Environmental Science; Estuarine sediments; ferruginous lake matano; Geochemistry & Geophysics; Geovetenskap och miljövetenskap; icp-ms; iron reduction; marine-sediments; Miljövetenskap; Mn speciation; organic-matter; ray-absorption spectroscopy; Rhodochrosite; sulfate soil runoff; sulfide; western finland; X-ray absorption spectroscopy
• ISSN: 0009-2541; 1872-6836
• DOI: 10.1016/j.chemgeo.2016.05.016

In sediments, manganese (Mn) is typically enriched in the form of authigenic Mn hydroxides at the water-sediment interface where intensive redox cycling of Mn occurs. Here we show, based on existing hydrochemical and geochemical (sediment core) data and new detailed chemical and mineralogical characterization of a 3.5 m long sediment core from a Boreal estuary, that the behavior of Mn can be profoundly different and more complex in estuarine settings receiving an abundance of terrestrial Mn. The most notable feature in the 3.5 m long sediment core is two depth intervals (60-155 cm and 181-230 cm) where there are strong fine-scale variations in Mn concentrations with peaks episodically reaching up to 10-25 g kg(-1) and 6.7-12 g kg(-1), respectively. X-ray absorption spectroscopy and sequential chemical extraction show that Mn occurs mainly as authigenic rhodochrosite at these two depth intervals and is mainly surface-sorbed in other sections with relatively low and stable Mn concentrations. The data suggests that the strong fine-scale variations in Mn concentrations are a reflection of the extent of formation and settling of Mn hydroxides, the precursors of the authigenic rhodochrosite (and also of the surface-sorbed Mn), rather than Mn input to the estuary or redox-related Mn translocation within the sediment. There was agreement between the results of linear combination fitting of extended X-ray absorption fine structure data and a 7-step sequential chemical extraction (SCE) in terms of quantification of surface-sorbed Mn species, whereas the SCE experiment failed to fractionate a majority of rhodochrosite into SCE step-2 (1M NH4-acetate at pH 6), which is frequently employed to dissolve carbonate. We ascribe this discrepancy to only partial dissolution of rhodochrosite in the weakly acidic (pH = 6) NH4-acetate leach. (C) 2016 Elsevier B.V. All rights reserved.