A new particulate Mn-Fe-P-shuttle at the redoxcline of anoxic basins

Pelagic redoxclines of anoxic basins and deeps form the suboxic transition between oxygenated surface and anoxic or even sulfidic bottom waters. Intense element cycling, favoured by elevated microbial activity, causes steep gradients of physicochemical parameters, nutrients and redox-sensitive trace metals. This study presents a conceptual model for authigenic particle formation at pelagic redoxclines, which is based on the tight coupling of Mn, Fe, and P cycles. Besides the well-known occurrence of Mn-oxides, textural (SEM-EDX) and geochemical (ICP-OES, ICP-MS) analyses of particles from the redoxclines of the Black Sea and the Baltic Sea (Gotland Basin, Landsort Deep) evidence the existence of earlier postulated Fe-oxyhydroxo-phosphates and emphasize mixed phases consisting of Mn-oxides and Fe-oxyhydroxo-phosphates as a new solid species. Most of the analyzed particles are star-shaped, of about 5 mu m in size, and occur as single particles or aggregates without any morphological differences between Mn-oxides, Fe-oxyhydroxo-phosphates, and mixed phases. Throughout the redoxcline, these minerals show a general succession with maximum abundance of Mn-oxides above the redoxcline followed by mixed phases and almost pure Fe-phosphates within and below the redoxcline, respectively. Molar Fe/P ratios of single particles argue against the formation of known pure Fe-phosphates like vivianite or strengite at the lower end of the redox transition zone, but are consistent with recent experimental findings for colloidal P-bearing hydrous ferric oxides. Moreover, morphological similarities suggest the formation of irregular Fe-oxyhydroxo coatings due to oxidation of upward diffusing Fe2+ by oxygen and stepwise replacement of Mn(IV) by Fe(III) on sinking MnOx particles followed by immediate adsorption or even co-precipitation of phosphate. Batch-type experiments using biogenic MnOx particles demonstrate the efficient potential of Fe2+ oxidation by sinking MnOx particles. When entering sulfidic waters MnOx particles are progressively reduced leading to an increasing relative abundance of Fe- and P-rich particles. In deeper parts of the water column these particles are also reductively dissolved, thereby releasing Fe2+ and phosphate to the water column. This Mn-Fe-P-shuttle likely affects phosphate transport throughout the water column and thus impacts primary production at least over longer time scales. Furthermore, the particulate Mn-Fe-P-shuttle must have played an important role for the cycling of P and certain trace metals in ancient ocean basins, e.g., during certain periods of Cretaceous black shale formation and should be considered in future mass balances and modeling approaches dealing with oxic/anoxic interfaces of aquatic ecosystems. (C) 2010 Elsevier Ltd. All rights reserved.