Dissolved reactive manganese at pelagic redoxclines (part II): Hydrodynamic conditions for accumulation

Abstract

Dissolved reactive Mn (dMnreact) has been determined at the redoxclines of two anoxic deeps from the Baltic Sea (Landsort Deep and Gotland Basin) and two seasonally anoxic freshwater lakes (Lake Dagow and Fuchskuhle, Germany). This dMnreact fraction is rapidly oxidised under oxygen atmosphere and is assumed to consist predominantly of Mn(III). There is a distinct increase of dMnreact from the outer regions towards the central part of the Landsort Deep. Although the presence of MnOx particles in the Gotland Basin is evidence of ongoing oxidation of reduced Mn species, almost no dMnreact was detected. Since completely different processes of Mn oxidation appear rather unlikely, we suggest oceanographic properties are responsible. Lateral currents and intrusions in the Gotland Basin seem to prevent the formation of a stable suboxic zone, a prerequisite necessary for accumulation of dMnreact. Such perturbations supply trace amounts of O2 and H2S, causing either immediate oxidation/reduction of dMnreact or deterioration of its stabilising ligands. dMnreact has also been determined in Lake Dagow with values significantly exceeding the level of the Landsort Deep due to stable stratification of this lake. In contrast, H2S appearance throughout the entire water column and a pH <5 prevent accumulation of dMnreact in Lake Fuchskuhle.