Novel manganese cycling at very low ionic strengths in the Columbia River Estuary.

Mixing of waters of different ionic strengths induces the geochemical cycling of reactive elements. The most reactive zone is where the gradient in ionic strength is steepest. In oxygenated systems, the redox-active metal manganese cycles between soluble and particulate fractions through three oxidation states, manganese(II), manganese(III) and manganese(IV). This cycling strongly affects the mobility of inorganic and organic chemicals. The most accessible environmental system where waters with different ionic strengths mix are estuaries. During six Eulerian studies in the Columbia River Estuary, each up to 26 h, we measured manganese speciation and concentration across a salinity (S) gradient centred around S = 0.06-6, equivalent to a seawater ionic strength (I) of 1.2-120 mM. This zone, representing the region between freshwater and the more intensively studied estuarine turbidity maximum, presents a highly dynamic geochemical environment in which the manganese cycle propagates through four steps as I increases due to mixing: 1. Before a measurable change in I, manganese, as particulate manganese(III/IV) oxides (MnO), undergoes reduction, independent of photochemical processes, to soluble manganese(III) stabilized in organic complexes (Mn(III)-L) and manganese(II); 2. As I increases between 5 and 80 mM, Mn(III)-L reduction continues and manganese(II) adsorbs onto particle surfaces; 3. As I increases further, though remaining below 80 mM (S ≈ 4), adsorbed manganese(II) desorbs and/or is oxidized and is released as Mn(III)-L or oxidises further to MnO; 4. The breakdown of Mn(III)-L complexes leads to higher manganese(II) and MnO, which at Mid-Estuary-Salinities (I = 320-480 mM) precipitates. This manganese cycling in low I waters directly affects a system's redox chemistry and provides a window into understanding the extensive, yet hidden, freshwater/saline water interface in aquifers, soils, sediments and estuaries.