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 (SP) gradient centred around SP = 0.06-6, equivalent to a seawater ionic strength (ISp) 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 ISp increases due to mixing: 1. Before a measurable change in ISp, manganese, as particulate manganese(III/IV) oxides (MnOx), undergoes reduction, independent of photochemical processes, to soluble manganese(III) stabilized in organic complexes (Mn(III)-L) and manganese(II); 2. As ISp increases between 5 and 80 mM, Mn(III)-L reduction continues and manganese(II) adsorbs onto particle surfaces; 3. As ISp increases further, though remaining below 80 mM (SP approximate to 4), adsorbed manganese(II) desorbs and/or is oxidized and is released as Mn(III)-L or oxidises further to MnOx; 4. The breakdown of Mn(III)-L complexes leads to higher manganese(II) and MnOx, which at Mid-Estuary-Salinities (ISp = 320-480 mM) precipitates. This manganese cycling in low ISp 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.

Automated summary

Mixing waters of different ionic strengths facilitates the geochemical cycling of reactive elements, with the most reactive zone being where the gradient in ionic strength is steepest. The redox-active metal manganese cycles between different oxidation states in oxygenated systems, influencing the mobility of inorganic and organic chemicals. Estuaries serve as accessible environmental systems where waters of different ionic strengths mix, providing insights into the dynamic geochemical environment and manganese cycle propagation.