Formation of a distinct class of Fe–Ca(–Corg)-rich particles in a complex peat–karst system

The Vallée-des-Ponts in the Jura mountains (Switzerland) is characterized by a complex hydraulic system, where drained peat bogs are crossed by a river (Bied Brook) which sinks into a karstic aquifer prior to rise as a spring (Noiraigue spring). A general chemical and microscopic survey of the particulate species throughout the basin highlighted the complex behaviour of iron-rich entities. Bulk chemical analyses show that iron is present as its dissolved and humics-complexed ferrous form in the anoxic–acidic peat porewater; the oxidation step of ferrous iron mainly occurs into drains, where conditions change from anoxic–acidic to oxic-neutral. Analytical electron microscopic characterization of individual colloids in the different compartments of this complex system suggests that iron is slowly oxidized to an abundant particulate form while it is transported by drains to the river, but these particles are then mostly retained when travelling through the karst. Small organic-rich granules, present as dominant species in the peat porewater, could act as condensation nuclei for the formation of iron-rich globular colloids containing significant amounts of calcium. These distinct particles have sizes in the 100–500 nm range and eventually contain a carbon-enriched core surrounded by a Fe–Ca-enriched outer layer.