The Iron Wheel in Lac Pavin: Interaction with Phosphorus Cycle

Lac Pavin is a crater lake, characterized by water column stratification, with oxygenated shallow waters lying above anoxic and ferruginous deep waters. In the deep waters, ferrous iron, Fe(II)aq, is the main dissolved cation, with concentrations up to 1 mM. Iron is efficiently confined below the oxic-anoxic boundary due to the formation of insoluble ferric iron species, Fe(III)s, by oxidation with O2 and other oxidants (e.g., NO3−, Mn(IV)). The Fe(III)s particles settle down and are reduced in the anoxic waters and at the lake bottom by reaction with organic matter to soluble Fe(II)aq. It then diffuses upward in the water column and finally is re-oxidized to Fe(III) at the redox boundary. This process, known as the “iron wheel”, is described in the present paper that reviews available data for dissolved and particulate matter in the water column, settling particles collected by sediment traps and sediment cores. Detailed analyses for some major and trace element concentrations, along with iron speciation and isotope composition, high-resolution microscopy, and geochemical modeling provide a picture of biogeochemical cycling in this Fe-rich aqueous system. At Lac Pavin the P and Fe cycles are tightly coupled. Orthophosphate is sorbed onto Fe oxyhydroxides and/or precipitated as Fe(II)-Fe(III)-phosphates at the redox interface, confining P ions in the deep anoxic waters. Deeper in the water column, particulate Fe concentrations progressively increase due to Fe(II) phosphate (vivianite) formation. In the sediment, Fe is buried as various ferrous minerals, such as vivianite, pyrite and siderite.