The occurrence, identification and environmental relevance of vivianite in waterlogged soils and aquatic sediments

In this article, we review the nature, occurrence and environmental relevance of the authigenic ferrous iron phosphate mineral vivianite (Fe3(PO4)2·8H2O) in waterlogged soils and aquatic sediments. We critically discuss existing work from freshwater and marine systems, laboratory studies and microbial batch culture experiments aiming to deduce common characteristics of the mineral's occurrence, and the processes governing its formation. Vivianite regularly occurs in close association with organic remains in iron-rich sediments. Simultaneously, it is a biogenic mineral product of metal reducing bacteria. These findings suggest that vivianite nucleation in natural systems is directed by the activity of such bacteria and crystal growth is particularly favoured within protected microzones. Taking into account recent findings from coastal marine sediments where vivianite authigenesis has been shown to be coupled to the anaerobic oxidation of methane, small-scale microbially mediated reactions appear to be crucial for the formation of vivianite. Small-scale heterogeneity within the sediment matrix may also explain why saturation calculations based upon bulk pore water constitutions often fail to accurately predict the occurrence of the mineral. Vivianite is not restricted to a specific trophic state of a system. The mineral forms in oligotrophic- as well as in eutrophic waters. However, depending on the iron inventory, the production, supply and degradation of organic matter determine the relative contribution of iron sulphide formation to the iron pool, and the concentration of inorganic phosphate and Fe2+ in pore waters. Thus, vivianite authigenesis is also governed by bulk chemical conditions such as the rate of sulphide formation relative to that of Fe2+ production. This situation allows stimulation of vivianite formation by iron supplementation aimed at restoring eutrophic lakes. Recent results from coastal marine sediments suggest that vivianite authigenesis is of significance for P burial in the marine realm. Vivianite authigenesis is likely important at the global scale, but has so far largely been ignored.