Phosphorus burial in vivianite-type minerals in methane-rich coastal sediments

Sediments are a key sink for phosphorus (P) in coastal systems. This allows coastal areas to act as a filter for P that is transported from land to sea. Recent work suggests that vivianite-type ferrous iron (Fe(II))-P minerals may be more important as a sink for P in coastal sediments than previously thought. Here, we investigate the occurrence of such vivianite-type minerals in sediments of three eutrophic coastal sites with contrasting dynamics with respect to iron (Fe) and sulfur (S), covering a salinity range of 0 to 7. We only find authigenic vivianite-type minerals at the low and intermediate salinity sites, where Fe is available in excess over sulfide production. Sequential extractions combined with SEM-EDS and mu XRF analysis point towards substitution of Fe in vivianite-type minerals by other transition metal cations such as magnesium and manganese, suggesting potentially different formation pathways modulated by metal cation availability. Our results suggest that vivianite-type minerals may act as a key sink for P in sediments of many other brackish coastal systems. Climate change-driven modulations of coastal bottom water salinity, and hence, Fe versus S availability in the sediment, may alter the role of vivianite-type minerals as a P burial sink over the coming decades. Model projections for the Baltic Sea point towards increased river input and freshening of coastal waters, which could enhance P burial. In contrast, sea level rise in the Chesapeake Bay area is expected to lead to an increase in bottom water salinity and this could lower rates of P burial or even liberate currently buried P, thereby enhancing eutrophication.