Vivianite is a major sink for phosphorus in methanogenic coastal surface sediments

Studies of authigenic phosphorus (P) minerals in marine sediments typically focus on authigenic carbonate fluorapatite, which is considered to be the major sink for P in marine environments. The role of other potentially important authigenic P phases, such as the reduced iron (Fe) phosphate mineral vivianite (Fe(II)3(PO4)*8H2O) has so far largely been ignored.
By combining the SEDEX method with more direct analytical tools, we show that vivianite represents a major burial sink for P below a shallow sulfate/methane transition zone in Bothnian Sea sediments, accounting for up to 50 % of total P burial. Our results demonstrate that anaerobic oxidation of methane drives a sink-switching from Fe-oxide bound P to vivianite. Methane thus plays a key role for P burial in these coastal surface sediments. We further suggest that vivianite formation may provide an important burial sink for P in many brackish coastal environments worldwide.