Vivianite [(Fe3(PO4)2.8H2O)] may precipitate in anoxic lake sediments affecting the porewater orthophosphate concentration, and thereby the trophic status of lakes. We have investigated changes in lake diagenesis of Fe and P (1969-2009), with particular attention focused on vivianite formation with sediment depth (0 – 20 cm) in an iron-silica-carbon rich lake sediment (Ørnsø, Denmark). Porewaters were supersaturated for vivianite by two to five orders of magnitude (upper 10 cm) with porewater phosphate concentrations ranging between 0.69 and 10 μmol l- 1, in winter, and summer concentrations ranging between 9.8 and 40 μmol l- 1. Significant formation of vivianite was confirmed by X-ray diffraction while scanning electron microscopy and electron dispersive X-ray spectroscopy indicated an increase in vivianite crystal size with depth (~ 20 to ~ 70 μm across). Variations in elemental composition of vivianite crystals in relation to atomic % P and Fe were especially seen going from 9.5cm to 24.5cm. The total sediment Fe pool was very large ~ 3000 μmol g- 1 and total P increased from 200 μmol g- 1 to 400 μmol g- 1 descending down the sediment profile. Differential extraction experiments of P release at pH 3 estimated that vivianite amounts to between 3 and 5 % of the total Fe pool. The total P burial fluxes estimate that ~ 38 μmol cm- 2 yr- 1 or ~ 26% of sedimentary P in the lower sediments is sequestered as vivianite. There are seasonal variations in the porewater composition with lower Fe, orthophosphate and higher sulfate concentrations during winter (5°C), than during summer (15°C). This suggests that temperature modulates the rate of organic matter degradation which in turns affects the rate of Fe(III) phase reduction, release of phosphate, and thereby the porewater Fe2 + and orthophosphate concentrations and hence vivianite formation. This work highlights the role vivianite can play for P retention in a Si-Fe-C rich lake sediment.