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 (0–20cm) in an iron–silica–carbon rich lake sediment (Ørnsø, Denmark). Porewaters were supersaturated for vivianite by two to five orders of magnitude (upper 10cm) with porewater phosphate concentrations ranging between 0.69 and 10μmoll−1, in winter, and summer concentrations ranging between 9.8 and 40μmoll−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 ~70μm across). Variations in elemental composition of vivianite crystals in relation to at.% P and Fe were especially seen going from 9.5cm to 24.5cm. The total sediment Fe pool was very large ~3000μmolg−1 and total P increased from 200μmolg−1 to 400μmolg−1 descending down the sediment profile. Differential extraction experiments of P release at pH3 estimated that vivianite amounts to between 3 and 5% of the total Fe pool. The total P burial fluxes estimate that ~38μmolcm−2yr−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. •Investigation of changes in lake diagenesis of Fe and P (1969–2009) focused on vivianite formation.•Differential extraction experiments for the estimation of vivianite•Temperature modulates the rate of organic matter degradation and eventual vivianite formation.•Work highlights the role vivianite can play for P retention in a Si–Fe–C rich lake sediment.