Early diagenesis in anaerobic lake sediments—II. Thermodynamic and kinetic factors controlling the formation of iron phosphate

Abstract

The ion activity product of Fe and phosphate in interstitial waters from four sediment cores taken from Greifensee, Switzerland indicate the presence of vivianite [Fe 3(PO 4) 2 · 8 H 2O] in the solid phase. Analysis of the sediment using an electron microprobe and by electron microscopy revealed P-rich grains to be also enriched in Fe. The combined methods provide strong evidence that vivianite is forming authigenically in the sediments. Thermodynamic stability calculations demonstrate that the most stable Fe and phosphate minerals (pyrite, siderite and apatite) are not the ones controlling the pore water chemistry. The results emphasize the importance of rate processes of mineral formation in early diagenesis. Calculations based on the sediment phosphate concentration, and the degree of supersaturation of Fe and phosphate in the upper portion (0–15 cm) of the pore waters, indicate that the rate of vivianite mineral growth is controlled by a surface reaction rather than a diffusion mechanism. The response time of dissolved phosphate in the sediment pore waters with respect to mineral precipitation is on the order of 1–20 days. Less than 15% of the phosphate released by organic matter degradation at the sediment-water interface and below is retained in the sediments.