Sediment cores were collected in an estuarine mixing zone from the Seine river (in northern France) and their extracted interstitial fluids were analyzed for the determination of total concentrations of dissolved elements using inductively coupled plasma optical atomic emission spectroscopy and potentiometry. Thermodynamic equilibrium calculations on these waters have allowed us to draw stability fields of some minerals that may exist in these anoxic sediments as a result of diagenesis processes. The findings reveal that throughout the anoxic sediment cores studied the interstitial fluids are oversaturated with respect to FeCO₃, FeS and/or FeS₂ and undersaturated with respect to Fe(OH)₂ and, to a lesser extent, Fe₃(PO₄)₂·8H₂O. To prove the existence of such newly generated compounds in anoxic Seine river estuary sediments, micro-Raman spectroscopic analyses were performed on these solids in order to visualize and localize micro-particles of authigenic iron sulfide minerals and iron oxides. In addition, iron-57 Mössbauer spectroscopy was used to evaluate the weight proportion of iron(II) (with reference to the total sedimentary iron) bound to carbonates at different sediment depths. This investigation shows that the total Fe(II) component wt.% increases significantly at depths of ca. 2–3 cm, then decreases sharply at depths of ca. 5–10 cm, and finally stabilises at depths 10 cm. This profile is comparable with the downcore variations/fluctuations of magnetic susceptibility measurements carried out on these sliced sediments. This particular magnetic behavior of these anoxic sediments is intimately related to the preferential generation of highly magnetizable minerals (Fe^IICO₃) in upper layers and weakly magnetizable minerals (pyrite) in deeper layers. The formation of such magnetic compounds is strongly dependent upon the variable physico-chemical conditions observed in the cores vs. depth.