Thermodynamic, spectroscopic and magnetic studies on anoxic sediments fromthe Seine river estuary

Sediment cores were collected in an estuarine mixing zone from the Seine ri ver (in northern France) and their extracted interstitial fluids were analy zed for the determination of total concentrations of dissolved elements usi ng inductively coupled plasma optical atomic emission spectroscopy and pote ntiometry. Thermodynamic equilibrium calculations on these waters have allo wed us to draw stability fields of some minerals that may exist in these an oxic sediments as a result of diagenesis processes. The findings reveal tha t throughout the anoxic sediment cores studied the interstitial fluids are oversaturated with respect to FeCO3, FeS and/or FeS2 and undersaturated wit h respect to Fe(OH)(2) and, to a lesser extent, Fe-3(PO4)(2). 8H(2)O. To pr ove the existence of such newly generated compounds in anoxic Seine river e stuary sediments, micro-Raman spectroscopic analyses were performed on thes e solids in order to visualize and localize micro-particles of authigenic i ron sulfide minerals and iron oxides. In addition, iron-57 Mossbauer spectr oscopy was used to evaluate the weight proportion of iron(II) (with referen ce to the total sedimentary iron) bound to carbonates at different sediment depths. This investigation shows that the total Fe(II) component wt.% incr eases significantly at depths of ca. 2-3 cm, then decreases sharply at dept hs of ca. 5-10 cm, and finally stabilises at depths greater than or equal t o 10 cm. This profile is comparable with the downcore variations/fluctuatio ns of magnetic susceptibility measurements carried out on these sliced sedi ments. This particular magnetic behavior of these anoxic sediments is intim ately related to the preferential generation of highly magnetizable mineral s ((FeCO3)-C-II) in upper layers and weakly magnetizable minerals (pyrite) in deeper layers. The formation of such magnetic compounds is strongly depe ndent upon the variable physico-chemical conditions observed in the cores v s. depth.