Depth Variability and some Geochemical Characteristics of Fe, Mn, Ca, Mg, Sr, S, P, Cd and Zn in Anoxic Sediments from Authie Bay (Northern France)

A study of an anoxic sediment core from Authie Bay in northern France has been undertaken; porewaters and sedimentary solid phases have been analysed by several techniques (inductively coupled plasma atomic emission spectroscopy; graphite-furnace atomic absorption spectroscopy; X-ray diffraction; and electron spin resonance spectroscopy). The chemical speciation versus depth of the major elements (Ca, Fe, Mg, Mn, P and Sr), some trace elements (Cd and Zn) and especially sulfur [i.e., dissolved sulfur as Acid Volatile Sulfide (AVS) and Chromium Reducible Sulfur (CRS)] have also been performed using sequential extraction procedures. Together with these analytical data, thermodynamic calculations have been used in order to gain information about the effects of microbial degradation of organic matter in these recent sediments on these elements during early diagenesis and/or the possible generation of new characteristic assemblages of authigenic mineral precipitates of carbonates, phosphates and/or sulfides. Thus, these calculations have shown that iron sulfide precipitates in interstitial waters to give the crystalline form greigite and, to a lesser extent, mackinawite, whereas different divalent metals (noted Me) as Mg, Mn, Sr, Cd, Zn and even Fe interact favourably with Ca2+ and CO2 3 [through a removal of these elements from anoxic porewaters by (co)precipitation and/or adsorption on to the sedimentary carbonate phase (calcite)] to give (1) a solid mineral: dolomite [CaMg(CO3)2]; and (2) solid solutions as MexCa1 xCO3. The distribution coefficients for these metals in calcite have been calculated and compared to those reported in the literature.