Pore-water and sediment geochemistry in the Marmara Sea (Turkey): Early diagenesis and diffusive fluxes

Pore waters and sediments were analysed in two gravity cores from the Çinarcik Basin and the southern shelf of the Marmara Sea. The sediments are organic-rich (1.1 and 1.3 wt% C_org) and have been deposited at rates of 1.9 and 0.45 m/ka, respectively. The oxic+suboxic zone is 0.50-0.80 m thick and the sulphate-methane interface (SMI; defined here as the depth at which pore-water sulphates diminish below 1 mM) is 3.4 and 2.4 metres below sea floor (mbsf) in the southern shelf and Çinarcik Basin cores, respectively. Changes in the phosphate and ammonium gradients suggest that the sulphate reduction occurs by simple microbial organic matter degradation. Steep sulphate gradients at both sites may suggest that a part of the sulphate reduction may occur by anaerobic methane oxidation. The flux computations, assuming a steady-state situation, show that the fluxes of sulphate, alkalinity, ammonium, phosphate and manganese near the top of the sediment column in the Çinarcik Basin are c. 1.7, 1.8, 2.1, 1.5 and 7.7 times greater than the respective fluxes in the southern shelf. In both cores, Mn²⁺ ions are released at 0.03-0.5 mbsf, and diffuse mainly upward. In the Çinarcik Basin core, a significant amount of dissolved Mn reprecipitates as Mn(IV) oxyhydroxides near the sediment top. Fe(II) produced from reduction of reactive Fe(III) minerals is precipitated as Fe sulphides in the sulphate reduction zone. Downward Mg flux into the sediments in the Çinarcik Basin is 2.5 times that in the southern shelf, whereas the Ca flux in both areas is similar, with a value of c. 26 mM m^-2 a^-1. The downward Ca and Mg fluxes are used in dolomite precipitation in the sediments near the SMI in both regions. The flux amounts and pore water profiles all indicate that the rates of early diagenetic reactions (sulphate reduction and probably AMO) in the Çinarcik Basin are considerably higher than those in the southern shelf. The differences between the two areas can be explained mainly by the considerably higher sedimentation rate in the former area.