Sediment chemistry and magnetic properties in an anomalously reducing core from the eastern Mediterranean Sea

In Core KC19C (19.6 m long), recovered in the abyssal plain between Crete and Cyprus in the eastern Mediterranean, a large number of organic-rich layers (sapropels) occur, which correlate to maxima in the insolation curve. In contrast to other sites in the eastern Mediterranean, porewaters contain sulfide below a few meters below seafloor (mbsf). Geochemical analyses were performed on the porewaters (ammonium, alkalinity, sulfate, manganese, iron, bisulfide, chloride, Eh, pH) and sediments (organic carbon, barium, total sulfur, total iron, manganese, pyrite, dithionite-extractable iron). In addition, a series of magnetic parameters and ratios (NRM, ARM, IRM, χ in, ARM/IRM, S-ratio, ARM 20mT/ARM in) were determined. In the top of the sediments, Fe and Mn (hydr)oxides occur at the oxic–suboxic boundary just above the youngest sapropel Si2; these (hydr)oxides appear to be more easily demagnetized by means of alternating fields than deeper (hydr)oxides which presumably are of detrital origin. The transition from suboxic to anoxic sediments is located at ∼2 mbsf. Sulfide is produced, possibly during sulfate reduction through anoxic methane oxidation, at ∼17.5 mbsf. From 17.5 mbsf sulfide migrates upward, titrating reactive Fe, resulting in pyrite formation in the entire sediment column up to ∼2 mbsf. At this depth, the upward sulfide flux has been totally consumed by reaction with solid phase Fe and dissolved Fe(II) diffusing downward from the suboxic zone above. From ∼2 mbsf downward magnetic intensities are significantly reduced, indicating reductive dissolution of Fe-oxide minerals and pyrite formation. As a consequence, no reliable NRM data can be obtained in the lower half of the core.