Sequential extraction of iron, manganese and related elements in S1 sapropel sediments, eastern Mediterranean

A detailed sequential extraction scheme (MESEX) was developed to distinguish Fe and Mn phases in eastern Mediterranean sediments. After calibration and validation using single natural minerals and a mixed mineral standard, MESEX was applied to Holocene eastern Mediterranean sediments from (1) a former brine basin, and (2) a normal setting, including the organic-rich sapropel S1. In both sediments the Mn(II)/Mn(III,IV) redox boundary is clearly distinguished with carbonate-related Mn being enriched below, and Mn oxides occurring only above it. A clear Fe(II)/Fe(III) redox boundary is present in the normal eastern Mediterranean sediments and is located just below the Mn(II)/Mn(III,IV) redox boundary. The Fe(II)/Fe(III) boundary closely coincides with the visible boundary between the lower unoxidised and the upper oxidised (burned-down) part of the original sapropel S1 interval. Pyrite is only present within the unoxidised interval, whereas Fe oxides are abundant above it. The Fe(II)/Fe(III) redox boundary in the former brine sediments resides on top of the organic-enriched sediments, as deduced from the distribution of Fe oxides and pyrite. As a result of burn-down of the sapropel S1 unit in the normal sediments, a vanadium peak has been formed below the oxidation front, whereas no such V peak was observed in the former brine sediments. This, together with the location of the Mn(II)/Mn(III,IV) redox boundary well above the organic-enriched sediments, suggests that no burn-down has taken place into the sapropelic sediments of the former brine basin, resulting in the preservation of the relationships of trace metal to organic carbon or pyrite. In these latter sediments, Mn and Co appear to be associated with pyrite, whereas Ni, Cu and Zn, as well as V, are more related to organic matter. On the basis of MESEX, variations have also been detected in the terrigenous component, i.e. aluminosilicates. (Fe/Al)clay, (Mg/Al)clay, and several trace metal to Al ratios are higher in sapropelic than in non-sapropelic sediments. These results indicate that sources of terrestrial material have been different during sapropel times compared to the period immediately after it. On the other hand, the fairly constant K/Alclay ratio suggests that no major changes may have occurred in the relative illite contribution.