Tracing water column euxinia in Eastern Mediterranean Sapropels S5 and S7

摘要

Sapropels S5 and S7 formed in the semi-enclosed Eastern Mediterranean Sea (EMS) during peak interglacial periods MIS5e and MIS7a, respectively, are considered among the most strongly developed Quaternary sapropels. This study investigates the redox dynamics of the water column during their formation, via Fe isotope and Fe speciation studies of cores taken at 2550 m depth at site ODP-967. Both sapropels show an inverse correlation between δ~(56)Fe and FeT/Al, with slopes mostly matching that found for the Black Sea, pointing to a benthic shelf to basin shuttle of Fe and subsequent precipitation of Fe sulphides in euxinic bottom waters. An exception to these Black Sea-type trends occurs during the later, peak, stages of S7, where the negative δ~(56)Fe - Fe_T/Al slope shallows. Fe speciation studies reveal that the dominant highly reactive Fe phase (Fe_(HR)) in the sapropels is pyrite, with Fe (oxyhydr)oxides forming the second major mineral component. Correspondingly, FeHR/FeT plots show increased strengthening of anoxic water conditions during the passage from pre-sapropel sediment into the sapropel. Nevertheless, despite the evidence for euxinic conditions from both Fe isotopes and high Mo concentrations in the sapropel, Fe_(py)/Fe_(HR) ratios remain below values commonly used to identify water column euxinia. This apparent contradiction is ascribed to the sedimentary preservation of a high flux of crystalline Fe (oxyhydr)oxide minerals to the basin, which resulted in a relatively low degree of sulphidation, despite the presence of euxinic bottom waters. Thus, the operationally defined ferruginous/euxinic boundary for EMS sapropels is better placed at Fe_(py)/Fe_(HR) = 0.6, which is somewhat below the usually ascribed lower limit of 0.7. Consistent with the significant presence of crystalline Fe (oxyhydr)oxides, the change in the δ~(56)Fe - Fe_T/Al slope during peak S7 is ascribed to an enhanced monsoon-driven flux of detrital Fe(III) oxides from the River Nile i