Molybdenum isotope, multiple sulfur isotope, and redox-sensitive element behavior in early Pleistocene Mediterranean sapropels

Organic-rich sediments (sapropels) deposited in the Mediterranean are presumed to have formed during periods of increased productivity, and/or deep water oxygen depletion, possibly including the development of sulfidic conditions (euxinia). Geochemical redox proxies (Re, Mo, Mo isotopes, V, Fe/Al, and multiple S isotopes) in 8 sapropels from the Pleistocene confirm water column euxinic conditions of varying intensity during sapropel deposition. These same proxies indicate an oxic origin for hemipelagic sediments deposited between sapropel-forming episodes.In one intensively sampled sapropel, deposited between 1.450 and 1.458 Ma, changing concentrations of organic carbon, Ba, Re, Mo, V, and Fe/Al track one another closely, reflecting coupling between water column euxinia and biological productivity. Multiple S isotope data from this sapropel suggest that the redox interface where oxidative sulfur cycling occurred was present in the sediments during hemipelagic sedimentation, but moved into the water column during sapropel deposition.Molybdenum isotopes of these 8 sapropels encompass a range of values (δ98Mo = + 0.2 to + 1.7), but are all 98Mo-depleted relative to seawater (δ98Mo = + 2.3‰), suggesting that quantitative removal of Mo did not occur. This finding contrasts with modern Black Sea sediments. In general, Re/Mo ratios in sapropels are greater than in modern seawater, implying that the water column was not sufficiently sulfidic during sapropel-forming episodes to induce complete removal of both these elements. Surprisingly, the heaviest δ98Mo values are found within hemipelagic sediments. Very few of the hemipelagic samples preserve the negative δ98Mo values commonly associated with modern oxic marine sediments. Many of the hemipelagic samples also contained higher concentrations of Re and Mo than are common in oxic sediments. These features may be attributable to diffusion from the sapropels of a 98Mo-enriched component into the hemipelagic sediments.

Research Highlights
► δ98Mo values in sapropels indicate non-quantitative removal of Mo from seawater.
► Sulfidic conditions are identified by proxies such as Fe/Al in the sapropels.
► Biological productivity appears closely linked to development of euxinic conditions.
► Heavy δ98Mo values in oxic sediments are the result of diffusional overprinting.
► S isotopes indicate a redox interface in the water column during sapropel deposition.