University of Bristol, United Kingdom

Molybdenum isotopes are a powerful proxy for paleoredox conditions in organic-rich sediments. Here, we apply this isotopic system to a Holocene S1 sapropel layer sampled at 2550m depth in the Eastern Mediterranean Sea (ODP core 967D). Our study utilizes the isotopic systems of Mo and Fe, together with geochemical tracers (Ba/Al, Fe (sub T) /Al, Mn/Al, S) to explore the relationship between the sapropel formation conditions and the Mo paleoredox proxy. Euxinic (sulphidic) bottom water conditions during the sapropel formation are supported by Fe isotopic composition, which shows negative correlations between delta (super 57) Fe and Fe (sub T) /Al and S wt%. This is consistent with the benthic Fe shuttle model whereby Fe is exported from the oxic shelf to the deep euxinic basin[1]. The Mo paleoredox model for highly sulphidic restricted marine systems envisages organic-rich sediments acquiring Mo isotope signatures of sea water (delta (super 98) Mo = 2.3ppm) due to the quantitative removal of Mo from the sulphidic water column. This scenario is not realized in the 967D profile, which shows a decrease in delta (super 98) Mo values from ca 0ppm in the pre-sapropel sediment to negative values of -0.8ppm within the sapropel, before gradually rising to values of 0 to 0.4ppm in the overlying post-sapropel sediment. Negative linear correlations between delta (super 98) Mo values and Fe (sub T) /Al, Ba/Al, Mo/Al and S indicate that maximum euxinic conditions are associated with the lowest Mo isotopic compositions. The low delta (super 98) Mo values are potentially compatible with sulphidization of Mo in mildly sulphidic euxinic conditions [2,3]. Nevertheless, increasing euxinia should lead to positive correlations between delta (super 98) Mo and the geochemical proxies. Thus, additional factors such as open system conditions for Mo uptake may be required to explain the full workings of the Mo paleoredox proxy.