Complex iron and sulphate reducing Cretaceous sedimentary system revealed by extreme isotope values

The iron and sulphur isotope compositions of sedimentary pyrites have been extensively used as a proxy for microbial metabolisms and redox evolution of the oceans. We illustrate the benefit of in-situ and coupled study of Fe and S isotopes on sedimentary pyrites from late mid-Cretaceous sediments from the Central Apennines, Italy. We report extremely low delta Fe-56 values for sedimentary pyrites (as low as -4.7 parts per thousand) and unusual high delta S-34 values (as high as +88.9 parts per thousand). We propose a model that explains these extreme values but also the large range of pyrite delta S-34 and delta Fe-56 values as well as the considerable microscale variations in this values. These isotopic compositions were likely produced by microbial dissimilatory iron reduction in the shelf sediment, transportation of Fe2+ via an iron shuttle, production of heavy SO4 by pyrite precipitation in the water column, diffusion and advection of SO4 and H2S, and finally bacterial sulphate reduction.