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.

Automated summary

The study highlights the significance of in-situ and coupled analysis of Fe and S isotopes in sedimentary pyrites, showing extreme values in Fe-56 and S-34 isotopes. The proposed model suggests microbial dissimilatory iron reduction and bacterial sulfate reduction as potential processes contributing to the isotopic compositions.