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

Abstract

AbstractThe 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 δ56Fe values for sedimentary pyrites (as low as −4.7‰) and unusual high δ34S values (as high as +88.9‰). We propose a model that explains these extreme values but also the large range of pyrite δ34S and δ56Fe 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.