Organic matter and pyritization relationship in recent sediments from a tropical and eutrophic bay

The degree of pyritization (DOP) and the extension of metals incorporation into pyrite was investigated at Guanabara Bay sediments. Maximum concentrations of total organic carbon (TOC), total sulfur, biopolymers and viable bacteria cells were observed in silted stations close to discharge points of sewage and minimum concentrations at sandy stations at the entrance of the bay. Pyrite iron concentrations (Fepy) was always lower than the reactive iron and Fepy were below the detection limit at sandy stations. The same trend was found to metals, which its degree of pyritization was Mn = Cu > As = Co > Ni > Cd > Zn ≫ Pb > Cr. The bay gathers all required factors to sulfate reduction and pyrite formation, once the C:S ratio express the reduced tendency conditions, almost half of the TOC present in its sediments is labile and both reactive sulfur and iron are available. However the degree of trace metals pyritization did not exceed 20%, consistent with the median DOP (29%).