The late Neogene marls from the Lorca, Murcia and Vera basins in S-E. Spain contain abundant dolomite nodules which were formed due to intense methane-rich fluid migration (Pierre et al, 2015). The pathways for these fluids are evidenced by dense networks of fractures that are crossing the sedimentary layers and eventually the dolomite nodules. The fractures are generally filled by secondary fibrous gypsum that form veins of a few cm thick. The oxygen and sulfur isotopic compositions of these gypsum veins exhibit wide ranges (-2.2 < δ18O ‰ VSMOW < 6.7 ; -22.3 < δ34S ‰ VCDT< 10.5) that are far away from the δ values of gypsum that would have precipitated from the Cenozoic seawater sulfate (δ18O = 11.5 to 15‰; δ34S = 22 to 23‰ for the Messinian gypsum of Sorbas basin). These low δ values clearly indicate the diagenetic origin of the gypsum veins that would have resulted from sulfide oxidation with 18O-depleted water. The isotopic compositions of the diagenetic dolomite nodules close to the gypsum veins (-1.9 < δ18O ‰ VPDB < 2.6 ; -11.9 < δ13C ‰ VPDB < 7.3) are indicative of their formation in methane-rich fluids. In this context, the formation of sulfide is related to the bacterial sulfate reduction that is associated with the anaerobic oxidation of methane. The late stage of sulfide oxidation occurred when oxygenated waters (probably of meteoric origin) circulated within the fractures after the subaerial exposure of the sedimentary deposits. Reference: Pierre et al, 2015. Mar. Petrol. Geol., 66 : 606-615.