Isotopic compositions of diagenetic dolomites in the Tortonian marls of the western Mediterranean margins: evidence of past gas hydrate formation and dissociation

Abstract

Previous studies in the Lorca basin, S.E. Spain (Sediment. Geol., 2002) have shown that the diagenetic dolomitic concretions from the Tortonian marls are related to the past occurrence of gas hydrates which were rapidly dissociated when the sea level dropped during the Messinian salinity crisis. These conclusions were based on the oxygen and carbon isotopic compositions of these dolomites which indicated that the carbonate diagenesis occurred either in 13C-rich/ 18O-poor solutions as it is typical in gas hydrate systems or in 13C-poor/ 18O-rich solutions generated during gas hydrate decomposition. We have recently discovered similar dolomitic nodules in other Tortonian marly successions from S.E. Spain and Northern Morocco. The oxygen and carbon isotopic compositions of the diagenetic dolomites from these areas in Spain and Morocco are very similar to those from the Lorca basin. This demonstrates that the dolomitic nodules from the Tortonian marls of the Betic and Rifian basins were all related to the same diagenetic environments where pore fluids were modified by gas hydrate formation–dissociation processes. These overall observations in S.E. Spain and Northern Morocco suggest that the dissociation of gas hydrates during the Messinian salinity crisis was not a local phenomenon but corresponded to a major event which affected both sides of the Western Mediterranean margins; it might have been initiated during the first major restriction step that started around 6.7 Ma in the Mediterranean Sea, and probably climaxed at 5.9 Ma when the Mediterranean sea level dropped by more than 1000 m during the salt deposition stage.