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A remarkable exposure of carbonate pipe-like and slab concretions occurs along the Enza riverbanks on the Northern Apennines foothills (Italy). Based upon geochemical and field evidences, their genesis has been attributed to microbial-governed carbonate precipitation from hydrocarbon-enriched fluids. The pipe-like concretions are thus interpreted as former conduits (chimneys) marking sites of methane ascent onto the seafloor. The resulting Enza River chimney field is arranged in a palisade fashion, a rare example of such. Mineralogical and petrographic analyses of some chimneys and slabs document that the dominant cement is dolomite. Although the chimneys show a rather homogeneous texture, a clear zonation is observed in the relative distribution of major and minor elements in their internal and external parts. The occurrence of sulphide minerals in the stratigraphically upper samples indicates possible renewal of fluids leakage after a major erosive event. [ABSTRACT FROM AUTHOR]

The comparison of methane-derived authigenic carbonates (MDACs) from four sites, located at different tectonic and stratigraphic settings of the Gulf of Cadiz, ranging from the inner central part of the Gibraltar Arc (the Betic-Rifean Domain, within the Mediterranean outflow -- MOW channel in front of the Gibraltar Strait), up to the external domain of the margin (on the recently discovered Michael Ivanov mud volcano, 4500m in the deep Portuguese Margin), allows the characterization and comparison of the different gas seepage records along the Gulf of Cadiz sedimentary wedge. The mineralogical and geochemical characterizations of these carbonates confirm their methane-derived origin, resultant of the anaerobic oxidation of methane in shallow sediments. Three distinctive groups of MDAC were identified, based on mineralogy and morphology: chimneys, crusts and concretions. These different groups reflect the different geochemical conditions of their formation environment, reinforcing previous results from similar studies. Chimneys indicate a past and older age of formation within the sediments and subsequent influence of the MOW on their exhumation and exposure to the seawater. These chimneys also show a distinct and heavier C stable isotopic composition (-25 up to -8‰ PDB) than the previously reported chimneys from the Gulf of Cadiz. The concretions have characteristics indicative of actively present-day formation, and their oxygen stable isotopic composition suggests a formation in close association with gas hydrates. [ABSTRACT FROM AUTHOR]