Stable isotopes and rare earth element compositions of ancient cold seep carbonates from Enza River, northern Apennines (Italy): Implications for fluids sources and carbonate chimney growth

Abstract

Abstract Cold seep carbonates are remarkable fossil records of hydrocarbon seepage and related conduits. In the present research, we analysed the carbon and oxygen isotopes, rare earth element (REE) and trace element content of carbonate chimneys and concretions in the Pleistocene Argille Azzurre Formation, northern Apennines, Italy. Strong 13 C depletions (values as low as −40.3‰) observed in both carbonate concretions and chimneys indicate that anaerobic oxidation of biogenic methane played a major role in the formation of these carbonates. Slightly higher δ 13 C values mostly found in the outer layers of the carbonate chimneys reflect a contribution of inorganic carbon from seawater. REE patterns reveal the carbonates formed under mostly anoxic pore water conditions. Combined REE and trace element data reveal the growth of chimneys occurred from the outside-in, with the outer chimney walls experiencing rapid growth. Excellent correlations between Ce/Ce*, REE contents, and La N /Sm N and Y/Ho ratios observed in the chimney walls provide additional evidence for the influence of seawater in the formation of the chimneys. Meanwhile, O isotopic compositions of the samples suggest an 18 O-depleted fluid was involved in carbonate precipitation. This study has major implications for tracking fluid sources of seep carbonates and patterns of carbonate chimney growth.