Hydrocarbon-derived ferromanganese nodules in carbonate-mud mounds from the Gulf of Cadiz: Mud-breccia sediments and clasts as nucleation sites

More than 500 Fe–Mn nodules were sampled during the Anastasya-01 cruise (TASYO project) along the continental margin of the Gulf of Cadiz (eastern Central Atlantic), at the confluence of the Mediterranean Sea with the Atlantic Ocean, where extensive nodule fields were discovered. Based on wide previous studies that included swath bathymetry, multi-channel and very high-resolution seismic reflection, gravimetry, magnetism and underwater photography surveys, nodules were collected at water depths ranging from 850 to 1000 m, associated with hydrocarbon-derived ankerite and dolomite chimneys and crusts. Thirty six selected samples among the various morphological types were used for the laboratory analysis of physical properties (morphology, colour, surface texture, sphericity, weight and size), mineralogy (XRD, optical and electronic microscopy) and geochemistry (XRF, AAS, ICP-MS, EPMA, GC-MS, S, C and O isotopes). Nodules show a wide range of sizes, densities, weights and morphologies. Nodules are formed of multiple millimetre-thick layers of Fe and Mn oxyhydroxides surrounding the nucleus composed of Early–Middle Miocene plastic marl. The textures developed by the Fe and Mn oxyhydroxides layers are massive, laminated, detrital and mottled to dendritic. Goethite, lepidocrocite, Mn-oxides (7 Å-manganates and 10 Å-manganates), quartz, and phyllosilicates are the main components. Accessory minerals are calcite, dolomite, siderite, rhodochrosite, kutnohorite, pyrite, chalcopyrite, potassium feldspar, zircon, rutile, ilmenite, and chlorite. Fe–Mn carbonates from the siderite–rhodochrosite continuous series are forming part of the marl nuclei. Framboidal, filamentous, and globular textures are observed in Fe–Mn oxides and pyrite suggesting biogenic origin. Nodules show a high mean abundance of Fe (38.6%) moderate Mn (6.0%), and low contents of trace metals and REEs compared to the average content of deep-seabed polymetallic nodules from other oceanic areas. The Mn/Fe ratio ranges from 0.07 to 0.25. The studied nodules hold in their oxide layers hydrocarbons (n-alkanes) derived from marine bacterial activity. Aromatic hydrocarbons in the form of phenanthrene, are also present which is characteristic of mature petroleum. The structure, mineralogy, and chemical composition in the studied nodules are similar to those of diagenetic–hydrogenetic continental margin nodules rather than deep-sea nodules. We suggest that the formation of this type of nodule occurred through