Tubotomaculum: fossil trace or bacterially-mediated polymetallic nodule?

Since the 50s of the last century, several authors have investigated the nature of Tubotomaculum: enigmatic Mn-Fe mineralizations occurring mainly in the latest Oligocene – Early Miocene varicoloured clays of the western Mediterranean area. Some authors regarded these structures as fossil traces (Garcia Ramos et al., 2014) or as fossil corals covered by goethite, hematite, and manganite (Hamoumi, 2006). In contrast, Pautot et al. (1975) considered these structures as inorganic Mn-Fe nodules. In addition, they recognized rhodocrosite, siderite, and non-crystalline Mn-oxides. Due to the lack of mineralogical and geochemical data the processes of crystallization and growth are still unclear and questionable. Mn-oxides are significant environmental indicators (e.g., composition, pH, Eh, and biological activities of the system) when the samples are properly characterized. Because of they occur typically as cryptocrystalline and fine-grained mixtures of different Mn-phases and additional minerals (Fe-oxides, carbonates, and silicates), their mineralogical characterization is a real challenge requiring a multimethodological approach. The Tubotomaculum analyzed in this work are from the western Rif in northern Morocco. The samples come from the pre-Numidian varicoloured clays of a section sampled close to the dam of the Reservoir lake 9th April. This work is a first contribution aimed at providing a proper mineralogical characterization of these Mn-oxides and to define the processes that led to the crystallization and growth of these mineralizations. The final aim of this work is to provide evidence to reconstruct the paleo-environmental conditions affecting the late Oligocene-early Miocene pre-Numidian sedimentary basin of northern Morocco. Crossing SEM-EDS, XRPD, FT-IR and Raman spectroscopy data, a proper characterisation of the samples was achieved. Tubotomaculum consists of a very fine mixture of birnessite [(Ca,Na)(Mn4+,Mn3+)2O4·1.5H2O] and todorokite [(Ca,Na,K)(Mn4+,Mn3+)6O12·nH2O] Mn-phases, which are commonly recognized in oceanic nodules (Burns & Burns, 1977). In particular, todorokite could be related to biological processes (Mandernack et al., 1995). Goethite, hematite, kaolinite, and quartz were also found. The presence of millimetric and sub millimetric Mn and Fe rich layers around central nuclei of clay minerals, quartz and/or bone fragments, together with the occurrence of 3-5 μm spheroidal structures (i.e., bacteria cells) characterizing the mm-scale Mn rich layers, allow to exclude the secondary diffusion of Mn and Fe from the surrounding sediment, suggesting a primary deposition of these mineralizations, possibly mediated by bacterial activity. According to these evidences, Tubotomaculum cannot be considered as a fossil trace or fragments of fossil coral. They are polymetallic nodules made up of a very fine grained mixture of birnessite and todorokite, whose growth was biologically mediated by bacterial activity, which played a key role in catalyzing the oxidation of Mn2+ on the late Oligocene-early Miocene sea floor of northern Morocco.