Tubotomaculum: a bacterially-mediated polymetallic nodule

Since the 50s of the last century, several authors have investigated the nature of Tubotomaculum: enigmatic Mn-Fe mineralizations occurring in the latest Oligocene – Early Miocene varicoloured clays of the western Mediterranean area. Some authors regarded these structures as fossil traces[1], as fossil corals covered by Fe-oxides and manganite[2], or as inorganic Mn-Fe nodules with rhodocrosite, siderite, and non-crystalline Mn-oxides[3]. 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 for composition, pH, Eh, and biological activities. However, since they occur typically as cryptocrystalline and fine-grained mixtures of different Mn-phases and additional minerals, like Fe-oxides, carbonates and silicates, their characterization is a real challenge that requires a multimethodological approach. The Tubotomaculum samples 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 characterization of these mineralizations and to define the processes that led to their crystallization and growth. The final aim of this work is to provide evidence of the paleo-environmental conditions affecting the late Oligocene-early Miocene pre-Numidian sedimentary basin of northern Morocco. Crossing SEM-EDS, XRPD, TEM, 3D ED and FT-IR and Raman spectroscopy data, a proper characterization of the samples was achieved. Tubotomaculum consists of a very fine mixture of birnessite [(Ca,Na)(Mn4+,Mn3+)O4·1.5HO] and todorokite [(Ca,Na,K)(Mn4+,Mn3+)6O12·nHO], Mn-phases commonly recognized in oceanic nodules[4]. In particular, todorokite could be related to biological processes[5]. Goethite, hematite, kaolinite, and quartz were also found. All these minerals have a clear spatial distribution in the samples: millimetric Mn-Fe rich layers grow around central nuclei of clay minerals, quartz and/or bone fragments. In these layers 3-5 μm Mn-rich spheroidal structures occur in a Fe-rich matrix. SEM analyses reveal the presence of rounded structures suggesting microbial colonies. Their occurrence points to a biofilm formed by a single species of bacteria. These features 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 evidence, 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 Mn-Fe oxides, whose growth was biologically mediated by bacterial activity. Allegedly, such activity played a key role in catalyzing the oxidation of Mn2+ on the late Oligocene-early Miocene sea floor of northern Morocco.