Les depots d'enduits manganoferriferes en environnement marin littoral; l'exemple de grottes sous-marines en Mediterranee nord-occidentale

Abstract

Abstract Black coating of hard substrates by Mn and Fe oxides has long been reported from shallow, dark, submarine caves. However, these littoral metallic deposits have never been studied in detail, despite expected analogies with deep-sea polymetallic crusts. Submarine caves are characterized by darkness and low rates of exchanges with the open sea. Lack of primary production and confinement of inner water bodies result in marked oligotrophy and extremely reduced biomass, i.e. conditions close to those prevailing in deep-sea habitats. Field evidences suggested that the formation of Mn-Fe coatings was closely tied to these particular environmental conditions. The goal of this study was to examine the detailed features of Mn-Fe coatings from dark caves with different local conditions, and to try to identify the processes responsible for their deposition. Study sites and methods: Three sublittoral, single-entrance, caves were sampled by scuba diving along the coasts of Provence (France, Mediterranean Sea). The first site is a large karstic cave (Tremies Cave, 16 m depth at entrance floor, 60 m long; Marseille-Cassis area) with an ascending profile which results in a buffered thermal regime and markedly oligotrophic conditions due to warm water trapping in its upper part. Wall fragments were sampled at 30 m (medium confinement: zone B) and 60 m (strong confinement: zone C) from the cave entrance. The second site is a large tubular cavity open in conglomerate formations (3PP Cave, 15 m depth at entrance floor, 120 m long; La Ciotat) with a descending profile which results in relative permanence of winter temperatures within the inner parts, complex water circulation and presumed greater input of sedimented particles than in the preceding cave. Wall samples were taken at 25 m, 70 m and 100 m from entrance. The third site is a small, horizontal, cave open in quartzite formations (Bagaud Cave, 7 m depth at entrance floor, about 10 m long; WNW of Port-Cros Island, bay of Hyeres). Sampling was performed on walls of a narrow corridor between an anterior room and a smaller inner room. A sporadic outflow of continental waters is located in the inner room. The samples were preserved in 50% ethylic alcohol or studied soon after their sampling. Before carbon coating and SEM examination, or microanalyses with SEM-associated spectrometers, they were treated in a 33% Chlorox solution and thereafter washed in demineralized water and dried. Micromorphology: At low-medium magnification (<20,000), the aspect of coatings varies between caves and, especially, between inner-cave locations. All the described structures are made up of Mn and Fe oxides. In Tremies Cave, coatings of walls from zone B are composed of irregular erected constructions (height: 10s to 100s mu m) formed by the aggregation of roughly ovoid primary concretions of about 10 mu m. The surface of those primary concretions displays numerous lacunose to reticulate films (pores, about 0.5 mu m in diameter, are often subrounded). Remnants of these films and organomorphic corpuscles occur also within the primary concretions. On younger substrates (broken wall exposed since 1970), primary concretions are poorly developed and no prominent construction is visible. In more confined conditions (zone C), the erected constructions of ancient coatings are smaller and less numerous than in zone B but are well individualized. In this zone C, besides some remnants of lacunose to reticulate films, there is an appearance of filaments and ovoid corpuscles (height/width: 10-30/5-15 mu m), which seem to be linked to filaments by a short stalk. In 3 PP Cave, at 25-70 m from entrance, wall coatings present porous heaps of primary concretions. The surface and the inside of the latter comprise remnants of lacunose to reticulate films that evoke those observed in Tremies Cave. On younger substrates (hard parts of sessile invertebrates), coatings are restricted to micrometric organomorphic corpuscles with some remnants of lacunose or fibrous films. At 100 m from the entrance, coatings are shaped by numerous erected constructions, more or less coalescing. Besides remnants of lacunose films, the primary concretions contain interlacing filaments (diameter: 0.2-0.3 mu m) forming cords or veils. In Bagaud Cave, the primary concretions are aggregated in irregular heaps. Lacunose films are particularly frequent and tend to form three-dimensional mamillated structures that were not observed in the other caves. In particular, there is an appearance of tubular structures and of numerous hemispheroidal structures (diameter: 4-5 mu m) with an upper orifice. At higher magnification (20,000), whatever the cave and inner-cave location, the aspect of oxide deposits is rather smooth or, especially, microgranular. Mineral composition: The composition of coatings is different between caves and according to their inner-cave location. In both large caves (Tremies and 3 PP), the Mn/Fe ratio increases with the distance from the cave entrance, i.e. when exchanges with the open sea diminish. This trend is particularly clear in Tremies Cave, where the confinement gradient is strongly marked.