Concept of Biogenic Ferromanganese Crust Formation: Coccoliths as Bio-seeds in Crusts from Central Atlantic Ocean (Senghor Seamount/Cape Verde)

Abstract

At depths of 2,000 to 3,000 m, seamounts from the Cape Verde archipelago (Central Atlantic Ocean) are largely covered with ferromanganese crusts. Here we studied 60 to 150 mm thick crusts from the Senghor Seamount (depth: 2257.4 m). The crusts have a non lamellated texture and are covered with spherical nodules. The chemical composition shows a dominance of MnO₂ (26.1%) and Fe₂O₃ (38.8%) with considerable amounts of Co (0.74%) and TiO₂ (2.1%). Analysis by scanning electron probe microanalyzer (EPMA) revealed a well defined compositional zonation of micro-layers; the distribution pattern of Mn does not match that of Fe. Analysis by high resolution scanning electron microscopy (SEM) revealed that coccospheres/coccoliths exist in the crust material as microfossils; most of the coccospheres/coccoliths are not intact. The almost circular coccoliths belong to the type of heterococcoliths and are taxonomically related to species of the family Calcidiscaceae. By energy dispersive X-ray spectroscopic analysis an accumulation of the coccoliths in the Mn- and Fe rich micronodules was detected. Focused ion beam assisted SEM mapping highlighted that the coccoliths in the crust are Mn rich, suggesting that the calcareous material of the algal skeleton has been replaced by Mn-minerals. We conclude that a biologically induced mechanism has been involved in the formation of the crusts, collected from the Cape Verde archipelago from depths of 2,000 to 3,000 m in the mixing region between the oxygen-minimum surface zone and the oxygen-rich deep waters; the deposition process might have been triggered by chemical reactions during the dissolution of the Ca-carbonate skeletons of the coccoliths allowing Mn(II) to oxidize to Mn(IV) and in turn to deposit this element in the crust material.