The occurrence of deep-sea ferromanganese nodules and crusts on the seafloor is widespread, providing an important resource for numerous metals such as Ni, Co, and Cu. Although they have been intensively studied in the past, the formation of micro-manganese nodules within carbonate rocks has received less attention, despite the considerable amounts of manganese released from the dissolution of the calcareous framework. The micro-petrographic and geochemical characteristics of reef carbonate rocks recovered from the Zhaoshu plateau in the Xisha uplift, north of the South China Sea, were studied using optical microscopy, scanning electron microscopy, confocal Raman spectrometry, and an electron probe micro-analyzer. The carbonate rocks are composed of biogenic debris, including frameworks of coralline algae and chambers of foraminifer, both of which are suffering strong micritization. Within the calcite micrite, numerous micro-manganese nodules were identified with laminated patterns. Mineral and elemental evidence showed that the Mn oxides in the carbonates are mixed with 10 Å vernadite, 7 Å vernadite and todorokite, both of which are closely associated with the carbonate matrix. The micro-nodules were found to have high Mn/Fe ratios, enriched in Ni and Cu and depleted in Co. We infer that theses nodules are mixed type with early diagenetic growth under oxic–suboxic conditions. The re-distribution of manganite within the rocks is likely influenced by micritization of the calcareous framework. We deduce that microbial-associated reduction of manganite induces the formation of diagenetic todorokite similar to nodules buried in marine sediments.

Geological observations have revealed a rapid evolution and obvious east–west difference in the North Sulawesi subduction zone. The Celebes Sea plate has inserted itself under the north arm of the Sulawesi Islands and the north arm of the Sulawesi Islands has rotated clockwise at the same time. The rotation of the north arm of the Sulawesi Islands may play an important role in facilitating tectonic processes like the slab rollback of the Celebes Sea plate. In view of the east–west differences along the north Sulawesi subduction zone, a numerical model with the convergence rate of the plates as the basic variable is established to quantitatively describe the evolution process of the north Sulawesi subduction zone. Our results reproduce the east–west differences of the subducting Celebes Sea plate, showing a shallow–deep–shallow subduction style. We suggest that the variable velocity ratio of the overriding plate to the subducting slab may be the main reason for the differential subduction along the strike of the North Sulawesi subduction zone. We therefore conclude that the residual slab and the rate of the eastern continental plate limit the downward movement of the subducted slabs of the eastern Sulawesi, and the tectonic location beyond the rotation radius influences the subduction morphology of the extreme western Sulawesi. Moreover, the widespread extension since Pliocene at the western Sulawesi is not affected by the rotating overriding plate.摘要 HTML。