Calcareous nannofossils identify the age and precipitation rates of manganese deposits of the Mozambique Ridge and Mozambique Basin, SW Indian Ocean

摘要

Calcareous nannofossil assemblages identified in manganese deposits dredged at three sites on the Mozambique Ridge and one on the Jaguar seamount in the Mozambique Basin comprise 61 species, including nine reworked ones. The samples represent cores of nodules or encrustations ranging from 4.04 to 0.00 Ma. Dating by nannofossils suggests bi-modal ages for the samples, the late Zanclean-Piacenzian (Pliocene) and the Calabrian (Pleistocene)-Holocene. The calculated rates of manganese precipitation range from 4.7 to 248.3 mm/Ma, which are generally typical for hydrothermal manganese accretion. The bi-modal age distribution points to oceanographic changes during the Pliocene-Holocene. The Pliocene manganese precipitation may reflect the closing of the Panama Isthmus, while the terminal Pleistocene-Holocene accretion may result from oceanic fluctuations caused by glacial and interglacial periods. Fe-Mn oxide precipitation rates at the Makarov Guyot, NW Pacific, re-calculated on the basis of the most recent calcareous nannofossil biostratigraphy, suggest hydrothermal processes of manganese accretion in that part of the West Pacific Seamount Province.