Ferromanganese crusts of the Vietnam margin, central South China Sea: Composition and genesis

Abstract

Ferromanganese (FeMn) crusts collected from two seamounts of the Vietnam continental margin, central South China Sea were analyzed for mineralogical and chemical compositions, and dated using 230Thexcees technique and cobalt chronometry. The studies of dissolved oxygen content in water column at the area by CTD-profiling display Oxygen Minimum Zone locates at the depth 420 to 1100 m with O2 concentration of 1.6–1.8 ml/l. The composition and morphology of studied crusts are different from Prime Crust Zone crusts in Pacific Ocean and partially similar to ferromanganese crusts and nodules from northern South China Sea due to unique environment of basin. The FeMn crusts from Vietnam margin show high ratios of Fe relative to Mn and Ni relative to Co, high Th and Pb concentrations and comparable to PCZ deposits low contents of REE. Few main mechanisms were dominated in crust formation at Vietnam margin including a permanent input of detrital material sourced by river discharge, shelf sediments and dust plume while Fe and Mn (oyxhydr)oxides slow precipitate from seawater as initially colloidal particles within oxygen-minimum zone predominantly at the surface of biogenic limestone substrate. Upwelling and high coastal primary productivity are important factors in Fe, Ni, Th and Pb transportation and accumulation. The main factor of high Fe/Mn ratio is the significant flux of Fe released by redox cycling in the sediment. SCS crusts are enriched in Ni relative to Co due to influx of Ni from coastal laterite weathering and optimum conditions for its transportation to crust formation area. Thorium and lead related to significant continental detrital input and eolian supply, especially Asian dust. Due to diversity of crust thicknesses local hydrographic current regime is also suggested as crucial factor in crust formation. Calculated growth rate using 230Thexcees technique is 11 ± 1 mm/Myr and 4.4 ± 0.5 mm/Myr for two measured crusts. The crust growth in South China Sea might be initiated around 1.5 Myr ago, the period which coincide with the Southern Ocean glacial-induced restructuring dated 1.6 Myr.