Depth profiles of 230Th excess, transition metals and mineralogy of ferromanganese crusts of the Central Indian basin and implications for palaeoceanographic influence on crust genesis

Abstract

Two ferromanganese encrustations of hydrogenetic origin from water depths well below the CCD (carbonate compensation depth ) from a seamount zone in the Central Indian basin were studied for growth rates using U-Th series isotopes and transition-metal fluxes. The 230Th excess inventories and transition-metal fluxes in depth profiles separated by a few cm on one crust (F-380) show insignificant variations. In another crust (SS-663X) this variation is within a factor of 2 as compared to variations by a factor of 5 in the Pacific crusts. The minor variations in growth rates, 230Th and transition-metal fluxes within a single crust as well as two adjacent crusts suggest that they are due to the contact of the accreting crust surface with a dynamic veneer of sediment in space and time. Nearly uniform Mn/Fe ratios (1–1.6), δ-MnO 2 as the main Mn mineral phase and a smooth exponential decay pattern of 230Th excess and 230Th excess/ 232Th activities with depth indicate that these crusts have not recorded any palaeoceanographic events of the past 0.4 Ma. The interpolated age of these crusts is between 10 and 13 Ma which is comparable to the period of increased carbonate dissolution due to enhanced activity of AABW (Antarctic Bottom Water) currents during the middle Miocene. Probably the middle Miocene oceanographic conditions, which were associated with increased carbonate dissolution, might have initiated the formation of crusts in the Central Indian basin.