Correlation of Co concentration and growth rate — A method for age determination of ferromanganese crusts☆

Abstract Based on the analysis and comparison of metal content and growth rate determination by the 10 Be method an empirical law has been developed to calculate the growth rate. Evaluating after 520 chemical analyses we have shown that the flux of Co into ferromanganese seamount crusts has been constant over the entire growth period of all crusts investigated. The amount of Co supplied per unit of time and seabottom area is nearly constant in the oceanic water column. The layered texture of ferromanganese crusts reflects the change of phases and hydrogenetically precipitated amounts as well as their different growth rates. The most important indicator revealing the growth rate is the Co enrichment of each crust layer: The higher the growth rate the lower the Co concentration. Therefore, it is possible to recalculate the age and growth rate of each ferromanganese crust layer by: G(mm Ma −1 )=1.28/[Co (%)-0.24] Based on the intense phosphorite impregnation of the lower part of the ferromanganese crust which has grown within the time period 18-12 Ma (so-called older crust) two additional effects have to be taken into account: (a) the intense phosphorite impregnation causing dilution; and (b) the chemical dissolution of the ferromanganese crusts caused by the phosphate-rich pore water. Both reduce the Co concentration and the relating Co contents, Co ( x ) ′ and Co ( x ) ″, have thus to be compensated: (a) Co (x)′ = Co (x)m Mn/Co (x) Mn/Co (b) (b) Co ( x )″ =Co ( x )′ (1-0.05 ΔP ) −1 where Co ( x ) ″ = double-corrected Co concentration of the layer ( x ) of the older crust generation; Co ( x ) ′ = corrected Co concentration of the layer ( x ) of the older crust generation concerning the phosphate dilution; Co ( x )m = measured Co concentration of the layer ( x ) of the older crust generation; Mn Co (x) = Mn Co ratio measured of the layer ( x ) of the older crust generation; Mn Co ( b ) = Mn Co ratio measured of the boundary layer between younger and older crust generation; and ΔP = difference between the apatite fraction of the older crust of the layer ( x ) and the average value of the younger crust generation. This simple cobalt calculation model makes it possible to recalculate the growth rate and age of each simple layer of ferromanganese crusts.