Ferromanganese nodules of the Baltic Sea: Composition,helium isotopes,and growth rate

Results of the study of shallow-water ferromanganese nodules in the Gulf of Finland of the Baltic Sea, which are of practical interest for metallurgical and chemical industries, are discussed. The nodules contain the following elements: Mn, Fe, Si, Al, Na, Mg, Ti, K, V, Cu, Ni, Zn, P, and Ba. Contents of Mn (～30%) and Fe (～10%) are virtually similar to those in deep-sea oceanic nodules. However, concentrations of Ti, Cu, and Ni are notably lower than average values in oceanic nodules. The helium isotopic composition was studied to reveal cosmic dust in the nodule substance. The measured ³He and ⁴He concentrations are ～10^?12 and ～10^?5 cm³/g, respectively. The isotope ratio ³He/⁴He is approximately 10^?7. More than 60% ³He is of cosmic (solar) origin, whereas ⁴He is of terrigenous (radiogenic) origin. Based on the cosmic duct concentration and the space tracer method, the FMN growth rate is estimated at 8–9 mm/ka at the nodule age varying from ～800 to 1500 yr. The growth rate of nodule has negative correlation with its size. Based on literature data, the growth rate of FMN from the western Baltic Sea is twice as high. An independent calculation of the FMN growth rate based on the diffusion-sorption mechanism (DSM) yielded 8.1 mm/ka, which is very close to the result based on the space tracer method. This value is proposed as the average growth rate of the studied nodules. Comparison with our previous measurements of growth rates for oceanic nodules showed that these values differ only slightly and are equal to n mm/ka, where n < 10. It is inferred that the formation mechanism of both marine and oceanic nodules is based on the same principles that control the generation of mobile forms of Mn in the bottom layer of sediments, i.e., principles related to bioproductivity of sea and ocean basins. Fluxes of lithogenic forms of Mn are of minor importance.