Uranium and thorium series nuclides in oriented ferromaganese nodules: growth rates, turnover times and nuclide behavior

Three ferromanganese nodules handpicked from the tops of 2500 cm2 area box cores taken from the north equatorial Pacific have been analysed for their U-Th series nuclides. 230Thsub(exc) concentrations in the surface 1-2 mm of the top side of the nodules indicate growth rates of 1.8-4.6 mm/106 yr. In two of the nodules a significant discontinuity in the 230Thsub(exc) depth profile has been observed at approximately 0.3 m.y. ago, suggesting that the nodule growth has been episodic. The concentration profiles of 231Pasub(exc) (measured via 227Th) yield growth rates similar to the 230Thsub(exc) data. The bottom sides of the nodules display exponential decrease of 230Thsub(exc)/232Th activity ratio with depth, yielding growth of 1.5-3.3 mm/106 yr. (Auth.)