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

Abstract

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. ²³⁰Th_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 ²³⁰Th_exc depth profile has been observed at ~0.3 m.y. ago, suggesting that the nodule growth has been episodic. The concentration profiles of ²³¹Pa_exc (measured via ²²⁷Th) yield growth rates similar to the ²³⁰Th_exc data. The bottom sides of the nodules display exponential decrease of ²³⁰Th_exc/²³²Th activity ratio with depth, yielding growth rates of 1.5-3.3 mm/10⁶ yr. The ²³⁰Th_exc and ²³¹Pa_exc concentrations in the outermost layer of the bottom face are significantly lower than in the outermost layer of the top face. Comparison of the extrapolated ²³⁰Th_exc/²³²Th and ²³⁰Th_exc/²³¹Pa_exc activity ratios for the top and bottom surfaces yields an "age" of (5-15) × 10⁴ yr for the bottom relative to the top. This "age" most probably represents the time elapsed since the nodules have attained the present orientation. The ²¹⁰Pb concentration in the surface ~0.1 mm of the top side is in large excess over its parent ²²⁶Ra. Elsewhere in the nodule, up to ~1 mm depth in both top and bottom sides, ²¹⁰Pb is deficient relative to ²²⁶Ra, probably due to ²²²Rn loss. The absence of ²¹⁰Pb_exc below the outermost layer of the top face rules out the possibility of a sampling artifact as the cause of the observed exponentially decreasing ²³⁰Th_exc and ²³¹Pa_exc concentration profiles. The flux of ²¹⁰Pb_exc to the nodules ranges between 0.31 and 0.58 dpm/cm² yr. The exhalation rate of ²²²Rn, estimated from the ²²⁶Ra-²¹⁰Pb disequilibrium is ~570 dpm/cm² yr from the top side and >2000 dpm/cm² yr from the bottom side. ²²⁶Ra is deficient in the top side relative to ²³⁰Th up to ~0.5-1 mm and is in large excess throughout the bottom. The data indicate a net gain of ²²⁶Ra into the nodule, corresponding to a flux of (24-46) × 10^-3 dpm/cm² yr. On a total area basis the gain of ²²⁶Ra into the nodules is <20% of the ²²⁶Ra escaping from the sediments. A similar gain of ²²⁸Ra into the bottom side of the nodules is reflected by the high ²²⁸Th/²³²Th activity ratios observed in the outermost layer in contact with sediments.