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 cm 2 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/10 6 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/10 6 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) × 10 4 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/cm 2 yr. The exhalation rate of 222Rn, estimated from the 226Ra- 210Pb disequilibrium is ∼570 dpm/cm 2 yr from the top side and >2000 dpm/cm 2 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/cm 2 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.