Uranium and thorium series nuclides in oriented ferromanganese nodules: growth rates,turnover times and nuclide behavior |
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| Authors: | S Krishnaswami J Kirk Cochran |
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| Institution: | Department of Geology and Geophysics, Yale University, New Haven, CT 06520 U.S.A. |
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| 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.230Thexc 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 the230Thexc depth profile has been observed at ～0.3 m.y. ago, suggesting that the nodule growth has been episodic. The concentration profiles of231Paexc (measured via227Th) yield growth rates similar to the230Thexc data. The bottom sides of the nodules display exponential decrease of230Thexc/232Th activity ratio with depth, yielding growth rates of 1.5–3.3 mm/106 yr.The230Thexc and231Paexc concentrations in the outermost layer of the bottom face are significantly lower than in the outermost layer of the top face. Comparison of the extrapolated230Thexc/232Th and230Thexc/231Paexc 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.The210Pb concentration in the surface ～0.1 mm of the top side is in large excess over its parent226Ra. Elsewhere in the nodule, up to ～1 mm depth in both top and bottom sides,210Pb is deficient relative to226Ra, probably due to222Rn loss. The absence of210Pbexc below the outermost layer of the top face rules out the possibility of a sampling artifact as the cause of the observed exponentially decreasing230Thexc and231Paexc concentration profiles. The flux of210Pbexc to the nodules ranges between 0.31 and 0.58 dpm/cm2 yr. The exhalation rate of222Rn, estimated from the226Ra-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 to230Th up to ～0.5–1 mm and is in large excess throughout the bottom. The data indicate a net gain of226Ra into the nodule, corresponding to a flux of (24?46) × 10?3 dpm/cm2 yr. On a total area basis the gain of226Ra into the nodules is <20% of the226Ra escaping from the sediments. A similar gain of228Ra into the bottom side of the nodules is reflected by the high228Th/232Th activity ratios observed in the outermost layer in contact with sediments. |
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