Radiochemical observations on manganese nodules from three sedimentary environments in the north Pacific

Details Radiochemical observations on manganese nodules from three sedimentary environments in the north Pacific Radiochemical observations on manganese nodules from three sedimentary environments in the north Pacific

May 1984

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1984gecoa..48..951h&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 48, Issue 5, pp.951-963

Computer Science

7

Scientific paper

Decay-series isotopes were measured on manganese nodules from three sedimentary environments, characterized by substrates of red clay (R), siliceous ooze (S) and hemipelagic clay (H). Growth rates of nodules are shown to be site dependent: 1-2 mm/Myr at site R, 3-8 mm/Myr at sites S, and 20-50 mm/ Myr at site H. Correlation between growth rate and the parameter Mn/Fe 2 suggests that regional diagenetic variations control the compositions and growth rates of the nodules. The frequency of nodule turnover and the period of their growth are assessed from the "top" vs . "bottom" distributions of several nuclides. Based on the 230 Th, 231 Pa and 226 Ra data, turnover times of 10 3 to 10 5 years are estimated and they vary with relative size and shape of the specimens at a given site. The presence of unsupported 210 Pb and 228 Th in the top surfaces but not in the bottom surfaces of the surface nodules suggests active growth during the last decades or years, despite their old ages. The data also indicate that turnover rates are not more frequent than once every several years or decades. The 230 Th- 226 Ra, 226 Ra- 210 Pb and 232 Th- 228 Th disequilibrium relationships in the nodules allow the migratory behavior of 226 Ra, 222 Rn and 228 Ra and their fluxes to be deduced: thus radium leaves the top sides but enters the bottom sides of the surface nodules at sites R and S. At site R there is a net loss of radium to the sea; the opposite is true at site S. Surface nodules at site H trap radium from both sides, probably due to more intense diagenetic input of radium from sediment pore water. The effective diffusivities of 226 Ra in nodules vary from 3 × 10 -12 to 8 × 10 -14 cm 2 /s, dependent on the textural variation of the nodule material, which crudely reflects the growth rate and hence sedimentary environment. 222 Rn diffuses out of nodules from all sides, with an effective diffusivity of ca . (2-6) × 10 -8 cm 2 / s . At all sites nodules serve as a more effective source of 222 Rn to sea water than their adjacent sediments. The outward flux of 222 Rn from nodules relative to that from adjacent sediments tends to be higher in more reduced environments, an effect caused by the fact that Mn-rich nodules from more reduced environment act as a more efficient trap for 226 Ra.

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