|Behaviour and mobility of U series radionuclides in Madeira Abyssal Plain turbidites over the past 750,000 years|
Colley, S.; Thomson, J. (1992). Behaviour and mobility of U series radionuclides in Madeira Abyssal Plain turbidites over the past 750,000 years. Mar. Geol. 109(1-2): 141-158
In: Marine Geology. Elsevier: Amsterdam. ISSN 0025-3227, more
Diffusion; Polonium isotopes; Pore water; Radionuclide kinetics; Radium isotopes; Sediment composition; Thorium isotopes; Turbidites; Uranium isotopes; Vertical profiles; Marine
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The irregular U profiles developed in Madeira Abyssal Plain turbidites shortly after emplacement are preserved thereafter in the sediments for at least 750 kyr, and can be correlated between widely-spaced cores. Such profiles at 22-29 m depth in units 650-730 kyr old have been exploited to study steady state diffusion of the longer-lived U decay-series radionuclides, and to derive effective in-situ diffusion coefficients applicable to post-oxic conditions. A model developed for comparison of parent-daughter pair data confirms that the only nuclide to exhibit convincing evidence of migration is 226Ra. The symmetrical distribution of 226Ra around the 230Th peaks indicates that pore water advection is absent. Other nuclides which might be expected to show evidence of migration are 234U and 222Rn. Although leaching experiments demonstrate that 234U is potentially preferentially mobile relative to 238U, this is not observed in the sediments, and suggests that the post-oxic geochemical environment immobilises 234U. 230Th is not expected to diffuse in pore waters. The profiles of 226Ra and 210Po show good agreement, which indicates that diffusion of 222Rn is limited by its short half life.