Using (U-234/U-238) to assess diffusion rates of isotope tracers in ferromanganese crusts

Ferromanganese crusts record seawater isotope history for several elements useful for paleoceanography. Interpreting crust isotope profiles, however, requires an assessment of the rate of diffusion of each element within the crust. We address this issue using U- and Th-isotope profiles, coupled with concentration measurements of some trace elements whose isotopes are often measured in crusts. A depth profile of (Th-230(xs)/U-234) for a North Atla ntic crust (Alv539) yields a growth rate of 3.05 mm/Myr, consistent with an existing Be-10/Be-9 record. But the growth rate implied by a (U-234/U-238) profile in the same crust is approximate to 6 times faster due to diffusio n of U in the crust. The difference between the observed (U-234/U-238) prof ile and that expected from (Th-230(xs)/U-234) enables the effective diffusi vity of U to be calculated as 1.2 x 10(-6) cm(2)/year. Recalculated literat ure data indicate that other crusts have diffusivities which range down to 4.7 x 10(-8) cm(2)/yr. This variation is to be expected given the range of porosity observed for ferromanganese crusts. Concentration measurements of various elements in the surface layer of the crust provide relative distrib ution coefficients for each element between seawater and crust. These distr ibution coefficients enable the effective diffusivity of each element to be calculated by comparison with the diffusivity of U. Such diffusivities are assessed for crust Alv539 and give a first indication of the expected pres ervation of isotope signals in ferromanganese crusts. Li, Os, and Sr have d iffusivities of 2 x 10(-4), 3 x 10(-5) and 2 x 10(-5) cm(2)/year respective ly and diffusion is expected to be sufficiently fast that crusts will not a ccurately preserve past seawater compositions. Th, Nd, Pb and Be have diffu sivities less than 10(-9) cm(2)/year and are highly immobile in crusts, a r eassuring result for Th and Be crust chronologies and for Nd and Pb-isotope tracing of past water masses using crusts. Hf has a diffusivity of 3 x 10( -8) in Alv539 - fast enough to perturb but not destroy its isotope history. For studies of Hf in this and other crusts, diffusion should therefore be considered. Measuring U-isotope profiles provides a potential way of assess ing such diffusion for Hf or for other elements with relatively low distrib ution coefficients into ferromanganese crusts. (C) 1999 Elsevier Science B. V. All rights reserved.