10Be and Th isotopes in manganese nodules and adjacent sediments: Nodule growth histories and nuclide behavior

Abstract

The concentration profiles of 9Be, 10Be, 230Th, 232Th, 231Pa (via 227Th) and 238U have been measured in three manganese nodules, one each from the North Pacific (A47-16(4)), the South Pacific (TF-5) and the Indian Ocean (R/V Vitiaz). In addition the 10Be concentration in deep water from the GEOSECS reoccupation station 500 of the North Pacific, and in ☐ cores raised from the manganese nodule field in the North Pacific have been measured. The 10Be concentration in nodule and seawater samples was measured by the accelerator masss spectrometric technique employing the Yale Tandem Van de Graaff accelerator. The concentrations of 10Be, 230Th exc and 231Pa exc and ratios of 10Be/ 9Be and 230Th exc/ 232Th all decrease with depth in the nodules. This decrease, interpreted in terms of nodule growth, yields “average” growth rates of a few millimeters per million years for the nodules. The growth rates of the nodules exhibit temporal variations, both on short time (∼ 50,000 years) and long time (several million years) scales. Of the three nodules studied, only in TF-5 is the short-term average growth rate based on 230Th exc in the top 0–0.5 mm the same as the long-term average rate based on Be isotope data for the 0.5–17 mm interval. For the other two nodules, the recent average growth rates based on 230Th exc data differ significantly from the long-term average growth rates based on Be isotopes. In A47-16(4) the 10Be based rate is less than the 230Th exc rate and in R/V Vitiaz the 10Be based rate is greater than the 230Th exc rate. This observation, coupled with measurable changes in growth rates even during the past few hundred thousand years, suggests, but does not prove, that the discordant growth rates deduced from 230Th exc and 10Be profiles document changes in nodule growth rate with time rather than mixing effects on 230Th exc profiles. The 10Be concentration in the GEOSECS North Pacific deep water is 6100±1200 atoms/g. This value coupled with the average surface 10Be/ 9Be ratio of North Pacific nodules predicts a 9Be concentration within the limits of measured values. The inventory of 10Be and 230Th exc in the nodules is only ∼ 10% of the total, the remaining being in sediments. The 10Be concentrations in the upper portions of two adjacent cores studied are nearly the same, but the deposition fluxes of both 10Be and 230Th based on 230Th dating vary by a factor of two. This difference is attributable to local redistribution of sediment at the time of deposition prior to accumulation.