Be10 and U-series isotopes in manganese from the Central North Pacific

It is observed in one specimen (Mn 139): 10Be-based rate 230Th-based rate < 231Pa-based rate. This suggests that diffusion-mixing may have modified the depth gradients of these nuclides, such that the true rate would be close to or even lower than the observed 10Be rate of 1.3 mm/10-6 yr.

Quantitative assessment using a diffusion-decay model indicates that in Mn 139, 230Th and 231Pa could have been subjected to diffusion-like transport with an effective diffusion coefficient of about 1 10power8xcm-2/yr. After correction for this effect, the 230Th and 231Pa data give a concordant rate of 1.9 mm/10-6 yr, about 2-3 times lower than their uncorrected values. The small and variable amounts of integrated dpm/cmSUP-2 of 10Be, 230Th excess and 231Pa excess found in nodules are best explained by their incorporating only a fraction of the nuclides supplied due to frequent coverage of sediments.