Growth rate of manganese nodule measured with ¹⁰Be and ²⁶Al

Abstract

MANGANESE nodules are considered to have accumulated in general very slowly: surface layers of nodules contain a large excess of ²³⁰Th, more than would be produced by uranium decay. The decrease of excess ²³⁰Th with depth within a nodule is generally interpreted to be due to the radioactive decay, and nodule accumulation rates were then estimated from this to be a few mm Myr⁻¹ (refs 1–4). How then do these nodules escape from burial by associated marine sediments which accumulate 3 orders of magnitude faster than the nodules? One possible explanation^5,6 is that the radionuclides of interest were not incorporated in the nodule matrix during its growth but were adsorbed later. Subsequent inward diffusion of the radionuclides might result in an apparent radioactive decay. We have tested this hypothesis by measuring simultaneously two radionuclides of different half lives. If the gradients of the radionuclides are really due to the radioactive decay, then, assuming a constant growth rate for a nodule, we expect the profile of the shortlived nuclide to be steeper (because of its faster decay) than that of the longlived nuclide.