Rare earth elements and U uptake by fish remains in seawater: how fast?

During diagenesis, biogenic phosphates are considerably enriched in trace elements, particularly in U and rare earth elements (REEs), which makes them palaeoceanographic proxies for the composition of past seawater. The rate at which this enrichment occurs is unknown. Here, we report on present day ichthyoliths that have spent the last 50 years in contact with seawater and show that their uptake of REEs is largely dominated by quantitative incorporation without fractionation, probably via the development of authigenic phases, in particular Fe oxides. Enrichments take place rapidly, for example at a rate of around 1.2 ng/g per year for Nd and 300 ng/g per year for U. Transposed to known fossils displaying the highest REE and U abundances, these results suggest that contact with seawater may have persisted for several thousand years, implying very slow sedimentation rates. Alternatively, post-burial enrichment processes could be much more efficient, which would imply that the signatures shown by these kind of fossils, rich in rare earth elements and yttrium, may largely reflect diagenetic overprinting rather than pristine marine signatures.