Diffusion of rare-earth elements in fish teeth from deep-sea sediments

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THE distribution of rare-earth elements (REEs) in biogenic phosphate phases in sediments (such as the remains of fish teeth) has been proposed as an indicator of the depositional environment in the water column, recording for example palaeo-redox conditions1 and isotope geochemical2–5 and hydrothermal6,7 processes. Enrichment of these phases in REEs takes place only during early diagenesis8,9 for which reason the REE signal has often been assumed to represent the unfractionated record of REE distribution in the water column overlying the sediment surface1,4,5,10. Here we report on studies of REE zoning in fish teeth from Pacific sediments, from which we extract a REE diffusion coefficient of ∼70 mm2 Myr–1. This value suggests that diffusion of REEs from sea water into the fish teeth before burial is too slow to produce the enrichments observed. Instead, the REE distributions must derive from the surrounding sediment pore fluids. The use of these distributions as palaeoceanographic indicators is therefore called into question.