Rare earth elements in ichthyoliths: Variations with redox conditions and depositional environment

Abstract

A study has been made of the rare earth element (REE) geochemistry of ichthyoliths from six oceanic regions (Peru/Chile Shelf, Tyrrhenian Sea, Namibian Shelf, Cook Islands, Pacific Ocean and the Cape Basin), including both oxic and anoxic sedimentary depositional environments. The REE contents of vertebrae and scales of modern fish are extremely low (e.g. 0.01–0.06 μg g −1 Nd), but REE are significantly enriched in ichthyoliths recovered from sediments (e.g. 0.78–1460 μg g −1 Nd) Ichthyoliths from anoxic shelf sediments have low REE contents and positive or weakly negative Ce anomalies. Ichthyoliths from oxic deep-sea sediments have high REE contents and strongly negative Ce anomalies. The REE patterns reflect the heavy REE enrichment of sea water, but do not record the inter-oceanic differences shown by sea water. The Ce anomalies of deep-sea ichthyoliths both resemble sea waters and generally reflect inter-oceanic differences (more negative Ce anomalies in the Pacific than in the Atlantic ocean). The REE uptake by ichthyoliths appears to occur during very early diagenesis. The REE patterns are useful indicators of the redox status of the sedimentary depositional environments, and, for deep-sea ichthyoliths, provide an approximate record of the redox condition of contemporaneous sea water.