Paleoceanography of Conodonts
Published:January 01, 1984
Modifications of standard instrumental neutron activation analysis procedures have produced quantitative results for REE, Th, and U contents of the biogenically precipitated apatite of conodonts and ichthyoliths. These results are in agreement with isotopic studies of four of the samples. Apatite of modern fish debris apparently concentrates REE during early diagenesis in the same proportions as seawater, and therefore carries a signature of the oceanic REE pattern, suggesting the same relationship between fossil apatite and ancient seas. REE patterns of conodont samples from Cambrian through Triassic times show significant secular variations. Further, these data show that REE vary among depositional settings; e.g., fauna from continental shelf biofacies vs. epicratonic seas, and are consistent among fauna from the same depositional framework.
In the modern oceanic environment, cerium is preferentially scavenged by hydrous iron oxides resulting in a negative Ce anomaly in seawater REE patterns. This Ce-Fe affinity leads to an analysis of changing oxidation-reduction conditions in ancient seas that explains the variations in Ceanom of conodonts. During times of widespread anoxia in the water column, dissolution of Fe oxides would release Ce to produce an enrichment of Ce in the water column, effectively removing the negative Ce anomaly, and at times even causing a positive Ce anomaly. These anoxic events are recorded by a small or absent Ceanom of conodonts during the Lower Paleozoic and again in the Triassic. Variations in Ceanom appear to be related to Th/U variations, another potential indicator of redox changes. Fossil apatite of conodonts records Nd and Sr isotopic variations in ancient seawater.