Evaluation of the application of rare-earth elements to paleoceanography

Anomalous departures of rare-earth elements (REE) from similarity of chemical behaviour are most evident in a comparison of Ce with its neighbors. Under reducing conditions Ce3+ acts like other REE, but oxidized Ce4+ is adsorbed on Mn-, Fe-oxyhydroxides, or organic particulates, and consequently the remaining solution will tend to exhibit a negative Ce anomaly. In addition, more subtle fractionations may emphasize the light, or the heavy, or the middle REE. A pattern of REE may be recorded by immediate diagenesis on a suitable chemical substrate, such as apatite (fish debris, conodonts), carbonate or chert. This record was originally thought to reflect the paleoredox conditions of primary deposition. However, any such primary record may be overprinted by late diagenetic alteration. Early analyses of REE in conodonts from a wide range of sediments suggested a prevalence of reducing conditions (no Ce anomaly) in pre-Carboniferous oceans, and more common oxidation (negative Ce anomaly) in the late Paleozoic. However, recent research by other investigators indicates that variations in Cean are more diverse in origin, and in particular many are distorted to an unknown extent by late diagenesis. In any case it is now clear that geological age does not show a consistent relationship to the level of the Ce anomaly. The Ce anomaly apparently does not play any regular role in the isotopic events that accompany prominent mass extinctions. Further research is required to understand, and thus make useful, the controls on the Ce anomaly.