Paleoenvironmental and geological applications

Fossil bone chemistry as a paleoredox indicator. Uniquely among the REE, cerium has the capability to adopt a tetravalent ion under the appropriate oxidizing conditions. The oxidized form of Ce (Ce ) is relatively insoluble compared to Ce (de Baar et al. 1985), so that cerium takes part in active redox cycling. Under oxic conditions, Ce exists in the tetravalent state, and is readily removed from solution either onto particle surface coatings, or into authigenic minerals (Sholkovitz et al. 1993, Koeppenkastrop and 

DeCarlo 1992). Under reducing conditions, Ce may be released back into the water column or into pore waters. The anomalous behavior of Ce compared to its neighboring REEs (the cerium anomaly) is quantified by the ratio of the measured abundance of Ce to an expected value (Ce ) interpolated from the neighboring trivalent REEs (after de Baar etal. 1985)  

Ce/Ce values that are significantly greater or less than 1 imply the presence of Ce and therefore oxic conditions. Similarly, the lack of a cerium anomaly in concentrations of dissolved REE implies suboxic or anoxic conditions. 

Wright et al. (1984) determined REE composition of hundreds of individual conodonts and ichthyoliths, and found consistent REE shapes in pre-Carboniferous conodonts, with enrichment of MREE, lesser enrichment of TREE, and no cerium anomaly. This pattern is fundamentally different to the REE pattern found in modem ocean bottom waters, and led Wright et al. (1984) to suggest that the REE chemistry and particularly Ce anomaly in ancient marine apatites could be used to infer widespread anoxia in pre-Carboniferous ocean basins. 

In summary, preservation of a seawater redox signal is favored by rapid mineralization and stabilization at the sediment/water interface and uptake of REE from pore waters (rather than from seawater) is likely to reduce or eliminate any inherited cerium anomaly. Negative cerium anomalies in ancient marine biogenic apatite therefore suggest oxic conditions in the water column and possibly in the upper pore waters, but the lack of a negative cerium anomaly in biogenic apatite does not necessarily indicate sub-oxic or anoxic conditions in the water column (Kemp and Trueman in press). 

---