Cerium anomalies

Vacuum evaporation of REE-bearing melts leads to residues with large negative cerium anomalies (Wang et al., 2001). Evaporation of REE-bearing CAI melts in the presence of hydrogen does not produce cerium anomalies, because conditions remain reducing (Davis et al.,... 

A few hibonite-rich CAIs have been found with significant isotopic mass fractionation effects and large negative cerium anomalies. Since both features result from evaporation in vacuum or under oxidizing conditions, it is likely that such CAIs are evaporation residues produced under these special conditions (Davis et al., 1982 Hinton et al., 1988 Ireland et al., 1992). We should note that this is different from the very... 

Wilde P., Quinby-Hunt M. S., and Erdtmann B.-D. (1996) The whole-rock cerium anomaly a potential indicator of eustatic sea-level changes in shales of the anoxic facies. Sedim.Geol. 101, 43-53. 

Pan Y. and Stauffer M. R. (2000) Cerium anomaly and Th/U fractionation in the 1.85 Ga Elin Flon Paleosol clues from REE- and U-rich accessory minerals and implications for paleoatmospheric reconstruction. Am. Mineral. 85,898 -911. 

Davranche. M. et al.. Adsorption of REE(III)-humate complexes onto MnOj Experimental evidence for cerium anomaly and lanthanide tetrad effect suppression, Geochim. Cosmochim. Acta. 69, 4825, 2005. 

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). 

Elderfield H. and Greaves M.J., 1981, Neptive cerium anomalies in the rare earth element patterns of oceanic ferromanganese nodules. Earth Planet. Sci. Lett., 55, 163-170. 

A direct origin from seawater for cerium in both Mn nodules and associated sediment is inferred by Elderfield et al. (1981) and by Elderfield and Greaves (1981) from the negative correlation between cerium anomalies and Mn/Fe ratios in the nodules. Inter-element relationships and leaching experiments point to the presence of lanthanides in two major phases, one Fe-rich and the other P-rich. The surfaces of these phases are thought to be the sites for the diagenetic reactions. 

Cerium anomalies develop under different types of conditions. The development of sub-oxic and anoxic conditions leads to the preferential release of Ce to estuarine waters. Chesapeake Bay is a classic example of this process (Sholkovitz et al. 1992). In regions of high biological productivity, there is biologically-mediated removal of dissolved cerium through an oxidative process. This has been observed in the Amazon estuary (Sholkovitz 1993). 

In summary, the lanthanides undergo large scale diagenetic reactions under sub-oxic and anoxic conditions. Pore water concentrations increase greatly over those of oxic seawater. Large cerium anomalies develop and large scale fractionation occurs as the strictly-trivalent lanthanides are added to and removed from pore waters. These features develop in vertical profiles within sediments and in surface sediments exposed to seasonally varying redox conditions. 

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