Our study emphasises the complexity of understanding REE (Rare Earth Element) variations in fossil biogenic apatite (such as from conodonts and fish). To relate these variations to ancient seas and depositional environments, it is crucial first to understand how these factors interact in modern ocean systems.

When biogenic apatite forms within living organisms, it acts as a minor site where major and trace elements from the ocean are incorporated. However, once the organism dies, its biogenic apatite undergoes significant chemical changes. These changes are driven by processes like:

• Decomposition of the surrounding organic material,
• Sedimentation and early diagenesis, where the apatite becomes part of the sediment,
• Deep burial and later diagenesis, which subject the material to further chemical alterations,
• Lithification, which transforms the sediment into rock,
• Uplift and fracturing, where geological processes expose or alter the fossil-containing rocks.

__________________________________________________________________________________________________________

Several lines of evidence suggest that the most important reactions occur during primary sedimentation and early diagenesis of biogenic apatite, essentially locking in the trace element characteristics of oceanic bottom waters.

Recent successes in radiometric age dating of conodonts by two separate methods, fission tracks (Sachs et al., 1980) and U-Pb (Kovach and Zartman, 1981), support the hypothesis that early diagenetic processes closely related to seawater composition are the last major influence on trace element contents of conodonts. Furthermore:

1. 87Sr/86Sr in conodonts (Kovach, 1981) fits the worldwide age curve already determined for calcite of fossil shells (Peterman et al., 1970) and for whole rock limestone (Burke et al., 1982; Veizer and Compston, 1974).
2. Sr contents from a widespread geographic distribution show a consistent secular decrease during the Paleozoic (Kovach, 1980).

Figure 4 illustrates the relationship between REE in Pacific (Goldberg et al., 1963) and Atlantic (Elderfield and Greaves, 1982) seawater, biogenic apatite from living fish (this study), and fish debris in sedimentary horizons from 4 to 600 cm on the Pacific abyssal plain (Bernat, 1975). The few analyses available for REE in biogenic apatite in modern living fish (Table 1, sample JWC 20-6, Fig. 4; Table 2, sample Ph-02) indicate that REE is significantly enriched during the primary biological process.

_____________________________________________________________________________________________________________

Key Points on Cerium Anomaly and REE Behavior

1. Cerium Behavior in Oceans and Sediments:
   • Modern Ocean: Cerium (Ce) is preferentially removed in oxidizing conditions by hydrous iron oxides (e.g., ferromanganese nodules), up to 10 times more than other REEs. This removal creates a negative Ce anomaly in seawater and biogenic apatite under oxic conditions.
   • Anoxic Conditions: In anoxic environments, iron oxyhydroxide flocs dissolve, releasing Ce into deeper water masses. This results in normal or non-depleted Ce concentrations, indicating the absence of a negative anomaly in sediments and fossils.
2. Biogenic Apatite and REE Adsorption:
   • Biogenic apatite, during early diagenesis, concentrates REEs and reproduces the Ce anomaly of the overlying water mass. This occurs without significant fractionation, meaning the REE pattern of the water is preserved in the apatite.
   • The REE patterns in Pacific fish debris closely resemble those of Pacific bottom water, supporting this preservation hypothesis.
3. Anoxic Events in the Paleozoic:
   • Palaeozoic anoxic events, such as those during the Lower Ordovician, are linked to REE patterns without a negative Ce anomaly in conodonts. These patterns reflect widespread anoxia in water masses.
   • Episodic anoxia and black shale deposition in the Devonian also indicate fluctuating redox conditions, with conodonts from epicratonic seas showing variations in Ce due to changes in water circulation and freshwater/marine input.

_______________________________________________________________________________________________________________

Reference paper

Wright, J., Seymour, R. S., & Shaw, H. F. (1984). REE and Nd isotopes in conodont apatite: variations with geological age and depositional environment.