Limits of calcium isotopes diagenesis in fossil bone and enamel

Diagenesis has been recognized for decades to significantly alter the trace elements biogenic signatures in fossil tooth enamel and bone that are routinely used for paleobiological and paleoenvironmental reconstructions. This signature is modified during diagenesis according to a complex continuum between two main processes, addition and substitution. For an additive-like, or early diagenesis, the trace elements biogenic profiles can be restored by leaching secondary minerals, but this technique is inefficient for a substitutive-like, or extensive diagenesis for which secondary trace elements are incorporated into the biogenic mineral. This scheme is however unclear for Ca, the major cation in tooth enamel and bone hydroxylapatite, whose stable isotope composition (δ^44/42Ca) also conveys biological and environmental information. We present a suite of leaching experiments for monitoring δ^44/42Ca values in artificial and natural fossil enamel and bone from different settings. The results show that enamel δ^44/42Ca values are insensitive to an additive-like diagenesis that involves the formation of secondary Ca-carbonate mineral phases, while bone shows a consistent offset toward ⁴⁴Ca-enriched values, that can be restored to the biogenic baseline by a leaching procedure. In the context of a substitutive-like diagenesis, bone exhibits constant δ^44/42Ca values, insensitive to leaching, and shows a REE pattern symptomatic of extensive diagenesis. Such a REE pattern can be observed in fossil enamel for which δ^44/42Ca values are still fluctuating and follow a trophic pattern. We conclude that Ca isotopes in fossil enamel are probably not prone to extensive diagenesis and argue that this immunity is due to the very low porosity of enamel that cannot accommodate enough secondary minerals to significantly modify the isotopic composition of the enamel Ca pool.

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Fig. 4. (A) δ^44/42Ca values of leached bone at Strud. Trophic levels are assessed based on ecomorphological features. (B) (La/Sm)n vs (La/Yb) n distribution of fossil samples analyzed in the study. Morocco and Niger samples are from Hassler et al. (2018). REE have been normalized to PAAS. (C) (ΣREE) n vs δ^44/42Ca distribution of fossil samples analyzed in the study. Morocco and Niger samples are from Hassler et al. (2018). REE have been normalized to PAAS. The δ^44/42Ca value is reported relative to ICP-Ca Lyon.

References

Dodat J.-P., Martin J. E., Olive S., Hassler A., Albalat E., Boisserie J.-R., Merceron G., Souron A., Maureille B., Balter V. 2023 – Limits of calcium isotopes diagenesis in fossil bone and enamel – Geochimica et Cosmochimica Acta 351 (2023) 45–50 – https://doi.org/10.1016/j.gca.2023.04.012