TIMESCALES OF REE AND ELEMENT UPTAKE INTO FOSSIL BONES – IMPLICATIONS FOR PALEOENVIRONMENTAL AND TAPHONOMIC RECONSTRUCTIONS

The element and isotope compositions of fossil bones and teeth can yield important information about the paleodiet, paleobiology, and paleoecology of extinct vertebrates and their ambient climate and environment. However, for such reconstructions an understanding of the chemical and mineralogical changes of bones and teeth during fossilization processes is fundamental.

Fossil bones contain significant amounts of trace elements (e.g. REE, U, Fe and F), which they have incorporated from their burial environment. These may allow inferences about the degree of diagenetic alteration, the taphonomic setting and fluid compositions, age of deposition as well as fossil reworking. In a classic view trace element uptake is generally restricted to the early diagenesis of bones (~10⁴ to 10⁵ years), which is characterized by rapid loss of the organic protein matrix, recrystallization of the nanocrystalline bioapatite and formation of secondary minerals. However, little is know about the late diagenetic alteration of trace elements in fossil bones after recrystallisation.

We measured LA-ICPMS and EMPA trace element profiles across Mesozoic to Cenozoic fossil bones from different diagenetic settings to assess chemical changes during early and late diagenesis. Both intra-bone REE concentration profiles and radiometric bone Lu-Hf dating (yielding younger Lu-Hf than chronostratigraphic ages) provides clear evidence that fossil bones and teeth behave as open systems with respect to REE and Hf over geological timescales, thus millions of years, rather than thousands of years, as previously suggested. The same may also apply to other trace elements.

Furthermore, significant intra-bone REE fractionation occurs during REE uptake into fossil bones. Rare earth element ratios (e.g. La_N/Yb_N, Nd_N/Yb_N and Y/Ho) can be fractionated within one bone specimen across the cortex over two orders of magnitude. This may also lead to the formation of an apparent Ce anomaly unrelated to the ambient redox conditions of the diagenetic fluid, limiting its use as a redox proxy.

Studies using REE patterns and trace element ratios of fossil biogenic apatite to constrain provenance, taphonomy, paleoenvironment or paleoceanography have thus to account for late diagenetic REE uptake as well as REE fractionation processes within skeletal remains.