NEODYMIUM ISOTOPIC COMPOSITION OF FOSSIL BONES–A DIAGENETIC TRACER

In vivo concentrations of REE in recent bone and tooth phosphate are at the ppb level. In contrast, REE are enriched postmortem by up to 5 orders of magnitude in fossil bones and teeth. Therefore, Neodymium (Nd) is a sensitive tracer for diagenetic alteration of biogenic phosphate and its isotopic composition reflects that of the early diagenetic fluid (sea, ground or pore water). Fossil bones, teeth and surrounding sediment from different marine and terrestrial diagenetic settings were analyzed with TIMS for their Nd isotopic composition (e_Nd value).

The early diagenetic incorporation of Nd in fossil bones correlates with the decay of the organic collagen phase. Zoned Late Pleistocene mammal bones with a collagen-free rim and a collagen-bearing center are enriched in the rim by up to 3 orders of magnitude compared to the bone center, which still has Nd concentrations in the ppb level. Despite the large Nd concentration gradients the Nd isotopic composition in these bone profiles are the same.

In general, inter-bone variations of Nd isotopic compositions within each of the investigated Jurassic to Pleistocene fossil sites are in a narrow range of less than ±1 e_Nd units, while all sites cover a total range of e_Nd values from +0.4 to -13.2. Often e_Nd values of fossil bone phosphate and surrounding sediment are similar and may indicate fossilization in the presence of a sediment influenced diagenetic pore fluid and thus local or even in situ fossilization. In contrast, different e_Nd values of bone-sediment pairs, such as for Pleistocene terrestrial mammal bones from North Sea deposits, indicate reworking and/or a diagenetic fluid with e_Nd values different from the embedding sediment. Thus, Nd isotopic composition of fossil bones and teeth is a valuable taphonomic tracer for bone provenance, diagenesis, embedding history, and diagenetic fluid composition.