Nd and Sr isotope compositions in modern and fossil bones – Proxies for vertebrate provenance and taphonomy

Abstract

Rare earth elements (REE), while not essential for the physiologic functions of animals, are ingested and incorporated in ppb concentrations in bones and teeth. Nd isotope compositions of modern bones of animals from isotopically distinct habitats demonstrate that the 143Nd/ 144Nd of the apatite can be used as a fingerprint for bedrock geology or ambient water mass. This potentially allows the provenance and migration of extant vertebrates to be traced, similar to the use of Sr isotopes. Although REE may be enriched by up to 5 orders of magnitude during diagenesis and recrystallization of bone apatite, in vivo 143Nd/ 144Nd may be preserved in the inner cortex of fossil bones or enamel. However, tracking the provenance of ancient or extinct vertebrates is possible only for well-preserved archeological and paleontological skeletal remains with in vivo-like Nd contents at the ppb-level. Intra-bone and -tooth REE analysis can be used to screen for appropriate areas. Large intra-bone Nd concentration gradients of 10 1–10 3 are often measured. Nd concentrations in the inner bone cortex increase over timescales of millions of years, while bone rims may be enriched over millenial timescales. Nevertheless, ε Nd values are often similar within one ε Nd unit within a single bone. Larger intra-bone differences in specimens may either reflect a partial preservation of in vivo values or changing ε Nd values of the diagenetic fluid during fossilization. However, most fossil specimens and the outer rims of bones will record taphonomic 143Nd/ 144Nd incorporated post mortem during diagenesis. Unlike REE patterns, 143Nd/ 144Nd are not biased by fractionation processes during REE-uptake into the apatite crystal lattice, hence the ε Nd value is an important tracer for taphonomy and reworking. Bones and teeth from autochthonous fossil assemblages have small variations of ±1 ε Nd unit only. In contrast, fossil bones and teeth from over 20 different marine and terrestrial fossil sites have a total range of ε Nd values from –13.0 to 4.9 ( n = 80), often matching the composition of the embedding sediment. This implies that the surrounding sediment is the source of Nd in the fossil bones and that the specimens of this study seem not to have been reworked. Differences in ε Nd values between skeletal remains and embedding sediment may either indicate reworking of fossils and/or a REE-uptake from a diagenetic fluid with non-sediment derived ε Nd values. The latter often applies to fossil shark teeth, which may preserve paleo-seawater values. Complementary to ε Nd values, 87Sr/ 86Sr can help to further constrain the fossil provenance and reworking.