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


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Nd and Sr isotope compositions in modern and fossil bones - Proxies for vertebrate provenance and taphonomy
Geochimica et Cosmochimica Acta
Tuetken T., Vennemann T.W., Pfretzschner H.U.
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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 (143)Nd/(144)Nd 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 (143)Nd/(144)Nd 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, epsilon(Nd)
values are often similar within one epsilon(Nd) unit within a single
bone. Larger intra-bone differences in specimens may either reflect a
partial preservation of in vivo values or changing epsilon(Nd) values of
the diagenetic fluid during fossilization. However, most fossil
specimens and the outer rims of bones will record taphonomic
(143)Nd/(144)Nd incorporated post mortem during diagenesis. Unlike REE
patterns, (143)Nd/(144)Nd are not biased by fractionation processes
during REE-uptake into the apatite crystal lattice, hence the
epsilon(Nd) value is an important tracer for taphonomy and reworking.
Bones and teeth from autochthonous fossil assemblages have small
variations of +/- 1 epsilon(Nd) unit only. In contrast, fossil bones and
teeth from over 20 different marine and terrestrial fossil sites have a
total range of epsilon(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 epsilon(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 epsilon(Nd) values.
The latter often applies to fossil shark teeth, which may preserve
paleo-seawater values. Complementary to epsilon(Nd) values,
(87)Sr/(86)Sr can help to further constrain the fossil provenance and
reworking. (C) 2011 Elsevier Ltd. All rights reserved.
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29/09/2012 16:22
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20/08/2019 15:43
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