Early diagenesis of bone and tooth apatite in fluvial and marine settings: Constraints from combined oxygen isotope, nitrogen and REE analysis


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Early diagenesis of bone and tooth apatite in fluvial and marine settings: Constraints from combined oxygen isotope, nitrogen and REE analysis
Palaeogeography, Palaeoclimatology, Palaeoecology
Tuetken T., Vennemann T.W., Pfretzschner H.U.
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Fossil bones and teeth of Late Pleistocene terrestrial mammals from
Rhine River gravels (RS) and the North Sea (NS), that have been exposed
to chemically and isotopically distinct diagenetic fluids (fresh water
versus seawater), were investigated to study the effects of early
diagenesis on biogenic apatite. Changes in phosphate oxygen isotopic
composition (delta O-18(PO4)), nitrogen content (wt.% N) and rare earth
element (REE) concentrations were measured along profiles within bones
that have not been completely fossilized, and in skeletal tissues (bone,
dentine, enamel) with different susceptibilities to diagenetic
alteration. Early diagenetic changes of elemental and isotopic
compositions of apatite in fossil bone are related to the loss of the
stabilizing collagen matrix. The REE concentration is negatively
correlated with the nitrogen content, and therefore the amount of
collagen provides a sensitive proxy for early diagenetic alteration. REE
patterns of RS and NS bones indicate initial fossilization in a fresh
water fluid with similar REE compositions. Bones from both settings have
nearly collagen-free, REE-, U-, F- and Sr-enriched altered outer rims,
while the collagen-bearing bone compacta in the central part often
display early diagenetic pyrite void-fillings. However, NS bones exposed
to Holocene seawater have outer rim delta O-18(PO4) values that are 1.1
to 2.6 parts per thousand higher compared to the central part of the
same bones (delta O-18(PO4) = 18.2 +/- 0.9 parts per thousand, n = 19).
Surprisingly, even the collagen-rich bone compacta with low REE contents
and apatite crystallinity seems altered, as NS tooth enamel (delta
O-18(PO4) =15.0 +/- 0.3 parts per thousand, n=4) has about 3%. lower
delta O-18(PO4) values, values that are also similar to those of enamel
from RS teeth. Therefore, REE concentration, N content and apatite
crystallinity are in this case only poor proxies for the alteration of
delta O-18(PO4) values. Seawater exposure of a few years up to 8 kyr can
change the delta O-18(PO4) values of the bone apatite by > 3 parts per
thousand. Therefore, bones fossilized in marine settings must be treated
with caution for palaeoclimatic reconstructions. However, enamel seems
to preserve pristine delta O-18(PO4) values on this time scale. Using
species-specific calibrations for modern mammals, a mean delta O-18(H2O)
value can be reconstructed for Late Pleistocene mammalian drinking water
of around -9.2 +/- 0.5 parts per thousand, which is similar to that of
Late Pleistocene groundwater from central Europe. (c) 2008 Elsevier B.V.
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29/09/2012 16:22
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20/08/2019 15:48
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