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| Titel |
Chemical composition of modern and fossil Hippopotamid teeth and implications for paleoenvironmental reconstructions and enamel formation – Part 1: Major and minor element variation |
| VerfasserIn |
G. Brügmann, J. Krause, T. C. Brachert, O. Kullmer, F. Schrenk, I. Ssemmanda, D. F. Mertz |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1726-4170
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| Digitales Dokument |
URL |
| Erschienen |
In: Biogeosciences ; 9, no. 1 ; Nr. 9, no. 1 (2012-01-06), S.119-139 |
| Datensatznummer |
250006655
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| Publikation (Nr.) |
 copernicus.org/bg-9-119-2012.pdf |
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| Zusammenfassung |
| Bioapatite in mammalian teeth is readily preserved in continental sediments
and represents a very important archive for reconstructions of environment
and climate evolution. This project provides a comprehensive data base of
major, minor and trace element and isotope tracers for tooth apatite using a
variety of microanalytical techniques. The aim is to identify specific
sedimentary environments and to improve our understanding on the interaction
between internal metabolic processes during tooth formation and external
nutritional control and secondary alteration effects. Here, we use the
electron microprobe to determine the major and minor element contents of
fossil and modern molar enamel, cement and dentin from Hippopotamids. Most
of the studied specimens are from different ecosystems in Eastern Africa,
representing modern and fossil lacustrine (Lake Kikorongo, Lake Albert, and
Lake Malawi) and modern fluvial environments of the Nile River system.
Secondary alteration effects - in particular FeO, MnO, SO3 and F
concentrations – are 2 to 10 times higher in fossil than in modern enamel;
the secondary enrichment of these components in fossil dentin and cement is
even higher. In modern and fossil enamel, along sections perpendicular to
the enamel-dentin junction (EDJ) or along cervix-apex profiles,
P2O5 and CaO contents and the CaO/P2O5 ratios are very
constant (StdDev ∼1%). Linear regression analysis reveals tight
control of the MgO (R2∼0.6), Na2O and Cl variation (for both
R2>0.84) along EDJ-outer enamel rim profiles, despite large
concentration variations (40% to 300%) across the enamel. These minor
elements show well defined distribution patterns in enamel, similar in all
specimens regardless of their age and origin, as the concentration of MgO
and Na2O decrease from the enamel-dentin junction (EDJ) towards the
outer rim, whereas Cl displays the opposite trend.
Fossil enamel from Hippopotamids which lived in the saline Lake Kikorongo
have a much higher MgO/Na2O ratio (∼1.11) than those from the
Neogene fossils of Lake Albert (MgO/Na2O∼0.4), which was a large
fresh water lake like those in the western Branch of the East African Rift
System today. Similarly, the MgO/Na2O ratio in modern enamel from the
White Nile River (∼0.36), which has a Precambrian catchment of
dominantly granites and gneisses and passes through several saline zones, is
higher than that from the Blue Nile River, whose catchment is the Neogene
volcanic Ethiopian Highland (MgO/Na2O∼0.22). Thus, particularly
MgO/Na2O might be a sensitive fingerprint for environments where river
and lake water have suffered strong evaporation.
Enamel formation in mammals takes place at successive mineralization fronts
within a confined chamber where ion and molecule transport is controlled by
the surrounding enamel organ. During the secretion and maturation phases the
epithelium generates different fluid composition, which in principle, should
determine the final composition of enamel apatite. This is supported by
co-linear relationships between MgO, Cl and Na2O which can be
interpreted as binary mixing lines. However, if maturation starts after
secretion is completed, the observed element distribution can only be
explained by equilibration of existing and addition of new apatite during
maturation. It appears the initial enamel crystallites precipitating during
secretion and the newly formed bioapatite crystals during maturation
equilibrate with a continuously evolving fluid. During crystallization of
bioapatite the enamel fluid becomes continuously depleted in MgO and
Na2O, but enriched in Cl which results in the formation of MgO, and
Na2O-rich, but Cl-poor bioapatite near the EDJ and MgO- and
Na2O-poor, but Cl-rich bioapatite at the outer enamel rim.
The linkage between lake and river water compositions, bioavailability of
elements for plants, animal nutrition and tooth formation is complex and
multifaceted. The quality and limits of the MgO/Na2O and other proxies
have to be established with systematic investigations relating chemical
distribution patterns to sedimentary environment and to growth structures
developing as secretion and maturation proceed during tooth formation. |
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