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Titel |
Nanostructural and biogeochemical features of the crinoid stereom |
VerfasserIn |
P. Gorzelak, J. Stolarski, M. Mazur, Y. Marrocchi, A. Meibom, E. Chalmin |
Konferenz |
EGU General Assembly 2009
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Medientyp |
Artikel
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Sprache |
Englisch
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Digitales Dokument |
PDF |
Erschienen |
In: GRA - Volume 11 (2009) |
Datensatznummer |
250019787
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Zusammenfassung |
Representatives of all echinoderm clades (e.g., echinoids, holothuroids, ophiuroids, asteroids,
and crinoids) form elaborate calcitic (polymorph of calcium carbonate) skeletons composed
of numerous plates. Each plate consists of a three-dimensional meshwork of mineral
trabeculae (stereom) that results from precisely orchestrated biomineralization processes.
Individual skeletal plates behave as single calcite crystals as shown by X-ray diffraction and
polarizing microscopy, however, their physico-chemical properties differ significantly from
the properties of geologic or synthetic calcites. For example, echinoderm bio-calcite does not
show cleavage planes typical of calcite but reveals conchoidal fracture surfaces that reduce
the brittleness of the material. The unique properties of echinoderm bio-calcite result from
intimate involvement of organic molecules in the biomineralization process and their
incorporation into the crystal structure. Remnants of echinoderm skeleton are among the
most frequently found fossils in the Mesozoic and Palaeozoic rocks thus, in order
to use them as environmental proxies, it is necessary to understand the degree of
biological ("vital effect") and inorganic control over their formation. Here, we show first
nanoscale structural and biogeochemical properties of the stereom of extant and fossil
crinoids. Using FESEM and AFM imaging techniques we show that the skeleton
has nanocomposite structure: individual grains have ca. 100 nm in diameter and
occasionally form larger aggregates. Fine scale geobiochemical mappings of crinoid plates
(NanoSIMS microprobe) show that Mg is distributed heterogeneously in the stereom with
higher concentration in the middle part of the trabecular bars. Although organic
components constitute only ca. 0.10-0.26 wt% of modern echinoderm bio-calcite, in situ
synchrotron sulphur K-edge x-ray absorption near edge structure (XANES) spectra show
that the central parts of stereom bars contain higher levels of SO4 that in various
carbonate biominerals are associated with sulphated polysaccharides. These data
are consistent with modern models of biomineralization suggesting involvement
of magnesium ions and sulphated polysaccharides in early phases of biomineral
formation.
Financial support: Polish Ministry of Science and Higher Education, projects
N307-015733, 155/ESR/2006/03(EC-370), BW-120000-501/68-179210, and Museum
National d’Histoire Naturelle and the Agence National de la Recherche. |
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