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Titel |
Manganese speciation in Diplodon chilensis patagonicus shells: a XANES study |
VerfasserIn |
A. L. Soldati, V. Vicente-Vilas, J. Goettlicher, D. E. Jacob |
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 |
250026019
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Zusammenfassung |
In addition to other types of climate archives, biogenic skeletons of a variety of different
organisms (i.e. shells of bivalves, skeletal hard parts of corals or sponges) are increasingly
used for high-resolution climate reconstructions. Bivalves are particularly suited for such
analyses because they are geographically broadly distributed and have been shown to
record climate and environmental information reliably and over long time intervals.
Variation of environmental parameters such as food supply, substratum type, salinity,
illumination, temperature, concentration of dissolved oxygen or oxygen/carbon
dioxide ratio, among others, may affect growth pattern, shell structure, mineralogy,
isotopic fractionation and chemistry. Thus, shell features, minor and trace element
composition patterns and isotopic signals may serve as an archive of environmental
history.
In turn, palaeoclimatic parameters such as ambient temperature, precipitation gradients,
seawater salinity and primary production can be reconstructed from the shells by means of
sclerochronological and geochemical methods. However, the distribution of minor and trace
elements in the biominerals is not only influenced by the environment or vital effects, but also
by intrinsic biomineralisation parameters like the carbonate polymorphism and the
mineral habit (Soldati et al., 2008a). Generally, it is assumed that the X2+ ions are
replacing the Ca2+ ion in the calcium carbonate (CaCO3) structure, but newest
findings show that amorphous (or disordered) phases may play a role in hosting
some of the elements use as proxies (Meibom et al., 2008; and Finch and Allison,
2007).
In this work we focused on the freshwater clam Diplodon chilensis patagonicus, a widely
distributed inhabitant of lakes and rivers in southern South America. Thanks to its long life
span and seasonal growth Diplodon mussels exhibit excellent characteristics to construct an
accurate chronological archive, with time windows of up to around a century, resolving the
environmental signal annually and even seasonally (Soldati et al., 2008b). High resolution
trace elemental analysis by LA-ICPMS and EPMA in the shells show that elements like Mg
and Mn are related to the seasonal pattern and can be enriched along the organic-rich
annual shell growth lines. Thus, these elements could possibly be bound organically
instead of occupying a defined site in the crystal lattice of the calcium carbonate
phase.
LA-ICP-MS results show that Mn concentrations in these Diplodon shells range
between 1000-300 μg/g and 100-10 μg/g and that the areas of enrichment are in the
micrometer range. Raman and XRD measurements at high spatial resolution failed
in recognizing whether the Mn is in carbonate solid solution or not. Therefore,
speciation techniques like X-ray absorption fine structure (XAFS) spectroscopy
with a high lateral resolution are required to address this question. Prior to XAFS
spectroscopy the samples were mapped with the intensity of the Mn Kα fluorescence
emission line in order to locate the Mn rich areas of interest. Because of the Mn
concentrations in the sub % range the XAFS spectra at the positions of interest
have been recorded in fluorescence mode using a 7 element Si(Li) detector. This
study focuses on the near edge (XANES: X-ray absorption near edge structure) part
of the spectra. For data evaluation, XANES spectra of reference substances were
additionally measured in order to get first hints to Mn valence and bonding. As standards
were used Mn and Mn rich carbonates, Mn oxides with Mn in different oxidation
states, and Mn in organic compounds (Mn-porphyrin and Mn-acetate). The XAFS
measurements have been carried out at the SUL-X beamline of the synchrotron
radiation source ANKA of the Forschungszentrum Karlsruhe. Data evaluation is
ongoing.
References
MEIBOM, A., CUIF, J.P., HOULBREQUE, F., MOSTEFAOUI, S., DAUPHIN, Y.,
MEIBOM; K.L. & DUNBAR, R. (2008). Compositional variations at ultra-structure length
scales in coral skeleton. Geochimica et Cosmochimica Acta 72: 1555-1569.
FINCH, A.A. & ALLISON, N. (2007). Coordination of Sr and Mg in calcite and
aragonite. Mineralogical Magazine 71: 539-552.
SOLDATI A.L., JACOB D.E., WEHRMEISTER, U. & HOFMEISTER, W. (2008a).
Structural characterization and chemical composition of aragonite and vaterite in freshwater
cultured pearls. Mineralogical Magazine 72: 577-590.
SOLDATI A.L., JACOB D., SCHÖNE B.R., BIANCHI M.M. & HAJDUK A. (2008b).
Seasonal periodicity of growth and composition in valves of Diplodon chilensis patagonicus
(D’Orbigny, 1835). Journal of Molluscan Studies, doi:10.1093/mollus/eyn044. |
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