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| Titel |
Determination of trace element incorporation into tests of in vitro grown foraminifera by micro- SYXRF - a basis for the development of paleoproxies |
| VerfasserIn |
U. Kramar, D. Munsel, Z. Berner, J. Bijma, G. Nehrke |
| 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 |
250024174
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| Zusammenfassung |
| Trace element chemistry and isotopic composition of calcareous foraminiferal tests reflect the
environment in which they grow. Consequently, geochemical parameters of the tests are often
used as paleo-proxies to constrain the environmental conditions in ancient seas (e.g. Boyle,
1981).
Currently only a limited number of trace elements is used as proxies. Difficulties arise
from the fact that often a proxy depends on several parameters and that seawater-chemistry
may be influenced by local sources such as hydrothermalism or by changes in redox
conditions. Reliable experimental data on elements which can be considered as diagnostic
for hydrothermal activity and/or changing redox conditions are non-existent to
scarce.
We have cultured shallow benthic foraminifera (Ammonia tepida) under controlled
conditions at defined trace element levels (5, 10, 20-fold average seawater concentration)
representing two distinct environmental simulations, one for hydrothermal (Mn, Cu, Co, Ni)
and another for changing redox conditions (Mo, As, Cr, and V). The goal of our
investigations is to provide diagenetically unbiased experimental trace element data in
foraminiferal shells as a basis for a more complete understanding of trace element partition
between seawater and foraminifera shell calcite as a function of environmental
conditions.
The foraminifers did not reproduce in culture but grew new chambers as evidenced by
labelling with calcein (Bernhard et al., 2004). The trace element uptake into old (field grown)
and new chambers was subsequently analysed.
Using excitation energies of 12.5 keV and focusing by refractive lenses to a spot size of
2x5 μm at FLUO-beamline of ANKA and 25 keV and confocal poly capillary optics at
HASYLAB Beamline L, old chambers and new ones of the same foraminifera have been
analyzed separately for Ni, Cu, Zn, Mn, As, Cr, V, Sr and Mo. The data will be compared
with results from LA-ICP/MS analytics. Concentrations of the elements of interest in the
newly grown chambers are varying in the 0.x to 1xx μg/g range. Because of these low
concentrations long measuring times (~500 s) were necessary at each spot. The trace element
contents in the foraminiferal test were quantified by fundamental parameters. Path
length of the incoming beam was determined from the ratio of the primary and
secondary monitor. Since the foraminifera shells consist of nearly pure calcite, Ca
was used as internal standard to calculate the average path length of the outgoing
beam.
Arsenic, V and Ni concentrations in the test correlate with their concentration in
seawater. Nevertheless considerable variations, possibly reflecting vital effects, are
observed between newly grown chambers of individuals grown under identical culture
conditions.
Boyle, E.A. (1981): Cadmium, zinc, copper, and barium in foraminifera tests. Earth and
Planetary Science letters 53: 11-35.
Bernhard, J.M., Blanks, J.K. Hintz, C.J. & Chandler, G.T. (2004): Use of the fluorescent
calcite marker calcein to label foraminiferal tests. Journal of Foraminiferal Research 34:
96-101. |
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