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| Titel |
Determination of Cr isotopic composition in low-level carbonates by MC-ICP-MS: a sensitive proxy for redox changes? |
| VerfasserIn |
Pierre Bonnand, Ian Parkinson, Rachael James, Anne-Mari Karjalainen, Manuela Fehr, Ian Fairchild |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250032234
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| Zusammenfassung |
| Geochemical data suggest that atmospheric oxygen increased during two major steps: the
Great oxidation event (~2.4 Ga) and the Neoproterozoic (~1Ga-545Ma). The O2
concentration in the atmosphere is strongly linked to the redox condition of the oceans.
Therefore the study of redox sensitive elements in marine sediments can be used to evaluate
the evolution of O2 concentrations in the atmosphere. Chromium is a redox sensitive element
which significantly fractionates its isotopes during the reduction of Cr(VI) to Cr(III) (Ellis et
al., 2002). Thus, Cr isotopes can be used to assess redox changes in the past oceans.
Chromium isotopic compositions in sedimentary rocks (BIFs) have been used to determine
the evolution of the O2 concentration in the atmosphere during the Proterozoic (Frei et
al., 2009). We have developed a chemical procedure for the purification of Cr in
carbonates by using a single cation column to separate the Cr from the matrix, Fe, Ti
and V. Cr isotopic compositions are determined used a 50Cr-54Cr double spike
method and analysed on a ThermoFisher Neptune MC-ICP-MS using HR and MR
in order to be able to discriminate Ar interferences. Standards and samples are
analysed as 50ppb Cr solutions and yield an external reproducibility 50 and 70ppm.
This new method allowed us to analyse samples with a Cr concentrations as low as
1ppm.
We have analysed a suite of Neoproterozoic carbonates from Australia, but also modern ooids
and oolithic limestones through the Phanerozoic. The Cr isotopic data for carbonates record a
range of δ53Cr between -0.1 and +1.7. This range indicates that some of these carbonates
clearly reflect oxidising conditions in the ocean. By comparison, the Neoproterozoic
samples have Cr isotopic compositions close to the continental crust value (-0.1 to
0.1), indicating the Neoproterozoic samples reflect deposition under more reducing
conditions
These data suggests that the redox condition during the deposition of shallow-water
carbonates reflects the oxygen concentration in the atmosphere and therefore Cr
isotopes are potentially a sensitive proxy for understanding the evolution of the
atmosphere.
Ellis, A.S., Johnson, T.M., and Bullen, T.D., 2002, Chromium isotopes and the fate of
hexavalent chromium in the environment: Science, v. 295, p. 2060-2062.
Frei, R., Gaucher, C., Poulton, S.W., and Canfield, D.E., 2009, Fluctuations in Precambrian
atmospheric oxygenation recorded by chromium isotopes: Nature, v. 461, p. 250-U125. |
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