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| Titel |
Calcium and Oxygen Isotopic Composition of Calcium Carbonates |
| VerfasserIn |
Andrea Niedermayr, Anton Eisenhauer, Florian Böhm, Basak Kisakürek, Isabelle Balzer, Adrian Immenhauser, Stephan Jurgen Kohler, Martin Dietzel |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250136276
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| Publikation (Nr.) |
 EGU/EGU2016-17276.pdf |
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| Zusammenfassung |
| Different isotopic systems are influenced in multiple ways corresponding to the crystal
structure, dehydration, deprotonation, adsorption, desorption, isotope exchange and diffusion
processes.
In this study we investigated the structural and kinetic effects on fractionation of stable
Ca- and O-isotopes during CaCO3 precipitation. Calcite, aragonite and vaterite were
precipitated using the CO2 diffusion technique[1]at a constant pH of 8.3, but various
temperatures (6, 10, 25 and 40˚ C) and precipitation rates R (101.5 to 105 μmol h−1
m−2).
The calcium isotopic fractionation between solution and vaterite is lower (Δ44∕40Ca=
-0.10 to -0.55 ‰ ) compared to calcite (-0.69 to -2.04 ‰ ) and aragonite (-0.91 to -1.55
‰ ). In contrast the fractionation of oxygen isotopes is highest for vaterite (32.1 ‰ ),
followed by aragonite (29.2 ‰ ) and calcite (27.6 ‰ ) at 25˚ C and equilibrium. The
enrichment of 18O vs. 16O in all polymorphs decreases with increasing precipitation rate by
around -0.7 ‰ per log(R). The calcium isotopic fractionation between calcite/ vaterite and
aqueous Ca2+ increases with increasing precipitation rate by ∼0.45 ‰ per log(R) and ∼0.1
‰ per log(R) at 25˚ C and 40˚ C, respectively. In contrast the fractionation of
Ca-isotopes between aragonite and aqueous Ca2+ decreases with increasing precipitation
rates.
The large enrichment of 18O vs. 16O isotopes in carbonates is related to the strong
bond of oxygen to the small and highly charged C4+-ion. In contrast equilibrium
isotopic fractionation between solution and calcite or vaterite is nearly zero as the
Ca–O bond length is similar for calcite, vaterite and the hydrated Ca. Aragonite
incorporates preferentially the lighter 40Ca isotope as it has very large Ca-O bonds in
comparison to the hydrated Ca. At the crystal surface the lighter 40Ca isotopes are
preferentially incorporated as dehydration and diffusion of lighter isotopes are faster.
Consequently, the surface becomes enriched in 40Ca. On the other hand, 40Ca may desorb
more easily, especially if the bond strength is lower as in the case of aragonite. For
kinetic oxygen isotopic fractionation, the faster deprotonation of HC16O3− and
the faster incorporation of C16O32− at the surfaces causes a smaller enrichment
of 18O in all three polymorphs, which will be preserved at higher precipitation
rates.
In consequence to the different behavior of calcium and oxygen isotopes, they can be
useful for multiproxy applications. Thereby calcium isotopes can be used to identify kinetic
effects, especially if both aragonite and calcite, can be analyzed in one sample. Oxygen
isotopes are more strongly related to temperature.
[1]A. Niedermayr, S.J. Köhler and M. Dietzel (2013), Chemical Geology, 340, 105-120. |
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