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| Titel |
Heterogeneity of Cr in Mytilus edulis: Implications for the Cr isotope system as a paleo-redox proxy |
| VerfasserIn |
Sylvie Bruggmann, Robert Klaebe, Robert Frei |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250145185
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| Publikation (Nr.) |
 EGU/EGU2017-9099.pdf |
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| Zusammenfassung |
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Changes in $^{53}$Cr/$^{52}$Cr ($\delta$$^{53}$Cr) values recorded by biogenic carbonates are emerging as a proxy for variations in the redox state of the Earth's oceans and atmosphere (e.g. [1], [2]). We investigate the ability of modern carbonate shells (\textit{Mytilus edulis}) to record the $\delta$$^{53}$Cr composition of ambient seawater in order to assess their utility as a paleo-redox proxy. Samples of cultivated \textit{M. edulis} from the Kiel Fjord, Germany, were analyzed for their $\delta$$^{53}$Cr composition and Cr concentrations [Cr] using TIMS. To disentangle the pathway of Cr into the carbonate shell, a series of step-digestions of their organic outer sheaths (periostraca) and their intra-layer composition were performed.
Bulk analyses of specimens with intact periostraca returned 16 to 34 ppb Cr with $\delta$$^{53}$Cr values ranging from 0.28 to 0.65 $\pm$ 0.1 (2SE) and thus fall within the range of surface seawater from the Baltic Sea (0.3 - 0.6 \permil \thinspace[3]). Partial removal of periostraca resulted in lower [Cr] (5 to 17 ppb) and $\delta$$^{53}$Cr values (-0.05 $\pm$ 0.15 \permil). These results show a positive correlation between the amount of organic matter present in a sample and both [Cr] and $\delta$$^{53}$Cr (n = 9). With nearly complete removal of periostraca, the remaining [Cr] is significantly lower (less than 5 ppb) and can only be accessed by incineration of the carbonate shell.
The correlation between [Cr], $\delta$$^{53}$Cr and the amount of periostracum present in bulk samples indicates that a significant proportion of preserved Cr may be associated with the organic outer sheath. The Cr endmember accessed after incineration is less likely associated with the carbonate crystal lattice. Instead, the $\delta$$^{53}$Cr values obtained after incineration are similar to those reported from terrestrial rocks, suggesting the influence of detrital particles. Alternatively, Cr may be reduced and subsequently re-oxidized during the mineralization of biogenic carbonates [4]. Seasonal changes in primary productivity in seawater may further influence the shell's Cr isotopic composition [3].
A detailed understanding of the processes causing the observed heterogeneity of [Cr] and $\delta$$^{53}$Cr in modern carbonate shells is crucial for the use of the Cr isotope system as a potential paleo-redox proxy.\\
[1] Bonnand, P., James, R. H., Parkinson, I. J., Connelly, D. P., and Fairchild, I. J. (2013). The chromium isotopic composition of seawater and marine carbonates. Earth and Planetary Science Letters, 382:10-20.\\
[2] Holmden, C., Jacobsen, A. D., Sageman, B. B., Hurgten, M. T. (2016). Response of the Cr isotope proxy to Cretaceous Ocean Anoxic Event 2 in a pelagic carbonate succession from the Western Interior Seaway. Geochimica et Cosmochimica Acta, 186:227-295.\\
[3] Paulukat, C., Gilleaudeau, G. J., Chernyavskiy, P., Frei, R. (2016). The Cr-isotope signature of surface seawater -A global perspective. Chemical Geology, 444:101-109.\\
[4] Pereira, N. S., Voegelin, A. R., Paulukat, C., Sial, A. N., Ferreira, V. P., and Frei, R. (2015). Chromium-isotope signatures in scleractinian corals from the Rocas Atoll, Tropical South Atlantic. Geobiology 1-14.
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