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| Titel |
Exploring the impact of temperature and fluid chemistry on Ca and Sr isotope
composition of Arctica islandica shells via experimental alteration |
| VerfasserIn |
Manuela Goos, Anton Eisenhauer, Volker Liebetrau, Florian Böhm, Jan Fietzke, Gernot Nehrke, Dieter Buhl, Vasileios Mavromatis, 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 |
250132382
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| Publikation (Nr.) |
 EGU/EGU2016-12887.pdf |
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| Zusammenfassung |
| Carbonate shells of marine organisms record by their chemical and isotopic
composition environmental conditions like temperature, salinity and pH
during their formation. However, primary signals can be overprinted by
diagenesis, which has to be considered for the interpretation of
environmental proxy data and on the other side may provide additional
information about distinct reaction pathways throughout diagenesis (e.g.
Allison et al, 2007).
Aragonitic\textit{ Arctica islandica} shells widely used as proxy archives (e.g. Sch\"{o}ne et al,
2005) were chosen for hydrothermal alteration experiments in order to get a
more detailed insight into diagenetic effects.
Here we present results of experiments in which \textit{Arctica islandica} shell fragments were
exposed to North Sea seawater at temperatures of 100\r{ }C and 175\r{ }C at
distinct reaction times. Strontium isotope composition of the experimental
medium was adjusted from an original composition of $\delta
^{88/86}$Sr$_{SRM987}$= 0.391(1)\permil\ and
$^{87}$Sr/$^{86}$Sr= 0.70921(1) to $\delta ^{88/86}$Sr$_{SRM987}$=
0.179(1)\permil\ and $^{87}$Sr/$^{86}$Sr= 0.70789(1) by sulfate
extraction and Sr replacement procedures. The artificially altered shell
material (six sub-samples of six shell fragments) and the corresponding
solutions were prepared and analyzed.
Results show that at 100\r{ }C aragonite is still the predominant phase, but
show lower peak intensities with Raman and XRD near the surface of the shell
fragments. At 175\r{ }C the aragonitic shell material recrystallized almost
completely to calcite. In addition a rim of weakly ordered dolomite (10$\mu
$m) was found around these samples.
Analysis of element concentrations of initial and final solutions show that
at 175\r{ }C Sr and Ca concentrations increase (Sr: 79 to 183 $\mu $mol/l;
Ca: 8 to 30 mmol/l), while Mg concentration decreases (Mg: 42 to 22 mmol/l).
A corresponding Sr and Ca depletion along with an increase of Mg occurred in
the samples, especially in the dolomite rim. At 100\r{ }C only minor
increase of Sr, Ca and Mg concentrations were observed in the experimental
solutions.
Changes of Sr isotopic composition are observed in the solutions. At 175\r{
} $\delta ^{88/86}$Sr increased by up to 0.04\permil\ and
$^{87}$Sr/$^{86}$Sr increased by 7.5x10$^{-4}$. At 100\r{ }C $\delta
^{88/86}$Sr increased by up to 0.31\permil\ and
$^{87}$Sr/$^{86}$Sr increased by1.6 x10$^{-4}$. On the other hand shell
analyses show no significant changes in isotopic composition ($\delta
^{88/86}$Sr$_{SRM987}$ 0.25 to 0.3\permil\; $^{87}$Sr
/$^{86}$Sr= 0.70915 before and after experiments). This indicates that Sr is
primarily released by dissolution of shell material without isotopic
fractionation.
Ca isotopes show no significant changes in the solutions for 100\r{ }C
experiments ($\delta ^{44/40}$Ca$_{SRM915a}$=1.7 to 1.9$\pm
$0.1$\textperthousand{})$. However, for experiments at 175\r{ }C Ca isotopic
composition of the fluids decreases by around 1\permil\. In the
shell fragments no significant changes of the Ca isotopic composition were
detected. Similar to Sr isotopes results this indicates dissolution of Ca
without fractionation.
Under these experimental conditions Sr and Ca isotopes are relatively stable
proxies, while changes in elemental composition and strong changes in
mineralogy were observed.
Allison, N., et al. (2007). "Palaeoenvironmental records from fossil corals:
The effects of submarine diagenesis on temperature and climate estimates."
GCA 71: 4693-4703.
Sch\"{o}ne, B. R., et al. (2005). "Climate records from a bivalved
Methuselah (Arctica islandica, Mollusca; Iceland)." Palaeo3 228:
130-148 |
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