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| Titel |
A method for combined Sr-Nd-Hf isotopic analysis of <10 mg dust samples:
implication for ice core science |
| VerfasserIn |
Gábor Újvári, Wencke Wegner, Urs Klötzli |
| 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 |
250142866
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| Publikation (Nr.) |
 EGU/EGU2017-6539.pdf |
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| Zusammenfassung |
| Aeolian mineral dust particles below the size of 10-20 μm often experience longer distance
transport in the atmosphere, and thus Aeolian dust is considered an important tracer of
large-scale atmospheric circulation. Since ice core dust is purely Aeolian in origin,
discrimination of its potential source region(s) can contribute to a better understanding of past
dust activity and climatic/environmental causes. Furthermore, ice core dust source
information provides critical experimental constraints for model simulations of past
atmospheric circulation patterns [1,2]. However, to identify dust sources in past dust
archives such as ice cores, the mineralogy and geochemistry of the wind-blown
dust material must be characterized. While the amount of dust in marine cores or
common terrestrial archives is sufficient for different types of analyses and even for
multiple repeat measurements, dust content in ice cores is usually extremely low
even for the peak dusty periods such as the Last Glacial Maximum (LGM) (5-8 mg
dust/kg ice; [3]). Since the most powerful dust fingerprinting methods, such as REE
composition and Sr-Nd-Pb isotopic analyses are destructive there is a clear need to
establish sequential separation techniques of Sr, Nd, Pb and other REEs to get the
most information out of small (5-10 mg) dust samples recovered from ice cores.
Although Hf isotopes have recently been added as a robust tool of aerosol/dust source
discrimination (e.g. [4,5,6,7]), precise Hf isotopic measurements of small (<10 mg) dust
samples are still challenging due to the small Hf amounts (on the order of 1-10
ng) and often compromised by potential problems arising during ion exchange
chemistry.
In this pilot study an improved method for chemical separation of Sr, Nd and Hf by Bast
et al. [8] was applied, which allows the precise isotope analysis of sub-ng amounts of Hf by
MC-ICPMS. This ion exchange chromatography procedure has been combined with
established methods of separating and purifying Sr and Nd for subsequent TIMS
isotopic measurements. International rock standard BCR-2 (10 mg aliquot) gave
87Sr/86Sr, 143Nd/144Nd and 176Hf/177Hf isotopic ratios of 0.705127±0.000004 (2σ),
0.512625±0.000004 and 0.282853±0.000006, respectively. These data are in good
agreement with the certified values (0.705000±0.0000011, 0.512637±0.000013 and
0.282866±0.000011; [9]) demonstrating that the experimental procedures and setup allow for
valid 176Lu and 176Yb corrections and results in reliable 176Hf/177Hf (and also 87Sr/86Sr,
143Nd/144Nd) ratios. Along with the standards 50, 10 and 5 mg aliquots of the
fine (<10 μm) fractions of three wind-blown loess sediment samples from Central
Europe (NUS), China (BEI) and the US (JUD) were processed (all acetic acid treated
for carbonate removal, i.e. aluminosilicate fractions were analysed). Sr isotopic
compositions varied between the aliquots within a range of ∼0.00007 for the three
samples. Comparison of these values with previously obtained 87Sr/86Sr isotopic
ratios from the same samples (different acid/sample amounts) reveals that these
values are very sensitive to the acetic acid treatment (acid molarity and amount). By
contrast, Nd isotopic ratios do not seem to be affected by acid treatment and these
signatures are extremely stable both within run (0.000008 variability for the three
aliquots, i.e. 0.15 ɛNd unit) and between runs of different laboratories (0.00006,
ca 1 ɛNd unit). Hf isotopic compositions varied within 1 (NUS) and 5 ɛHf units
(BEI) between the three aliquots, but the BEI sample always gave more positive ɛHf
values (mostly below -4.7) than the Central European sample (NUS, above -6.5,
mostly around -8). JUD gave two values of -4.9 and -7.9. It should be further studied
using other dust samples if the slightly larger dispersion in 176Hf/177Hf for the
different aliquots of BEI is a unique feature or not. Nevertheless, the ca. 3-7 ɛHf unit
difference between the Central European and Chinese dust sample provides an
opportunity to better discriminate last glacial Greenland dust sources [7] in Nd-Hf
space.
This study was supported by the OMAA 92öu7 project, the OTKA PD-108639 grant and
the Bolyai János Research Fellowship (to GÚ).
References
[1] Biscaye et al., 1997. J. Geophys. Res. 102, 26765–26781; [2] Svensson et al., 2000. J.
Geophys. Res. 105, 4637–4656; [3] Ruth et al., 2003. J. Geophys. Res. 108(D3), 4098; [4]
Aciego et al., 2009. Chem. Geol. 266, 194–204; [5] Aarons et al., 2013. Chem. Geol.
349(350), 18–26; [6] Zhao et al., 2015. Geophys. Res. Lett. 42, 5661–5669; [7] Újvári et al.,
2015. Geophys. Res. Lett. 42, 10399–10408; [8] Bast et al., 2015. J. Anal. Atom.
Spectrom. 30, 2323; [9] Jweda et al., 2016. Geostand. Geoanal. Res. 40, 101–105. |
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