Geoscience Reference
In-Depth Information
18.2.4
Provenance Techniques
Isotopic fingerprinting is the most selective technique available for determining the
sources of dust deposited in polar ice cores. The technique is based on the variability
of isotope ratios that result from radioactive decay of elements in the Earth's crust,
as well as the varied ages and geochemistry of the continents. Radioactive decay
chains have half-lives of billions of years, and are unaffected by chemical or physical
transport processes such as dust deflation and dissolution. Two isotopic systems
have been used for investigating dust provenance: strontium (Sr) and neodymium
(Nd) (Grousset and Biscaye
2005
); and lead (Pb) (Rosman
2001
).
87
Sr and
143
Nd are
the respective radioactive decay products of rubidium and samarium, so the ratios of
87
Sr and
143
Nd to stable
86
Sr and
144
Nd, respectively, will change over time. For Pb,
three isotopes (
206
Pb,
207
Pb,
208
Pb) are respective end-products of radioactive decay
chains of
238
U,
235
Uand
232
Th. For these isotopic systems to be effective, there
must be sufficient variability between the different Potential Source Areas (PSAs)
and the ice core dust to enable the viable PSAs to be distinguished. In the case
of Pb, other emission sources such as volcanism or industrial activity can interfere
with the interpretation (Vallelonga et al.
2002b
). Isotope ratios are determined using
sensitive instruments capable of separating the isotopes of an element and measuring
their relative abundances. These measurements are routinely performed using TIMS
and Multicollector ICP-MS instruments.
Ice core dust provenance has also been evaluated using Rare Earth Elements
(REEs), determined by ICP-MS. REEs have the benefit of being relatively easy
to measure and difficult to contaminate, so they can be determined at higher
resolution and more quickly than Sr/Nd or Pb isotopes. Interpretation of REE
provenance is complicated by overlapping PSA signatures as well as sample
processing interferences; Rhodes et al. (
2011
) have found that REE signatures can
be altered by some sample preparation techniques.
18.2.5
Visual Stratigraphy
Ice core dust content is often visible to the naked eye as a layering or banding in the
ice (Alley et al.
1997
). A profile of the visual ice core stratigraphy is best obtained
after carefully preparing a clear flat ice core surface. Using an indirect light source
and a digital camera the visual stratigraphy or 'linescan' profile can be obtained
(Svensson et al.
2005
; McGwire et al.
2008
). Similar to dark field microscopy, in this
profile transparent ice will appear dark whereas any impurities in the ice, such as air
bubbles or dust particles, will cause light scattering and make the impurities appear
white in the line scan image (Fig.
18.2
). Continuous line scan profiles are available
for the glacial part of the Greenland NorthGRIP (NGRIP) ice core (Svensson et al.
2005
) and the Antarctic EPICA Dronning Maud Land (EDML) ice core (Faria et al.
2010
).
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