Publication Date:
2017-01-24
Description:
Advances in trace gas analysis allow localised,
non-atmospheric features to be resolved in ice cores, superimposed
on the coherent atmospheric signal. These highfrequency
signals could not have survived the low-pass filter
effect that gas diffusion in the firn exerts on the atmospheric
history and therefore do not result from changes in
the atmospheric composition at the ice sheet surface. Using
continuous methane (CH4) records obtained from five polar
ice cores, we characterise these non-atmospheric signals
and explore their origin. Isolated samples, enriched in CH4
in the Tunu13 (Greenland) record are linked to the presence
of melt layers. Melting can enrich the methane concentration
due to a solubility effect, but we find that an additional
in situ process is required to generate the full magnitude of
these anomalies. Furthermore, in all the ice cores studied
there is evidence of reproducible, decimetre-scale CH4 variability.
Through a series of tests, we demonstrate that this
is an artifact of layered bubble trapping in a heterogeneousdensity
firn column; we use the term “trapping signal” for
this phenomenon. The peak-to-peak amplitude of the trapping
signal is typically 5 ppb, but may exceed 40 ppb. Signal
magnitude increases with atmospheric CH4 growth rate
and seasonal density contrast, and decreases with accumulation
rate. Significant annual periodicity is present in the CH4
variability of two Greenland ice cores, suggesting that layered
gas trapping at these sites is controlled by regular, seasonal
variations in the physical properties of the firn. Future
analytical campaigns should anticipate high-frequency artifacts
at high-melt ice core sites or during time periods with
high atmospheric CH4 growth rate in order to avoid misinterpretation
of such features as past changes in atmospheric
composition.
Repository Name:
EPIC Alfred Wegener Institut
Type:
Article
,
isiRev
Format:
application/pdf
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