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| Titel |
CH4 and N2O concentrations from the deepest part of the NEEM ice core: affected by artifacts? |
| VerfasserIn |
Matthias Baumgartner, Adrian Schilt, Neem Gas Consortium |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250053287
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| Zusammenfassung |
| During the last years, a new 2,533 m long deep ice core in the frame of the North Greenland
Eemian Ice Drilling (NEEM) project was drilled in order to catch for a first time the last
interglacial period called the Eemian in Greenlandic ice. Here, we present 278 new
concentration measurements of the greenhouse gases methane (CH4) and nitrous oxide
(N2O) in the depth interval 2,200 – 2,520 m of the deep NEEM ice. In a melt-refreeze step
we extract the enclosed air bubbles and analyze the sample by gas chromatography yielding a
precision of about 10 ppbv for CH4 and 5 ppbv for N2O. The known natural variability of
CH4 is restricted to the range from 350 ppbv to 800 ppbv and of N2O from 200 ppbv to 300
ppbv by existing ice core measurements, e.g. from the Antarctic EPICA Dome C ice
core.
Our new NEEM data show well defined natural concentration variations of CH4 and N2O in
line with the stable water isotope temperature proxy δ18O, beside the two following
exceptions. First, in the depth-interval from 2,200 – 2,220 m where the δ18O signal points
to problems in the stratigraphy of the bottom ice, both CH4 and N2O show large
parallel variations, but clearly deviate from the temperature trend. Second, in the
depth-interval (2,360 – 2,450 m) supposed to be of Eemian origin, both the atmospheric
CH4 and N2O trends are interrupted by event-like spikes of extraordinary high
concentrations (CH4 up to 2,000 ppbv and N2O up to 400 ppbv). Since greenhouse
gases are well mixed over the whole globe (interhemispheric mixing time of 1-2
years) and since several Antarctic ice cores (EPICA Dome C, Talos Dome) show no
counterparts of these high concentration events, it is virtually impossible that the
measured signals are of atmospheric origin. We hypothesize that surface melt-layers
combined with in situ production could be responsible for the scattered concentration
pattern. However, neglecting these outliers and following the overall concentration
trend, our new measurements do not exclude that the ice could be of Eemian origin.
During the transition supposed to be Termination 2 CH4 rises from around 430
ppbv up to 750 ppbv followed by a slow decrease down to 600 ppbv and a rapid
drop to 510 ppbv. EPICA Dome C shows a slow increase from 360 ppbv to 530
ppbv and a rapid transition up to 725 ppbv followed by a slow decrease to 470
ppbv. Taken at face value this points to an interhemispheric methane gradient at the
beginning of the Eemian, which is somewhat smaller than for the early Holocene,
however, may still be in line with increasing boreal wetland sources at that time. |
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