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| Titel |
Interhemispheric concentration gradient of CH4 around the last glacial maximum (20-30 kyr BP) and attenuation of the atmospheric signal due to enclosure process in the ice |
| VerfasserIn |
Matthias Baumgartner, Adrian Schilt, Jakob Schwander, Renato Spahni, Hubertus Fischer, Thomas Stocker |
| 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 |
250053412
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| Zusammenfassung |
| The natural emissions of the greenhouse gas methane (CH4) occur to a major part in the
wetlands of the tropical regions. However, when the northern hemisphere is warming during
glacial/interglacial and stadial/interstadial transitions, a northern higher latitude source
becomes important too. On contrary, southern emissions are small since the ocean is no
important CH4 source. In the polar ice sheets of Greenland and Antarctica air is enclosed
providing a unique climate archive of the past atmospheric composition. The rapid and
large variations of the greenhouse gas CH4 are useful to perform synchronization
between different climate records, e.g. polar ice core records from Greenland and
Antarctica. This is possible because greenhouse gases are well mixed over the whole
globe with an interhemispheric mixing time of 1 – 2 years. However, the mean
atmospheric lifetime of CH4 (about 10 years) is only an order of magnitude higher than
the mixing time. Combined with the asymmetric source distribution this leads (in
certain time intervals) to a significant interhemispheric concentration gradient of
CH4.
We present 392 new CH4 measurements from the ice cores of the North Greenland Ice Core
Project (NGRIP) and the European Project for Ice Coring in Antarctica Dronning
Maud Land (EDML) in the time interval 20,000 – 30,000 years before present
(BP) around the Last Glacial Maximum (LGM) including the Dansgaard Oeschger
events (DO) 2, 3 and 4. The time resolution is better than 70 years on the EDML1
timescale for both, the NGRIP and EDML records. Since the measurements are
performed in one lab and Greenland and Antarctic ice samples have been measured in
parallel within the same measurement series, any systematic offsets between the two
records can be excluded. The measurements allow for a better synchronization in
this time interval, where CH4 variations are relatively small, as well as a precise
determination of the interhemispheric concentration gradient. At NGRIP, CH4 is
stabilized at 410 ppbv just after DO3 followed by a fast drop down to 380 ppbv
and a slow increase back to 400 ppbv before DO2. At EDML, a slow decrease
occurs after DO3 down to 365 ppbv where it stays stable until the onset of DO2,
apart from a small bump of about 5 ppbv. Therefore the gradient is about 25 ppbv
before DO4, decreases from 25 ppbv to 15 ppbv after DO3, then slowly increases up
to 35 ppbv just before DO2 and finally achieves a value of about 10 ppbv after
DO2.
In order to determine the gradient over fast variations like DO2, 3 and 4, it is necessary to
calculate the attenuation of the atmospheric signal due to diffusive mixing in the firn column
and gradual bubble close-off in both, the NGRIP and EDML sites. We apply a firn diffusion
model to our data to take care of this effect. Based on the information of the interhemispheric
concentration gradient, a three-box model is applied in order to estimate the source
distribution of CH4. |
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