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| Titel |
Inter-polar difference of CH₄, δD(CH₄) and δ¹³CH₄ during the Holocene |
| VerfasserIn |
Jonas Beck, Michael Bock, Jochen Schmitt, Barbara Seth, Thomas Blunier, Hubertus Fischer |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250097412
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| Publikation (Nr.) |
 EGU/EGU2014-12990.pdf |
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| Zusammenfassung |
| The past variation of the concentration of atmospheric methane (CH4), the third most
important greenhouse gas after water vapour and carbon dioxide (CO2), is observed to be
generally in phase with the northern summer insolation cycle driven by the precession of the
rotation axis of the Earth. However, in the mid-Holocene this regularity breaks down, and
atmospheric CH4 starts to rise again while the northern summer insolation continues to
decline. Despite different attempts to explain this feature (e.g. contrasting hypotheses on
early human influences and enhanced emissions in the southern tropics), a clear explanation
for the evolution of the atmospheric methane concentration has not been found
yet.
One possibility to get more information about the highly under-determined system
of the methane cycle is to measure the methane concentration in Greenland and
Antarctica and to calculate the relative inter-polar difference (rIPD) of methane, allowing
us to draw conclusions about the hemispherical imbalance of the source and sink
distribution.
More information on the involved production and sink processes of atmospheric CH4 can
be gained from the isotopic composition (δD and δ13C) of CH4. Each source emits
methane of a typical isotope signature, and each sink process leads to a certain
isotopic fractionation and thus influences the isotopic composition of atmospheric
methane.
To exploit for the first time the full parameter set, we also measured the inter-polar
difference of δD (IPDδD) and δ13C (IPDδ13C) of methane over the Holocene. The IPDδD
record results from a high resolution δD record from the NGRIP (Greenland) ice core and a
coarse resolution record of EDML (Antarctica) ice core samples. The NGRIP δD data show a
clear covariation with the long-term CH4 concentrations changes during the Holocene. The
δD variations of 8-10 oare significantly larger than our measurement error of 2.3 o which
is of significantly better precision than previous data. To avoid any systematic error, the
EDML data were measured on the same system and during the same measurement campaign
as the NGRIP samples. The resulting IPDδD is constant within the measurement error
at approximately -16.2 o(north-south) during the entire Holocene. The δ13C
measurements (NGRIP and Talos Dome) were done on another system with an measurement
error of 0.13 o supporting a long-term decrease in δ13C over the entire Holocene
as previously observed (Sowers, QSR, 2010). Our new measurements show that
also the IPDδ13C appears to be relatively constant over the last 12’000 years as
well. Given the significant signal in both isotopes of methane and in the methane
concentration over the Holocene, the rather constant offset between Greenland and
Antarctica is surprising and helps to constrain past changes in the global methane cycle. |
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