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| Titel |
Accurate age scale of the Dome Fuji ice core, Antarctica from O2/N2 ratio of trapped air |
| VerfasserIn |
K. Kawamura, S. Aoki, T. Nakazawa, K. Suzuki, F. Parrenin |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250066074
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| Zusammenfassung |
| Chronology of the first Dome Fuji deep ice core (core length: 2,500 m, ice thickness: 3,035
m) for the age range from 80 kyr to 340 kyr ago was established by orbital tuning of
measured O2/N2 ratios in trapped air to local summer insolation, with precision better than
about 2,000 years (Kawamura et al., 2007). The O2/N2 ratios found in polar ice cores are
slightly lower than the atmospheric ratio because of size-dependent molecular fractionation
during bubble close-off. The magnitude of this gas fractionation is believed to be governed by
the magnitude of snow metamorphism when the layer was originally at the surface, which in
turn is controlled by local summer insolation (Fujita et al., 2009). A strong advantage of the
O2/N2 chronology is that there is no need to assume a lag between climatic records in the ice
core and orbital forcings, becacuse O2/N2 ratios record local insolation through
physical processes. Accuracy of the chronology was validated by comparing the
O2/N2 chronology with U-Th radiometric chronology of speleothem records (Cheng
et al., 2009) for the ends of Terminations II, III and IV, as well as several large
climatic events, for which both ice-core CH4 and speleothem δ18O (a proxy for
precipitation) show abrupt shifts as seen in the last glacial period. All ages from O2/N2 and
U-Th chronology agreed with each other within ~2,000 yr. The O2/N2 chronology
permits comparisons between Antarctic climate, greenhouse gases, astronomically
calculated orbital parameters, and radiometrically-dated sea level and monsoon
records.
Here, we completed the measurements of O2/N2 ratios of the second Dome Fuji ice core,
which reached bedrock, for the range from 2,400 to 3,028 m (320 - 700 kyr ago) at
approximately 2,000-year time resolution. We made significant improvements in ice core
storage practices and mass spectrometry. In particular, the ice core samples were stored at
about -50 Ë C until the air extraction, except during short periods of transportation, in order to
prevent size-dependent fractionation due to gas loss during storage. The precision of the new
O2/N2 data set is improved by a factor of 3 over the previous data. Clear imprint of local
insolation is recognizable in the O2/N2 data towards the deepest depths, even around 400 kyr
ago when summer insolation wiggles are small due to small orbital eccentricity. A new
chronology using this O2/N2 data set will be established by applying the inverse
method for EDC3 age scale (Parrenin et al., 2007) for the entire 700 kyr, and climatic
implications will also be discussed especially on Terminations and interglacial
periods.
REFERENCES
Kawamura, K. et al., Northern Hemisphere forcing of climatic cycles in Antarctica over
the past 360,000 years, Nature, 448, 912-916, 2007.
Parrenin, F. et al., The EDC3 chronology for the EPICA Dome C ice core, Clim. Past, 3,
485–497, 2007.
Fujita, S., Okuyama, J., Hori, A., and Hondoh, T., Metamorphism of stratified
firn at Dome Fuji, Antarctica: A mechanism for local insolation modulation of
gas transport conditions during bubble close off, J. Geophys. Res., 114, F03023,
doi:10.1029/2008JF001143, 2009.
Cheng, H. et al. Ice Age Terminations. Science 326, 248-252, doi:10.1126/science.1177840,
2009. |
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