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Contraintes chronologiques et biogéochimiques grâce au méthane dans la glace naturelle : une application aux forages du projet EPICA

Abstract : With the start of the 21st century, it becomes obvious that human activities directly or indirectly impact the climate system. It is thus of high priority to improve future climate simulations and their consequences. Part of this priority relies on exploring the past.

Air extracted from polar ice in Greenland and Antarctica is a fantastic recorder of past changes in the Earth's atmospheric composition. Data obtained on several ice cores have shown a strong correlation between the mixing ratio of greenhouse gases (CO2, CH4, N2O) and temperature, on a time scale of hundreds of thousands of years. These data have revealed that the currently elevated greenhouse gas mixing ratios had no equivalent over the last 650'000 years, within the resolution limit of the records. Lastly, these records offer the opportunity to better understand the link between climate and the biogeochemical cycles responsible for past greenhouse gas changes, which may feedback on the future climate.

Our PhD work consisted first in measuring at high temporal resolution methane in air bubbles of the new drilling sites EPICA, at Dome C and at Dronning Maud Land. These measurements allowed us (i) to propose a scheme for the causes of the methane / climate link during the last 800'000 years, and (ii) to improve the chronologies of trapped gases in ice cores, and in parallel to discuss the sequence of climatic events recorded in both hemispheres and in Antarctic ice cores.

We conclude that the low frequency modulation of methane mixing ratios is probably linked to the variability of Asian monsoon and to the shift of the Intertropical Convergence Zone. Superimposed on this low frequency modulation, high latitudes wetlands may have switched on and off during major transitions between glacial and interglacial conditions. At millennial time scale, we show the ubiquity of rapid climate changes during the last eight glacial periods, and the probable persistence of the bipolar seesaw mechanism of energy redistribution between the two poles.

Calculations of the difference of age between trapped air and the surrounding ice (delta age) is affected by large errors for past climatic conditions. Using the synchronization of methane signals between the EPICA ice cores, and the beryllium-10 anomaly associated with the geomagnetic event of Laschamp, we have been able to re-evaluate delta age and to show that it is largely overestimated by the firn densification model during the last glacial period. This brings us to lower the delay of carbon dioxide over Antarctic temperature at the start of the last glacial-interglacial transition. The exact causes of this delta age overestimate in glacial conditions are still unknown at the end of this PhD ; but they probably involve together the uncertainties related with climate parameters (temperature, accumulation) used as boundary conditions for the model, and the physics of firn densification under glacial conditions.
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https://tel.archives-ouvertes.fr/tel-00329799
Contributor : Laetitia Loulergue <>
Submitted on : Monday, October 13, 2008 - 2:29:52 PM
Last modification on : Thursday, November 19, 2020 - 3:54:26 PM
Long-term archiving on: : Monday, June 7, 2010 - 7:31:19 PM

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  • HAL Id : tel-00329799, version 1

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Laetitia Loulergue. Contraintes chronologiques et biogéochimiques grâce au méthane dans la glace naturelle : une application aux forages du projet EPICA. Océan, Atmosphère. Université Joseph-Fourier - Grenoble I, 2007. Français. ⟨tel-00329799⟩

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