METHANE IN ICE CORES AT MILLENNIAL SCALE : WHICH PART OF THE CLIMATE SYSTEM DOES IT DESCRIBE?

Over the last 15 years, increasingly detailed records of atmospheric methane (CH₄) mixing ratio have been produced from Antarctic and Greenland ice cores. In addition to its long-term glacial-interglacial variability (typically in the range 350-700 ppbv or parts per billion in volume), these records have revealed millennial scale CH₄ changes associated with Dansgaard/Oeschger events in Greenland. As an end-product of a complex biogeochemical cycle, these CH₄ fluctuations are clear indicators that millennial climate events were not limited to the circum North Atlantic sector but indeed affected the climate system on a much larger scale.

We will present new CH₄ records from the North GRIP and GRIP (Greenland) cores reaching sub-centennial resolution in particular over Marine Isotope Stage 4 ; they show a remarkable correspondence with Greenland climate features, including unnumbered isotopic events. Parallel measurements of N₂ and Ar isotopes indicate that Greenland climate and atmospheric methane varied synchronously at sample resolution (~50 yr).

As Greenland ice cores do not yet allow to study older periods than the last interglacial, and thanks to the Greenland/CH₄ correspondence observed during the last glacial, detailed Antarctic CH₄ records provide a tool to extend the observation of millennial climate events back in time. We will present and discuss the status of such records obtained from the Vostok and Dome C deep ice cores. Although the strong smoothing of atmospheric variations at these low-accumulation sites tamper the amplitude of events, the data at hand do reveal Dansgaard/Oeschger variability during previous glaciations. Coherence and phase studies between CH₄ and Antarctic climate suggest that the bipolar see-saw mechanism remained a strong feature of the climate system over the last four climatic cycles.

Causes of natural CH₄ changes mostly include continental hydrology (wetland extent and temperature), the atmospheric oxidative capacity and possible hydrate destabilisation. We will discuss the arguments available about the relative contribution of these mechanisms.