NITROUS OXIDE MEASURED ALONG POLAR ICE CORES: INSIGHT INTO GLOBAL PROCESSES

Greenhouse gas measurements performed along polar ice cores show a close relation to the global temperature on glacial-interglacial time scales with high CO₂, CH₄ and N₂O concentrations during interglacial and low concentrations during glacial periods.

During climatic variations on millennial time scales in the last glacial and the transition to the Holocene, however, the evolutions of CO₂, CH₄, and N₂O experience significant differences. While CO₂ shows a close relation to the temperature reconstructed for the Southern Hemisphere, CH₄ and N₂O are linked to the temperature variations recorded in the Northern Hemisphere and vary in parallel to Dansgaard-Oeschger (DO) events. High resolution records of CH₄ and N₂O unveil further significant differences which give important insight into the processes during fast climatic variations and the response of different ecosystems to climatic changes during the last glacial.

1) The response of the two greenhouse gases CH₄ and N₂O to fast climatic changes does not show the same relative magnitude for all events and seems to be determined by different mechanisms. While the amplitude of CH₄ variations parallel to DO events is modulated with the Northern Hemisphere summer insolation, the N₂O concentration change depends on the duration of the DO event.

2) At the beginning of DO events N₂O starts to increase several hundred years earlier than CH₄ and the temperature reconstructed for the Northern Hemisphere. Therefore, N₂O seems to be a precursor to DO events.

Both CH₄ and N₂O have important soil sources, but in contrast to CH₄, N₂O also has a substantial oceanic source. The two records and their different behaviour at the beginning and during DO events suggest that the variations are at least partly caused by soil sources in different latitudes and that N₂O is further influenced by changes in the oceanic source.