Presentation アイスコアとフィルン空気の分析から探る氷期の南極内陸におけるフィルンの厚さ

川村, 賢二

Ice cores provide records of past climate, atmospheric composition and glaciological conditions. In order to establish the age difference between ice and gas (e.g. Antarctic temperature and CO2), firn densification models are generally employed for estimating the past firn thickness. However, N2 and Ar isotopes from the Antarctic interior (e.g. Vostok, Dome Fuji, Dome C) for glacial periods have shown significantly smaller gravitational fractionation (equivalent to 30-40 m of firn thickness) than predicted by models. This discrepancy may be explained if a deep air convection, which eliminates isotopic fractionation at the top part of firn, was extremely well developed during glacial periods. We show, however, by measuring isotopic ratios of Kr, Ar and N2 from the Dome Fuji ice core, that a deep convective zone was not developed during the last glacial maximum (LGM). Because Kr has smaller diffusivity than Ar and N2, it is less gravitationally fractionated in deep firn if strong convective mixing exists. We developed a method to measure these gases in ice-cores, and applied it to the Dome Fuji core over the last ~30,000 years. When normalized to unit mass difference and corrected for thermal signal, the differences between N2 and Kr isotopic ratios are similar for the Holocene and LGM, suggesting similar size of convection zone in those periods. The Vostok core showed the same results for the penultimate glacial maximum and the last interglacial. These results suggest that firn was thinner in glacial periods, and that densification rates at these sites are severely underestimated by models. Similar conclusion was obtained with a different method for the Dome C site by other researchers. This problem may be due to insufficient representation of effects of temperature and accumulation, or missing physics such as the potential effects of impurities on densification rate, in the current firn densification models.
Polar Meteorology and Glaciology Group seminar / 気水圏コロキウム 日時:6月22日(水)10:00-10:50 場所:C301(3階セミナー室)

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