XU Baiqing, YAO Tandong, TIAN Lide, HUO Wenmian, DUAN Keqin, J. Chappellaz
There are 12 samples from the upper 95 m of the Dunde ice core providing 800 a record of CH4 concentration in the air trapped in ice core bubbles. An anthropogenic increase of CH4 over the last200 years is revealed, and what is more the increaserate and level are quicker and higher than Antarcticand Greenland records. For a given interval in the20th century, the Dunde CH4 concentration is about0.5×10-6 0.6×10-6 mol/mol higher than Antarctic and Greenland ones. From our present-day knowledge of the latitudinal distribution of CH4, even inthe center of human activity, one will not expect a difference of more than 0.15×10-6 mol/mol with respect to the Antarctic level. But before the 19th century, there were two CH4 concentrations obtained fromthe Dunde ice core, dated to about 1280 AD and 1800 AD, which fit well with the Antarctic and Greenlandreferences, and the CH4 concentration fluctuatedwithin 0.7×10-6~0.8×10-6 mol/mol. It is clearthat the CH4 concentration recorded in the Dunde icecore bubbles was disturbed by the transformation process of snow to ice during this century.The elevation of Dunde ice cap is 5 200 m, justthe same as Mont Logan (5300 m). In Mont Loganice samples the total gas content is about 7cm3/100g, but in Dunde it is only about 1~5 cm3/100 g. Also most of the bubbles are angular and unevenly-distributed in Dunde ice samples. By all appearances, theDunde ice records can not be attributed to the transformation of dry-firn to ice, but to an intensemelting-refreezing process. The variation of CH4 concentration over the last 800 years recorded in the Dunde ice core can be divided into two stages: before the 19th century, Dunde CH4 records fit well with polar ice sheet references, reflecting the true concentration of atmospheric CH4.After 19th century, the Dunde ice core clearly revealsthe intense increase of CH4 concentration, about 0.5×10-6~0.6×10-6 mol/mol higher than Antarcticand Greenland records. The measurement of total gascontent reveals that the total gas contents of two samples dated to about 1800 AD and 1280 AD are 5.2 and 3.0 cm3/100 g respectively, higher than those ofthe upper samples, which means that the two samplescontain richer bubbles, and less melting occurred onthe snow surface at that time. We can conclude thatthe amplification of CH4 concentration recorded inthe Dunde ice core, compared with atmospheric level,may be attributed to the intensification of melting onthe snow surface during this century.
