Change of the Asian dust source region deduced from the composition of anthropogenic radionuclides in surface soil in Mongolia
Recent climate change, especially during the 2000s, may be the primary reason for the expansion of the Asian dust source region. The change in the dust source region was investigated by examining anthropogenic radionuclides contained in surface soil samples from Mongolia. Surface soil was globally labeled by radioactive fallout from nuclear testing during the late 1950s and early 1960s, but there are no current direct sources for anthropogenic radionuclides in the air (before the Fukushima nuclear power plant accident in 2011). Radionuclides in the atmosphere are therefore carried mainly by wind-blown dust from surface soil, that is, aeolian dust. Asian dust carries traces of (90)Sr, (137)Cs, and other anthropogenic radionuclides; the heaviest deposition occurs in spring and has been recorded in Japan since the early 1990s. The composition of anthropogenic radionuclides in atmospheric depositions would be affected by a change in the dust source. Previous studies of atmospheric deposition at long-term monitoring sites (e. g. in Tsukuba, Japan) have detected changes in the (137)Cs/(90)Sr ratio and in the specific activity of the radionuclides. These changes in the composition of observed atmospheric depositions are supposed to reflect changes in the climatic conditions of the dust source region. To investigate this dust source change, we conducted a field survey of radionuclides ((90)Sr and (137)Cs) in surface soil samples in September 2007 in the eastern and southern regions of Mongolia, where dust storms have occurred more frequently since 2000. The specific activities of both radionuclides as well as the (137)Cs/(90)Sr ratio in the surface soil were well cor-related with annual average precipitation in the Mongolian desert-steppe zone. Higher specific activities and a higher (137)Cs/(90)Sr ratio were found in grassland regions that experienced greater precipitation. These findings suggest that the increased specific activities and the activity ratio detected in atmospheric depositions in Japan during years with frequent Asian dust transport events in the 2000s are a sign of grassland degradation.