CARBON ISOTOPE OF SURFACE MATERIALS ALONG THE MEGA-TRANSECTION: BAOJI, CHINA¡ªHANHAYN HURYEE, MONGOLIA

Carbon isotope has been used as a proxy for reconstructing paleoclimate. Yet, the quantitative definition is still dubious. In order to reconstruct the past climate in arid regions of central Asia, Its bioclimatic dependency should be examined. We did the examination on carbon isotope (d¹³C) along a transect from the northern slope of Qinling Mountain, close to Baoji, China, though to Hanhayn Huryee at the north tip of Hovsgol Nuur, Mongolia, and report here the preliminary result. The N-S transect starts at 34°14'24"N (106°55'30"E) and ends at 51°35'08"N, (100°45'49"E). It crosses several climate and vegetation zones and is ideal for the proxy evaluation. The climate becomes moister from the Mongolia Gobi toward both south and north due to increase in precipitation. Vegetation well reflects the climate change. From the Gobi to the Siberia, it varies from desert steppe, cold steppe, and coniferous forest to subarctic meadow. Whereas southwards to the southern Chinese Loess Plateau, the vegetation varies from desert steppe, warm steppe to deciduous forest. The soil also closely follows the climate and the vegetation patterns. Histosols are found in subarctic meadow, spodosols in coniferous forest, mollisols in steppe, aridsols in desert and alfisolls in deciduous forest.

d¹³C of organic material in the surface soil (horizon A) has four plateaus along the transect. They are -28.2 ± 1.9 (‰ PDB), -25.4 ± 1.0 (‰ PDB), -24.0 ± 1.7 (‰ PDB), and -27.6 ± 1.3 (‰ PDB) from south to north. The first plateau corresponds to the Qinling Mountains; the second plateau is from 36°11'39"N (106°11'50"E) to 41°51'23"N (110°04'10"E), corresponding to the arid and semi-arid areas in Ordos Plateau and Inner Mongolia, China; the third plateau corresponds to the Gobi area in southern Mongolia, which is the driest place in the world; the last plateau corresponds to the mountain regions in northern Mongolia. The dry region has the highest d¹³C, while the humid area the lowest one. The d¹³C values in Qinling Mountains are similar to those in the northern Mongolia. It seems that climate controls the d¹³C values of the surface soils, but the type of the plant seems not a factor controlling the isotope ratios.