CHEMICAL AND COMPOUND-SPECIFIC CARBON ISOTOPIC CHARACTERISTICS OF n-ALKANES IN THE QINGSHI LACUSTRINE SEDIMENTS, WUDALIANCHI, NORTHEAST CHINA,AND THEIR PALEOVEGETATION SIGNIFICANCES
Wudalianchi is located in the transition zone of the Lesser Khingan Range and Songnen Plain, Heilongjiang Province, Northeast China. In order to investigate the paleovegetation in Wudalianchi during the typical warm and cool weather events, a 300cm-long core called Qingshi Core (48°37'8.66"N, 126°7'25.9"E) was obtained during a drill in 2012. The lithology is relatively uniform. The lithology is mainly lacustrine clay. After drilling, core sediments were stored at 4°C before sub-sampling. The chronology was established according to the AMS-~(14)C data of the sediments, extending back to about 35.2ka B.P. 29 samples were collected to obtain a representative organic record of the present study profile. The molecular biomarkers including n-alkanes were identified from the sediments of Qingshi Core. The distribution of n-alkanes and composition of delta~(13)C values were mainly discussed in this paper. According to the two distribution patterns of n-alkanes, the n-alkanes exhibited mixed sources, both the import of the algae and fungi and the terrestrial higher plants. The distribution of n-alkanes indicated that from 35.2ka B.P. to the present, the vegetation types in the study area are mainly grasses from the indexes of nC_(27)/nC_(31) and ACL_(25~31). In addition, the compound-specific carbon isotopic composition was determined for C_(27),C_(29) and C_(31) n-alkanes in the core, ranging from -27.3~-33.6,-28.8~-34.3 and -29.5~-34.3, respectively. Relative contribution of C_3 and C_4 plants was estimated by using a binary model, indicating that from 35.2ka B.P., C_3 plants was dominated in the study area. The relative biomass of C_3/C_4 plants varied obviously in the typical warm and cool weather events influenced by the temperature. While during the Holocene Megathermal ( 6.Oka B.P.),controlled by the high temperature, the biomass of C_4 plants increased (from 18.6% to 43.1% ) while that of C_3 plants decreased (from 81.4% to 56.9%). After the typical events of cold weather ( the cooling event of the Middle Holocene at 8.2ka B.P. and the Younger Dryas at 12.9ka B.P.),C_3 plants increased (from 69.5% to 81.4% and from 74.4% to 81.2% , respectively) , relatively C_4 plants decreased ( from 30.5% to 18.6% and from 25.6% to 18.8% ,respectively) . During the Last Glacial Maximum at 21ka B.P., affected by the low temperature, the biomass of C_3 plants was high (almost 80.0% ). This could be the result of the cold environment which inhibited the growth of C_4 plants. This study was to explore the distribution patterns of C_3/C_4 plants in Northeast China, and to provide scientific evidence for the vegetation patterns and agricultural production affected by the future climate change.