温度影响东北地区更新世植被变化的黄土记录

TEMPERATURE FORCED VEGETATION VARIATIONS IN GLACIAL-INTERGLACIAL CYCLES IN NORTHEASTERN CHINA REVEALED BY LOESS-PALEOSOL DEPOSIT

温度、降水（湿度）和大气CO2含量被认为是影响C3植物生长的主要因素.大量的现代植物和土壤有机质碳稳定同位素（δ13C）研究表明,温度升高可使C3植物的碳同位素变重（正）,降水增多（湿度增大）和大气CO2含量升高可使C3植物的碳同位素偏轻（负）;同时,C4植物可明显地影响土壤的有机碳同位素组成.基于这些认识,以前对黄土高原的黄土-古土壤序列有机质碳同位素和植被组成变化进行过不少研究.但是,相关的研究在我国东北地区的黄土中还未开展.本文对我国东北地区厚度36m的喀喇沁旗牛样子沟黄土-古土壤剖面进行了间隔10cm的采样和总有机碳含量（TOC）、有机质稳定碳同位素的测试分析.结果表明,在间冰期发育的古土壤有机质含量高、δ13C值偏正;反之,在冰期堆积的黄土有机质含量低、δ13C值偏负.通过分析表明,研究区的植被类型是以C3植物占主导地位,C4植物对土壤有机质δ13C变化的贡献有限,并且气候变化具有冰期-间冰期季风气候变化的特点,据此推断温度是决定东北地区植被碳同位素组成变化的主要因素,超过了降水（湿度）和大气CO2含量对植被（植物碳同位素组成）变化的反向影响.这一发现揭示了温度对我国东北地区长时间尺度植被变化的控制作用.这些认识对于在未来全球变暖背景下,东北地区的林木和小麦、大豆、水稻等C3作物的种植有借鉴意义.

Carbon isotopic composition of bulk organic matter and biomarkers have been extensively applied in reconstructing paleovegetation on the Chinese Loess Plateau （CLP）, which has the thickest loess-paleosol deposit in world. Previous studies generally considered low δ13C values in loess sections were due to reduced input of C4 plants as a result of lower temperature during glacial periods. However, much less is known about the paleovegetation record in loess deposits outside the CLP. Here we report a high resolution study of a 36-m thick loess-paleosol sequence in Niuyangzigou （NYZG） （41°55＇N, 118°43＇E） , Northeastern China. δ13C samples were measured for total organic matter content （TOC） and bulk organic δ13C. Our results show that TOC of the NYAG section has an average of 0.131% ; the maximum value is 0.435% which takes place in the S5 paleosol and the minimum value is 0.001% which occurs in the L5 loess layer. Value of δ13C has an average of -23.38‰, with the maximum -18.98‰ occurs in the S8 and S6 paleosols, and the minimum value of -26.99‰ appears in L7 loess. Most paleosol layers are characterized by higher TOC and more positive δ13C values, and loess units have lower TOC and more negative δ13C. Different from the CLP where both C3 and C4 plants contributed to soil organic matter, organic matter in this Northeast China loess-paleosol deposit is predominated by the input from pure C3 plants. Therefore, the observed δ13C variations in loess and paleosol are exclusively controlled by environmental effects on the δ13C of C3 plants. As higher precipitation or atmospheric CO2 during interglacials tend to decrease δ13C in C3 plants, the observed more positive δ13Cvalues in paleosol are most likely due to higher temperature. Variations in temperatures during glacial-interglacial cycles probably caused the δ13C variations in loess-paleosol sequence in Northeast China.