洛南黄土有机碳同位素组成及其与洛川、西峰黄土对比

以往黄土-古土壤碳同位素的研究主要集中在东亚季风控制的干旱-半干旱黄土高原中北部地区，而对于秦岭南部暖温带季风性半湿润气候区黄土-古土壤序列有机碳同位素的研究鲜有报道。对比两种气候条件下的黄土有机质碳同位素组成可更广泛和深入地认识黄土碳同位素所指示的生态环境变化。为此，本研究选取了位于秦岭南麓的洛南上白川、刘湾两剖面，剖面厚度分别为74m和10m，以20cm间距采集上白川剖面38个样品，刘湾剖面51个样品。采用高温灼烧氧化法对全部样品有机质碳同位素组成进行测定，并将结果与黄土高原中部洛川和西峰剖面有机质碳同位素进行了对比。结果表明：  上白川剖面有机碳同位素( δ¹³C_org.)的变化范围为-24.43‰～-21.30‰，刘湾剖面的变化范围为-24.70‰～-20.52‰，洛川剖面的变化范围为-23.37‰～-18.72‰， 西峰剖面的变化范围为-23.68‰～-19.47‰。末次间冰期位于半湿润地区的上白川和刘湾剖面有机碳同位素峰值较半干旱地区洛川、西峰剖面偏负1‰～2‰，即上白川和刘湾剖面有机碳同位素峰值分别较洛川剖面的峰值偏负1.8‰和1.9‰，较西峰剖面的峰值偏负1.3‰和1.4‰。因此，该区的植被覆盖应是以C₃植物为主的C₃和C₄混合草原植被类型；   从整体上来看，洛南剖面古土壤层碳同位素较相邻黄土层的碳同位素偏正，因此，古土壤形成时期C₄植被相对比例增加。末次间冰期洛南上白川和刘湾剖面有机碳同位素峰值较洛川、西峰剖面的峰值偏负1‰多，表明受东亚季风控制的半干旱-半湿润的黄土区，暖季节降水对C₄/C₃植被相对比例增加的趋势和幅度均具有明显的影响，主要表现在暖季节降水的增加有利于C₄植被相对比例的增高，同时降水的季节性分布和/或降水量的增多影响土壤有机碳同位素所记录的C₄植被相对比例及其增加幅度。

ORGANIC CARBON ISOTOPE COMPOSITION OF LUONAN LOESS COMPARED WITH THAT OF LUOCHUAN AND XIFENG LOESS

Previous organic carbon isotope studies on loess-paleo-sol sequence focused on the arid-semiarid Loess Plateau controlled by the East Asian monsoon with few reports on carbon isotope composition of loess-paleosol from Southern Qinling  Mountain that locates in semi-moist and temperate climate monsoon zone. The studies of different organic carbon isotope composition of loess-paleosol from these two climate types may help us to better understand the ecological environment changes. For this purpose，we sampled 38 soil samples from the Shangbaichuan profile with a thickness of 7.4m，and 51 samples from the Liuwan profile with a thickness of 10m，both of which are located in the Luonan Basin in Southern Shaanxi. We analyzed the organic carbon isotope compositions of loess-paleosol collected from these two profiles. The obtained δ¹³C results were compared with those of loess-paleosol from the Luochuan and Xifeng profiles in the center of the Loess Plateau. The results showed that the δ¹³C_org. values vary from -24.43‰ to -21.30‰ in Shangbaichuan profile，from -24.70‰ to  -20.52‰ in Liuwan profile，from -23.37‰ to -18.72‰ in Luochuan profile，and from -23.68‰ to -19.47‰ in Xifeng profile，respectively. The paleosol carbon isotope in Luonan was more positive than the adjacent loess one. The peak values carbon isotope of loess in semi humid region is about 1‰ more negative than those of loess-paleosol in arid semiarid region in the last interglacial. In detail，the peak values of carbon isotope from the Shangbaichuan and the Liuwan profiles are about 1.8‰ and 1.9‰ more negative than that of the Luochuan profile，and more negative about 1.3‰ and 1.4‰ than that of Xifeng profile. Therefore，the vegetation coverage in Luonan area was mainly a mixture of C₃ and C₄ grassland. From the overall point of view，the paleosol carbon isotope of Luonan profiles was more positive than the loess one，implying that the relative proportion of C₄ plant increased during the paleosol forming time. The peak values of carbon isotope of loess in semi humid region are about 1‰ more negative than those of loess-paleosol in arid semiarid region in the last interglacial，showed that warm season precipitation had obvious influence on both the increasing trend and the rate of C₄/C₃plants. In detail，the increase of warm season precipitation was beneficial to the increasing of C₄ vegetation，and the seasonal distribution of precipitation and/or the increase of warm season precipitation volume would influence the relative proportion of C₄/C₃ plants which can be recorded by the soil organic carbons.