西峰、洛川黄土碳酸盐根茎体碳同位素分布特征及古环境意义探讨

CARBON ISOTOPE COMPOSITION AND PALEOENVIRONMENT INFORMATION OF RHIZOLITH IN XIFENG AND LUOCHUAN LOESS

以往对于土壤碳酸盐根茎体(CR)的形态和碳同位素分析表明其具有纯的生物成因源的特征,被用于指示不同大陆的高分辨率气候变化。为此,本研究选取了位于黄土高原中部的西峰和洛川黄土-古土壤序列,分别采集65个和22个样品。通过湿筛法(80目)分离后,在显微镜下通过细针挑选出根茎体样品进行碳同位素(δ^(13)C_(CR))的分析,并将结果与总有机质碳同位素(δ^(13)C_(TOC))、总无机碳酸盐碳同位素(δ^(13)C_(TIC))、总有机碳含量(TOC%)和磁化率(MS)等多指标进行对比分析。结果表明:西峰剖面的土壤碳酸盐根茎体δ^(13)C_(CR)值的变化范围为-8.6‰～-3.7‰,δ^(13)C_(TOC)值的变化范围为-23.68‰～-19.47‰,δ^(13)C_(TIC)值的变化范围为-8.65%～-4.95‰;洛川剖面的土壤δ^(13)C_(CR)值的变化范围为-8.30‰～-3.58‰,δ^(13)C_(TOC)值的变化范围为-23.28‰～-18.72‰,δ^(13)C_(TIC)值的变化范围为-8.50‰～-3.78‰;两个剖面末次冰期以来的δ^(13)C_(CR)均没有完全响应MS,TOC和δ^(13)C_(TOC)的变化趋势,表现为在S_0呈现相反的变化趋势,在L_1呈现相似的变化趋势;而与δ^(13)C_(TIC)值在冰期-间冰期尺度上呈现一致的变化趋势和幅度。该结果表明西峰、洛川剖面碳酸盐根茎体碳同位素类似于总无机碳酸盐碳同位素,其影响因素可能较为复杂(如碳酸盐的淋溶迁移、植被的影响、原生碳酸盐等复杂因素的影响),特别是淋滤深度的不确定性。因此,在独立使用黄土碳酸盐根茎体δ^(13)C_(CR)值来重建该地区的古植被(C_4/C_3)变化信息时要慎重。

Previous morphological and carbon isotope studies have indicated the pure biogenicity of rhizolith that is applied in high-resolution modeling of climates of different continents. Therefore, the loess-paleosol sequences of Xifeng（35°47＇N, 107°36＇E） and Luochuan（35°47＇N, 109°26＇E） located in the middle of a loess plateau were chosen, at a profile depth of 11.0m and 9.3m,respectively. Both of the two sections contain L1 and SO loess-paleosol sequences. A total 205 samples in 10cm interval （111 samples for the Xifeng section, and 94 samples for the Luochuan section）were collected for magnetic susceptibility （MS）. In Xifeng and Luochuan sections, the high MS values occur in SO paleosol and LlSS1 weakly developed paleosol,varing from 60×10-a m3/kg to 160×10-8 m3/kg. The low MS values occur in LILL1 and L1LL2 loess,varing from 30×10-a m3/kg to 60×10-8 m3/kg. And the TOC content show similar variation with MS values in both sections. A total 87 Rhizoliths samples（65 samples for the Xifeng section in 20cm interval, and 22 samples for the Luochuan section in 40cm interval）were collected by wet sieving the loess samples through 80mesh screen using a sharp needle under a binocular microscope. The collected rhizoliths were subjected to carbon isotope analysis to compare the magnetic susceptibilities, stable isotopic compositions,and TOC content of the loess-paleosol sequences. The following variations were noted in the Xifeng profile： δ13CcR from -8.6‰ to -3.7‰; δ13CTOC from -23.68‰ to - 19.47‰; and δ13CTIC from -8. 65‰ to -4. 95‰. By contrast, the Luochuan profile showed the following variations： δ13CCR from -8.30‰ to -3.58‰; δ13CTOC from -23. 28‰ to -18. 72‰; and δ13CTic from -8. 50‰ to -3. 78‰. The δ13CcR values did not correspond completely to MS, TOC and δ13CToc variations in the Xifeng and Luochuan profiles since the last glaciation, indicating a contrasting trend in SO and a similar trend in L1. However, they showed similar variation trends and amplitudes to δ13CTIC in the glacial-interglacial interval. These results indicate that theδ13CcR values are similar to the δ13CTIC values. This phenomenon may be caused by complicated processes such as leaching and movement of carbonate, the interference of vegetation, and the influence of the primary carbonate, particularly in undetermined leaching depth. Therefore, a careful reconstruction of the paleovegetation （ C4/C3 ） using only the δ13CCR value was performed.