皋兰山红黏土-黄土记录的上新世-更新世转型期C3/C4植被演化

上新世-更新世转型是上新世温暖气候向更新世冰期-间冰期旋回过渡的重要时段，与此同时，青藏高原的强烈隆升也深刻改变了高原及周边地区的地貌格局和生态环境面貌。因此，开展青藏高原东北缘地区上新世-更新世转型期的古气候变化研究是理解地球各圈层相互作用的重要切入点。而兰州盆地地处中国三大自然区的衔接位置，对气候变化和构造活动响应较为敏感，是探讨构造-气候-生态相互作用的理想区域。以兰州皋兰山红黏土-黄土岩芯为研究对象，在已有黄土高精度磁性地层年代学基础上，综合开展了有机碳同位素和长链正构烷烃碳同位素分析，重建了西部黄土高原上新世-更新世转型期（3.0~2.2 Ma）的C₃/C₄植被相对丰度演化历史。结果表明：皋兰山上新世-更新世转型期风成沉积中正构烷烃多以C₃₁为主峰的单峰模式，可能指示了草本植被的主要贡献。而典型黄土层位（L32和L33）的正构烷烃多呈现微生物降解的双鼓包模式，表明饱和烃受外源输入影响。综合对比常规指标、总有机碳和长链正构烷烃碳同位素后认为，皋兰山底部黄土的总有机碳同位素不能直接用于重建区域C₃/C₄植被演化。长链正构烷烃碳同位素研究表明：在3.0~2.2 Ma时期，皋兰山古生态整体以C₃植被为主；C₄植被扩张发生在3.0 Ma之前，3.0~2.9 Ma发生C₄植被快速减少事件，初步推测这可能与降温有关。

The C3/C4 vegetation history during the Pliocene-Pleistocene transition:evidence from the Gaolanshan red clay-loess deposition

As the critical stage from the Pliocene to the Quaternary， the Pliocene-Pleistocene transition during the interval of 3.0~2.2 Ma obviously experienced climate change from warm climate to the Pleistocene glacial-interglacial cycles. Meanwhile， the Tibetan Plateau also experienced intensive tectonic uplift， the ecological environment and geomorphologic pattern of the Tibetan Plateau and its surrounding areas have been significantly shaped. So the detailed research on the paleoenvironment evolution during the Pliocene-Pleistocene transition can shed new lights on the coupled relationships among the atmosphere， biosphere and lithosphere. The Lanzhou Basin is the ideal place to discuss the interactions between the tectonic uplift， global cooling and regional ecosystem. Here the Gaolanshan red clay-loess sequence from the Lanzhou Basin were detailed analyzed via the n-alkanes distribution and organic geochemistry （total organic carbon and long-chain n-alkanes carbon isotopes） methods. Particularly， the C₃/C₄ relative abundance evolution history during the interval of 3.0~2.2 Ma in Lanzhou Basin were reconstructed. In detail， the n-alkanes of Gaolanshan red clay-loess sequence during the interval of 3.0~2.2 Ma mainly be characterized with unimodal pattern and the C₃₁ main peak， which implicates the contribution of grassland vegetation. In addition， the n-alkanes distribution of typical loess L32 and L33 show the existence of double bulge， obviously implicating the microbial activities. After the integration of total organic carbon isotope， long-chain n-alkanes carbon isotope and grain sizes， we proposed that organic carbon isotope could be affected by exogenous input via the mixing n-alkanes by the wind. Therefore， we suggested that the total organic carbon isotopes should not be used to reconstruct the C₃/C₄ relative abundance during the Pliocene-Pleistocene transition in the Lanzhou Basin. Based on the integrated evidence of classical proxies and leaf wax compound-specific carbon isotope， the Gaolanshan paleoecology during the interval of 3.0~2.2 Ma was dominated by the C₃ vegetation except the short C₄ expansion before 2.9 Ma. It is obvious that the C₄ vegetation sharply decreased near 3.0 Ma， possibly correlated with global cooling.