甘肃西峰红粘土~4.5Ma孢粉记录的植被转变及其与青藏高原隆升的关系

甘肃西峰红粘土剖面蕴涵了大量古环境和古气候变化信息,是重建黄土高原生态环境和气候演化的重要地质记录。通过对红粘土地层孢粉学研究,发现孢粉组合特征在~4.5Ma发生了明显转变,乔木植物花粉明显减少,草本植物花粉大量增加,尤其是藜科花粉百分含量显著增加,麻黄出现频率明显增高;孢粉类型C/A(Chenopodiaceae/Artemisia)和B/A(Betula/Artemisia)的比值发生显著变化,反映研究区从~4.5Ma开始植被类型由稀疏森林草原逐步转变为典型草原,甚至荒漠草原,揭示了亚洲内陆生态环境的一次显著变干事件。这一信息与前人研究红粘土中其他替代性指标指示的气候变干过程具有很好的一致性。孢粉组合中藜科植物含量在此时迅速递增,反映黄土高原气候的季节性反差明显加强。青藏高原在~4.5Ma发生的隆升可能是导致这次亚洲内陆干旱化的主要驱动因素,高原对暖湿气流的屏障和加强季节性反差可能是导致干旱化的重要途径。

Significant paleovegetation shift at～ 4.5 Ma recorded in the red clay sequence of the central China loess plateau and its link with the Qinghai-Tibet uplift

In northern China, the late Miocene-Pliocene red clay in the eastern Loess Plateau fills a gap of climate records between the well-known loess-soil sequences of the last 2.6 Ma and Miocene loess-soil sequences from the western Loess Plateau. Early studies indicated that the red clay is also of windblown origin. The red clay, therefore, serves as a good archive for reconstruction of paleoecological succession and paleoclimate change. In this study, a palynological investigation was conducted on the late Miocene-Pliocene red clay sequence at Xifeng, central Loess Plateau,northwest China, which provides new insights into the nature of the vegetation evolution and climate change from 6.2 to 2.4 Ma. The results show that during this time interval the central Loess Plateau was covered mainly by a steppe vegetation, indicating a lasting dry climatic condition. A marked ecological shift towards drier climate, recorded by pollen assemblages, occurred at ~4.5 Ma, when a sparse wood grassland (wood-steppe) gradually changed into typical grassland. Firstly, the mean percent of arbor pollen decreased from 27% to 12%, along with the disappearance of some arboreal taxa. In addition, the mean percent of birch pollen decreased from 10% to 0.6%; robur, from 1.6% to 0.5%; and Pinus, from 15% to 10%. Secondly, there is an apparent increase of herbaceous vegetation from mean percentages of 71% to 85%. The mean percent of Chenopodiaceae increased from 8% to 20% with a peak value of 54%. The extremely drought-tolerant species Ephedra frequently occurs, also with a sudden increase in its mean percent. Thirdly, the ratios of Chenopodiaceae/Artemisia and Betula/Artemisia, which are good indices for moisture, exhibit a significant increase and a sudden decrease, respectively, at ~4.5 Ma. These characteristics suggest an expansion of drought-tolerant species, signaling a prominent drying event during that period, which coincides with other proxies and other place records. A significant increase in herbaceous Chenopodiaceae indicates that the seasonal reversal was intensified at ~4.5 Ma. The uplift of the Qinghai-Tibet plateau at about 4.5 Ma, which resulted in intensification of the monsoon reversal, is thought to have played an important role in this significant ecological change.