河北坝上丰宁黄土剖面记录的230 ka BP以来的环境演化*

河北坝上地区位于东亚季风气候边缘区,对气候变化响应敏感。丰宁黄土剖面为研究该区域长时间尺度的环境演化提供了理想材料。通过丰宁黄土剖面地球化学元素和磁化率分析,结合光释光测年结果,恢复了该地区230,ka BP以来的环境演化历史; 通过野外观察,识别出了S₁古土壤之下发育的风化淋滤黄土层,探讨了S₁古土壤发育时期的风化淋滤特征及其所揭示的气候变化问题。结果表明: (1)230,ka BP以来的磁化率记录可与深海氧同位素3—8阶段进行细节上的对比,表明丰宁黄土堆积区对全球气候变化有着积极的响应;(2)S₁古土壤发育时期,由于受到强降水的影响,土壤中元素受到强烈淋溶从而向下迁移到L₂黄土中,造成L₂上部地球化学记录与磁化率的差异;(3)从S₁古土壤顶部到钙结核层之间土壤属于酸性淋溶土,可能相当于现今中国北亚热带的黄棕壤,其剖面由土壤层、风化淋滤黄土层和CaCO₃淀积层构成;(4)S₁古土壤发育时期的温度和降水量与现今江苏泗洪和六合地区相当,表明当时研究区为亚热带气候。

Environmental evolution since 230 ka BP recorded by Fengning loess section in Bashang area of Hebei Province

Bashang area of Hebei Province,located in the peripheral region of the East Asian Monsoon,is sensitive to climate change. The Fengning loess section provides ideal materials for studying long-time environmental changes in this region. Combined with OSL dating results,elemental concentrations and magnetic susceptibility were determined to obtain the record of environmental evolution since 230 ka BP. Based on identification of weathering and leaching loess under S₁ in the field,we also discussed weathering features in S₁ paleosol and the relevant climate change. The results showed that:(1)Magnetic susceptibility record of the Fengning loess section coincided with marine isotope stage 3-8 perfectly,that indicated that the study area had given a sensitive response to globe palaeoclimatic changes since 230 ka BP;(2)Due to the impact of heavy rainfall in the period of S₁ paleosol development, elements were strongly leached and migrated downward to L₂ loess,resulting in the difference between geochemical records and magnetic susceptibility in the upper of L₂ loess;(3)The Layer from S₁ paleosol to calcrete belongs to acidic leached soil type that can be equivalent to yellow-brown soil developed at modern northern subtropics, and is consist of soil layer,weathered and leached loess layer and CaCO₃ illuvium; (4)The temperature and precipitation in the period of S₁ paleosol development were quite similar to those in Sihong area and Liuhe area of Jiangsu Province,that indicated that the climate of the study area was subtropical during the forming period of S₁ paleosol.