山西娘子关泉钙华记录的MIS12/11以来的气候和植被历史

对山西娘子关绵河不同阶地上沉积的泉钙华进行了高精度的230Th定年和碳氧稳定同位素组成测定。结果发现,绵河II级阶地沉积的娘子关泉钙华的最老年龄在407～466ka,远老于早前通过钙华中的石英砂热发光法(TL)获得的年龄,即绵河II级阶地上的娘子关泉钙华是在中更新世MIS12/11阶段形成的,而非原来认为的是晚更新世的产物。由此推测,绵河III级阶地娘子关泉钙华形成的年代更早,可能是中更新世的MIS14/13阶段。230Th测年获得的绵河I级阶地顶部的钙华形成于5ka前,即是在全新世中期以前形成的。结合钙华规模及其碳氧稳定同位素组成分析进一步发现,上述绵河三级阶地上的娘子关泉钙华均主要是湿热气候下的产物。然而,自II级阶地钙华形成至今,气候总体上在向干冷方向发展,泉域植被则呈现退化的趋势。

Changes in climate and vegetation in Niangziguan Spring Watershed of Shanxi Province since MIS12/11 recorded by the spring tafa

High precision 230Th-dating of the tufas deposited on different terraces of Mian River at Niangziguan shows that the ages of the oldest tufas on the terrace II are between～406 ka and～466 ka,which are much higher than those dated by the thermoluminescence method earlier,indicating that the tufas on the terrace II deposited during the Marine Isotope Stage(MIS) 12/11 of the middle Pleistocene but not in the late Pleistocene as suggested in early studies.It was inferred that the tufas on the terrace III of Mian River deposited earlier,possibly during the MIS 14/13 of the middle Pleistocene.The 230Th-dating of the top tufa on the terrace I shows that the tufa deposited 5 ka ago,i.e.,before the middle Holocene.Combined dimensions of tufa deposits with their carbon and oxygen isotopic compositions(δ13C,δ18O),it shows that all the tufas on the terraces of Mian River were formed mainly under hot and humid climate.However,from the time when the tufa on the terrace II was formed till now,the tendency of climate change was towards drier and colder,and the vegetation in Nangziguan spring watershed degraded gradually.