我国东北地区黄土堆积的磁性地层年代与古气候变化

Magnetostratigraphy of loess in northeastern China and paleoclimatic changes

我国东北地区有较广泛的黄土分布,是环境变化的重要地质记录.长期以来,由于缺乏可靠的年代学证据,对这里黄土沉积及其记录的古气候变化了解不多.在广泛野外踏勘的基础上,对具有代表性的科尔沁沙地南缘牛样子沟和平安村黄土剖面进行了磁性地层与光释光年代等测试,结果表明,牛样子沟剖面的B/M界限位于深度27m处,平安村剖面处于布容正极性期.基于光释光年龄和磁性地层年代计算的黄土沉积速率外推,科尔沁沙地南缘的黄土堆积应早于1.0Ma.黄土是半干旱环境下的粉尘堆积,指示东北地区的半干旱气候至少在早更新世晚期已经开始.这里黄土开始堆积的年代对应于中更新世全球气候转型,可能为全球气候变冷驱动了亚洲内陆干旱化和较快粉尘堆积的假说提供了证据.

Loess is widely distributed in northeastern China, and is an important geological record of past environmental change. For many years, the lack of a reliable chronology for this set of eolian dust-derived sediments has impeded our understanding of the paleoclimatic evolution and the land surface processes in this region. In this study, an extensive field survey of loess deposits was carried out, and two typical profiles at Niuyangzigou （NYZG） and Pinancun （PAC） were selected for detailed magnetostratigraphy and optically stimulated luminescence dating （OSL）. Results showed that the Brunhes/Matuyama magnetic reversal boundary may be pinpointed at a depth of 27 m for the NYZG profiles. The PAC profile does not contain this polarity shift. Thus, the age of the base of these loess deposits should be older than 1.0 Ma, by extrapolating the deposition rate, which was calculated by the magnetostratigraphic control point and the OSL age constraints. Loess is a dust accumulation that forms under semi-arid climate conditions and therefore indicates that a semiarid climate existed in northeastern China at least since the Early Pleistocene. This environmental change corresponds to the global Middle Pleistocene Climate transition, and it may provide evidence that global cooling drove aridity development in central Asia and rapid dust accumulation in northeastern China.