Environmental magnetic record of the fluvial sediments from the Tianzhu borehole in Beijing for the last 800 ka

摘要

A high-resolution environmental magnetic investigation has been carried out on a 186.3-m-thick sequence of fluvial sediments from the Tianzhu borehole in the Beijing Plain. Magnetic stratigraphie analysis revealed that the sediment sequence spans the last 800 ka without significant hiatuses. Comparison of the magnetic susceptibility (k) with pollen records suggests that the magnetic susceptibility serves as a good proxy for climate changes. The high k values of the sediments reflect warm climate conditions, and the low k values match cold periods. Magnetic susceptibility records correlate well with the ODP 677 oxygen isotope proxy, with high/low k values corresponding to the interglacial/glacial stages, implying a climate alternation of glacial-interglacial cycles in the Beijing Plain. The lithologie analysis of the Tianzhu borehole cores suggests that climate change was the primary factor in controlling the fluvial sedimentary cycles in the studied area. A fluvial sedimentary model in which precipitation affected the fluvial sediment by controlling the discharge and transport capacity of rivers in different stages may be applied to our research. The rock magnetic records of the Tianzhu fluvial sequence display two extreme events at 750-720 ka and 270-250 ka, respectively, which are regarded as a two-period cycle of aridification and cooling in the Asian interior. The extreme event at 750-720 ka likely resulted from the fast uplift of the Himalayan-Tibetan Plateau and the expansion of Arctic ice-sheet.