西宁黄土堆积记录的最近13万年高原季风气候变化

PLATEAU MONSOON VARIATIONS DURING THE PAST 0.13 MA INDICATED BY LOESS DEPOSIT AT XINING, NORTHEAST QINGHAI, CHINA

青藏高原对于大气的热力和动力作用形成了高原季风气候现象。青藏高原东北部西宁盆地位于高原季风控制区 ,末次间冰期以来的黄土堆积记录了高原季风气候变化过程。对西宁盆地湟水阶地上的黄土堆积进行了地貌观查、地层对比和探槽及探井采样 ,完成了热释光和光释光测年 ,测量了古地磁定向样品以及磁化率、频率磁化率、CaCO3含量和粒度等古气候代用指标。结果表明 ,西宁黄土堆积记录的高原夏季风环流在相当于深海氧同位素阶段5e特别强 ,在 5a和 5c阶段接近于阶段 3。高原夏季风和冬季风变化存在位相差以及冬季风强的时候夏季风不一定弱 。

Climate variations at Xining area, northeast Qinghai Xizang Plateau is characteristic of alternations of plateau summer and winter monsoon circulation, which is generated by thermodynamic and kinetic effect of the immense Qinghai Xizang Plateau. The plateau monsoon variation in geological time was paid less attention previously. However, changes of the plateau monsoon is important for our understanding of climate change above and around the plateau, the process of the plateau uplift as well as the mechanism of East Asian monsoon changes.\;Loess deposits at Xining, northeast Qinghai Xizang Plateau, present an excellent record of the plateau monsoon variations in the Pleistocene and even in the Pliocene Epochs. The loess sediment is more than 200 meters thick, interceded by paleosols. It was showed that loess was deposited in cold dry time, where the paleosol was developed during warm humid episodes. The loess and paleosol formations are correlated to the glacial and interglacial periods, respectively.\;The loess paleosol sequence of the last 0.13Ma were sampled by digging ditch and well in order to obtain un contaminated samples, since secondary loess cover in this region is quite thicker. The loess deposit, located at low terrace of Huangshui River, was sampled at interval of 5～10 centimeter. A 44 m loess paleosol profile at Tuxiangdao was sampled. Black loam is found in upper part of the exposure, and it is regarded as typical sediment formed during the Holocene Optimums in North China. Malan loess, which was formed during the last glacial period and correlated to marine isotope stage 2～4,is interceded with very thinner, weak paleosols. Underlying the Malan loess is reddish paleosol, correlated to the time of marine isotope stage 5. Thermoluminescence (TL) and optically stimulated luminescence (OSL) dating of the loess deposit proved that the loess was formed during the last glacial cycle. In order to get parallel records, we sampled a loess deposit at high altitude terrace at Panzishan, Xining, from a well. The well is 17 meters in depth, and the bottom of the last interglacial paleosol is at 13.5m deep. Stratigraphic correlation between two sites is very good and shows the loess paleosol deposit was formed during the last glacial cycle. A detailed palaeomagnetic investigation hasn′t found the Blake excursion. However, stratigaphic correlation, absolute dating and geomorphologic observations tend to suggest that the loess pa leosol deposits were formed during the last glacial cycle.\;We measured magnetic susceptibility, frequency dependent susceptibility, calcium carbonate content, which are regarded as good proxy of the plateau summer monsoon (PSM) strength. The grain size distribution is regarded as a sensitive proxy index of the plateau winter monsoon (PWM) strength. The results show that the summer monsoon was unusually strengthened during the period correlating to the marine isotope stage 5e, and strength of the PSM in 5a and 5c was close to the stage 3. The PWM was frequently changed in the stage 3 indicated by the grain size record. There are existed phase lag and lead between the PWM and PSM records, the two systems were not co variations during the last 0.13Ma.\;There are sub orbital time scale variations in both the grain size and frequency depend ent susceptibility records, but we can not separate the local and regional climate signals from these records. We believe that, mixture of local and regional climatic signals, lower time reso lution as well as non detailed dating of the loess paleosol sequences might have caused many uncertainties in the long distance climatic correlation in the Pleistocene period. The previous conclusion that the climatic correlation in East Asia and the North Atlantic should be further examined.\;Our ongoing investigations in northeast Qinghai Xizang Plateau will spatially and temporally extend the environmental records. A preliminary result of grain size and magnetic susceptibility measurement of a 2 Ma continuous loess paleosol sequence