轨道调谐建立西宁厚层黄土堆积时间标尺的探索

青藏高原东北部西宁-互助地区堆积的厚层黄土是认识高原第四纪环境演化和气候变化的重要信息载体。对西宁泮子山厚层黄土钻孔（181.7m）进行了间隔5cm的采样，在实验室测量了全部样品的粒度，以粒度和北半球高纬太阳辐射数据为材料，选择古地磁年龄作为独立年代控制点，采用改进的自动轨道调谐方法，建立了西宁黄土早更新世以来天文轨道调谐的时间标尺。调谐后的粒度时间序列与ETP曲线在轨道周期上高度相关，在一定程度上证明了轨道调谐方法建立西宁厚层黄土堆积年代标尺的可行性。泮子山厚层黄土的沉积速率在1.0Ma前后发生了明显变化，1.0Ma以来沉积速率的相对高值可能反映了粉尘源区的干旱化程度增强，也指示了亚洲中部和青藏高原干旱环境的急剧发展。西宁厚层黄土年代标尺的建立，对深入认识晚新生代亚洲干旱气候发展过程和青藏高原的环境演化具有积极意义。

A PRELIMINARY ORBIT-TUNED TIME SCALE FOR THICK LOESS DEPOSIT AT XINING,THE NORTH EASTERN QINGHAI-TIBETAN PLATEAU

The thick loess-palaeosol deposit in North Eastern Qinghai-Tibetan Plateau (NQTP) is sensitive to reflect paleoclimatic changes,thus providing a good opportunity to investigate regional palaeoclimatic and palaeoenvironmental variations and the relationship with the global climatic changes.During August-November of 2004,we undertook a drill project at the site with the thickest loess deposit in the NETP,this drilling penetrates the loess and reaches fluvial gravels and pebbles underlaid the loess.The Panzishan core(181.7m) was sampled every 5cm,all samples were measured for grain size distribution which is regarded as an excellent proxy index of East Asian monsoon variations during Pleistocene.In this paper,paleomagnetic ages are chosen as independent control points instead of establishing an initial age framework by comparing the grain size variations with the previous typical grain size time series.And,the ETP curve is selected as target and the APC automatically orbital tuning method is used.A preliminary orbit-tuned time scale for Xining thick loess has been developed based on comparison between the mean grain size variations and the insolation of 65°N since the Early Pleistocene.After tuning,the new time scale is tested by cross spectral analysis.The results show that either the grain size record or the ETP curve has changed at a lager amplitude and higher frequency in orbital periods.In addition,the grain size record and ETP curve are highly correlated in the orbital periods.To a certain extent,this proves that our method to establish time scale for the Xining thick loess deposit in the past 2.0Ma is feasible.The sedimentation rate of Panzishan loess deposit shifts sharply at 1.0Ma,the average sedimentation rate is twice after 1.0Ma than that of the before one.The relative high sedimentation rate during the past 1.0Ma reveal aridity of dust resource was strong,probably imply a rapid development of the arid environment in Central Asia.We believe that this orbit-tuned time scale for Xining thick loess will bring further awareness for understanding paleoclimatic changes in Qinghai-Tibetan Plateau.