Loess Stratigraphy And Palaeoclimatic Records Since 1.2 Ma In Northeastern China

Little is known about the long-term climatic evolution of northeastern(NE)China,because well-preserved terrestrial paleoenvironmental archives are scarce in this vast region.The extensive loess deposits in the Chifeng region are the thickest and most complete loess-paleosol sequence in NE China,and have significant potential for reconstructing the paleoclimate of the region.Here,we present new high-resolution magnetostratigraphic results from three loess-paleosol sections in the Chifeng region:the Sanbahuo(SBH),Niuyangzigou(NYZG)and Toudaojingzi(TDJZ)sections.The results demonstrate that all the sections record the Brunhes normal polarity chron and the upper portion of the Matuyama reversed polarity chron.Based on these new results,the base of the SBH and NYZG section are dated at ca.1.0 Ma,and the base of the TDJZ loess sequence is dated at ca.1.2 Ma,representing the oldest known loess deposits in NE China.The magnetostratigraphic results of the three sections demonstrate that the loess deposits in NE China have preserved a complete loess-paleosol sequence(L1-L14)since the late Early Pleistocene.The sequence comprises the equivalent of the Malan Formation(L1),the Upper Lishi Formation(S1-L5)and the Lower Lishi Formation(S5-L15)on the Chinese Loess Plateau(CLP).However,there are some significant differences between the stratigraphy of our sections and that of typical sections on the CLP.It is noteworthy that in contrast to the classical stratigraphy of the CLP,S5 is not composed of a distinct paleosol triplet,and the L9 unit('sandy loess' marker bed)is not characterized by the highest proportion of coarse silt and fine sand.The most prominent marker bed of the loess deposits in NE China is the paleosol group S9-S13,which consists of an interval of densely-packed paleosols intercalated with weathered loess layers containing layers of large carbonate nodules similar to those on the CLP.In terms of its reddish color,and the abundance of large carbonate nodules,this paleosol complex resembles the four major paleosol groups of the Wucheng formationon the CLPThe grain size analysis suggests that overall the grain-size characteristics of the loess deposits in NE China are very similar to those of the sandy loess in the northern CLP,and that the sand-sized component dominates the grain-size record of the entire section of the SBH and NYZG section.Because the SBH section is more close to the sandy deserts in NE China,and has been demonstrated to be sourced from these areas,the loess samples are enriched in sand-sized particles associated with short-distance transport,which accords with their proximal provenance.Therefore,we infer that variations in the content of the sandfraction(>63?m)in the NE China should be sensitive to migration of the desert margin and to wet-dry climate oscillations within the dust sources areas,as is the case for the loess deposits in the northern CLP.We also noted that the MS values of the mid-Pleistocene paleosols from S5 to S1 were generally lower than the early Pleistocene paleosols from S11 to S6 in the SBH and NYZG sections,which was contrary to the MS records of the representative sections on the CLP.We propose that the increased flux of sand-sized quartz grains(with a diamagnetic susceptibility)and coarse-grained lithogenic magnetic minerals probably led to the dilution of the magnetic susceptibility signal derived from the pedogenic production of ferrimagnets at the NYZG site during these interglacial phases.The initiation of loess accumulation in NE China was roughly synchronous with the large expansion of the Eurasian loess belt during the late Early Pleistocene(1.2-0.7 Ma)and the initiation of the Mid-Pleistocene climate transition(MPT),probably reflecting a widespread intensification of aridity in northern middle latitudes.The grain-size record reveals a long-term drying trend in NE China since ca.1.2 Ma,punctuated by two significant abrupt drying events at?0.9 Ma and?0.7 Ma,which is corresponding to the extremely cold event(MIS22-24)during the MPT and the time of the largest ice volume estabilshment at the termination of the MPT,respectively.In addition,the wavelet analysis shows that the grain size and magnetic susceptibility time series of the loess deposits in NE China have their strongest power in frequency-time space with a quasi-periodicity of?100 ka.Thus,all the periodicity,drying trend and events are consistent with the variability of global ice volume and global cooling since the MPT.We suggest that the global ice volume increase and global cooling during the MPT was probably responsible for the aridification and dust accumulation in NE China.Our observations provide direct evidence for a linkage between global cooling and aridification in the Eurasian interior during the Quaternary,particularly during the MPT.