Material Source Changes In Harbin Loess Since The Middle Pleiocene And Their Genesis Mechanisms

Loess contains rich paleoclimatic and palaeoenvironmental information,and understanding its physical source changes is crucial to understanding the aridification of inland Asia,atmospheric circulation patterns,and the evolution of the Asian monsoon.The Harbin loess,located at the forefront of Asian inland aridification,is an ideal vehicle for studying climatic aridification in northeastern China,and the study of its physical source can help explain the coupled relationship between wind and dust accumulation and tectonic-geomorphic-climate change in the region.The automatic mineral identification system is fast in analysis and rich in data acquisition.Therefore,in this paper,the TIMA（TESCAN Integrated Mineral Analyzer）automated quantitative mineral and chronological(OSL,ESR and ¹⁴C)tests of the Harbin loess-paleosol sequence were investigated with the following results:（1）The age of the bottom boundary of Harbin loess was inferred from OSL and ESR dating data and ¹⁴C data to be about 0.46 Ma,and the age of Harbin loess-paleosol sequence at 15.1 m was 234.2 ka.（2）There are 15 kinds of heavy minerals identified by the traditional optical method,such as zircon,apatite,rutile,anatase,albite,titanite,monazite,hornblende,tourmaline,garnet,chlorite,etc.In addition to the 15 kinds of minerals identified by the traditional optical method,the TIMA method also includes neonite,crossite,corundum,chromite,calcite,etc.,a total of 61 kinds.（3）The TIMA method has absolute advantages in heavy mineral identification species,large sample analysis and identification efficiency,however,it performs poorly in the identification of some homogeneous polymorphic minerals（e.g.,Ti O₂group polymorphic minerals,hematite/chalcopyrite and magnetite.）There are some differences in the heavy mineral composition identified by TIMA and traditional methods,but the revealed heavy mineral assemblages（ilmenite,cordite and amphibole types）are basically the same,indicating that both methods can effectively reveal the heavy mineral composition of Harbin loess.（4）The heavy mineral contents and dominant mineral assemblages of the 20-63μm fraction of the loess-paleosol sequence differ significantly in the upper and lower strata at 15.1 m（234.2 ka）,specifically in the upper strata（0-15.1 m）where the contents of ferroplate titanite,zircon,rutile,ilmenite,chromite and titanite are significantly lower than those in the lower strata（15.1-29.9 m）,while the contents of amphiboles,The content of amphibole,apatite and pyroxene is significantly higher than that of the lower stratum.The combination of heavy minerals in the upper stratum is titanite+ilmenite+hematite+amphibole,while the combination of heavy minerals in the lower stratum is ilmenite+titanite+hematite+zircon.The differences in heavy mineral characterization indices（ZTR and GZi）and PCA analysis further indicate the different material sources in the upper and lower strata.The<20μm fractions that are well and uniformly mixed after long-distance transport do not clearly reflect the difference in the source of the upper and lower strata.（5）The comparative analysis of the heavy mineral characteristics of Harbin loess and northeastern sands shows that the lower stratum of Harbin loess receives contributions from the Sonnen sands in addition to the distant dust contributions from the Hunsandak and Horqin sands.However,with the increasing aridification of the Songnun Plain and the gradual expansion of the area of the Songnun sands,the contribution of distant sources is significantly reduced under the effect of enhanced winter winds,which provide more dust particles to the upper stratum of the Harbin loess.The gradually increasing climatic aridification is the root cause of the physical source change of Harbin loess.