Dust Accumulation Process And Provenance Indicated By Grain Size Characteristics Of Huahonggou Loess In Dalian

The grain size characteristics of sediments and endmember analysis models are of great significance in tracing provenance,exploring sedimentary dynamics,and reconstructing paleoclimate environments,the surface microstructure characteristics of quartz grains play an important role in inferring the genesis of sedimentary environments.Previous research has suggested that the Liaoning coastal loess is mainly composed of dual material sources,which are shelf sandy deposits from the Bohai Sea and dust particles from the northwest inland.However,whether the provenance of the loess in southern of Liaoning is consistent and the detailed sedimentation dynamics characteristics and transport mechanisms during its accumulation have not been intensively discussed.The special geographical position of Huahonggou loess,which is adjacent to the Yellow Sea and close to the Malan River basin,gives it important research significance.In this study,the Huahonggou loess was used as an object of study.Firstly,the OSL dating method was used to date the deposition of the profile and to infer the paleoclimatic environment at the time of its accumulation.The differences between the material source and the Liaoning coastal loess were analysed by comparing the material source with that of the Liaoning coastal loess;the sedimentation dynamics and source-sink processes of the Huahonggou loess were recovered from the different end member components selected by the end element analysis model;finally,the depositional environment and transport dynamics indicated by the surface features of the quartz grains were analysed by SEM.The OSL dating,grain size and SEM analyses of the Huahonggou loess yielded the following conclusions.(1)The section dating results show that the OSL samples are extremely close in age all around 124 ka and belong to the early Late Pleistocene,an abrupt rapid cooling event during the transition from MIS5 e to MIS5 d in the last interglacial.The environmental characteristics of this period show a trend of weakening summer monsoon and gradually increasing winter monsoon.As a result,the depositional environment was dominated by dry and cold climatic conditions,with strong physical weathering and rapid deposition rates.At the same time,due to the scarcity of precipitation,soil formation is weak,which is conducive to the deposition of coarse-grained loess.(2)In terms of particle size composition,there are obvious differences between the Huahonggou loess and the Liaoning coastal loess.Huahonggou loess has the highest fines content and is dominated by coarse fines.However,the coastal loess is composed of coarse chalk and fine sand,with a coarser grain size composition.The frequency distribution curve of the profile shows a bimodal distribution structure with a complex source and poor sorting,but the main peak is obvious(4-6?),mainly concentrated in the coarse chalk,indicating that the source is more concentrated and dominated by near-source deposition,while a small amount of distant fine-grained deposition exists between the secondary peaks of 0-2?.The grain size parameters indicate that the loess deposits are typical of aeolian deposition,with larger mean and median grain sizes and coarser grain sizes,and therefore require stronger depositional dynamics for transport and accumulation.The grain size fractional dimension of the profile calculated using the power index method is 2.22.When compared with the coastal loess,riverbed sediments and seafloor sediments,the grain size fractional dimension of the Huahonggou loess is significantly smaller than that of the Dalian Qidingshan loess and seafloor sediments,but is very close to that of the riverbed sediments.This shows that the Huahonggou loess has some differences compared to the shelf sandy deposits of the coastal loess which mainly originated from the Bohai Sea.The near-source deposits of the profile probably originated mainly from the nearby Malan River tributary basin.The dry and cold climatic conditions exposed the riverbed,with strong near-surface winds thus transporting and accumulating sandy deposits on the riverbed substrate,while the distal deposits are similar to those of the coastal loess,originating from fine-grained dusty material from the north-western interior.(3)The surface micromorphological features of the quartz grains were observed using scanning electron microscopy(SEM)and revealed that the quartz sand grains are poorly rounded,with sub-angular grains being the most abundant,followed by sub-rounded grains,and the surface undulations of the quartz grains are predominantly medium protrusions,all of which imply that the quartz sand grains in the profile were transported over a relatively short distance.The surface of the quartz grains is not only characterised by typical wind-formed environmental features such as disc-shaped pits,crescent-shaped impact pits and pockmarks/facets,but also by water-formed environmental features such as V-pits,submerged polished surfaces and shell-like fractures,with the two features appearing in combination with each other.At the same time,the chemical and mechanochemical effects on the surface of the quartz sand grains are also evident,with the surface of the grains becoming rough and uneven due to chemical dissolution and precipitation,and in some cases the entire surface of the grains is even completely covered by a siliceous film,indicating that the quartz sand grains were affected by the underwater high-energy environment and chemical effects in a hot and humid climate.(4)The parametric end member analysis model extracted three end member components representing the depositional dynamics,and reconstructed the dust accumulation process and the transport dynamics of different intensities of the Huahonggou loess.EM1(10.75?m)belongs to the clay component,which is the suspended component of long-distance transport under the action of high-altitude westerly circulation.EM2(30.07?m)belongs to the fine silt sand component,which is the close suspended transport component under the blowing of low-altitude monsoon.EM3(52.63?m)is a coarse component of coarse sands,which is a typical grain size component of loess composed of river sediments.The end element frequency distribution curve is the highest and narrowest of the three end elements,and the sorting coefficient is the smallest,indicating that this end element is a concentrated source of material,which generally requires strong transport power for transport and a short transport distance.The coarse particulate material is transported close to the surface under dust storm conditions and is transported by near-surface suspending or leaping.