| The middle subtropical zone is an important distribution area of Quaternary red earth in South China.There are many researches on laterite types,genesis and environmental records.The typical aggradation laterite section is usually covered with a layer of "yellow cap",which is generally considered as the dust deposition since the late Pleistocene,called yellow-brown soil.The yellow brown soil layer in the upper part of laterite in South China is the most widely distributed in the middle and lower reaches of the Yangtze River,with the thickest sequence and aggradation attribute.It is one of the important carriers of Paleoenvironmental Research in the middle subtropical zone.Previous studies have made important achievements in grain size,magnetic susceptibility,geochemical elements and other aspects,previous studies on sporopollen are still relatively weak.In this paper,the typical yellow brown soil deposits in the middle subtropics are selected.Based on the discussion of their sedimentary characteristics,the study of palynoflora inversion paleoclimate is carried out to enrich the research results of laterite Paleoenvironment in the middle subtropics.In this paper,76 samples of palynological analysis were selected for the study of palynological paleovegetation reconstruction in the yellow brown soil sedimentary section of six additional laterite sedimentary sections,i.e.FLZ,FJW,JL,SH,HHZC,andPJZF.At the same time,28 samples of Xiashu loess and 11 samples of sporopollen analysis of homogeneous laterite were selected from the XG profile,Tangxi profile and YD profile in the middle of Zhejiang Province,in order to make clear the environmental change characteristics and internal relations of Xiashu loess,yellow brown soil and homogeneous laterite,combined with the data of grain size,geochemistry and sporopollen The temporal and spatial framework of environmental change and the development environment of yellow brown soil in the middle subtropical region since late Pleistocene are discussed.The preliminary conclusions are as follows:(1)The sedimentary parent material of the yellow brown soil in the middle subtropical zone shows the aeolian characteristics and is an important part of the aggradation red earth in the south of China.The test results of 680 grain size analysis samples of the yellow brown soil section of six accretion laterite profiles show that the average content of the yellow brown soil is 66.71% ? 73.95% with silt(4 ? 63 ? m)as the dominant grain group,of which the average content of fine silt(4 ? 10 ? m)is 14.24% ? 28.72%,and the average content of coarse silt(10 ? 63 ? m)is 46.00% ? 54.24%,which is rich in silt The average content of "basic particle size of wind dust"(10-50 ? m)is 41.82%-49.74%;the next is clay(< 4 ? m),17.48%-27.48%;sand(> 63 ? m)is the lowest,1.02%-11.19%.Compared with Xiashu loess and homogeneous laterite,the content of coarse silt is the highest(76.72%)and that of homogeneous laterite is the highest(30.28% ? 35.52%).The average content of the basic particle group of wind dust decreases from 53.21% of Xiashu loess to 41.82% ? 49.74% of yellow brown soil and 35.01% ? 37.23% of homogeneous laterite in turn.The average particle size(m ?)shows the characteristics of South thinning in space.Although there are weathering differences among Xiashu loess,yellowish brown soil and homogeneous laterite,the signal of wind separation among them under the pattern of winter wind field can not be ruled out,and the three and their forming environment may have internal relations.According to the longitudinal variation of grain size parameters of typical sections,the grain size composition of yellowish brown soil is coarser than that of reticulated laterite,reflecting the trend of cooling and drying of paleoenvironment after the gradual termination of reticulation process.Therefore,it is of great significance to study the sedimentary characteristics and development environment of yellow brown soil for understanding the information of monsoon evolution in the whole subtropical region.(2)A total of 115 samples,including 76 samples of yellow brown soil,28 samples of Xiashu loess and 11 samples of homogeneous laterite,identified 82 sporopollen types.The main woody plants are Pinus,Querus-evergreen,Querus-deciduous,Juglans,Moraceae,Altingia,Carpinus,Castanopsis,Anacardiaceae,Rosaceae,etc.,and the main desert steppe plants are Gramineae,Compositae,Chenopodiaceae,Artemisia,Aster,Cruciferae,Labiatae,etc The fern are mainly Hipriopteris,Polypodium and Hymenophyllaceae,and the algae are mainly Concentricystis.On the whole,the pollen assemblages of the yellow brown soil,Xiashu loess and homogeneous laterite are dominated by woody plants,and there are few desert steppe plant.But from the section,the desert steppe plant in the yellow brown soil increased from the bottom to the top along the section,and the Juglans pollen in the middle and upper part of the Loess in XG section appeared,which indicated that the climate changed gradually from the late Pleistocene to the cold and dry.In contrast,in the homogeneous laterite samples,there are Querus-evergreen and some ferns which mainly contain Polypodium spores of tropical and subtropical attributes.The environment is relatively warm and humid,and the trend of cooling and drying along the profile environment is not obvious.(3)The yellow-brown soil spore-pollen combination showed certain regional differences.First,there are more ferns in the lower part of the Pujiang(PJZF)section at the south latitude than in the Fulu Town(FLZ)and Fujiawan(FJW)sections at the north of the latitude;the middle of the PJZF section is more frequent than the FLZ and FJW sections Querus-evergreen pollen;the proportion of desert steppe plants in the upper part of the PJZF profile is significantly less than the proportion of desert steppe plants in the upper part of the FLZ and FJW profiles.The above characteristics well reflect the existence of a latitudinal gradient in the paleoclimate,that is,the period of yellow-brown soil development,and the climate in southern Anhui is warmer than that in central Zhejiang.Secondly,the sporopollen profiles of the three yellow-brown soil profiles in Jiujiang(JL),Shahe(SH),and Haihui(HHZC)in northern Jiangxi are consistent.However,due to the peculiarities of the geographical environment of the Poyang Lake area,they are in line with PJZF,FLZ,FJW and other profiles do not seem to have the spatial law of latitude or longitude,which is another manifestation of the regional differences of the yellow-brown soil spore-pollen combination.(4)The appearance of the looped algae in the yellow-brown soil and its changes along the profile have certain environmental indicators.Taking typical profiles such as FJW and SH as examples,the yellow-brown soil spore-pollen combination can be divided into two stages.In the first stage(in the lower and middle layers of the yellow-brown soil),only a few samples contained the Concentricystis,the content was very low;in the second stage(in the upper and lower layers of the yellow-brown soil),the content of the Concentricystis was significantly increased.After a comparative analysis,it was found that the changes in the pollen content of Concentricystis and desert steppe plants were relatively consistent,and the period during which the proportion of Concentricystis increased,corresponding to the period when the desert steppe plants content was higher.The period of abundant Concentricystis algae may correspond to the period when the water level of the Yangtze River and Poyang Lake decreased during the last glacial period.The decrease of water level of the river and lake resulted in a large area of river floodplain and lake bed exposed.Under the action of strong winter wind,the near source river lake facies materials and flood plain sediments with Concentricystis algae pollen contributed dust to the deposition of yellow brown soil.This conclusion has also been confirmed in some source studies.(5)Based on the pollen records of FJW,SH and other typical sections,combined with the comprehensive analysis of grain size,geochemistry,carbon chip statistics and other data,the climate of the yellow brown soil in the development period is cool and dry,which better responds to the trend of climate deterioration after the transformation of the Middle Pleistocene.Along the yellow brown soil profile,from the bottom to the top,the contents of ring algae and xerophytes increased,the concentration of fine and medium-sized carbon chips increased,the content of clay particles(< 4 ? m)decreased,the CIA value showing the weakening weathering degree decreased,and the SA value increased,all of which indicated that the climate became drier and colder since late Pleistocene.From another point of view,the appearance of cricoid algae also shows that the exposed lacustrine sediments during the ice age have a great contribution to the material source of the yellow brown dust deposition. |
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