Characteristics Of Loess Weathering And Its Response To Climate Change In Shangdan Basin

Revealing the mechanism and causes of past global changes is the basic task of the research of global environment changes, the climate variability of last glaciation and the Holocene is the focus part. The Danjiang River Valley is located in the north subtropical zone, which is a transition regional between the North Temperate zone and northern subtropical region, belong to a sensitive climate change regional. It plays an irreplaceable role in environment change study in epipleistocene-Holocene. The loess palaeosol sequences is one of the best records of information carrier on the environments and climate variations. However, the studies of climate records for the area mainly recorded by peat and stalagmite, there have few climate researches recorded by loess-paleosol. Therefore, we should pay more attention to carry out the climate change research with different ancient induction magnet. The loess palaeosol sequences were found on the different levels of river terraces land in the Danjiang River valley. However, what are the loess-palaeosol sequences in this region? How does the weathering and pedogenic intensity under the north subtroptical monsoon cliamtes? Compared with the loess-palaeosol in Weihe River Valley, what are the different between wreathing and pedogenic intensity? and so on. The eolian loess-palaeosol sequences were found at the Chafang Village in the Danfeng country of the Danjiang River Valley. Combined with field observation and indoor analysis, characteristics of macro and microstructure, physical and chemical properties were system studied in the Chafang site profile. The research project related with this thesis was subsidized by the National Nature Science Foundation (number:4124118). These results show that:(1) The loess-palaeosol sequences are MS-L0-S0-Lt-L1 in the Chafang Villay profile on the Shangdan Basin. They are similar to the loess-palaeosol sequences in the Weihe River Valley. Detailed stratigraphy describe were presented as follow:Modern soil (MS, 0-60 cm), dull brown (7.5YR6/3), clay silt, granular structure; Modern loess (L0,60-130 cm), dull yellow orange (10YR6/4), silt, block-massive structure; Plalaesol (S0,130～260 cm), dull brown (5YR3/4), clay silt, columnar-blocky structure, relative fiem, abundant dull brown clay film; Transitional loess (Lt,260～300 cm), dull yellow orange (7.5YR7/4), silt, block-massive structure, little dull brown clay film in the crevices; Malan loess (Li, 300～420 cm), dull yellow orange (7.5YR5/4), silt, block-massive structure. The Malan loess Li were dated to about 11500 a B.P.; palaeosol So were dated to 8500～3100 a B.P., which has a good stratigraphic comparison in the Weihe River valley.(2) Physical and chemical properties in the eolian loess deposits were different at the Chafang site. The variability of redness、redness/yellowness、magnetic susceptibility, loess-on-ignition, clay, clay/silt sand were similar to each other in the Chafang site. The highest values present in the palaeosol So, but the lower values present in the loess (L1、 L0、Lt). However, lightness、median size、silty sand have an opposite changing direction. The lowest values present in the palaeosol So and the higher values present in the loess (L1、L0、Lt). These Physical and chemical properties results in the Chafang Village profile shou that during the Mid-Holocene climatic optimum (8500～3100 a B.P.) developed palaeosol So.(3) The migration variability of major elements which are normalized by Al in the palaeosol So presents different trend at Chafang profile. Major element Fe is obvious enrich in the palaeosol So, but other elements Ca, Na, Mg, Si and K are leached out. The migration trend of major elements is Ca>Na>Mg>Si>K>Al>Fe.(4) Chemical weathering intensity in the loess-palaeosol sequences is different at the Chafang profile. Chemical index of Na/K, Rb/Sr, CIA, Si/Al, SiO2/(Al2O3+Fe2O3), (CaO+K2O+Na2O)/Al2O3 were analyzed in the Chafang profile. The results show that the trend of chemical weathering is S0>Lt>L1, L0. And the A-CN-K diagram show that major elements Ca, Na, rapidly leach out, while K relative stability. The Chafang profile falls into the intermediate weathering stage.(5) Micro-configuration properties in the eolian loess deposits were different at Chafang Village profile. On the whole, much more clay content, dense, abundant, clay film and dull brown Fe-Mn film in the palaeosol So and much more silt content, loess, little clay film and dull brown Fe-Mn film in the loess. The micro-morphology in different deposits are obviously different. More clay content, small pores, and clay grain tightly cemented in the palaeosol So, while in the loess, there are more silt content, simpler bridged and easily elevated between clay and silt grain.(6) The data show that the chemical composition of the Chafang loess in Danfeng is very similar to that of the loess in Weihe Valley. At the same time, the CV (Coefficient of variance) value of the major elements in the Chafang loess profile is extremely low, which suggests that the composition of either major elements or trace elements is very consistent within the whole profile. This can be regarded as an important proof to support the view that the Chafang loess is aeolian origin. Compared with the mean chemical composition of the Weihe River Valley, the distribution patterns of the major elements except Ca and Na are almost flat and close to those of Weihe River Valley loess. It indicates that the chemical composition of the Chafang loess is similar to that of the Weihe River Valley loess, which implies that the Chafang loess has multiple sources which have been mixed completely. As a whole, The Chafang loess profile underwent moderate chemical weathering, which was stronger than that of the loess and paleosol in Weihe River Valley.(7) The chemical weathering differences among the studied aeolian-dust deposits were mainly induced by the distinction of the climate condition, which imposed important influence on the geochemical environment through the mean annual temperature and annual precipitation. It seems that the annual precipitation has more influence on the chemical weathering process. The Chafang loess have finished the primary process weathering of chemical weathering and almost reached the secondary process as a whole. The aeolian sediments went through different degree of pedogenic modification in different periods with the sequence So>Lt>Li, Lo. These Physical and chemical properties results indicate that the climate "cool-dry (before 11500 a B.P.)→cool-wet (11500-8500 a B.P.)→warm-wet (8500-3100 a B.P.)→cool-dry (since 3100 a B.P.)". The characteristics of regional climate change to global climate change has good comparability consistent with the Weihe River Valley region on both sides of the Qinling mountains.