| 其他摘要 | The Luochuan loess section is well known in the world as great thickness and continuous dust deposition where much information of continental environmental and climatic change was completely recorded during the Quaternary period. The Luochuan loess section was more deeply researched than other continental sediment sequences and was accepted as the standard loess section in the world. The Loess sections have been studied by experts for many years and many valuable results have been obtained, including establishment of proxy records(magnetic susceptibility, grain size, etc.), discussion on material source of the loess and paleosols, reconstruction of the Eastern Asian monsoon evolution, and atmosphere circulation model, etc.. It is importantly significant for these results to explain reasons of paleoclimatic change on the global scale and predict trends of climatic change in the future. The results, mentioned above were emphasized on physics and chemistry. However, how to research local climatic change and its response to global climatic change by using biogeochemistry ideas so that we can better understand mechanism, periodicity and influences to predict climatic change in the future. Consequently, on the basis of the former data on loess, nutrient elements are first introduced into the research on loess and paleoclimate in this thesis, and distribution and evolution of nutrient elements (C,N,P,K) in the Luochuan loess section is discussed. Several main results are obtained as follow: Corg content varies from 0.14% to 0.54%in the Luochuan loess section during the last 130ka, and its mean content is 0.26%. Corg content changes with climate in the Luochuan loess section. There is low content of Corg in the loess L] formed under the dry and cold climate whereas high content of Corg occurs in the paleosol Si developed under the humid and warm climate. Corg content in the loess LjSSi developed during the interstade period is higher than in the loess LjLLi formed during the late stade period and loess L1LL2 formed during the early stade period. Distribution of Corg represents these characteristics that low value of Gorg corresponds to the cold period, and high value of Corg parallels to the warm period. Variation: of Corg is consistent with magnetic susceptibility, reflecting the development of vegetation in different periods in some degree. Comparison of the Luochuan section (109°25'E, 35°45r N) with the Duanjiapo section (109°12' E, 34°25' N) indicates that Corg content of the different loess sections during the same period gradually increases from northern west to southern east in the Loess plateau, which is in accordance with climatic change in geography. Therefore, variation of Corg reflects the East Asian summer monsoon change spatially and temporally in the Loess Plateau. Nitrogen in the Luochuan loess section is composed of organic; nitrogen, fixed-NHN and NH4+-M, of which, Norg and fixed- NH4+-N are main fractions and account for above 95% of total nitrogen. Norg varies from lSlu-g-g1 to 568u.g:g- and its mean value is 299ug-g-1 about 55%-76% of total nitrogen. Distribution of Norg is very similar to Cwg in the Luochuan loess section. There is low content of Norg in the loess Li, but high content of Norg occurs in the paleosol Si. What's more, Norg content in the loess LiSS, is higher than in the loess LiLL, and LiLL2. Fluctuation of Norg is consistent with loess-paleosol alternations. Variation of Norg reflects development of vegetation in some degree, and indicates influences of the East Asian summer monsoon on the Chinese Loess Plateau. NH4+-N is low in content in the Luochuan section, and varies from Ojxg-g"1 to 11.5y.g-g~', which is higher in the paleosol Si and the loess LiSS, than in the other units, reflecting a good correlation between NH4 -N and soil humidity, clay fraction. Total nitrogen varies consistently with organic nitrogen, and is mainly controlled by Norg, because NH4+-N oscillates in amplitude as little as fixed-NH4+-N. 3. Fixed- NH4+-N make up to 24%-43% of total nitrogen in loess-paleosol sequences, ranging from 140u,g-g-1 to 210u.g-g-1. Fixed NH4+-N content in the loess is lower than in the interstratified paleosols. The content of fixed NH/-N is the highest in the paleosol S5 developed under the most humid and warm climate, about 201.87u.g-g-1 (mean value). Fixed NH4+-N content is the lowest in the loess L2 formed under the most cold and dry climate, with the mean value of about 145ng-g-1. The fixed NH/-N content varies from 134u.g-g-1 to Wp-g-g-1 in the S0-LrSi sequence with the mean value of around 160u.g-g''. The spatial distribution curve of fixed NH4+-N in the S0-Li-Si sequence exhibits three significant fluctuations between low and high values. That is, the fixed NH4+-N is low in the loess Li, medium in the paleosol So and high in the paleosol Si. Similarly, the spatial distribution curve of fixed NH4+-N in the Malan Loess section exhibits three small fluctuations between low and high values. That is to say, the fixed NH/-N in the loess LiLLi is low, medium in the loess-L-1LL2 and high in the weakly pedogenic loess LJSSJ. The curves of fixed NH4+-N and magnetic susceptibility and the <2m fraction exhibit a similar oscillation both in amplitude and in frequency, and there is a good correlation between them. The original content of fixed NH4+-N in aeolian dust during the period the paleosol Sj developed Is recovered by using correlation between particle size and fixed NH4+-N in the Malan loess. As a whole, it is found that the release rate of fixed NH/-N is greater than its fixation rate in the paleosol Si and fixed-NH4+-N is in loss in the paleosol Si. Despite th,e fact mentioned above, fixed NH4+-N content in the paleosol Si measured is higher,than in the loess L,. Therefore, fluctuation of fixed NH4+-N in the Luochuan loess section is closely related to particle sizes of aeolian dust during the last 130,000 years, which reflects periodically strong-weak variation of the East Asian winter monsoon in the Loess Plateau. Attentively, abrupt increase of fixed-NH4+-N in the Si/Li transition was possibly affected by YTT event. 4.Corg varies consistently with Norg in the Luochuan loess section, and both are well correlated. Corg/Norg ratio is high in the L1SS1, and well differentiates the cold period from the warm period. The second is Corg/Norg ratio in the paleosol Si. Corg/Norg ratio is low in the loess L1LL1 and LjLL2. The organic matter is easily preserved,'but vegetation developed poorly under the dry and cold climate (L/), especially in the late and early stade. However, there is a more warm and humid climate, and vegetation developed better in interstade than in late and early stade, and degradation speed of Corg in this period is slower than in the paleosol Si, consequently, there is high Corg/Norg ratio in the interstade period. Vegetation developed well during the warm and humid climate ( Si) whereas the organic matter is not easily preserved because of its great degradation rate, especially in the bottom part of the paleosol Si, thus, COrg/Norg ratio is lower in the paleosol St than in the loess L1SS1. Consequently, Corg/Norg ratio reflects source of the organic matter in some degree, and indirectly indicates evolution; of vegetation in the Loess Plateau. Inorganic phosphorus varies from 324 ug-g-1 to 560 ug-g-1 and its mean value is 499 ug-g-1, about 79% of total phosphorus in the S0-Li-Si sequence. Inorganic phosphorus varies from 493 ug-g-1 to 554 ug-g-1 and its mean value is 515 ug-g-1 in the Malan loess. Inorganic phosphorus is very stable in the Malan loess, which indicates inorganic phosphorus is not related with grain size of dust but chemical composition of dust. Although Pjnog is stable in the Malan loess, which indicates inorganic phosphorus is not related with grain size of dust but chemical composition of dust. Although Pinorg is stable in the Malan loess, there are still slightly differences. These differences range from 20 ug-g-1 to 40 ug-g-1, affected by local fixation of Porg by calcium. According to mean values of three phases where Pinorg wasnot accumulated, Pjnorg content in parent materials of the loess is calculated, about 504.7 ug-g-1. PjnOrg in the paleosol Si varies from 324u,g-g"' to 476u.g-g'1, and its mean content is 395 ug-g-1, Compared with Pinorg in the Malan loess, Pinorg in the paleosol Si is evidently lower because Pinorg is easily leached under the warm and humid climate. In the paleosol Si existed the relatively most warm and humid climate phase in which variation of Pinorg could indicate climatic change quantitatively. There are two parameters to reflect climatic change: one is the maximum loss ratio of PjnOrg, which represents climatic change, 0.135 in the paleosol So and 0.352 in the paleosol Si respectively. The other is the maximum time difference of PjnOrg loss, which represents duration of the most warm and humid climate. The most warm and humid climate lasted for a short time in the paleosol So but for about 22 ka in the paleosol S.i. Organic phosphorus varies from 59 ug-g-1 to 233 ug-1 and its mean value is 132 ug-g-1, about 21% of total phosphorus in the So-Li-Si sequence. Mean content of Porg in the Malan loess is tightly correlated with the 2urn -8u,m fraction, which indicates that Porg in the loess basically inherited Porg in parent material of the loess. Variation of Porg resulted from the transformation of Porg to Pinorg and transportation of Porg induced by different climates, ranging from 2-OfA-g-g-1 to 4-Of xg-g-1. As a whole, mineralization and leaching led to loss of Porg in the paleosol Si. |
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