7.0Ma以来中国黄土粉尘源区的化学风化历史研究-Sr同位素比值的证据

To investigate the changes in the chemical weathering intensity of dust source region in Asia continent, ~(87)Sr/~(86)Sr ratios of acid-washed residues from the loess deposits in China and Tajikistan were analyzed, respectively. The results and conclusions are listed as below. 1. The oscillation of ~(87)Sr/~(86)Sr ratios of acid-insoluble residues in the Chinese Loess Plateau was mainly attributed to the chemical weathering intensity of the source region and the grain size. Counteracted the effect of particle size, the calibrated 87Sr/S6Sr ratio can be used as a proxy for the chemical weathering intensity of the source region. 2. The Sr/ Sr ratios of red clay-loess sequence from the Loess Plateau indicate that the chemical weathering intensity of the dust source region between 7.0 and 2.6 Ma is stronger than that in the Quaternary period. This also suggests a general decline in chemical weathering intensity of the source region from 2.6 Ma to the present. Such pattern is more remarkable since 1.0 Ma BP. 3. The ~(87)Sr/~(86)Sr ratios of the Tajik loess during 0.8-1.8 Ma is much more higher than those from 0.8 Ma to the present. This implies that the chemical weathering intensity of the source region in Central Asia is much stronger during 0.8-1.8 Ma than the period since 0.8 Ma. 4. The record of Sr isotope ratios from both sections shows an accelerating course of aridity of the Asian dust source region over the Quaternary period.

黄土高原22万年来的黑碳记录和古火灾害历史重建

The term black carbon is used to describe a relatively inert and ubiquitous form of carbon, comprising a range of materials from char and charcoal to element or graphite carbon produced by the incomplete combustion of fossil fuels and biomass. Due to its inertness, the BC in soils, lacustrine and marine sediments and ice can persist over a long period of time. So BC signatures in geological deposits can be used as evidence of natural fires happened in their surroundings. To study the temporal and spatial changes in paleofires over the Chinese Loess Plateau, black carbon concentrations were analyzed on the loess-paleosol samples from three sections including Lijiayuan, Lingtai and Weinan along a north-south transect. Using the orbitally-tuned time-scales of the sections, the black carbon sedimentation rates (BCSR) were calculated. Meanwhile, with objective to document fine resolution fire history during late Pleistocene and Holocene periods, we measured BC concentrations of loess-paleosol samples at dense sampling intervals since 28 ka BP. in Lijiayuan section. The BCSR of the samples were also calculated. In addition, we also conducted observation on black carbon morphologies to examine their sources. Based on the results, the following remarks can be concluded: 1. In the last two glacial cycles, the BCSR values in glacial periods are 2-3 times higher than in interglacial periods, and the BCSR variability has a relatively strong precession-associated 23 kyr period, suggesting that the glacial cold-dry climate conditions were apt to induce natural fires over the Loess Plateau, 2. Comparison of the BCSR records among the three loess sections demonstrates that natural fire occurrence was much more intensive and frequent in the northern and interglacial periods. 3. Pollen records and carbon isotope analyses of organic matter have shown that the Loess Plateau was covered by an Artemisia-dominated grassland vegetation both during glacial and interglacial periods, So grassland fires were the dominant fire types in the Plateau, which is also corroborated by the observation of black carbon morphology. In addition, statistics and comparison of BC particle sizes among the sections demonstrated that BC records probably reflected local fires. 4. According to previous studies about the effect of fires on vegetation changes, we considered that the fires might play an important role in the expansion of grassland during glacial periods, besides the control of climate changes. 5. The high resolution black carbon record in Lijiayuan section has shown that the BCSR series well documented Younger dryas (YD) and Heinrich (HI和H2) events, suggesting that natural fires in the northwestern part of Chinese Loess Plateau could regularly respond to the millennial scale climate oscillation.

中新世以来中国北方风尘堆积的粘土矿物组成及其古环境意义

In northern China, the loess-soil sequence of the last 2.6 Ma, the Hipparion Red-Earth of eolian origin and recently reported Pliocene-Miocene loess-soil sequence provide a near continuous continental eolian record of climatic history for the past 22.0 Ma. This work aims to investigate the composition and structure of clay minerals contained in deposits, and to explore their implications for environmental evolutions over the last 22.0 Ma. Clay minerals, which were extracted from eolian samples collected at Xifeng (0-6.2 MaBP) and Qinan (6.2-22.0 MaBP) sections, were analyzed qualitatively and semi-quantitatively by using X-ray diffraction (XRD), differential thermal analysis (DTA), thermogravimetric analysis (TG) and chemical analysis. The main conclusions are as follows: Over the last 22.0 Ma, the clay mineral assemblage among Quaternary loess-soils, Hipparion Red-Earth, and Miocene loess-soils shows similar components, mainly consisting of illite (55-80%), kaolinite (7-20%), chlorite (4-13%), smectite (2-23%) as results calculated by comparing major peak areas. There are no obvious differences in both types and amounts of clay minerals between loess and interbedded soils, suggesting that overwhelming part of the clay minerals is derived from the source. According to the components of clay minerals, the whole sequence of eolian deposits in the Loess Plateau can be divided into ten clay mineral assemblage zones over the last 22.0 Ma, whose corresponding ages are: 22.0-21.0 MaBP, 21.0-18.0 MaBP, 18-16.2 MaBP, 16.2-13.0 MaBP, 13.0-10.0 MaBP, 10.0-5.5 MaBP, 5.5-4.4 MaBP, 4.4-2.8 MaBP, 2.8-1.0 MaBP, 1.0-0 MaBP, respectively. This may imply that dust supply changed at least nine times over the past 22.0 Ma. The loess illite has a better crystaliinity, higher value of the FWHM and IC, than the interbedd soils. Previous studies indicated that irregular mixed layer minerals could form under relatively warm and humid conditions (Han, 1982). According to the general distribution of clay minerals of zonal soil (Chamley, 1989), the clay mineral assemblage of eolian deposits in Xifeng and Qinan sections is typical of temperature-humid and warm-subarid environment. Therefore, our results indicate climatic environment in Loess Plateau did not change remarkably since 22.0 Ma, and fluctuated between temperature-humid and warm-subarid climate. 4. The illite generally presents poorer crystaliinity during the period of 22.0 to 2.8 MaBP than in the last 2.8 Ma BP, especially at the intervals of 3.5-4.5 Ma BP, 14.0-17.0 MaBP and 20.0-22.0 Ma BP, which indicates that the weathering intensity was stronger in Neogene than in Quaternary. 5. The relatively low ice volume and high global temperature may be responsible for the strange weathering intensity during the interval of the 3.5-4.5 Ma BP, 14.0-17.0 Ma BP and 20.0-22.0 Ma BP.

泥河湾盆地磁性地层学研究——以东谷索和岑家湾剖面为例

The Nihewan Basin is a key area for research into human occupation at high northern latitudes in northeast Asia after the initial expansion of early humans out of Africa. Well-developed late Cenozoic lacustrine deposit sequence in this basin offers a unique opportunity to address this issue. In this thesis, detailed magnetostratigraphic investigation coupled with mineral magnetism was conducted on the Donggutuo and Cenjiawan sections in the eastern basin, where lacustrine deposits sequences containing the Donggutuo, Maliang and Cenjiawan Paleolithic sites are well developed. The sequences are mainly composed of grayish-white clays, grayish-green clayey silts, grayish-yellow silts and fine-grained brown sands, which have recorded reliable polarity variations of geomagnetic field.Characteristics of the anisotropy of magnetic susceptibility show that the sediments have preserved typical original magnetic fabric for sediments, indicating that the strata were developed in a low-energy lake environment and were never perturbed by tectonic stress since deposition. High-temperature magnetic susceptibility measurements (x-T) of representative specimens and demagnetization experiments indicate that the dominant magnetic mineral and remanent carrier at the two sections is magnetite. In addition, hematite and possibly minor maghemite exists in some portions of the sequences. The majority of the samples have relatively simple demagnetization behaviors. After removal of soft magnetic components, the stable characteristic remanent magnetizations (ChRMs) are isolated, which can represent the original remanences.The Donggutuo section mainly records the Brunhes chron, the Matuyama chron and the Jaramillo subchron. The Maliang stone artifact layer occurs just below the Brunhes/Matuyama boundary; and the Donggutuo artifact layer, just below the Jaramillo onset. Accordingly, the Maliang and Donggutuo sites can be dated at about 0.78 Ma and 1.1 Ma, respectively. The Cenjiawan section has recorded a portion of the Matuyama chron. After correlations with the magnetic polarity sequence of the Majuangou section adjacent to this section, the Cenjiawan stone artifact layer is determined below the Jaramillo onset, with an estimated age of 1.1 Ma.To establish the magnetic stratigraphy framework for the lacustrine sediments in the eastern Nihewan Basin, this thesis draws on the magnetic polarity sequences of the Donggutuo and Cenjiawan sections as well as previously obtained results from the Majuangou, Haojiatai, Xiaochangliang and Donggou sections for magnetostratigraphic correlations. The accumulation of the lacustrine sequences at the east margin of the basin commenced from about 2.0 Ma. These sequences record not only the coarse magnetostratigraphy of the Brunhes normal chron and the middle to late Matuyama reverse chron (that is, the Jaramillo and Olduvai subchrons) but also some of the fine structure (that is, the Kamikatsura, Santa Rosa, Punaruu and Cobb Mountain geomagnetic events). The development of the Nihewan paleolake experienced at least twice large expansion periods, split by a large-scale shrinking event in the middle period of the paleolake development. The accumulation of the lacustrine strata was controlled by fault activities.After temporal control for the Donggutuo, Maliang and Cenjiawan Paleolithic sites were established, the three sites along with other well-dated Paleolithic/hominin sites of the Early Pleistocene in North China were combined to construct a chronological sequence of early human occupation in northeastern Asia. Furthermore, after incorporation of paleoclimate changes retrieved from Chinese loess/paleosol sequences and marine sediments, it could be possibly proposed that human groups of the Early Pleistocene in North China might have survived repeated warm/humid interglacials and cold/dry glacials, which were paced by earth orbital variations of the Eastern paleomonsoon.

西部黄土高原中新世风尘沉积剖面的磁性地层学和沉积学研究

Observation of the Miocene aeolian dust deposits in Qin'an area provide a long-timescale, almost continuous geological record for understanding significant environmental changes, such as the environmental evolution of Asian monsoons and the inland Asian aridification, and make possible new explorations of global and regional environmental evolutions and other relevant studies. However, the QA-I and QA-II sections that are previously investigated are near to each other, and there is a lack in understanding the spatial characteristics of changes in paleoenvironment. Therefore, new sections of Miocene aeolian dust deposits are needed to complete the present spatial framework. Meanwhile, it is necessary to carry out in-depth studies, which are already belated, on the depositional characteristics of these Miocene aeolian dust deposits.This thesis choose the Miziwan section and Shaogou section that are west to Liupan Mountain for magnetostratigraphic studies, results of which are compared with the QA-I section with respect to the lithology, magnetic susceptibility and paleomagnetic polarity stratigraphy. Quartz-fraction grain size of 300 samples from the QA-I section was analyzed. In addition, 30 out of these samples were determined in terms of the quartz morphological characteristics by using scanning electronic microscopy. On the basis of these data and the correlation with the Xifeng sequence of aeolian dust deposits, the following 3 conclusions are drawn as below:The upper and the lower boundary age of the Miziwan section are 11.6 Ma B.Rand 18.5 Ma B.R, respectively. The Shaogou section spans from 15.1 Ma B.P, to20.8 Ma B.R The lower boundary age of both sections is no older than that of theQA-I section. The Miziwan and the Shaogou sections can be well correlated withthe QA-I section in terms of the field lithological features, magnetic susceptibilityand magnetic polarity stratigraphy.Sedimentological evidence of the QA-I quartz grain size and quartz morphologicalcharacteristics further supports the previous conclusion that the QA-Iloess-paleosol sequence is of aeolian origin, and also verifies the accretionalfeatures in paleosols of this sequence in terms of the grain-size composition.Modifications of the grain-size composition of the original dusts due to later-stageweathering and pedogenesis are common characteristic of Miocene and Quaternary aeolian dust depositions. The above-mentioned processes further rework and refine the bulk grain size, which is more evident in paleosol layers. 3, Like the hipparion Red-Earth and the Quaternary aeolian deposits, the Miocene loess deposits are transported by near-ground winter monsoons. However, the average wind intensity evidenced by the quartz median grain size of Miocene loess, together with the maximum wind intensity by the quartz maximum grain size, is weaker than those of the Quaternary.

甘肃西峰晚第三纪红粘土地层中的蜗牛化石记录及其古环境意义

In this study, 260 mollusk fossil samples from a Red Clay sequence at Xifeng, Gansu province, in the northern China were analyzed quantitatively. 12 fossil species and four fossil zones have been identified. Three main ecological groups were determined based on ecological requirement of each mollusk taxon. According to fossil composition and succession of three ecological groups, the author discussed the origin and sedimentary environment of the red clay deposits, and the process of ecological environmental changes as well as the variations of the East Asia monsoons during 6.2-2.4 Ma in the Loess Plateau. A preliminary study on periodicity of paleoclimatic changes was also conducted by using spectral analysis method. The main results and conclusions are presented as follows:A continuous land mollusk fossil sequence of 6.2-2.4 Ma from Xifeng Red Clay Formation has been established, which provided a basic data for studying the environmental changes during late Miocene to Pliocene.The study of composition and preservation condition of mollusk fossils reveals a terrestrial in situ ecological population in the Red Clay Formation. All of identifiable mollusk species are composed of terrestrial taxa, which support the view that the Red Clay is an eolian origin, similar to the overlying Quaternary loess deposits.The mollusk record reveals the processes of ecological and environmental changes during 6.2-2.4 Ma in the Loess Plateau. Climatic changes experienced cold and dry from 6.2-5.4 Ma, warm and wet during 5.4-4.5 Ma, mild and moderate from 4.5-3-4 Ma, to rapid cooling and drying after 3.4 Ma. From '5.4- 2.4 Ma, climate was stepwise cooling. The cooling trend is in good agreement with a general1 0global cooling trend during this period, as documented by marine 5 0 records.4. Three remarked ecological shifts took place in mollusk assemblages from 6.2-2.4 Ma, focused on about 5.4, 4.5 and 3.4 Ma. The warming shift around 5.4 Ma was probably related to the rising of the global temperature. The cooling shifts around 4,5 and 3.4 Ma however might be closely linked to the uplift of Tibet Plateau and the development of Northern Hemisphere ice sheet.The succession in mollusk ecological groups also recorded the variability of the East Asian winter and summer monsoon. The winter monsoon dominated two periods from 6.2-5.4 Ma and from 3.4-2.4 Ma, while the summer monsoon was strong during 5.4-4.5 Ma. The variations in winter and summer monsoons were in phase during 4.5-3.4 Ma. Monsoon regimes changed with the duration about 1 Ma, which roughly corresponds to the cycle driven by tectonic activity on the time scales of ICP-IO7 years. In addition, mollusk fossils recorded the large amplitude and high frequency fluctuations overlapped on 105-107 years climate cycle.The maximum entropy spectral analysis and filter-band analysis of total mollusk individuals and three typical ecological groups suggest that the climate changes controlled mainly by solar insolation had periods about 70 ka and 40 ka on the time scales of 105 during late Miocene-Pliocene. Climatic periodicity intensified from 4.0 Ma, which reflected strengthened forcing by high latitude ice volume.

陆相砂泥岩层序的高精度检测方法研究及应用

Ordos Basin is a typical cratonic petroliferous basin with 40 oil-gas bearing bed sets. It is featured as stable multicycle sedimentation, gentle formation, and less structures. The reservoir beds in Upper Paleozoic and Mesozoicare are mainly low density, low permeability, strong lateral change, and strong vertical heterogeneous. The well-known Loess Plateau in the southern area and Maowusu Desert, Kubuqi Desert and Ordos Grasslands in the northern area cover the basin, so seismic data acquisition in this area is very difficult and the data often takes on inadequate precision, strong interference, low signal-noise ratio, and low resolution. Because of the complicated condition of the surface and the underground, it is very difficult to distinguish the thin beds and study the land facies high-resolution lithologic sequence stratigraphy according to routine seismic profile. Therefore, a method, which have clearly physical significance, based on advanced mathematical physics theory and algorithmic and can improve the precision of the detection on the thin sand-peat interbed configurations of land facies, is in demand to put forward.Generalized S Transform (GST) processing method provides a new method of phase space analysis for seismic data. Compared with wavelet transform, both of them have very good localization characteristics; however, directly related to the Fourier spectra, GST has clearer physical significance, moreover, GST adopts a technology to best approach seismic wavelets and transforms the seismic data into time-scale domain, and breaks through the limit of the fixed wavelet in S transform, so GST has extensive adaptability. Based on tracing the development of the ideas and theories from wavelet transform, S transform to GST, we studied how to improve the precision of the detection on the thin stratum by GST.Noise has strong influence on sequence detecting in GST, especially in the low signal-noise ratio data. We studied the distribution rule of colored noise in GST domain, and proposed a technology to distinguish the signal and noise in GST domain. We discussed two types of noises: white noise and red noise, in which noise satisfy statistical autoregression model. For these two model, the noise-signal detection technology based on GST all get good result. It proved that the GST domain noise-signal detection technology could be used to real seismic data, and could effectively avoid noise influence on seismic sequence detecting.On the seismic profile after GST processing, high amplitude energy intensive zone, schollen, strip and lentoid dead zone and disarray zone maybe represent specifically geologic meanings according to given geologic background. Using seismic sequence detection profile and combining other seismic interpretation technologies, we can elaborate depict the shape of palaeo-geomorphology, effectively estimate sand stretch, distinguish sedimentary facies, determine target area, and directly guide oil-gas exploration.In the lateral reservoir prediction in XF oilfield of Ordos Basin, it played very important role in the estimation of sand stretch that the study of palaeo-geomorphology of Triassic System and the partition of inner sequence of the stratum group. According to the high-resolution seismic profile after GST processing, we pointed out that the C8 Member of Yanchang Formation in DZ area and C8 Member in BM area are the same deposit. It provided the foundation for getting 430 million tons predicting reserves and unite building 3 million tons off-take potential.In tackling key problem study for SLG gas-field, according to the high-resolution seismic sequence profile, we determined that the deposit direction of H8 member is approximately N-S or NNE-SS W. Using the seismic sequence profile, combining with layer-level profile, we can interpret the shape of entrenched stream. The sunken lenticle indicates the high-energy stream channel, which has stronger hydropower. By this way we drew out three high-energy stream channels' outline, and determined the target areas for exploitation. Finding high-energy braided river by high-resolution sequence processing is the key technology in SLG area.In ZZ area, we studied the distribution of the main reservoir bed-S23, which is shallow delta thin sand bed, by GST processing. From the seismic sequence profile, we discovered that the schollen thick sand beds are only local distributed, and most of them are distributary channel sand and distributary bar deposit. Then we determined that the S23 sand deposit direction is NW-SE in west, N-S in central and NE-SW in east. The high detecting seismic sequence interpretation profiles have been tested by 14 wells, 2 wells mismatch and the coincidence rate is 85.7%. Based on the profiles we suggested 3 predicted wells, one well (Yu54) completed and the other two is still drilling. The completed on Is coincident with the forecastThe paper testified that GST is a effective technology to get high- resolution seismic sequence profile, compartmentalize deposit microfacies, confirm strike direction of sandstone and make sure of the distribution range of oil-gas bearing sandstone, and is the gordian technique for the exploration of lithologic gas-oil pool in complicated areas.

中国黄土的磁组构特征及其古风向指示意义

The transportation and deposition of eolian materials of Chinese loess is correlated and effected by the monsoon from the mid-high latitude. Therefore study of the winter monsoon evolution can help us to understand the dynamic mechanism to climate changes in the east-Asian areas. The anisotropy of magnetic susceptibility (AMS) measurements have been carried out on the samples from the last 250ka wind -blown loess-paleosol sequences at Baicaoyuan and Luochuan. And the main conclusions are following:The magnetic foliation is almost horizontal of the two sections. AMS canthus be represented by an oblate ellipsoid with average K3 perpendicular to thebedding plane and Ki within the bedding plane. It has also shown that the ^-factor isless than 0.5 of the majority of samples. So the two sections are normal magneticfabric for sediments.The degree of anisotropy always shows a strong correlation with the foliationrather than with lineation, therefore the anisotropy is controlled by the foliation.Furthermore the foliation is nearly less than 1.02 and shows the typical wind-blownsediments anisotropy.The intensity of winter monsoon, grain size of the eolian inputs, the foliationand the degree of anisotropy are somewhat inter-related. Generally, the higherintensity of the winter monsoon will carry coarser-grained eolian material, therebyresulting in a larger foliation during deposition. Also the post-depositional compactioncontributes to the anisotropy.The AMS features between loess and paleosol are somewhat different. Wefound that the F, P values of paleosol are lower than that of its parent loess respectively. Moreover, the difference does also exists between the two sections. The anisotropy of Baicaoyuan is more significant than Luochuan section, which maybe related with the location and the intensity of the post-deposition reworks.5. We note that the declination of the long axis is NWW in Baicaoyuan section and the observed NWW direction of the winter monsoon winds based on AMS is consistent with the view that the winter monsoons prevail along the NW-SE direction. But at the Luochuan section, because of the strong affection of the post-deposition reworks, the direction of the long axis is nearly random in the foliation and hardly recognizes the paleowind direction since the last two interglacials.Correlation between the two loess-paleosol sequences implies that it is available in arid or semi-arid areas to take AMS to recognize the paleowind directions on the Loess Plateau.

黄土高原全新世风成沉积的岩石磁学性质

Chinese eolian deposits are especially suitable for the studies of paleoclimatic changes, environmental magnetism and remanence acquisition mechanisms. In the past two decades, many studies have documented their magnetic properties. However, some important problems, such as the origin of magnetic minerals, the mechanisms for enhancing magnetic susceptibility and the lock-in effect, remain debatable. Therefore, it is essential to detail the rock-magnetic properties of the eolian deposits. This study shows thermomagnetic analyses, petrographic measurements and soil chemistry methods can be combined to obtain a better understanding of the sequence of magnetic mineral alterations during thermal treatment and of the pedogenic mechanism responsible for the susceptibility enhancement. This helps to further develop the interpretation of paleoclimate records in the Holocene eolian deposits along a NW-SE transect of the loess plateau. A partial heating/cooling method and X-ray diffraction (XRD) analysis were performed on representative samples of the present-day loess, in order to investigate mineralogical changes during thermal treatment. The temperature-dependent susceptibility (TDS) and XRD results show complex alteration of magnetic phases during heating and cooling. The 300 ℃ susceptibility hump in heating curves might be due to the production of maghemite from less magnetic lepidocrocite during heating. Goethite is transformed into hematite when heating to above 300 ℃. The susceptibility decrease from 300 ℃ to 450 ℃ can be interpreted as the conversion of maghemite to hematite. This thermal instability makes it possible to quantatively estimate the maghemite contribution to the pedogenically-enhanced susceptibility in loess or paleosols. Minor occurrence of thermally-stable maghemite in the present-day loess is possible; nevertheless, the TDS measurements show that the degree of the thermally-induced alteration is closely related to pedogenesis. The TDS measurement and XRD analysis results demonstrate that although magnetite and hematite both exist in the Holocene loess eolian deposits and their modern source area, magnetite is the predominant contributor to magnetic susceptibility. Both magnetite and hematite are the primary carriers of the remanent magnetization. Fine-grained maghemite, mainly produced by pedogenesis, is significantly responsible for enhancement of the magnetic susceptibility in the Chinese loess and paleosols. Since the degree of oxidation of magnetite grains depends on climate, the presence of maghemite has paleoclimatic significance, and variations in climate could be reflected as variations in the amount of low-temperature oxidation. If that is the case, the TDS curves can be used to compare the effects of climate at different sampling sites. The TDS results along the studied NW-SE transect suggest that stronger pedogenesis results in higher content of maghemite and greater susceptibility decrease during thermal treatment. This behavior seems to indicate that the final product of pedogenic magnetite in Chinese loess and paleosols is maghemite, which makes significant contributions to the enhanced magnetic susceptibility of Chinese eolian deposits. It is interesting to note that the 510 ℃ Hopkinson/alteration peak is larger in the present-day loess than in the black loam for each section. Obiviously, the Hopkinson/alteration peak of the Holocene eolian deposits is closely related to the degree of pedogenesis, which is a function of climate, and thus the peak itself could be a useful climate indicator. There are three effects that may be important in producing this trend. First, low-temperature oxidation preferentially affects the finer single-domain magnetites responsible for the Hopkinson peak, which is therefore suppressed in the more oxidized loams. Second, the possible production of uniaxial magnetite with shape anisotropy can also lead to a relatively muted Hopkinson peak. There is, additionally, a third alternative, and the one preferred here, that the natural alteration processes involved in pedogenic susceptibility enhancement have probably depleted the supply of iron-bearing precursor phases, so that less new magnetite is formed on heating. In summary, the TDS method is very reliable and highly sensitive in detecting magnetic phase changes in eolian deposits during thermal treatment, which are closely related to pedogenic processes. Thus, the studied NW-SE transect clearly exhibits paleoclimatically-induced mineral- and rock-magnetic variations. It is suggested that TDS can be used as a new method for the analysis of pedogenesis and climatic change.

13万年以来洛川黄土剖面中营养元素的分布及演化规律

洛川剖面以其厚度大，沉积连续，并完整地记录了第四纪以来陆相环境、气候变化的信息而举世闻名，是陆相沉积序列中研究最为成熟的黄土剖面，被定为国际标准的黄土剖面。学者们对黄土剖面经过多年的研究，获得了许多宝贵的成果，例如，磁化率、粒度、地球化学等指标的建立，黄土物质来源问题的探讨，东亚季风变迁的研究，及大气环流模型的建立等等。这些成果在解释全球性尺度上的古气候变化以及在预测未来气候变化趋势上具有重要的理论指导意义。上述成果着重从物理和化学的角度来展开研究的，而如何从生物地球化学的角度来研究局部的气候环境变化及其对全球变化的响应问题，以便更好地弄清楚气候变化的机制、周期性及影响因素，预测未来气候的变化，科学防治环境污染，合理利用自然环境，促进人类社会进步呢。这是目前黄土研究中较薄弱的地方。因此，本文在前人工作的基础上，首次将营养元素的概念较系统地引入到黄土古气候的研究中来，初步讨论了营养元素（C，N，P，K）在洛川黄土剖面中的分布和演化规律，获得了如下凡点新的认识：1．13万年来洛川黄土剖面中有机碳的含量在0.14％-0.54％之间变化，平均含量为0.26％。在时间上，有机碳高一低值的变化特征对应不同的气候时期。干冷气候条件下形成的黄土L1有较低的有机碳含量，湿热气候条件下发育的古土壤有较高的有”机碳含量。在马兰黄土中，间冰阶形成的L1SS1中有机碳含量高于始冰阶和盛冰阶形成的L1LL1、L1LL2中的有机碳含量，表现出低的有机碳含量对应于冷期，高的有机碳含量对应于暖期。有机碳的这种变化特征，与磁化率的变化趋势一致，在一定程度上反映了不同时期植被的发育程度。通过洛川剖面（109°25′E，35°45·N）与段家坡剖面（109°12′E，34°25′N）有机碳的对比可知，在空间上，从黄土高原的西北到东南，同一时期黄土剖面中有机碳的含量呈逐渐增加的变化趋势，这与地理上的气候变化是一致的。因此，时空上有机碳的变化特征反映黄土高原地区东亚夏季风强弱周期性变化的特点。2．洛川黄土剖面中的氮由有机氮、固定钱态氮和速效态氮组成，其中前两者为主要部分，占总氮的95％以上。有机氮占总氮的55％-76％，在181μg·g-15689μg-1之，间变化平均值为299μg-1彭，其在剖面中的分布与有机碳极为相似，在黄土LI中有较低的有机氮，在古土壤S1中有较高的有机氮，并且在马兰黄土中，间冰阶形成的L1SS1中有机氮含量高于始冰阶和盛冰阶形成的L1LL1、L1LL2中的有机氮，其高一低值的分布与黄土一古土壤中冷一暖变化相互对应。有机氮的含量变化在一定程度上也反映了植被的生长发育程度，同样揭示了东亚夏季风对黄土高原的影响。速效性氮（交换性钱态氮和水溶性钱态氮）在洛川黄土剖面中的含量低，主要在Oμg·g-1-11.5μg·-1之间变化，在古土壤51和间冰阶时期形成的弱土壤化黄土LISSI中有较高的值，反映了速效性氮与土壤湿度和粘粒的含量有关。由于速效性氮和固定按态氮在黄土剖面中的振荡幅度不大，总氮的分布基本与有机氮一致，其在黄土剖面中的变化主要受有机氮变化的制约。3．黄土一古土壤层位中固定钱态氮占总氮的24％-43％，其含量在140μg·210μg·g-1之间变化。黄土层中的固定钱态氮含量低于相邻的古土壤。