•Submerged plants had relatively positive δ13C values similar to terrigenous C4 plants.•Sedimentary n-C27/n-C29 δ13C changes related to water depth variation.•These δ13C values may create bias in ...estimating C4 land plant % contribution.•δ13C values of n-C31 in surface sediments agreed with modern terrigenous vegetation.•δ13C values of n-C31 might be a more reliable terrigenous vegetation proxy.
The carbon isotope composition (δ13C values) of long chain n-alkanes in lake sediments has been considered a reliable means of tracking changes in the terrigenous contribution of plants with C3 and C4 photosynthetic pathways. A key premise is that long chain leaf wax components used for isotope analysis are derived primarily from terrigenous higher plants. The role of aquatic plants in affecting δ13C values of long chain n-alkanes in lacustrine sediments may, however, have long been underestimated. In this study, we found that a large portion of long chain n-alkanes (C27 and C29) in nearshore sediments of the Lake Qinghai catchment was contributed by submerged aquatic plants, which displayed a relatively positive carbon isotope composition (e.g. −26.7‰ to −15.7‰ for C29) similar to that of terrestrial C4 plants. Thus, the use of δ13C values of sedimentary C27 and C29n-alkanes for tracing terrigenous vegetation composition may create a bias toward significant overestimation/underestimation of the proportion of terrestrial C4 plants. For sedimentary C31, however, the contribution from submerged plants was minor, so that the δ13C values for C31n-alkane in surface sediments were in accord with those of the modern terrestrial vegetation in the Lake Qinghai region. Moreover, we found that changes in the δ13C values of sedimentary C27 and C29n-alkanes were closely related to water depth variation. Downcore analysis further demonstrated the significant influence of endogenous lipids in lake sediments for the interpretation of terrestrial C4 vegetation and associated environment/climate reconstruction. In conclusion, our results suggest that the δ13C values of sedimentary long chain n-alkanes (C27, C29 and C31) may carry different environmental signals. While the δ13C values of C31 were a reliable proxy for C4/C3 terrestrial vegetation composition, the δ13C values of C27 and C29n-alkanes may have recorded lake ecological conditions and sources of organic carbon, which might be affected by lake water depth.
The isotopic compositions of total organic carbon (TOC) in lakes have been widely used to interpret paleoclimatic changes and the depositional environments of lake sediments. However, the main ...factors that affect the carbon isotopes of TOC (source of organic material, water condition and others) may vary in different lake sediment records, which have limited the applicability of organic carbon isotopes in explaining biogeochemical and environmental changes in lakes. In this study, the organic carbon isotopic compositions of aquatic plants and surface sediments from Lake Qinghai and the living terrestrial plants and surface soils around the lake were systematically investigated to identify the sources of TOC in the sediments and the significance of the organic carbon isotopes of sedimentary TOC. We found that the aquatic plants in the deep water areas (>10m) were primarily dominated by Cladophora, but submerged plants (Potamogeton and Ruppia L.) are the dominant species in shallow water (<10m). The Cladophora have negative δ13Corg values (−33.6‰ to −28.6‰) that are caused by C3-like photosynthesis, but the submerged plants have enriched δ13Corg values (−17.8‰ to −15.4‰) that are caused by C4-like photosynthesis. In addition, the δ13Corg values of Cladophora become more negative with increasing water depth because of the slow photosynthetic rate caused by the weak light intensity at depth. The isotopic data indicate that the carbon isotopes of organic material in the surface sediments are primarily controlled by the types of aquatic plant and that the δ13Corg values of TOC can be used to indicate the variation of the water depth (lake level). The organic carbon isotopic data from the 1F core showed that the water was shallow (<10m) because of intense evaporation related to high temperatures even though precipitation sharply increased in the warm period during the early-mid Holocene. The lake level reached its maximum level at 3ka.
•Isotope of total organic carbon in the Lake Qinghai, Qinghai–Tibet Plateau, China.•The Cladophora have low δ13Corg values caused by C3-like photosynthesis.•The submerged plants have positive δ13Corg values caused by C4-like photosynthesis.•The δ13Corg values of lake surface sediments indicate the variation of water depth.•The core δ13Corg reveals change of lake level in Lake Qinghai since early Holocene.
Over the past century, fossil fuels have provided the majority of China's energy. However, their extensive utilization leads to a shortage and environmental pollution. Recently, submarine and ...permafrost gas hydrate deposits have been investigated as a possible clean and sustainable energy source by governmental institutions, research organizations, and energy industries in China. The primary objective of this paper is to review the potential studies pertaining to gas hydrate exploration and resource assessment, the safe and efficient exploitation of gas hydrates and the basic properties of gas hydrates. To date, there are over 20 institutions and organizations in China committed to gas hydrate investigation, among which the Guangzhou Marine Geological Survey (GMGS) and the Chinese Academy of Geological Sciences (CAGS) etc. primarily focus on gas hydrate exploration research, while the China National Offshore Oil Corporation (CNOOC) Research Center, Guangzhou Institute of Energy Conversion (GIEC) and China University of Petroleum-Beijing (CUPB) etc. concentrate on gas hydrate mining technologies. In this paper, the occurrence and exploration of gas hydrates in both permafrost regions and the continental slope of China have been determined from numerous research contributions and are presented. Moreover, the latest progress in gas hydrate fundamental studies, including hydrate phase equilibria, hydrate formation mechanisms, hydrate thermal physical properties and the acoustics and resistivity characteristics of gas hydrates are briefly reviewed, and relevant data are gathered and compared. Emphasis is also placed on gas hydrate mining technologies and gas production using depressurization methods, thermal stimulation methods or other methods. Furthermore, the security of natural gas hydrate-bearing sediments during gas production and the environmental impacts of gas hydrate are identified. With additional financial and political support and advanced research facilities, research on gas hydrates in China is progressing rapidly but is still in its early developing stage, thus, future work should be undertaken with greater diligence.
Glacial-Interglacial Indian Summer Monsoon Dynamics Zhisheng, An; Clemens, Steven C.; Shen, Ji ...
Science (American Association for the Advancement of Science),
08/2011, Letnik:
333, Številka:
6043
Journal Article
Recenzirano
The modern Indian summer monsoon (ISM) is characterized by exceptionally strong interhemispheric transport, indicating the importance of both Northern and Southern Hemisphere processes driving ...monsoon variability. Here, we present a high-resolution continental record from southwestern China that demonstrates the importance of interhemispheric forcing in driving ISM variability at the glacial-interglacial time scale as well. Interglacial ISM maxima are dominated by an enhanced Indian low associated with global ice volume minima. In contrast, the glacial ISM reaches a minimum, and actually begins to increase, before global ice volume reaches a maximum. We attribute this early strengthening to an increased cross-equatorial pressure gradient derived from Southern Hemisphere high-latitude cooling. This mechanism explains much of the nonorbital scale variance in the Pleistocene ISM record.
In this paper, the performances of a surface-mounted permanent magnet (SPM) motor and a hybrid magnetization interior permanent magnet (IPM) motor are compared, with particular reference to losses ...and temperature rise. The analysis reveals that the gross loss of the IPM motor is higher than that of the SPM motor. However, thermal analysis by the analytical lumped-circuit method shows that the rotor temperature rise of the SPM motor is higher than that of the IPM motor due to significant eddy-current loss and bad heat dissipation capability in the sleeve of the SPM motor. Predicted temperature rises are compared with measured results, and good agreement is achieved.
Genetic algorithm (GA) has strong global searching ability and is easy to operate, but its disadvantages such as poor convergence speed, unstable and easy to fall into local optimal value restrict ...its application. In order to overcome these disadvantages, an improved genetic algorithm based on the deep reinforcement learning model SAC (soft actor-critic) is proposed in this paper, which is applied to the resolution of traveling salesman problem (TSP). The improved algorithm regards the population as agent's interaction environment, meanwhile greedy algorithm is used to initialize this environment for improving the quality of initial populations. For controlling the evolution of the population, the improved crossover and mutation operations are used as agent's action space. With the goal of maximizing the cumulative rewards of population evolution, the improved algorithm treats the evolution of the population as a whole and uses a policy gradient algorithm based on SAC to generate evolution controlling action
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•Si boosts lignin biosynthetic enzyme activity, mainly phenylalanine ammonia lyase.•Si up-regulates lignin biosynthetic genes.•Si enhances photosynthesis by increasing chlorophyll ...content and stomatal conductance under low light.•Si decreases the drastic shading effects on stem stability.•Si application in intercropping could be effective in decreasing lodging.
In order to improve soybean’s resistance to lodging, silicon (Si) solutions at concentrations of 0,100, 200,300 mg kg−1 were applied during the seedling stage. The Si accumulation in different parts of the plants, the photosynthetic parameters of leaves and chlorophyll content, the stem bending resistance, the expression of genes of lignin biosynthesis and associated enzyme activity and sap flow rates were measured at early and late growth stages. The potential mechanisms for how Si improve growth and shade tolerance, enhances lodging resistance and improves photosynthesis were analyzed to provide a theoretical basis for the use of Si amendments in agriculture. After application of Si at 200 mg kg−1, the net photosynthetic rate of soybeans increased by 46.4 % in the light and 33.3 % under shade. The application of Si increased chlorophyll content, and fresh weight of leaves, reduced leaf area and enhanced photosynthesis by increasing stomatal conductance. The activity of peroxidase (POD), 4-coumarate:CoA ligase (4CL), cinnamyl alcohol dehydrogenase (CAD) and phenylalanine ammonia-lyase (PAL) increased during pre-and post-growth periods, whereas Si also increased lignin accumulation and inhibited lodging. We concluded that Si affects the composition of plant cell walls components, mostly by altering linkages of non-cellulosic polymers and lignin. The modifications of the cell wall network through Si application could be a useful strategy to reduce shading stress in intercropping.
The Tibetan Plateau uplift and Cenozoic global cooling are thought to induce enhanced aridification in the Asian interior. Although the onset of Asian desertification is proposed to have started in ...the earliest Miocene, prevailing desert environment in the Tarim Basin, currently providing much of the Asian eolian dust sources, is only a geologically recent phenomenon. Here we report episodic occurrences of lacustrine environments during the Late Miocene and investigate how the episodic lakes vanished in the basin. Our oxygen isotopic (δ ¹⁸O) record demonstrates that before the prevailing desert environment, episodic changes frequently alternating between lacustrine and fluvial-eolian environments can be linked to orbital variations. Wetter lacustrine phases generally corresponded to periods of high eccentricity and possibly high obliquity, and vice versa, suggesting a temperature control on the regional moisture level on orbital timescales. Boron isotopic (δ ¹¹B) and δ ¹⁸O records, together with other geochemical indicators, consistently show that the episodic lakes finally dried up at ∼4.9 million years ago (Ma), permanently and irreversibly. Although the episodic occurrences of lakes appear to be linked to orbitally induced global climatic changes, the plateau (Tibetan, Pamir, and Tianshan) uplift was primarily responsible for the final vanishing of the episodic lakes in the Tarim Basin, occurring at a relatively warm, stable climate period.
Significance The relative roles of the two plausible causes, Cenozoic global cooling and Tibetan Plateau uplift, for the Asian interior aridification/desertification are often difficult to disentangle. High-quality terrestrial records from the Tarim Basin document how enhanced aridification occurred over the last 7 my, from episodic lacustrine environments to the currently prevailing desert environments. During the Late Miocene–Early Pliocene period, episodic occurrences of great lakes and thus wetter climate closely corresponded to periods of high eccentricity and possibly high obliquity, suggesting that the uplifted plateau by then did not effectively block the moisture availability at optimum climatic conditions. The finally irreversible drying up of great lakes at ∼4.9 Ma could be attributed to the plateau uplift.
In this paper, a series of experiments was conducted on remoulded hydrate-bearing marine sediments from a gas hydrate core drilling site located in the Pearl River Estuary of the South China Sea. The ...composition and particle size distribution of marine sediments were analysed, and triaxial loading experiments were performed to study the mechanical properties of hydrate-bearing marine sediments. The strength properties of hydrate-bearing marine sediments were compared with those of hydrate-bearing kaolin clay, ice-marine sediment mixtures and hydrate-dissociated marine sediments under the same porosity (40%), temperature (8 °C), confining pressures (2, 3, and 4 MPa), and strain rate (1%/min) conditions. Several important results were obtained: The composition and particle size distribution analysis indicated the marine sediment and kaolin clay are both sandy clays, and the stress-strain behaviours and strength properties of hydrate-bearing marine sediments are similar to those of hydrate-bearing kaolin clay. Under different confining pressures, the failure strengths of all ice-marine sediment mixtures were lower than those of methane hydrate-bearing sediments, confirming that hydrate particles enhance the cementation of sediment particles. Hydrate dissociation leads to strength reduction of gas hydrate-bearing marine sediments, as was demonstrated in our previous research of hydrate-bearing kaolin clay. Through the analysis of Mohr circles and failure envelopes of three types of mixtures under different confining pressures, it can be inferred that the cohesive strength played an important role in the failure strength of hydrate-bearing marine sediments following hydrate dissociation.
•Triaxial strength experiments were conducted on hydrate-bearing marine sediments of South China Sea.•Strength property of hydrate-bearing marine sediment was found to be similar to hydrate-bearing kaolin clay.•Hydrate between sediment particles enhances the cohesion of sediment.•Cohesion reduction accompanying with hydrate dissociation greatly affects the failure strength of hydrate-bearing sediment.
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