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1.
Holocene climate variability 总被引：17，自引：0，他引：17
2.
A late Pleistocene and Holocene mineral magnetic record from sediments of Lake Aibi,Dzungarian Basin,NW China 总被引：1，自引：0，他引：1
Alina Tudryn Piotr Tucholka Elisabeth Gibert Francoise Gasse Keiquin Wei 《Journal of Paleolimnology》2010,44(1):109-121
We studied mineral magnetic properties of a 6-m-long, late Pleistocene through Holocene sediment sequence from Lake Aibi in
Dzungaria (Zunggary, Junggar), northern Xinjiang, China. Results were used to infer environmental changes and are compared
with previously studied cores from Lake Manas. Both water bodies occupy the deepest parts of the Dzungarian Basin and are
remnants of large Holocene lakes. During the Late Pleistocene, the magnetic mineralogy in both lakes was dominated by detrital,
iron oxide minerals. Oxic conditions, which dominated during sedimentation and early diagenesis, persisted over the Pleistocene–Holocene
transition. Later, during the middle Holocene, lake bottom conditions enabled authigenic formation of iron sulphide minerals
such as pyrite (FeS2) in Lake Aibi, and pyrite and greigite (Fe3S4) in Lake Manas. This iron sulphide mineralogy suggests increased biological activity in stagnant, anoxic bottom waters. Anoxic
bottom conditions started about 9.8 cal kyr BP in Lake Manas and at about 7.2 cal kyr BP in Lake Aibi. A short dry event recorded
in Lake Manas between 6.8 and 5.2 cal kyr BP is not clearly observed in Lake Aibi. In the late Holocene, i.e. the last 2.8 cal
kyr, sediments of both lakes are again characterised by iron oxides, suggesting well-mixed, shallow water bodies. For this
recent period, it seems that the detrital material in the two lakes had a common origin. Magnetic properties of sediments
in Lakes Aibi and Manas show broadly similar environmental evolution during the late Pleistocene and Holocene. Nevertheless,
despite the close proximity of the two lakes (~200 km) in the same basin, they display some different magnetic properties
and record environmental changes at different times. 相似文献
3.
Catherine Rio Jean-Yves Grandpeix Frédéric Hourdin Francoise Guichard Fleur Couvreux Jean-Philippe Lafore Ann Fridlind Agnieszka Mrowiec Romain Roehrig Nicolas Rochetin Marie-Pierre Lefebvre Abderrahmane Idelkadi 《Climate Dynamics》2013,40(9-10):2271-2292
Recently, a new conceptual framework for deep convection scheme triggering and closure has been developed and implemented in the LMDZ5B general circulation model, based on the idea that deep convection is controlled by sub-cloud lifting processes. Such processes include boundary-layer thermals and evaporatively-driven cold pools (wakes), which provide an available lifting energy that is compared to the convective inhibition to trigger deep convection, and an available lifting power (ALP) at cloud base, which is used to compute the convective mass flux assuming the updraft vertical velocity at the level of free convection. While the ALP closure was shown to delay the local hour of maximum precipitation over land in better agreement with observations, it results in an underestimation of the convection intensity over the tropical ocean both in the 1D and 3D configurations of the model. The specification of the updraft vertical velocity at the level of free convection appears to be a key aspect of the closure formulation, as it is weaker over tropical ocean than over land and weaker in moist mid-latitudes than semi-arid regions. We propose a formulation making this velocity increase with the level of free convection, so that the ALP closure is adapted to various environments. Cloud-resolving model simulations of observed oceanic and continental case studies are used to evaluate the representation of lifting processes and test the assumptions at the basis of the ALP closure formulation. Results favor closures based on the lifting power of sub-grid sub-cloud processes rather than those involving quasi-equilibrium with the large-scale environment. The new version of the model including boundary-layer thermals and cold pools coupled together with the deep convection scheme via the ALP closure significantly improves the representation of various observed case studies in 1D mode. It also substantially modifies precipitation patterns in the full 3D version of the model, including seasonal means, diurnal cycle and intraseasonal variability. 相似文献
4.
暴露岩石中宇生核素的浓度与暴露年代、侵蚀速率相关,暴露年代和侵蚀速率可相互求证,但两者均为未知数时,可求算出最大侵蚀速率和最小暴露年代。据此得出张八岭地区的最大侵蚀速率为7 12m/Ma,最小暴露年代为(85700±1500)a,这一年代远小于统一夷平面的形成年代(7～3 6Ma),造成这种差距的主要原因是:①张八岭地区侵蚀速率大,岩石中宇生核素的浓度较早达到平衡,即达到该地区的测年上限;②采样地区曾被土壤覆盖,造成计算年代值过于年轻。 相似文献
5.
Francoise Combes 《Celestial Mechanics and Dynamical Astronomy》1998,72(1-2):91-127
In the interstellar medium, as well as in the Universe, large density fluctuations are observed, that obey power-law density
distributions and correlation functions. These structures are hierarchical, chaotic, turbulent, but are also self-organizing.
The apparent disorder is not random noise, but can be described by a fractal, with a deterministic fractal dimension. We present
a new theory of the self-gravity thermodynamics, that could explain the existence of these fractal structures, and predict
their fractal dimension. The media obeying scaling laws can be considered critical, as in second order phase transitions for
instance.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
6.
地貌学与第四纪研究的新手段——陆地宇生核素研究 总被引：3，自引：1，他引：3
暴露岩石表层中的宇生核素的浓度，是产生速率、衰变常数及侵蚀速率的函数。根据该原理，在宇生核素形成速率为已知的条件下，建立宇生核素浓度与时间及侵蚀速率的函数关系，根据该模式可对岩面的暴露时间及侵蚀速度进行计算，这使得该测年技术在地貌学与第四纪研究中有着极大的运用前景。在众多的宇生核素中，取自于石英的^26Al和^10Be是目前该领域中运用最广泛、最成熟的宇生核素。 相似文献
7.
MENG Xianghu GE Ming LIU Yanxue KUANG Hongwei LIU Weifu Francoise G. BOURROUILH-LE JAN Robert BOURROUILH Institute of Sedimentary Basins China University of Geosciences Beijing Chinese Academy of Geological Sciences Beijing Department of Geophysics Yangzi University Jingzhou Hubei Laboratoire Cibamar Universite Bordeaux Avenue des Facultes TALENCE Cedex France 《《地质学报》英文版》2004,78(3)
Based on a study of Neoproterozoic carbonates in the Jilin-Liaoning-Xuzhou-Huaiyang area, especially its cyclic sequence stratigraphy and Sr isotopes, two maximum sea flooding events (at 820 Ma and 835 Ma) have been identified. The resulting isochronous stratigraphic correlation proves that these Precambrian strata were connected between the Qingbaikou and the Nanhuan systems with a time range from 750 Ma to 850 Ma. The disappearance of microsparite carbonate and coming of a glacial stage offer important evidence for worldwide stratigraphic correlation and open a window for further correlation of the stratigraphic successions across the Sino-Korean and Yangtze Plates. A new correlation scheme is therefore provided based on our work. 相似文献
8.
Global lopsided instability in a purely stellar galactic disc 总被引：1，自引：0，他引：1
Kanak Saha Francoise Combes Chanda J. Jog 《Monthly notices of the Royal Astronomical Society》2007,382(1):419-432
It is shown that pure exponential discs in spiral galaxies are capable of supporting slowly varying discrete global lopsided modes, which can explain the observed features of lopsidedness in the stellar discs. Using linearized fluid dynamical equations with the softened self-gravity and pressure of the perturbation as the collective effect, we derive self-consistently a quadratic eigenvalue equation for the lopsided perturbation in the galactic disc. On solving this, we find that the ground-state mode shows the observed characteristics of the lopsidedness in a galactic disc, namely the fractional Fourier amplitude A 1 , increases smoothly with the radius. These lopsided patterns precess in the disc with a very slow pattern speed with no preferred sense of precession. We show that the lopsided modes in the stellar disc are long-lived because of a substantial reduction (approximately a factor of 10 compared to the local free precession rate) in the differential precession. The numerical solution of the equations shows that the ground-state lopsided modes are either very slowly precessing stationary normal mode oscillations of the disc or growing modes with a slow growth rate depending on the relative importance of the collective effect of the self-gravity. N -body simulations are performed to test the spontaneous growth of lopsidedness in a pure stellar disc. Both approaches are then compared and interpreted in terms of long-lived global m = 1 instabilities, with almost zero pattern speed. 相似文献
9.
Paleo-hydrologic and -vegetation proxy data from the Tanganyika basin are integrated in energy and water balance equations to infer past evaporation and precipitation during the last glacial maximum (LGM). Our approach is first validated on the modern system. Large variations are assigned to input variables to simulate the interannual precipitation variability. Equations are then applied to the LGM. We first change those input parameters inferred from proxies (basin and lake surfaces, temperature, and land albedo). Our LGM simulation suggests (in percent of modern mean values) decreases in evaporation from the lake [El: −5% (between −13% and +3%)] and land [Ec: −8% (−19/+5)] bodies, in precipitation [P: −11% (−21/0)] and (P − Ec): −42% (−44/−40). Decreases in P and E are amplified [El: −8% (−16/0); Ec: −14% (−24/−2); P: −17% (−26/−6)] when including empirical changes in atmospheric transmission coefficient and Bowen ratio. Sensitivity runs suggest that even large changes in cloud cover and air humidity should not modify these trends. The results suggest that the Earth's glacial/interglacial boundary conditions play a significant role on climate of subequatorial southern Africa. 相似文献
10.
Pollen- and Diatom-Inferred Climatic and Hydrological Changes in Sumxi Co Basin (Western Tibet) since 13,000 yr B.P. 总被引：1，自引：0，他引：1
Although the Tibetan Plateau greatly influences the atmospheric circulation of the Nortbern Hemisphere, few continuous paleoclimatic records are available from the plateau. A 13,000-yr pollen and diatom record from the Sumxi-Longmu Co basin in western Tibet gives information on major changes both in regional vegetation and in local hydrology. After the basin first filled ca. 13,000 yr B.P., a dry spell occurred about 10,500 yr B.P. within the interval spanned by the European Younger Dryas chronozone. A major environmental change occurred suddenly at ≈10,000 yr B.P., with the establishment of wet conditions, and was followed by a long-term trend toward maximum aridity, which lasted approximately 6000 yr. Short-term oscillations are superimposed on this general climatic change with a major reversal event about 8000 yr B.P. and a second wet pulse leading to a maximum lake volume ca. 7500-6000 yr B.P. Maximum aridity occurred 4300 yr B.P. The major environmental fluctuations recorded at Sumxi-Longmu Co appear in phase with climatic changes recognized in north tropical Africa, suggesting that the 8000 to 7000-yr-B.P. event was caused by an abrupt disequilibrium in the climatic system, as was the Younger Dryas and possibly the 4300-yr-B.P. event. 相似文献