云南先峰中新世木乃伊化木化石α-纤维素的提取

本文对云南先锋中新世木乃伊化木化石进行α-纤维素实验提取,以验证化石纤维素的可得性。实验利用有机溶剂分离、碱液分离、无机酸分离等方法分别对研究区5件木乃伊化化石样品进行处理,并采用红外光谱检测方法与现代木材α-纤维素和国际标准α-纤维素进行对比试验。实验结果显示,3种提取方法均可以从化石木材中提取出α-纤维素,但过程特征各有不同。其中,有机溶剂分离实验过程耗时最长,碱液分离方法样品损失最大。相比之下,无机酸分离方法对于木乃伊化木化石α-纤维素提取的效率最高。     

北京地区大气污染分布的“南北两重天”现象

利用2006年北京地区空气质量监测站和自动气象站网资料,以及华北地区中尺度气象观测资料,分析了北京地区大气污染分布的“南北两重天”现象.通过对此类现象发生过程中天气形势和北京地区气象要素的分析,指出了造成该现象的天气学成因和气象要素特征.研究结果表明,2006年北京共发生47次“南北两重天”现象,其中大部分发生在秋、冬季的午夜至次日上午,且多为南差北好的污染物分布情景.该现象的发生与北京地区中-α尺度天气系统活动的一些特征有关,特别是与干冷空气进入北京地区的路径以及移动速度的区域差异有关.另外,在弱天气系统控制下,北京西南部地区经常出现的小尺度辐合型流场,也是形成北京地区“南北两重天”现象的重要原因.     

Organic geochemistry of Cuban oils—I. The northern geological province

The isoprenoid, hopanoid and steroid compositions of 15 oils from the most productive oil fields in Cuba were studied to determine source-rock depositional environments and organic matter sources. The oils, which are from the northern geological province of Cuba and can be defined by the position with respect to the overthrust belt, can be grouped into two families: those from the Remedios (1) and those from the Placetas (2) tectonostratigraphic units.Remedios oils contain 17α(H)-diahopane, high relative amounts of 18α(H)-22,29,30-trisnorneohopane and diasteranes, which is indicative of generation from clay-rich source rocks. The crude oils of the Placetas Unit exhibit a sterane/hopane composition consistent with a carbonate origin. Nevertheless, these polycyclic hydrocarbons exhibit significant changes in composition, indicating that several organic matter sources, e.g. a carbonate/evaporitic origin of Varadero and Varadero Sur oils, have contributed to the oils from this Unit. The Remedios oils are more mature [evaluated from the and sterane indices] than the Placetas oils.A wide range of biodegradation levels are encountered in these oils (from 0 to 7–8 using the scale derived by Volkman et al. (1983). The high relative abundance of 25-norhopanes is a distinctive feature of Remedios oils. The presence of these compounds in lightly biodegraded or nondegraded oils corresponds to a mixing of paleobiodegraded oil with more recently sourced nondegraded oil in the reservoir. The most biodegraded oil, Cantel, exhibits and sterane ratios, and hopane ratios that have been altered by microbial attack.     

Processes controlling 234U and 238U isotope fractionation and helium in the groundwater of the St. Lawrence Lowlands,Quebec: The potential role of natural rock fracturing

The goal of this study is to explain the origin of ²³⁴U–²³⁸U fractionation in groundwater from sedimentary aquifers of the St. Lawrence Lowlands (Quebec, Canada), and its relationship with ³He/⁴He ratios, to gain insight regarding the evolution of groundwater in the region. (²³⁴U/²³⁸U) activity ratios, or (²³⁴U/²³⁸U)_act, were measured in 23 groundwater samples from shallow Quaternary unconsolidated sediments and from the deeper fractured regional aquifer of the Becancour River watershed. The lowest (²³⁴U/²³⁸U)_act, 1.14 ± 0.01, was measured in Ca–HCO₃-type freshwater from the Quaternary Shallower Aquifer, where bulk dissolution of the carbonate allows U to migrate into water with little ²³⁴U–²³⁸U isotopic fractionation. The (²³⁴U/²³⁸U)_act increases to 6.07 ± 0.14 in Na–HCO₃–Cl-type groundwater. Preferential migration of ²³⁴U into water by α-recoil is the underlying process responsible for this isotopic fractionation. An inverse relationship between (²³⁴U/²³⁸U)_act and ³He/⁴He ratios has been observed. This relationship reflects the mixing of newly recharged water, with (²³⁴U/²³⁸U)_act close to the secular equilibrium and containing atmospheric/tritiogenic helium, and mildly-mineralized older water (¹⁴C ages of 6.6 kyrs), with (²³⁴U/²³⁸U)_act of ≥6.07 and large amounts of radiogenic ⁴He, in excess of the steady-state amount produced in situ. The simultaneous fractionation of (²³⁴U/²³⁸U)_act and the addition of excess ⁴He could be locally controlled by stress-induced rock fracturing. This process increases the surface area of the aquifer matrix exposed to pore water, from which produced ⁴He and ²³⁴U can be released by α-recoil and diffusion. This process would also facilitate the release of radiogenic helium at rates greater than those supported by steady-state U–Th production in the rock. Consequently, sources internal to the aquifers could cause the radiogenic ⁴He excesses measured in groundwater.     

Buoyancy-driven perturbations in a rapidly rotating,electrically conducting fluid: part II – dynamo action

The helicity, electromotive force and α-effect produced in a homogeneous, rapidly rotating, electrically conducting fluid by an isolated source of buoyancy at small Elsasser number are calculated, visualized and analyzed. Due to physical symmetries of the system, the integrals of helicity and electromotive force over all space are zero. However, each has a significant non-zero value when integrated over the cross section of the Taylor column. The local α-effect is found to be significantly anisotropic; it is strongest when the applied magnetic field is toroidal and the resulting EMF is parallel to the applied field.     

Maximally-efficient-generation approach in the dynamo theory

Abstract

We propose a method of derivation of global asymptotic solutions of the hydromagnetic dynamo problem at large magnetic Reynolds number. The procedure reduces to matching the local asymptotic forms for the magnetic field generated near individual extrema of generation strength. The basis of the proposed method, named here the Maximally-Efficient-Generation Approach (MEGA), is the assertion that properties of global asymptotic solutions of the kinematic dynamo are determined by the distribution of the generation strength near its leading extrema and by the number and distribution of the extrema.

The general method is illustrated by the global asymptotic solution of the α²-dynamo problem in a slab. The nature of oscillatory solutions revealed earlier in numerical simulations and the reasons for the dominance of even magnetic modes in slab geometry are clarified.

Applicability of the asymptotic solutions at moderate values of the asymptotic parameter is also discussed. We confirm this applicability using comparisons with complementary asymptotic expansions and numerical simulations. In particular, this justifies application of the MEGA solutions to estimation of the generation threshold.     

Non-local memory effects of the electromotive force by fluid motion with helicity and two-dimensional periodicity

In mean-field dynamo theory, the electromotive force term 〈u′ × B′〉 due to small-scale fields connects the small-scale magnetic field with the large-scale field. This term is usually approximated as the α-effect, assumed to be instantaneous in time and local in space. However, the approximation is valid only when the magnetic Reynolds number Rm is much less than unity, and is inappropriate when Rm ? 1, which is the condition satisfied in the Earth's core or solar convection zone. We introduce a function φ _qr as a non-local and non-instantaneous generalization of the usual α-effect and examine its behaviour as a function of Rm in the range 1/64 ≤ Rm ≤ 10 for a kinematic dynamo model. We use the flow of G.O. Roberts 1972 Roberts, GO. 1972. Dynamo action of fluid motions with two-dimensional periodicity. Phil. Trans. Roy. Soc. London Ser. A, 271: 411–454. [Crossref], [Web of Science ®] , [Google Scholar] (Phil, Trans. Roy. Soc. London Ser. A, 1972, 271, 411–454), which is steady and has non-zero helicities and two-dimensional periodicity. As a result, we identify three regions in Rm space according to the behaviour of the function φ _qr : (i) Rm ? 1/4, where the function φ _qr is local and instantaneous and can be approximated by the traditional α and β effects, (ii) 1/4 ? Rm ? 4, where the deviation from the traditional α and β effects increases and non-localness and non-instantaneousness increase, and (iii) Rm ? 4, where boundary layers develop fully and non-localness and non-instantaneousness are prominent. We show that the non-local memory effect for Rm ? 4 strongly affects the dynamo action and explains an observed augmentation of the growth rate in the dispersion relation. The results imply that the non-local memory effect of the electromotive force should be important in the geodynamo or the solar dynamo.     

Weakly nonlinear analysis of the α effect

We address mathematical issues raised by the so-called α?effect of dynamo theory, which is a dynamo mechanism arising in conducting flows with small scale fluctuations. Analytical results on the α?effect concern the linear induction equation, and are usually claimed to hold for the whole magnetohydrodynamics (MHD) system, as long as the amplitude of the perturbations is small. We discuss the justification of that claim, in the case of periodic fluctuations of the fields. We show a nonlinear instability result on the MHD system, that predicts dynamo action for a large class of high frequency periodic flows, up to the fully nonlinear regime.     

α径迹测量在非洲特殊环境下的应用及效果

在非洲纳米比亚开展了α径迹测量。针对非洲地区与国内迥然不同的特殊地质和恶劣气候环境,采取了各种应对措施,保证了测量的高灵敏度和准确性;经与钻探资料对比,径迹密度与放射性异常对应很好,测量效果显著。使α径迹测量这一物探找铀矿方法在非洲特殊环境的条件下得到全新的阐释。