碳酸岩的地质地球化学特征及其大地构造意义

从已知碳酸岩的地质产状、岩石学特征、Nd-Sr-Pb-O-C同位素及痕量元素地球化学特征数据,结合高温高压实验岩石学资料,论述了其地幔源区的物质成分、交代过程软流圈地幔部分熔融机制和碳酸岩岩浆的演化模型。碳酸岩既可以产生于拉张岩石圈构造背景,也能够产生于挤压而派生的引张岩石圈构造背景。前者以产于裂谷环境、与硅酸不饱和过碱性岩构成环状碳酸岩—碱性杂岩为特征,主要由起源于软流圈地幔的霞石质超基性—基性岩浆经液态不混溶作用而形成;后者产于碰撞造山过程中派生的引张岩石圈断裂带,以单一的透镜状、条带状和似层状碳酸岩体为标志,直接由导源岩石圈富集地幔的低程度部分熔融作用而产生的碳酸岩浆侵入或喷发所形成。     

Modelling of thermochemical plumes and implications for the origin of the Siberian traps

Thermochemical plumes form at the base of the lower mantle as a consequence of heat flow from the outer core and the presence of local chemical doping that decreases the melting temperature. Theoretical and experimental modelling of thermochemical plumes show that the diameter of a plume conduit remains practically constant during plume ascent. However, when the top of a plume reaches a refractory layer, whose melting temperature is higher than the melt temperature in the plume conduit, a mushroom-shaped plume head develops. Main parameters (melt viscosity, ascent time, ascent velocity, temperature differences in the plume conduit, and thermal power) are presented for a thermochemical plume ascending from the core–mantle boundary. In addition, the following relationships are developed: the pressure distribution in the plume conduit during the ascent of a plume, conditions for eruption-conduit formation, the effect of the P–T conditions and controls on the shape and size of a plume top, heat transfer between a thermochemical plume and the lithosphere (when the plume reaches the bottom of a refractory layer in the lithosphere), and eruption volume versus the time interval t₁ between plume formation and eruption. These relationships are used to determine thermal power and time t₁ for the Tunguska syneclise and the Siberian traps as a whole.

The Siberian and other trap provinces are characterized by giant volumes of lavas and sills formed a very short time period. Data permit a model for superplumes with three stages of formation: early (variable picrites and alkali basalts), main (tholeiite plateau basalts), and final (ultrabasic and alkaline lavas and intrusions). These stages reflect the evolution of a superplume from the ascent of one or several independent plumes, through the formation of thick lenses of mantle melts underplating the lithosphere and, finally, intrusion and extrusion of differentiated mantle melts. Synchronous syenite–granite intrusions and bimodal volcanism abundant in the margins of the Siberian traps are the result of melting of the lower crust at depths of 65–70 km under the effect of plume melts.     

Early Paleozoic tectonics and geodynamics of Central Asia: role of mantle plumes

There were two key stages in the history of Paleozoids that formed in the place of the Paleoasian ocean, one in the Cambrian–Ordovician and the other in the Permian–Triassic. Both time spans were characterized by a combination of similar geodynamic, magmatic, and geomagnetic events: closure and opening of oceanic basins, intense plume magmatism associated with Earth's core cooling, and absence of geomagnetic reversals (superchrons). Three superchrons about 490–460, 260–300, and 124–86 Ma correlate with major events of plume magmatism. Plume reconstructions have to be updated for the period 490–460 Ma, which corresponded to the third superchron and was marked by ocean opening. The previous superplume, about 800–740 Ma, requires further justification but fits the global periodicity with 240 Ma major cycles and smaller ones of 120 (or also 30) Ma.In the Late Cambrian–Ordovician, large-scale accretion and collision events acted, in similar tectonic settings, upon the vast territory that currently extends from the Polar Urals to Lake Baikal (and was times larger in the past). As a result, Gondwanian microcontinents (Kokchetav, Altai–Mongolia, Tuva–Mongolia, etc.) and island arcs joined into the Kazakhstan–Tuva–Mongolia system. The formation of the Late Cambrian–Ordovician orogen in Central Asia was synchronous with opening of the Ural, Ob–Zaisan, Turkestan, and Paleotethys oceans. The plume pulses (520–500 and 490–460 Ma) may have been responsible for opening of new oceans, accelerated amalgamation of terranes, and synchronicity in geodynamic events from the Urals to Transbaikalia.     

Carbon and biogenic silica export influenced by the Amazon River Plume: Patterns of remineralization in deep-sea sediments

The Amazon River Plume delivers freshwater and nutrients to an otherwise oligotrophic western tropical North Atlantic (WTNA) Ocean. Plume waters create conditions favorable for carbon and nitrogen fixation, and blooms of diatoms and their diazotrophic cyanobacterial symbionts have been credited with significant CO₂ uptake from the atmosphere. The fate of the carbon, however, has been measured previously by just a few moored or drifting sediment traps, allowing only speculation about the full extent of the plume's impact on carbon flux to the deep sea. Here, we used surface (0.5 m) sediment cores collected throughout the Demerara Slope and Abyssal Plain, at depths ranging from 1800 to 5000 m, to document benthic diagenetic processes indicative of carbon flux. Pore waters were extracted from sediments using both mm- and cm-scale extraction techniques. Profiles of nitrate (NO₃) and silicate (Si(OH)₄) were modeled with a diffusion-reaction equation to determine particulate organic carbon (POC) degradation and biogenic silica (bSi) remineralization rates. Model output was used to determine the spatial patterns of POC and bSi arrival at the sea floor. Our estimates of POC and Si remineralization fluxes ranged from 0.16 to 1.92 and 0.14 to 1.35 mmol m⁻² d⁻¹, respectively. A distinct axis of POC and bSi deposition on the deep sea floor aligned with the NW axis of the plume during peak springtime flood. POC flux showed a gradient along this axis with highest fluxes closest to the river mouth. bSi had a more diffuse zone of deposition and remineralization. The impact of the Amazon plume on benthic fluxes can be detected northward to 10°N and eastward to 47°W, indicating a footprint of nearly 1 million km². We estimate that 0.15 Tmol C y⁻¹ is remineralized in abyssal sediments underlying waters influenced by the Amazon River. This constitutes a relatively high fraction (~7%) of the estimated C export from the region.; the plume thus has a demonstrable impact on C_org export in the western Atlantic. Benthic fluxes under the plume were comparable to and in some cases greater than those observed in the eastern equatorial Atlantic, the southeastern Atlantic, and the Southern Ocean.     

地铁站点有害气体扩散三维可视化研究

本文运用高斯烟羽模型对地铁站点有害气体扩散事故进行三维模拟,计算出了有害气体在各个方向上的扩散距离和任意特定点的的气体浓度,并在三维场景中显示出了等浓度有害气体的扩散范围,以及有害气体扩散事故的三维影响范围。研究结果表明,运用高斯烟羽模型和GIS技术,能够在三维场景中更加直观地表示出有害气体在三维空间上的扩散范围。同时,论文实现并扩展了高斯烟羽模型在三维中的应用。     

The effects of mixing and spreading on density in near-field river plumes

Water mass modification in surface-trapped, near-field river plumes is examined using a 1.5-layer reduced gravity model and a three-dimensional numerical model. Solutions to the layer model are shown to be qualitatively similar to previous observations and three-dimensional simulations of near-field plumes. Analytic analysis of the layer model demonstrates how the near-field plume is controlled by the competing processes of mixing and spreading. The two models are then used to explore the parameter space dependence of density changes within the near-field plume and their associated cross-shore length scales. Both the magnitude of density changes and their length scales are proportional to either estuarine discharge or fresh water discharge; density changes are also inversely proportional to the estuary mouth width. One surprising feature of the parameter space solutions is that the density of water exiting the near-field plume, a measure of the net dilution of the entire near-field plume, is shown to be inversely proportional to local mixing rates. This is because when local mixing is lower, the influence of plume spreading becomes greater; this spreading accelerates the plume, requiring more net mixing to bring the plume back to subcritical flow.     

Uplifted crust in parts of western India

During northward movement of the Indian sub-continent, after its breakup from the Gondwanaland in the Late Cretaceous, the western part of India traversed over the Reunion plume. The Saurashtra peninsula and the Cambay Basin are two important geological regions in this part. Two and half dimensional density models, based on the crustal seismic structure, were generated to establish a relationship between these two regions. These models indicate that the crust is 32–33 km thick in the eastern Saurashtra and the northern part of the Cambay Basin. The shallower crust is in a triangular region formed by the extension of the western limb of the Proterozoic Aravalli trend in Saurashtra, its eastern limb and the Narmada fault in the south. Compared to 36–37 km thick crust to the west and 38–40 km to the east of this region the crust in the above triangular region is uplifted by 4 to 6 km. This uplift took place either after the deposition of Mesozoic sediments or was concomitant with the rise of Reunion plume prior to the extrusion of the Deccan volcanics as the region was close to the axis of the plume.     

Modeling of an MTBE plume at Pascoag, Rhode Island

A numerical groundwater flow and mass transport model was developed to predict the extent of impact from methyl tertiary butyl ether (MTBE) release on a down-gradient drinking water well field. An MTBE incident in Pascoag, Rhode Island, was used as a case study and the plume’s past and future development was simulated using scenario analysis. The numerical code used was GeoSys/Rockflow, which permits a coupled flow and transport simulation as one object, thus alleviating the need for simulating the MTBE fate with separate flow and transport codes. The numerical model was built on available hydrogeological and chemical data as well as on GIS information of the site. By comparing the simulated results with observed field data, it was found that the model could provide reliable results even when the simulated aquifer was simplified to a two-dimensional flow and transport domain. Finally, the calibrated model was used for exploring a location that may be suitable for a new well field. Despite the model limitations associated with uncertainties of data and simplifying assumptions, numerical modeling of this MTBE contaminated site proved a useful tool and provided guidance for future municipal well field operation strategies and aquifer remediation alternatives.     

Laboratory Simulation Of Vertical Plume Dispersion Within A Convective Boundary Layer

Mesoscale measurements of the vertical dispersion coefficient ₂ by using a composite turbulence water tank were validated through a comparison with CONDORS (Convective Diffusion Observed with Remote Sensors) field data, and were analysed with respect to the intensity of the thermal flux, mechanical turbulence, and plume release height.It seems possible to correct the plume _z values for different release heights below 0.5z_i (z_i is the mixing height) by applying an equation expressing the height dependency of turbulence intensity. The downwind distance where the plume's mass centre height approaches its final level was also analysed with respect to the above three parameters, and an empirical equation to estimate the downwinddistance derived.     

地震层析成像板块构造及地幔演化动力学

地震层析成像技术的飞速发展给人类提供了一个探测地球内部结构的强有力的工具，也给人类展示了一片新的天地，使人类更加清晰地在两个层次之上（地球的层圈构造和内部横向不均匀性）了解地球内部，特别是对幔内部非均匀结构，从全球和区域的尺度上揭示出地球内部特别是地壳和上地幔横向不均匀结构。呈现出全新的地球，特别是地幔结构的三维图像框架，很容易理解，这一基本框架正是地幔演化热动力学过程的现代表现，如此，我们可以利用该框架去追溯、探讨全球构造特别是地幔构造演化的历史，从而深化对于我们这个星球演化过程的理解。