The effect of dispersion on the stability of density-driven flows in saturated homogeneous porous media     

Jude L. Musuuza  Florin A. Radu  Sabine Attinger 《Advances in water resources》2011,34(3):417-432

This work is a continuation of Musuuza et al. [37] in which a stability criterion for density-driven flow in a saturated homogeneous medium was derived. The criterion predicted the stability of a system as a function of the density and viscosity contrasts, the magnitude of the flow velocity and the concentration gradients for flow aligned orthogonal to gravity. It could not accurately predict stability transition with increasing velocity, a failure we attributed to dispersion effects that were not included. Small-scale dispersion and molecular diffusion, the main stabilising mechanisms in homogeneous media can stabilise flow if the instability wavelengths are below a certain cutoff. The width of the mixing zone is also central in controlling the range of wavelengths that persist into fingers. We propose a method of quantifying the cutoff wavelength and the width of the mixing zone, which are incorporated into the earlier criterion as constituents of the dispersive part. The earlier criterion is reformulated in terms of the Rayleigh number and with the dispersive part added, we attempt to predict the number of fingers formed which is directly related to the physical stability of the system. The inclusion of the Rayleigh number and dispersion into a single stability criterion provides new insight in the way dispersion affects vertical flow systems. Stability numbers computed with the new criterion are in reasonable agreement with numerical simulations for a range of physical variables. The numerical computations are performed with the software package d³f, which uses the cell-centred finite volume and the implicit Euler methods for the spatial and temporal discretisations, respectively. The admission of the density and dispersivities as inputs into the criterion makes it usable in practical problems.  相似文献   

The stability of density-driven flows in saturated heterogeneous porous media     

Jude L. Musuuza  Florin A. Radu  Sabine Attinger 《Advances in water resources》2011,34(11):1464-1482

This work concludes the investigations into the stability of haline flows in saturated porous media. In the first part [33] a stability criterion for density-driven flow in a saturated homogeneous medium was derived excluding dispersion. In the second part [34], the effects of dispersion were included. The latter criterion made reasonable predictions of the stability regimes (indicated by the number of fingers present) as a function of density and dispersivity variations. We found out that destabilising variables caused an increase in the number of fingers and vice versa. The investigation is extended here for the effects of the medium heterogeneity. The cell problem derived via homogenization theory [20] is solved and its solution used to evaluate the elements of the macrodispersion tensor as functions of time for flow aligned parallel to gravity. The longitudinal coefficient exhibits asymptotic behaviour for favourable and moderately unfavourable density contrasts while it grows indefinitely for higher density contrasts. The range of densities stabilised by medium heterogeneities can thus be estimated from the behaviour of the coefficient. The d³f software program is used for the numerical simulations. The code uses the cell-centred finite volume and the implicit Euler techniques for the spatial and temporal discretisations respectively.  相似文献   

Ecological stability during the LGM and the mid-Holocene in the Alpine Steppes of Tibet?     

Georg Miehe  Sabine Miehe  Kerstin Bach  Jürgen Kluge  Karsten Wesche  Yang Yongping  Liu Jianquan 《Quaternary Research》2011,76(2):243-252

Arid and Alpine ecosystems are known for extreme environmental changes during the Late Quaternary. We hypothesize that the world's largest Alpine arid ecosystem however, the Alpine Steppes of the Tibetan highlands, remained ecologically stable during the LGM and the mid-Holocene. This hypothesis is tested by distributional range of plant species, plant life forms and rate of endemism. The set of character species has a precipitation gradient between 50 and 350 mm/a, testifying for resilience to precipitation changes. 83% of the species have a wider vertical range than 1000 m used as a proxy for resilience to temperature changes. 30% of the species are endemic with 10 endemic genera, including plate-shaped cushions as a unique plant life form. These findings are in line with palaeo-ecological proxies (δ¹⁸O, pollen) allowing the assumption that Alpine Steppes persisted during the LGM with 3 to 4 K lower summer temperatures.During the mid-Holocene, forests could have replaced Alpine Steppes in the upper catchments of the Huang He, Yangtze, Mekong, Salween and Yarlung Zhangbo, but not in the interior basins of the north-western highlands, because the basins were then flooded, suppressing forests and supporting the environmental stability of this arid Alpine grassland biome.  相似文献   

Insights into cation exchange selectivity of a natural clinoptilolite by means of dielectric relaxation spectroscopy     

Gerardo Rodríguez-Fuentes  Sabine Devautour-Vinot  Sekou Diaby  François Henn 《Physics and Chemistry of Minerals》2011,38(8):613-621

Purified natural clinoptilolite from the Tasajeras deposit, Cuba, and some of its metal exchanged forms are studied, at the dehydrated state, by means of dielectric relaxation spectroscopy (DRS) using two different modus operandi: complex impedance spectroscopy and dielectric dynamic thermal analysis. Data analysis yields the determination of the extra-framework cation (EFC) population into the various possible crystallographic sites of the zeolitic framework as well as of the activation energy characterizing the localized hopping mechanism of EFC. First, it is shown that the DRS responses obtained here match well with the previous reported data, which were previously localized EFCs in positions close to M1 and M2 sites when the clinoptilolite is modified to almost homoionic form. From this outcome, it can be concluded that all EFCs are in the same crystallographic situation regarding solvation or, in other terms, that no steric effect can be taken into account to explain cationic selectivity. Second, based on the assumption that the activation energy for EFC hopping is directly connected to the EFC/framework interaction and on simple thermodynamics consideration, we show this interaction does not govern the EFC exchange reaction. So, it is emphasized that EFC/H₂O interaction is the key factor for cation exchange selectivity.  相似文献   

Formation of todorokite from vernadite in Ni-rich hemipelagic sediments   总被引：1，自引：0，他引：1  

Sabine Bodeï  Nicolas Geoffroy  Martine Buatier 《Geochimica et cosmochimica acta》2007,71(23):5698-5716

Todorokite is considered to form from vernadite in nature and commonly concentrates nickel. However, this mineralogical transformation has never been imaged nor explained mechanistically, and its effect on the uptake of nickel has never been quantified at the molecular-level. We have characterized these reactions at the macroscopic, microscopic, nanoscopic and atomic scales in a marine manganese concretion by combining transmission electron microscopy, electron and X-ray microprobe analysis, powder and micro X-ray diffraction, and Mn and Ni K-edge EXAFS spectroscopy. The concretion was collected during the Ticoflux II expedition near the Nicoya Peninsula, Costa Rica, and is representative of Mn deposits in hemipelagic sediments. It consists of 5 to 25 μm aggregates, shaped like sea-urchins, with a core of 7Å-vernadite (1.0 wt% Ni), a rim of 10Å-vernadite (3.8 wt% Ni), and an outermost region of todorokite fibers (1.9 wt% Ni) that extend outwards. The crystallites of 7Å-vernadite are single- to bi-layered, with hexagonal layer symmetry (a = b = 2.83 Å), and an average structural formula of . The crystallites of 10Å-vernadite contain 10 to 20 layers semi-coherently stacked in the ab plane and uniformly separated in the [0 0 1] direction by ∼9 Å due to the intercalation of hydrated Mg²⁺ cations. The average structural formula of 10Å-vernadite is if the layers contain vacancy sites, or alternately , if they contain Mn³⁺. The average formula of todorokite is .A genetic model is proposed based on combining these new data with previously published results. The thermodynamically unstable 7Å-vernadite transforms via dissolution-recrystallization to semi-ordered Mg-rich 10Å-vernadite. Nickel is released from dissolved biogenic silica or reduced organic matter, and taken up mainly in the Mn layer of 10Å-vernadite. Interlayer magnesium serves as a template to the further topotactic transformation of 10Å-vernadite to todorokite. The dimension of the todorokite tunnels in the [0 0 1] direction is uniform and determined by the size of the hydrated Mg²⁺ ion (8.6 Å). The tunnel dimension in the [1 0 0] direction depends on the density of Mg²⁺ in the interlayer and the superstructure of the phyllomanganate layer. If the parent phyllomanganate contains high amounts of Mg²⁺ (i.e., high layer charge), or Mn³⁺ and Mn⁴⁺ cations ordered following the Mn³⁺-Mn⁴⁺-Mn⁴⁺ sequence as in synthetic triclinic birnessite, then the tunnel dimension is ideally 3 × 3 octahedral chain widths in both crystallographic directions. Otherwise, the tunnel dimension is incoherent in the [1 0 0] direction (i.e., T(3,n) tunnel structure), as has been observed in all natural todorokites. Natural todorokite is defective because the precursor natural phyllomanganates either have a layer charge deficit below 0.33e per octahedral site, or rarely are triclinic birnessite. The abundance of Mg in seawater and its key role in converting phyllomanganate to tectomanganate with T(3,n) tunnel structure explain why todorokite is common in marine ferromanganese oxides, and seldom present in terrestrial environments. The topotactic phase transformation described here is the only known route to todorokite crystallization. This implies that all natural todorokites may be authigenic because they are formed in situ from a phyllomanganate precursor.  相似文献   

Unusual catchment runoff in a high alpine karst environment influenced by a complex geological setting (Northern Calcareous Alps,Tyrol, Austria)     

Krainer  Karl  Winkler  Gerfried  Pernreiter  Sabine  Wagner  Thomas 《Hydrogeology Journal》2021,29(8):2837-2852

Garber Schlag (Q-GS) is one of the major springs of the Karwendel Mountains, Tyrol, Austria. This spring has a unique runoff pattern that is mainly controlled by the tectonic setting. The main aquifer is a moderately karstified and jointed limestone of the Wetterstein Formation that is underlain by nonkarstified limestone of the Reifling Formation, which acts as an aquitard. The aquifer and aquitard of the catchment of spring Q-GS form a large anticline that is bound by a major fault (aquitard) to the north. Discharge of this spring shows strong seasonal variations with three recharge origins, based on δ¹⁸O and electrical conductivity values. A clear seasonal trend is observed, caused by the continuously changing portions of water derived from snowmelt, rainfall and groundwater. At the onset of the snowmelt period in May, the discharge is composed mainly of groundwater. During the maximum snowmelt period, the water is dominantly composed of water derived from snowmelt and subordinately from rainfall. During July and August, water derived from snowmelt continuously decreases and water derived from rainfall increases. During September and October, the water released at the spring is mainly derived from groundwater and subordinately from rainfall. The distinct discharge plateau from August to December and the following recession until March is likely related to the large regional groundwater body in the fissured and moderately karstified aquifer of the Wetterstein Formation and the tectonic structures (anticline, major fault). Only a small portion of the water released at spring Q-GS is derived from permafrost.

  相似文献   

Intraplate volcanism in the Danube Basin of NW Hungary: 3D geophysical modelling of the Late Miocene Pásztori volcano     

Pánisová  Jaroslava  Balázs  Attila  Zalai  Zsófia  Bielik  Miroslav  Horváth  Ferenc  Harangi  Szabolcs  Schmidt  Sabine  Götze  Hans-Jürgen 《International Journal of Earth Sciences》2018,107(5):1713-1730

International Journal of Earth Sciences - Three-dimensional geophysical modelling of the early Late Miocene Pásztori volcano (ca. 11–10 Ma) and adjacent area in the Little...  相似文献   

The occurrence of cyanobacteria and microcystins in the Hoan Kiem Lake and the Nui Coc reservoir (North Vietnam)     

Thi Thuy Duong  Sabine Jähnichen  Thi Phuong Quynh Le  Cuong Tu Ho  Trung Kien Hoang  Trung Kien Nguyen  Thi Nguyet Vu  Dinh Kim Dang 《Environmental Earth Sciences》2014,71(5):2419-2427

The occurrence of increasing blooms of toxic cyanobacteria in freshwaters has received much attention due to the ability of many cyanobacteria to produce potent cyanotoxins. In this paper, the occurrence of dominant cyanobacteria and the concentration of microcystins (MCs) analysis were investigated monthly from July 2008 to April 2009 in the Hoan Kiem Lake and from February to April 2009 in the Nui Coc reservoir. Concentrations of intracellular MCs from water, bloom samples, and isolated strains were quantified by using high performance liquid chromatography (HPLC). During the study period, the microscopic examination of the phytoplankton samples showed the dominance of the genus Microcystis in the water environment of the Hoan Kiem Lake and the Nui Coc reservoir. The toxin analysis by HPLC demonstrated the presence of two MC variants: MC-LR and MC-RR in water samples. Total concentrations of the toxins in filtered samples from surface water ranged from non-detected to 0.91 μg L^?1 at Nui Coc reservoir and they ranged from 2.1 to 46.0 μg L^?1 at Hoan Kiem Lake. The results of the HPLC analysis confirmed the production of MCs in bloom samples (ranged from 115.9 to 184.6 μg L^?1 in the Hoan Kiem Lake and from 726.5 to 1116 μg L^?1 in the Nui Coc reservoir) and isolated strains of Anabaena sp. and Microcystis with the concentration of MC ranging from 152 to 396.2 μg g^?1 dry mass, respectively.  相似文献   

Exploring aberrant bivalve shell ultrastructure and geochemistry as proxies for past sea water acidification     

Sabine Hahn  Erika Griesshaber  Wolfgang W. Schmahl  Rolf D. Neuser  Ann‐Christine Ritter  Rene Hoffmann  Dieter Buhl  Andrea Niedermayr  Anna Geske  Adrian Immenhauser 《Sedimentology》2014,61(6):1625-1658

Throughout much of Earth's history, marine carbonates have represented one of the most important geological archives of environmental change. Several pivotal events during the Phanerozoic, such as mass extinctions or hyperthermal events have recently been associated with ocean acidification. Nevertheless, well‐defined geological proxies for past ocean acidification events are, at best, scarce. Here, experimental work explores the response of bivalve shell ultrastructure and isotope geochemistry (δ¹³C, δ¹⁸O and δ²⁶Mg) to stressful environments, in particular to sea water acidification. In this study, the common blue mussel, Mytilus edulis, was cultured (from early juvenile stages to one year of age) at four pH regimes (pH_NBS 7·2 to pH 8·0). Shell growth rate and ultrastructure of mainly the calcitic portion of the shells were compared between experimental treatments. Specimens exposed to low‐pH environments show patches of disordered calcitic fibre orientation in otherwise well‐structured shells. Furthermore, the electron backscattered diffraction analyses reveal that, under acidified conditions, the c‐axis of the calcite prisms exhibits a bimodal or multi‐modal distribution pattern. Similar shell disorder patterns have been reported from mytilids kept under naturally acidified sea water conditions. In contrast, this study found no evidence that different pH regimes affect shell carbon, oxygen or magnesium isotope ratios. Based on these observations, it is proposed that: (i) stressful environments, in this case low sea water pH, predictably affect bivalve biomineralization patterns; and (ii) these findings bear potential as a novel (petrographic) proxy for ancient sea water acidification. An assessment of the applicability of these data to well‐preserved fossil shell material from selected time intervals requires additional work.  相似文献   

Multiple Factors driving Variability of CO2 Exchange Between the Ocean and Atmosphere in a Tropical Coral Reef Environment     

Rachel F. S. Massaro  Eric Heinen De Carlo  Patrick S. Drupp  Fred T. Mackenzie  Stacy Maenner Jones  Katie E. Shamberger  Christopher L. Sabine  Richard A. Feely 《Aquatic Geochemistry》2012,18(4):357-386

In this paper, we present the results of the first automated continuous multi-year high temporal frequency study of CO₂ dynamics in a coastal coral reef ecosystem. The data cover 2.5?years of nearly continuous operation of the CRIMP-CO₂ buoy spanning particularly wet and dry seasons in southern Kaneohe Bay, a semi-enclosed tropical coral reef ecosystem in Hawaii. We interpret our observational results in the context of how rapidly changing physical and biogeochemical conditions affect the pCO₂ of surface waters and the magnitude and direction of air–sea exchange of CO₂. Local climatic forcing strongly affects the biogeochemistry, water column properties, and gas exchange between the ocean and atmosphere in Kaneohe Bay. Rainfall driven by trade winds and other localized storms generates pulses of nutrient-rich water, which exert a strong control on primary productivity and impact carbon cycling in the water column of the bay. The “La Ni?a” winter of 2005–2006 was one of the wettest winters in Hawaii in 30?years and contrasted sharply with preceding and subsequent drier winter seasons. In addition, short-term variability in physical forcing adds complexity and helps drive the response of the CO₂–carbonic acid system of the bay. Freshwater pulses to Kaneohe Bay provide nutrient subsidies to bay waters, relieving the normal nitrogen limitation of this system and driving phytoplankton productivity. Seawater pCO₂ responds to the blooms as well as to physical forcing mechanisms, leading to a relatively wide range of pCO₂ in seawater from about 250 to 650?μatm, depending on conditions. Large drawdowns in pCO₂ following storms occasionally cause bay waters to switch from being a source of CO₂ to the atmosphere to being a sink. Yet, during our study period, the southern sector of Kaneohe Bay remained a net source of CO₂ to the atmosphere on an annualized basis. The integrated net annual flux of CO₂ from the bay to the atmosphere varied between years by a factor of more than two and was lower during the wet “La Ni?a” year, than during the following year. Over the study period, the net annualized flux was 1.80?mol?C?m^?2?year^?1. Our CO₂ flux estimates are consistent with prior synoptic work in Kaneohe Bay and with estimates in other tropical coral reef ecosystems studied to date. The high degree of climatological, physical, and biogeochemical variability observed in this study suggests that automated high-frequency observations are needed to capture the short-, intermediate-, and long-term variability of CO₂ and other properties of these highly dynamic coastal coral reef ecosystems.  相似文献