Density gradient Research Papers - Academia.edu (original) (raw)

. Geographic and hydrodynamic characteristics of shallow coastal lagoons. In: L.G. Ward and G.M. Ashley (Editors), Physical Processes and Sedimentology of Siliciclastic-Dominated Lagoonal Systems. Mar. Geol.,. Coastal lagoons can be conveniently separated based on geomorphology into leaky, restricted or choked systems. This subdivision orders coastal lagoons hydrodynamically according to water exchange with the coastal ocean. Leaky systems have the shortest turnover time, restricted lagoons have a longer turnover time, and choked lagoons have the longest turnover time. In leaky coastal lagoons, oscillating tidal currents account for most of the flow variance and mask residual currents due to wind-and freshwater-induced density gradients. In restricted lagoons, tides, winds, and freshwater runoff drive components of the circulation. Winds are usually dominant in causing circulation and mixing in choked lagoons, and water exchange with the coastal ocean largely depends on the hydrologic cycle. Coastal lagoons are usually well mixed, but can exhibit significant horizontal gradients due to freshwater runoff and vertical gradients due to solar radiation or surface cooling. In general, river inflow and heat balance induce stratification in coastal lagoons, whereas winds and tides cause mixing. Coastal lagoons usually trap inorganic sediment and organic matter, thus serving as material sinks. On a geologic time scale, they are short-lived landscape features, with an existence intrinsically linked to filtering efficiency and relative sea-level change as a result of glacioeustatic cycles and local tectonic activity.

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- Earth Sciences, Marine Geology, Stratification, Tides

The hypothesis that fronts are sites of active subduction is examined using density, temperature, salinity, and horizontal velocity data from a trio of surveys of the Azores Front done in May 1991 and March 1992. These surveys were made using a SeaSoar equipped with a conductivity-temperature-depth profiler and a shipboard acoustic Doppler current profiler. The potential density and potential vorticity indicate that dense water from the north side of the front may be sliding down beneath the surface outcrop. This apparently subducting isopycnal has a great deal of temperature and salinity variability. Horizontal velocity is nearly parallel to isopycnals, indicating that the time rate of change and vertical advection must be small. The thermal wind balance is observed to be valid, especially in the region of the largest horizontal density gradients. Shear at the base of the mixed layer is likely due to near-inertial motions. The potential vorticity is dominated by the planetary vorticity, except at the front, where vertical shears (the tilting term) become large. The tilting term acts to reduce the magnitude of the potential vorticity at the front, in agreement with simple theoretical models. The magnitude of the tilting term is similar to the total vorticity in the seasonal thermocline.

- by James Luyten
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- Multidisciplinary, Seasonality, Density gradient, Potential Vorticity

We show that a liquid mixture in the thermodynamically stable homogeneous phase can undergo a phaseseparation transition when rotated at sufficiently high frequency ω. This phase-transition is different from the usual case where two liquids are immiscible or where the slow sedimentation process of one component (e.g. a polymer) is accelerated due to centrifugation. For a binary mixture, the main coupling is due to a term ∝ ∆ρ(ωr) 2 , where ∆ρ is the difference between the two liquid densities and r the distance from the rotation axis. Below the critical temperature there is a critical rotation frequency ωc, below which smooth density gradients occur. When ω > ωc, we find a sharp interface between the low density liquid close to the center of the centrifuge and a high density liquid far from the center. These findings may be relevant to various separation processes and to the control of chemical reactions, in particular their kinetics. Séparation de phases des liquides miscibles dans une centrifugeuse Résumé Nous prouvons qu'un mélange de liquides dans une phase homogène thermodynamiquement stable peut subir une transition de phase une fois centrifugéá une fréquence ω suffisammentélevée. Cette transition de phase est différente du cas habituel où deux liquides sont non-miscibles ou du cas où un processus lent de sédimentation d'un composant (par exemple un polymère) est accéléré par une centrifugation. Pour un mélange binaire, le couplage principal est dûà un terme ∝ ∆ρ(ωr) 2 , où ∆ρ est la différence des densités entre les deux liquides et r est la distance de l'axe de rotation. Au-dessous de la température critique, il y a une rotation critique de fréquence ωc ; au-dessous d'ωc, de faibles gradients de densité prennent naissance. Quand ω > ωc, nous trouvons une interface mince entre le liquide de faible densité près du centre de la centrifugeuse et un liquideà haute densité loin du centre. Ces résultats peuventêtre appropriésà divers processus de séparation et au contrôle de réactions chimiques, en particulier de leur cinétique.

- by Yoav Tsori
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- Thermodynamics, Kinetics, Density, High Frequency

Helicon discharges produce plasmas with a density gradient across the confining magnetic field. Such plasmas can create a radial potential well for nonaxisymmetric whistlers, allowing radially localized helicon ͑RLH͒ waves. This work presents new evidence that RLH waves play a significant role in helicon plasma sources. An experimentally measured plasma density profile in an argon helicon discharge is used to calculate the rf field structure. The calculations are performed using a two-dimensional field solver under the assumption that the density profile is axisymmetric. It is found that RLH waves with an azimuthal wave number m = 1 form a standing wave structure in the axial direction and that the frequency of the RLH eigenmode is close to the driving frequency of the rf antenna. The calculated resonant power absorption, associated with the RLH eigenmode, accounts for most of the rf power deposited into the plasma in the experiment.

- by Guangye Chen
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- Magnetic field, High Frequency, Electromagnetic Radiation, Classical Physics

CyP is a lipopolysaccharide (LPS)-like molecule extracted from the freshwater cyanobacterium Oscillatoria planktothrix FP1, which has been reported to be a potent competitive inhibitor of bacterial LPS. In the present study the ability of CyP to affect human polymorphonuclear leukocyte (PMN) function was investigated. PMNs were isolated from venous blood by standard density-gradient centrifugation. Cell migration was measured by use of the Boyden chamber assay. Interleukin (IL)-8 and tumor necrosis factor (TNF)-α production was measured using a sandwich-type enzyme-linked immunosorbent assay. PMN intracellular reactive oxygen species (ROS) levels were assessed by the use of a fluorescent probe coupled to spectrophotometry. CyP 10-100 μg/ml was chemotactic for PMNs without affecting the chemotactic response to either E. coli LPS or N-formyl-Met-Leu-Phe (fMLP). CyP per se did not affect PMN production of either IL-8 or TNF-α, but concentration-dependently reduced LPS-induced production of both cytokines. On the contrary, CyP had no effect either on fMLP-induced production of IL-8 or on PMN oxidative burst (at rest and after stimulation with fMLP), a response which is known to be independent from LPS-operated pathways. In human PMNs CyP behaves as a selective and effective LPS antagonist. These findings support the therapeutic potential of CyP in endotoxin-dependent disease.

- by Franca Marino and +2
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- Cell Migration, Calcium, Cyanobacteria, Immunopharmacology

The mitochondrion and the apicoplast of the malaria parasite, Plasmodium spp. is microscopically observed in a close proximity to each other. In this study, we tested the suitability of two different separation techniques -Percoll density gradient centrifugation and fluorescence-activated organelle sorting -for improving the purity of mitochondria isolated from the crude organelle preparation of Plasmodium falciparum. To our surprise, the apicoplast was inseparable from the plasmodial mitochondrion by each method. This implies these two plasmodial organelles are bound each other. This is the first experimental evidence of a physical binding between the two organelles in Plasmodium.

- by Yoh-ichi Watanabe
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- Genetics, Electron Microscopy, Mitochondria, Organelles

Significant progress has been made on the Mega Ampere Spherical Tokamak (MAST) towards a fundamental understanding of transport, stability and edge physics and addressing technological issues for future large devices. Collaborative studies of the L-H transition with NSTX and ASDEX Upgrade confirm that operation in a connected double-null configuration significantly reduces the threshold power, P thr . The MAST data provide support for a theory for the transition based on finite β drift wave turbulence suppression by self-generated zonal flows. Analysis of low and high field side density gradients in the H-mode pedestal provides support for an analytical model of the density pedestal width dependent on the neutral penetration depth. Adding MAST data to international confinement databases has enhanced confidence in scalings for ITER by significantly expanding the range of β and ε explored and indicates a slightly stronger ε dependence than in current scalings. Studies of core transport have been conducted for well-diagnosed L-mode, H-mode and internal transport barrier (ITB) discharges using TRANSP, and microstability and turbulence studies have been carried out using GS2. Linear micro-stability analysis indicates that ITG modes are typically unstable on all flux surfaces with growth rates that are comparable to the equilibrium E × B flow shearing rate. Mixing length estimates of transport coefficients from ITG (neglecting flow shear) give diffusion coefficients that are broadly comparable with observed thermal diffusivities. Non-linear, collisionless ETG calculations have been performed and suggest radially extended electrostatic streamers up to 100ρ e across in radius. Transport from ITG could easily be suppressed in regions where the E × B shear flow rate,

- by Ken Mcclements and +1
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- Wave turbulence, Nuclear, Nuclear Fusion, THERMAL DIFFUSIVITY

In the present study, we have examined the effect of density gradient preparations BoviPure ® and Percoll ® on bull sperm separation and the in vitro fertilization (IVF) and culture (IVC) results. Frozen/thawed semen from five simmental bulls were pooled. Sperm quality parameters such as sperm motility, concentration, membrane activity (HOS assay), membrane integrity (SYBR-14/PI assay) and acrosomal status (EthD-1/FITC-PSA assay) were evaluated before and after sperm processing for IVF using BoviPure ® and Percoll ® density gradient separations. The results of the evaluated parameters before sperm processing were: motility 50%, concentration 82.33 × 10 6 spz/mL, membrane activity 39.05%, membrane integrity 42.97% and the acrosomal status 46.90% of the live spermatozoa with intact acrosomes. After sperm processing with BoviPure ® and Percoll ® the motility was 66.67 and 64.17%, the concentration was 25.50 × 10 6 and 27.67 × 10 6 spz/mL, the membrane activity was 53.78 and 56.58%, the membrane integrity was 70.85 and 68.76% of and the acrosomal status was 74.16 and 67.46% of the live spermatozoa with intact acrosomes, respectively. Percentages were referred to the total number of spermatozoa. There were significant differences (P < 0.05) between the evaluated parameters before and after sperm processing for both separation protocols. We found no significant differences (P > 0.05) regarding sperm evaluation parameters between the protocols. A total of 492 oocytes were matured and fertilized in vitro and cultured in SOFaaBSA in six replicates. The cleavage (D2) and blastocysts (D7) rate were significantly higher (P < 0.05) for the BoviPure ® group compared hatched blastocysts (D10) did not differ significantly between sperm separation protocols (P > 0.05).

- by J. Gi and +1
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- Animal reproduction, Cell separation, Biological Sciences, In Vitro Fertilization

The Fast Ignition Scenario (FIS) for Inertial Confinement Fusion (ICF) has prompted in recent years many theoretical , numerical and experimental works on relativistic beam plasma instabilities . Due to the density gradient of the pre-compressed target (the center is about 10 4 denser than the border), the beam-plasma interaction is collisionless near the REB emitting region, and collisional near the center.

- by Antoine Bret
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- Density gradient, Inertial Confinement Fusion, Electron Beam

The purpose of this study was to evaluate the ability of spermatozoa and leukocytes in semen to produce reactive oxygen species (ROS) by using nitroblue tetrazolium (NBT) staining and to examine the association between NBT staining and levels of ROS as measured by chemiluminescence. Twenty-one infertility patients (leukocytospermia; n = 8; nonleukocytospermia, n = 13) and 9 healthy donors were included. Standard semen analysis and density gradient centrifugation were performed to test NBT staining, ROS, and total antioxidant capacity. A ROS-total antioxidant capacity (ROS-TAC) score was calculated by using principal component analysis. In the leukocytospermic group, after separation on a density gradient, the percentage of NBT-positive staining was significantly higher in sperm suspensions contaminated with leukocytes (median [25th, 75th percentiles]; 70% [61%, 79%]) compared to the nonleukocytospermic group (14.5% [9%, 25.5%]; P =.03) and donors (7% [3%, 11%]; P =.02), respectively...

- by Rakesh Kumar Sharma
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- Andrology, Principal Component Analysis, Antioxidants, Peroxidase

SUMMARY A virus with isometric particles ca. 30 nm in diameter and angular contour was recovered by mechanical trans- mission from a grapevine with fanleaf-like symptoms growing at Nevsheir (Cappadocia, Turkey). In sucrose density gradient centrifugation this virus (laboratory code: isolate N66) sedimented as three components, T (empty shells), M, and B, both of which consisted of ap- parently intact particles. Virus preparations contained two RNA species with mol. wt. 2.6·10 6 Da (RNA-1) and 1.3·10 6 Da (RNA-2). The coat protein (CP) subunits were of a single type with M r of c. 53,000. An antiserum with a titre of 1:1024 was raised, which did not react with healthy plant antigens. A fragment 1,175 nt in size from the 3' terminal region of RNA-2 was sequenced, which comprised part of the CP cistron. Comparison analysis with GenBank sequences from the same region revealed variable levels of homology with other grapevine nepoviruses, the closest being Arabis mosaic virus (ArMV, ...

- by M. Digiaro
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- Plant Biology, Density gradient, Amino Acid Profile, Plant Pathology

In this paper, we report on the solid phase crystallisation of carbon-free HfO 2 thin films deposited by plasma ion assisted deposition (PIAD). After deposition, the HfO 2 films were annealed in N 2 ambient for 3 h at 350, 550 and 750 • C. Several characterisation techniques including X-ray reflectometry (XRR), X-ray diffraction (XRD), spectroscopic ellipsometry (SE) and atomic force microscopy (AFM) were used for the physical characterisation of as-deposited and annealed HfO 2 . XRD has revealed that the as-deposited HfO 2 film is in an amorphous-like state with only traces of crystalline phase and that the annealed films are in a highly crystalline state. These results are in good agreement with the SE results showing an increase of refractive index by increasing the annealing temperature. XRR results show a significant density gradient over the as-deposited film thickness, which is characteristic of the PIAD method. The AFM measurements show that the HfO 2 layers have a smooth surface even after annealing at 750 • C. The present study demonstrates that the solid phase crystallisation of HfO 2 PIAD thin films starts at a temperature as low as 550 • C.

- by Mircea Modreanu
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- Engineering, Atomic Force Microscopy, Thin Film, Physical sciences

During July-September 1998 a comprehensive survey was undertaken of the Celtic Sea and St. George's Channel of the Irish Sea in order to resolve the three-dimensional density field and circulation. Towed undulating CTD data revealed that isolated below the seasonal thermocline was a dense (cold and saline) pool flanked by strong near-bed density gradients. The trajectories of 23 satellite-tracked Argos drifters demonstrated the existence of a cyclonic (anticlockwise) circulation pattern. This was in accord with persistent and narrow (ca. 20 km) cores of cyclonic flow (dense water to the left) as revealed by geostrophic current estimates and moored acoustic Doppler current profiler data, with velocities >0.1 m s À1 and exceeding 0.3 m s À1 in St. George's Channel.

- by Liam Fernand
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- Earth Sciences, Environmental Management, Biological Sciences, Seasonality

We have carried out a comparison study of hydrodynamical codes by investigating their performance in modelling interacting multiphase fluids. The two commonly used techniques of grid and smoothed particle hydrodynamics (SPH) show striking... more

- by Romain Teyssier
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- Galaxy Formation and Evolution, Density gradient

Two algorithms for in-situ detection and identification of vertical free convective and double-diffusive flows in groundwater monitoring wells or boreholes are proposed. With one algorithm the causes (driving forces) and with the other one the effects (convection or doublediffusion) of vertical transport processes can be detected based on geophysical borehole measurements in the water column. Five density-driven flow processes are identified: thermal, solutal, and thermosolutal convection leading to an equalization, as well as saltfingers and diffusive layering leading to an intensification of a vertical density gradient. The occurrence of density-driven transport processes could be proven in many groundwater monitoring wells and boreholes; especially shallow sections of boreholes or groundwater monitoring wells are affected dramatically by such vertical flows. Deep sections are also impaired as the critical threshold for the onset of a density-driven flow is considerably low. In monitoring wells or boreholes, several sections with different types of density-driven vertical flows may exist at the same time. Results from experimental investigations in a medium-scale testing facility with high aspect ratio (height/radius = 19) and from numerical modeling of a water column agree well with paramters of in-situ detected convection cells.

The onset of clinical assisted reproduction, a quarter of a century ago, required the isolation of motile spermatozoa. As the indication of assisted reproduction shifted from mere gynaecological indications to andrological indications during the years, this urged andrological research to understand the physiology of male germ cell better and develop more sophisticated techniques to separate functional spermatozoa from those that are immotile, have poor morphology or are not capable to fertilize oocytes. Initially, starting from simple washing of spermatozoa, separation techniques, based on different principles like migration, filtration or density gradient centrifugation evolved. The most simple and cheapest is the conventional swim-up procedure. A more sophisticated and most gentle migration method is migration-sedimentation. However, its yield is relatively small and the technique is therefore normally only limited to ejaculates with a high number of motile spermatozoa. Recently, ...

- by Ralf Henkel
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- Endocrinology, Biology, Reproductive Biology, Cell separation

In order to elucidate the discriminating power of various soil analytical techniques, over 100 soils samples were analyzed using conventional analysis (i.e., color, percent organic and density gradient) and a novel FTIR technique. The FTIR technique involves collecting a spectrum of a soil sample that has been oxidatively pyrolysed, and therefore all organic have been degraded. This spectrum is subtracted from the spectrum of the same sample that contained the organic prior to pyrolysis. This resultant IR spectrum represents the organic portion of the sample. The use of organic components increases the discrimination in soils that are otherwise similar. We have illustrated the usefulness of this technique by selecting four soil samples, which have identical Munsel color values but can be discriminated by subtractive ETIR. We propose that the ETIR spectra of the organic portion of soil serves a useful purpose in forensic investigations.

- by Edgard Espinoza
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- Forensic Science, Soil, Forensic Medicine, Multidisciplinary

A lthough density functional theory is widely used in the computational chemistry community, the most popular density functional, B3LYP, has some serious shortcomings: (i) it is better for main-group chemistry than for transition metals; (ii) it systematically underestimates reaction barrier heights; (iii) it is inaccurate for interactions dominated by mediumrange correlation energy, such as van der Waals attraction, aromatic-aromatic stacking, and alkane isomerization energies. We have developed a variety of databases for testing and designing new density functionals. We used these data to design new density functionals, called M06-class (and, earlier, M05-class) functionals, for which we enforced some fundamental exact constraints such as the uniform-electron-gas limit and the absence of self-correlation energy. Our M06-class functionals depend on spin-up and spin-down electron densities (i.e., spin densities), spin density gradients, spin kinetic energy densities, and, for nonlocal (also called hybrid) functionals, Hartree-Fock exchange. We have developed four new functionals that overcome the above-mentioned difficulties: (a) M06, a hybrid meta functional, is a functional with good accuracy "across-theboard" for transition metals, main group thermochemistry, medium-range correlation energy, and barrier heights; (b) M06-2X, another hybrid meta functional, is not good for transition metals but has excellent performance for main group chemistry, predicts accurate valence and Rydberg electronic excitation energies, and is an excellent functional for aromatic-aromatic stacking interactions; (c) M06-L is not as accurate as M06 for barrier heights but is the most accurate functional for transition metals and is the only local functional (no Hartree-Fock exchange) with better across-the-board average performance than B3LYP; this is very important because only local functionals are affordable for many demanding applications on very large systems; (d) M06-HF has good performance for valence, Rydberg, and charge transfer excited states with minimal sacrifice of ground-state accuracy. In this Account, we compared the performance of the M06-class functionals and one M05-class functional (M05-2X) to that of some popular functionals for diverse databases and their performance on several difficult cases. The tests include barrier heights, conformational energy, and the trend in bond dissociation energies of Grubbs' ruthenium catalysts for olefin metathesis. Based on these tests, we recommend (1) the M06-2X, BMK, and M05-2X functionals for main-group thermochemistry and kinetics, (2) M06-2X and M06 for systems where main-group thermochemistry, kinetics, and noncovalent interactions are all important, (3) M06-L and M06 for transition metal thermochemistry, (4) M06 for problems involving multireference rearrangements or reactions where both organic and transition-metal bonds are formed or broken, (5) M06-2X, M05-2X, M06-HF, M06, and M06-L for the study of noncovalent interactions, (6) M06-HF when the use of full Hartree-Fock exchange is important, for example, to avoid the error of self-interaction at longrange, (7) M06-L when a local functional is required, because a local functional has much lower cost for large systems.

- by Donald Truhlar
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- Analytical Chemistry, Computational Chemistry, Kinetics, Transition-Metal Oxides

It is well known that a dilute suspension of spheroids sedimenting under gravity at low Reynolds number is unstable to density fluctuations as a result of hydrodynamic interactions [D. L. Koch and E. S. G. Shaqfeh, J. Fluid Mech. 209, 521 (1989)]. Using a linear stability analysis, it is shown that a vertical density gradient in such a suspension can

- by Eric Darve and +1
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- Engineering, Stratification, Numerical Simulation, Mathematical Sciences

In this paper, we propose a unified framework for Quantum-corrected drift-diffusion (QCDD) models in nanoscale semiconductor device simulation. QCDD models are presented as a suitable generalization of the classical drift-diffusion (DD) system, each particular model being identified by the constitutive relation for the quantum-correction to the electric potential. We examine two special, and relevant, examples of QCDD models; the first one is the modified DD model named Schrö dinger-Poisson-drift-diffusion, and the second one is the quantum-drift-diffusion (QDD) model. For the decoupled solution of the two models, we introduce a functional iteration technique that extends the classical Gummel algorithm widely used in the iterative solution of the DD system. We discuss the finite element discretization of the various differential subsystems, with special emphasis on their stability properties, and illustrate the performance of the proposed algorithms and models on the numerical simulation of nanoscale devices in two spatial dimensions. channel potential barrier . As these effects cannot be appropriately dealt with by the classical drift-diffusion (DD) model, alternative and more sophisticated mathematical models must be adopted. In this respect, two main approaches can be taken:

- by Riccardo Sacco and +1
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- Engineering, Computational Physics, Finite element method, Semiconductor Devices

Lake Hallstättersee is a holomictic alpine lake, which is influenced by salt mining since the middle Bronze Age. Beside the constant saline waste water load, two massive brine spills loaded the lake with additional 16,900 tons sodium chloride (≈10,250 tons Cl−) from 1977 to 1979 and 3,000 tons salt (≈1,820 tons Cl−) in 2005. The effect of waste water intrusions from salt mining on stratification of Lake Hallstättersee was analysed over a period of 40 years. Water density, dissolved oxygen and total phosphorus (TP) concentrations were measured and an exponential model was fitted to describe the wash-out of chloride from Lake Hallstättersee after the brine spills. Furthermore, the time required returning to holomixis and steady chloride content after the second brine spill was estimated. During the whole sampling period the minimum and maximum volume-weighted annual mean chloride concentrations were 23.58 mg/L in 1979 and 3.19 mg/L in 1998. However, the mixing regime of Lake Hallstättersee, as well as the chloride concentrations, varied considerably and exhibited three holomictic and three meromictic periods between 1970 and 2009. Holomictic periods were observed when the yearly density gradient was below 0.06 kg/m3, deepwater oxygen in spring above 4 mg/L and consequently declining TP concentration in the deepest water layer below 60 mg/m3, otherwise meromictic periods were observed. Our study showed that Hallstättersee was 13 years ectogenic meromictic and 27 years holomictic during the study period.

- by Robert Schabetsberger
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- Stratification, Multidisciplinary, Sampling, Bronze Age

For assisted reproduction technologies (ART), numerous techniques were developed to isolate spermatozoa capable of fertilizing oocytes. While early methodologies only focused on isolating viable, motile spermatozoa, with progress of ART, particularly intracytoplasmic sperm injection (ICSI), it became clear that these parameters are insufficient for the identification of the most suitable spermatozoon for fertilization. Conventional sperm preparation techniques, namely, swim-up, density gradient centrifugation and glass wool filtration, are not efficient enough to produce sperm populations free of DNA damage, because these techniques are not physiological and not modeled on the stringent sperm selection processes taking place in the female genital tract. These processes only allow one male germ cell out of tens of millions to fuse with the oocyte. Sites of sperm selection in the female genital tract are the cervix, uterus, uterotubal junction, oviduct, cumulus oophorus and the zona pellucida. Newer strategies of sperm preparation are founded on: (i) morphological assessment by means of 'motile sperm organelle morphological examination (MSOME)'; (ii) electrical charge; and (iii) molecular binding characteristics of the sperm cell. Whereas separation methods based on electrical charge take advantage of the sperm's adherence to a test tube surface or separate in an electrophoresis, molecular binding techniques use Annexin V or hyaluronic acid (HA) as substrates. Techniques in this category are magnet-activated cell sorting, Annexin V-activated glass wool filtration, flow cytometry and picked spermatozoa for ICSI (PICSI) from HA-coated dishes and HA-containing media. Future developments may include Raman microspectrometry, confocal light absorption and scattering spectroscopic microscopy and polarization microscopy.

- by Ralf Henkel
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- Flow Cytometry, DNA damage, Cell separation, Intracytoplasmic Sperm Injection

Fourteen shallow seismic refraction profiles were conducted in Wadi Wardan, northeast Gulf of Suez, Sinai to delineate some of the shallow soil engineering characteristics for construction purposes. Both compressional (P) and shear (S) waves were acquired and interpreted using Generalized Reciprocal Method (GRM) then the resulted velocity-depth model is verified using a Finite Difference (FD) method to improve the final velocitydepth model. A number of engineering parameters such as Concentration Index, Material Index, Density Gradient, and Stress Ratios are calculated. The seismic velocity values, engineering, consolidation, and strength parameters show that the bedrock at northern and southern parts of the study area is characterized by less competent rock quality, while the central part is characterized by more competent rock quality. Hence, the central part is suggested for construction activities.

- by Sherif Hanafy
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- Geophysics, Stress, Density, Concentration

Retroviral vectors from both oncoretroviral and lentiviral origins have a great potential as gene delivery vehicles. A number of research groups have devoted considerable effort to the development of large-scale production strategies for retroviral vectors. However, the manufacturing of clinical-grade vectors for gene therapy, especially for in vivo applications, additionally requires scaleable purification strategies to remove the contaminants present in the harvested supernatants while preserving the functionality of the vectors. In this article, we review recent advances made in the field of downstream processing of retroviral vectors. The methods currently described in the literature for clarification, concentration and purification of retroviral vectors will be presented, with special emphasis on novel chromatography methods that open up the possibility to selectively and efficiently purify retroviruses on a large-scale. Problems associated with stability and quantification of retroviral particles will be outlined and future challenges will be discussed. D

- by Amine Kamen
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- Engineering, Technology, Biotechnology, Gene Therapy

This paper addresses the effect of thermocompression pressure, shear deformation of green laminates, and postsinter HIPing on the microstructural homogeneity of cast tapes and laminates prepared from fine AI,O, and AI,O,/ZrO, powders. Green density increases with increasing thermocompression pressure. Sintered densities, however, depend more on the macroscopic uniformity in the green tapes. When density gradients develop within the individual green tapes (because of improper drying), sintering is constrained in two dimensions and densities remain low. Postsinter HIPing does not significantly increase the sintered densities because of the retention of open porosity within the individual tape-cast layers. The use of a revised thermocompression process involving shear deformation results in higher sintered densities and complete densification after HIPing. Sintered densities increase with the degree of shear strain during green-state deformation processing. Thus, greenstate deformation can improve homogeneity in laminates. A further variation of the shear deformation process has also been developed that allows the formation of complex shapes from tape-cast laminates in the green state, while retaining layer integrity.

- by Kevin Plucknett
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- Materials Engineering, Mechanical Engineering, Manufacturing, Fabrication
- by jose antonio gavilanes blanco
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- Nuclear, Nuclear Fusion, Magnetic Resonance, Density gradient

Conventional ohmic boundary conditions are shown to be inconsistent with density-gradient (DG) theory. New ohmic conditions that are consistent with DG theory are then derived and illustrated with two device examples. The first example uses a short p-n diode to understand the basic situation while the second treats a MOSFET contact and studies the "insulator proximity effect" seen at the point/edge where the ohmic contact abuts an insulator.

- by Bryan Biegel
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- Proximity Effect, Density gradient, Computational electronics, Ohmic Contact

A large schlieren system with exceptional sensitivity and a high-speed digital camera are used to visualize loud sounds and a variety of common phenomena that produce weak shock waves in the atmosphere. Frame rates varied from 10,000 to 30,000 frames/s with microsecond frame exposures. Sound waves become visible to this instrumentation at frequencies above 10 kHz and sound pressure levels in the 110 dB (6.3 Pa) range and above. The density gradient produced by a weak shock wave is examined and found to depend upon the profile and thickness of the shock as well as the density difference across it. Schlieren visualizations of weak shock waves from common phenomena include loud trumpet notes, various impact phenomena that compress a bubble of air, bursting a toy balloon, popping a champagne cork, snapping a wooden stick, and snapping a wet towel. The balloon burst, snapping a ruler on a table, and snapping the towel and a leather belt all produced readily visible shockwave phenomena. In contrast, clapping the hands, snapping the stick, and the champagne cork all produced wave trains that were near the weak limit of visibility. Overall, with sensitive optics and a modern high-speed camera, many nonlinear acoustic phenomena in the air can be observed and studied. Communicated by B. W. Skews.

- by Michael Hargather
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- Sound, Shock Waves, Noise, Classical Physics

We study the effect of buoyancy on pressure-driven flow of two miscible fluids in inclined channels via direct numerical simulations. The flow dynamics are governed by the continuity and Navier-Stokes equations, without the Boussinesq approximation, coupled to a convective-diffusion equation for the concentration of the more viscous fluid through a concentration-dependent viscosity and density. The effect of varying the density ratio, Froude number, and channel inclination on the flow dynamics is examined, for moderate Reynolds numbers. We present results showing the spatiotemporal evolution of the flow together with an integral measure of mixing.

- by Kirti Sahu
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- Engineering, Direct Numerical Simulation, Fluid Dynamics, Mathematical Sciences

A new method for numerical simulation of flame propagation in turbulent premixed combustible gaseous mixtures is proposed and tested. The method combines (I) a modified eikonal equation, employed to model the flame front dynamics; (2) a method of front tracking using a strip of computational cells containing the flame; and (3) a formula for turbulent flame propagation speed. The method includes two separate models. The first one obviates the necessity to solve equations for heat release, temperature and enthalpy, and utilizes a model equation of state to accurately render the volumetric effect and related instabilities. The second one provides a model for direct heal release and temperature calculation in the presence of heat-conducting boundaries (walls), in the multi-component combustible mixtures with variable composition and temperature-dependent heat capacities of all species. This model can be used when the heat transport on the walls is of interest, effects of flame quenching are essential and for the especially important case of combustion in a closed volume, e.g., in a cylinder of an internal combustion engine or chemical reactor. This modification is also effective for simulation of ramjet engines. The new method proves to be self-consistent, robust and highly effective.

Iste magistrorum locus est simul et puerorum, mittunt quando volunt hic res quas perdere nolunt .

- by M. Di Toro
- •
- Mathematical Sciences, Physical sciences, Phase transition, Density gradient

A new model is developed for the transient thermal response of a packed bed. using the method of spatial averaging. Equations for the average temperature of the fluid and the solid phase are derived from the point equations for thermal energy in each phase. The new model exhibits some unusual convective and dispersive coupling between the equations for the average fluid and solid temperatures. The response of the model equations to a pulse disturbance is analyzed. It is found that after a sufficiently long time has elapsed, the temperature pulses for the fluid and solid phases will be separated by a constant distance and will spread or disperse about their centroids at an equal rate. The pulse separation predicted by the new model equations is larger than that predicted using more conventional analyses of heat transfer in packed beds. Effective thermal conductivities measured under steady state conditions can differ significantly from those observed in transient experiments due to the spread in temperature pulses caused by heat exchange between phases. Estimates are made of the magnitude of the more important terms affecting longitudinal and lateral effective thermal conductivities under flow conditions, in order to make possible a direct comparison between theory and experiment in a companion paper.

- by Ruben Carbonell
- •
- Chemical Engineering, Mass Transfer, Fluid Mechanics, Heat Transfer
- by Aspi Maneckjee
- •
- Kinetics, Biology, Ecology, Medicine

Caveolae, a specialized form of lipid rafts, are cholesterol-and sphingolipid-rich membrane microdomains implicated in potocytosis, endocytosis, transcytosis, and as platforms for signal transduction. One of the major constituents of caveolae are three highly homologous caveolin isoforms (caveolin-1, caveolin-2, and caveolin-3). The present study expands the analysis of caveolin isoform expression in C6 glioma cells. Three complementary approaches were used to assess their differential expression during the dibutyryl-cyclic AMP-induced differentiation of C6 cells into an astrocyte-like phenotype. Immunoblotting, conventional RT-PCR, and real-time RT-PCR analysis established the expression of the caveolin-3 isoform in C6 cells, in addition to caveolin-1 and caveolin-2. Similar to the other isoforms, caveolin-3 was associated with light-density, detergent-insoluble caveolae membrane fractions obtained using sucrose-density gradient centrifugation. The three caveolin isoforms display different temporal patterns of mRNA/protein expression during the differentiation of C6 cells. Western blot and real-time RT-PCR analysis demonstrate that caveolin-1 and caveolin-2 are up-regulated during the late stages of the differentiation of C6 cells. Meanwhile, caveolin-3 is gradually down-regulated during the differentiation process. Indirect immunofluorescence analysis via laser-scanning confocal microscopy reveals that the three caveolin isoforms display similar subcellular distribution patterns. In addition, co-localization of caveolin-1/caveolin-2 and caveolin-1/caveolin-3 was detected in both C6 glioma phenotypes. The findings reveal a differential temporal pattern of caveolin gene expression during phenotypic differentiation of C6 glioma cells, with potential implications to developmental and degenerative events in the brain.

- by M. Rubio-Dávila and +2
- •
- Cognitive Science, Developmental neuroscience, Confocal Microscopy, Gene expression

In design of ground-source energy systems the thermal performance of the borehole heat exchangers is important. In Scandinavia, boreholes are usually not grouted but left with groundwater to fill the space between heat exchanger pipes and borehole wall. The common U-pipe arrangement in a groundwaterfilled BHE has been studied by a three-dimensional, steady-state CFD model. The model consists of a 3 m long borehole containing a single U-pipe with surrounding bedrock. A constant temperature is imposed on the U-pipe wall and the outer bedrock wall is held at a lower constant temperature. The occurring temperature gradient induces a velocity flow in the groundwater-filled borehole due to density differences. This increases the heat transfer compared to stagnant water. The numerical model agrees well with theoretical studies and laboratory experiments. The result shows that the induced natural convective heat flow significantly decreases the thermal resistance in the borehole. The density gradient in the borehole is a result of the heat transfer rate and the mean temperature level in the borehole water. Therefore in calculations of the thermal resistance in groundwater-filled boreholes convective heat flow should be included and the actual injection heat transfer rate and mean borehole temperature should be considered.

- by Göran Hellström
- •
- Mechanical Engineering, Computational Fluid Dynamics, Modeling, Heat Transfer

587͔ plasma with high power density ͑up to 8 MW/ m 3 ͒ core deposited electron cyclotron resonance heating at significant plasma densities ͑ഛ7 ϫ 10 19 m −3 ͒ is analyzed for the electron thermal transport. The discharge distinguishes itself as it has four distinct high confinement mode ͑H-mode͒ phases. An Ohmic H-mode with type III edge localized modes ͑ELMs͒, which turns into a type I ELMy H-mode when the ECRH is switched on. The ELMs then vanish, which gives rise to a quasistationary ELM-free H-mode. This ELM-free phase can be divided into two, one without magnetohydrodynamics ͑MHD͒ and one with. The MHD mode in the latter case causes the confinement to drop by ϳ15%. For all four phases both large-scale trapped electron ͑TEM͒ and ion temperature gradient ͑ITG͒ modes and small-scale electron temperature gradient ͑ETG͒ modes are analyzed. The analytical TEM formulas have difficulty in explaining both the magnitude and the radial profile of the electron thermal flux. Collisionality governs the drive of the TEM, which for the discharge in question implies it can be driven by either the temperature or density gradient. The TEM response function is derived and it is shown to be relatively small and to have sharp resonances in its energy dependence. The ETG turbulence, predicted by the Institute for Fusion Studies electron gyrofluid code, is on the other hand driven solely by the electron temperature gradient. Both trapped and passing electrons add to the ETG instability and turbulent thermal flux. For easy comparison of the results of the above approaches and also with the Weiland model, a dimensionless error measure, the so-called average relative variance is introduced. According to this method the ETG model explains 70% of the variation in the electron heat diffusivity whereas the predictive capabilities of the TEM-ITG models are poor. These results for TCV support the conclusion that the ETG model is able to explain a wide range of anomalous electron transport data, in addition to existing evidence from ASDEX ͓F.

- by Elina Ottaviani
- •
- Physics, Thermodynamics, Magnetohydrodynamics, Transport Properties

Semilocal density functionals for the exchange-correlation energy are needed for large electronic systems. The Tao-Perdew-Staroverov-Scuseria (TPSS) meta-generalized gradient approximation (meta-GGA) is semilocal and usefully accurate, but predicts too-long lattice constants. Recent "GGA's for solids" yield good lattice constants but poor atomization energies of molecules. We show that the construction principle for one of them (restoring the density gradient expansion for exchange over a wide range of densities) can be used to construct a "revised TPSS" meta-GGA with accurate lattice constants, surface energies, and atomization energies for ordinary matter.

Abstract. Variation of wood specific gravity from the base to the top of tree and from pith to bark was investigated for Gmelina arborea Roxb. plantations in western Venezuela. Increment cores were taken from thirty trees at five different sections up the stem: 0.1 m; 1.3 m, half total height, 3/4 total height and at the top. These trees were chosen from commercial plantations located at three different sites. Specific gravity was obtained using an X-ray densitometer. The results show that there is an increase in specific gravity from pith to bark. It was also evident that specific gravity decreased from stump to half of the total height, then increased towards the top of the stem. No correlation between specific gravity and height of the tree was found. Correlations in specific gravity with height categories were generally small, and non-significant at level α=0.05. Palabras clave: Gmelina arborea, Propiedades de la madera, Gravedad específica Resumen. En la Gmelina arborea Roxb se estudió la variación de la gravedad específica desde la base hasta el tope del árbol y desde la médula hasta la corteza. Se tomaron muestras de madera provenientes de treinta árboles a cinco alturas diferentes a lo largo del fuste a 0.1 m; 1.3 m, mitad de la altura total, 3/4 de la altura total, y del tope, las cuales fueron seleccionadas de plantaciones comerciales localizadas en tres sitios distintos al occidente de Venezuela. La gravedad específica fue determinada con la ayuda de un densitómetro de rayos-x. Los resultados muestran un incremento de la gravedad especifica desde la médula hacia la corteza. También se encontró que la gravedad específica disminuye de la base a la mitad del fuste y luego aumenta hacia el tope del fuste. No se encontró correlación entre la gravedad específica y la altura del árbol. La correlación entre la gravedad específica y las categorías de altura donde las muestras fueron tomadas son generalmente pequeñas y poco significativas a un nivel de probabilidad α=0.05.

- by Jesus Espinoza
- •
- X Rays, Density gradient, Forestry Sciences, Public Administration and Policy

Subcellular fractionation has been an important tool in the investigation of neutrophil structural organization including granule heterogeneity, composition and mobilization. The resolution of organelles obtained by subcellular fractionation was improved considerably after the introduction of nitrogen cavitation as an efficient but gentle means of disrupting neutrophils and with Percoll as a density medium. This paper describes in detail the methodology of subcellular fractionation of nitrogen cavitated neutrophils on one-, two-, and three-layer Percoll density gradients. Appropriate marker proteins are presented for neutrophil organelles including azurophil, specific and gelatinase granules, in addition to secretory vesicles and plasma membranes. The dynamics of granule and secretory vesicle exocytosis is demonstrated by subcellular fractionation of resting and activated human neutrophils. Finally, the paper describes the applications of subcellular fractionation in the investigation of the localization of neutrophil constituents, in protein purification schemes and in the study of translocation of cytosolic proteins to isolated neutrophil organelles.

- by Lars Kjeldsen
- •
- Immunology, Protein Purification, Method, Cell Fractionation

Extracting and purifying a representative fraction of bacteria from soil is necessary for the application of many techniques of microbial ecology. Here the influence of different soil types on the quantitative and qualitative recovery of bacteria by soil grinding and Nycodenz density gradient centrifugation was investigated. Three soils presenting contrasted physicochemical characteristics were used for this study. For each soil, the total (AODC: acridine orange direct count) and culturable (cfu: colonyforming units) bacterial densities were measured in three distinct fractions: (i) the primary soil, (ii) the soil pellet (soil remaining after centrifugation), and (iii) the extracted cells. The automated-ribosomal intergenic spacer analysis (A-RISA) was used to characterize the community structure directly from the DNA extracted from each fraction. The physicochemical characteristics of soils were found to influence both the efficiency of bacterial cell recovery and the representativeness of the extracted cells in term of community structures between the different fractions. Surprisingly, the most representative extracted cells were obtained from the soil exhibiting the lowest efficiency of cell recovery. Our results demonstrated that quantitative and qualitative cell recovery using Nycodenz density gradient centrifugation are not necessarily related and could be differentially biased according to soil type.

- by dr ravimohan mavuduru
- •
- Soil Science, Microbial Ecology, Biological Sciences, DNA

On étudie, par simulation des grosses structures, un écoulement turbulent stratifié autour d'un obstacle. L'étude dynamique relative à la formation des structures tourbillonnaires cohérentes a permis de déterminer l'allure de l'écoulement et de préciser ses principales caractéristiques (couche cisaillée, recirculation et recollement). En présence d'une discontinuité de température (stratification stable), l'écoulement est instable en aval de l'obstacle. On note la présence des appariements caractérisant les interactions entre la couche cisaillée et la zone de recirculation. Ces phénomènes s'expliquent par le forçage induit par la recirculation. L'écoulement devient de plus en plus organisé lorsqu'on augmente l'intensité de stratification. On montre le blocage de la couche cisaillée par inhibition du processus d'appariements et la perte de l'intensité des instabilités de Kelvin-Helmholtz. Un intérêt particulier a été réservé au développement des instabilités de type barocline. Ces instabilités provenant des gradients horizontaux de température sont transportées par l'écoulement moyen sur une couche de mélange entre les couches chaude et froide. On note la propagation des ondes de gravité sur cette couche. Ce mécanisme assure une homogénéisation du champ de température et contribue à la propriété de mélange turbulent à petite échelle. La dynamique de l'écoulement stratifié en aval de l'obstacle est tout à fait similaire à la couche de mélange libre stratifiée. 2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved.

- by Ali Belghith
- •
- Mechanical Engineering, Applied Mathematics, Turbulence, Large Eddy Simulation

The biosynthesis of hormones and neuropeptides involves post-translational cleavage of precursors at basic amino acids by prohormone convertases (PCs) predominantly in secretory granules that bud from the trans-Golgi Network. This study reports that the amino acid sequence of PC3 (aa617-638), previously identified as a novel transmembrane (TM) domain, confers lipid raft association and facilitates sorting of the enzyme to the secretory granules of Neuro2A cells for prohormone cleavage. Floatation analysis on sucrose density gradients showed that a proportion of full length (PC3-FL) and carboxyl terminus-truncated PC3 1-638 (PC3-638) containing the TM domain were associated with lipid rafts in Neuro2A cells, while PC3 1-616 (PC3-616) and PC3-ΔTM lacking the TM domain were not. Secondly, PC3-FL and PC3-638 underwent stimulated secretion and were shown to be colocalized with a secretory granule marker, chromogranin A, by immunocytochemistry. In contrast, PC3-616 and PC3-ΔTM were constitutively secreted and primarily localized in the Golgi. These data indicate that the transmembrane domain of PC3 plays a key role in sorting the enzyme to the regulated secretory pathway.

- by Nigel Birch and +1
- •
- Biological Sciences, Molecular and cellular biology, Mice, Secretory Pathway

In the field of Inertial Confinement Fusion (ICF), a major goal of spectroscopy is to extract as much information as possible from a limited set of data. Two revealing physical quantities to study are electron temperature and density. Spectroscopy is commonly used in the field to generate spatially-averaged values for the temperature and density. This work is focused on extracting temperature and density gradients, or spatial profiles, from the data. Innovative spectroscopic techniques have been applied to a series of stable indirect and direct drive experiments at the University of Rochester's Laboratory for Laser Energetics. In these experiments, Ar-doped and D 2 -filled plastic microballoon targets were imploded. Imaging of the implosion cores was provided by the Multi-Monochromatic X-ray Imager (MMI), which recorded Ar Lyβ, Heβ, and Lyα narrow-band line and continuum images, as well as x-ray line spectra. This data is used to achieve the level of detail necessary to extract spatial gradient structure. Objective techniques have been developed in an IDL environment to process the data. Three different spectroscopic methods are used to characterize the core gradients. One technique is a multi-objective search and reconstruction which utilizes a genetic algorithm to search efficiently for the temperature and density gradients that yield the best simultaneous and self-consistent fits to both an x-ray line spectrum and emissivity profiles from optically-thin narrow-band images. The other techniques are analyses involving the emissivity ratio of Lyβ/Heβ to extract temperature, followed by the solution of either emissivity or intensity equations based on the Lyβ or Heβ line to infer density. In particular, the idea of solving intensity equations permits the consideration of the opacity effect, which turns out to be size-ii able even for quasi-optically thin lines (i.e. Lyβ and Heβ). In order to further extract the mixing spatial profile and account for its effect on density, the optically-thick Lyα line is brought into the analysis. This is a novel approach which addresses the issue of uniqueness and facilitates the extraction of information on the spatial extent and level of mix directly from the experimental data. To connect this mixing analysis with a theoretical methodology, two independent mix models, Youngs' model and the Haan saturation model, have been used to estimate the level of mixing in these experiments.

- by Nobuhiko Izumi
- •
- Engineering, X-ray imaging, Mathematical Sciences, Physical sciences

A pulsed Nd:Yag laser, at intensities of the order of 10 10 W/cm 2 , is employed to irradiate different thick metallic targets (Ti, Fe, Ag, and Ni) placed in vacuum. The obtained non-equilibrium plasmas are investigated with various analytical techniques. An electrostatic ion energy analyzer and different ion collectors are employed to monitor in situ the ions ejected from the plasma and to determine the core plasma temperature, the ion energy distributions and the ion angular emission. An optical spectrometer is employed to analyze the plasma corona emitted light vs. wavelength and to identify the emitted characteristic lines. The optical spectroscopy permitted to evaluate the electron temperatures and densities. Results show strong temperature and density gradients occurring in the laser-generated plasma plume.

- by Lorenzo Torrisi
- •
- Optical Spectroscopy, Physical sciences, Density gradient, Energy Distribution

Observations from the York River Estuary, Virginia, demonstrate that the along-channel wind plays a dominant role in governing the estuarine exchange flow and the corresponding increase or decrease in vertical density stratification. Contrary to previous findings that suggest wind stress acts predominantly as a source of energy to mix away estuarine stratification, our results demonstrate that the wind can play a more important role in straining the alongchannel estuarine density gradient. Down-estuary winds enhance the tidally-averaged vertical shear, which interacts with the along-channel density gradient to increase vertical stratification. Up-estuary winds tend to reduce, or even reverse the vertical shear, reducing vertical stratification. In two experiments each lasting approximately a month, the estuarine exchange flow was highly correlated with the along-channel component of the wind. The changes in stratification caused by the exchange flow appear to control the amount of vertical mixing as parameterized by the vertical eddy viscosity. The degree of stratification induced by wind straining also appears to play an important role in controlling the effectiveness of wind and tidal mixing.

- by Malcolm Scully
- •
- Earth Sciences, Stratification, Biological Sciences, Environmental Sciences

We study the rheology of a granular paste, i.e. a dense suspension of non-Brownian particles, quantitatively at steady state, in a cylindrical Couette cell. Previous studies have shown a discrepancy between local and global measurements of the viscosity for these materials, making it impossible to predict their resistance to flow. Using both MRI investigation techniques and classical rheology studies, we show that agreement between local and global measurements can be obtained, provided the migration of particles inside the gap is taken into account. As found by Leighton & Acrivos (J. Fluid Mech. vol. 181, 1987, p. 415), the migration leads to a particle density gradient in the flow, the highly sheared regions being less dense in particles. Here, by comparing the local viscosity and particle density measurements from MRI with the macroscopic relation between viscosity and the volume fraction, it is shown that global and local measurements agree with each other. This consequently allows us to define a viscosity for dense suspensions.

- by Daniel Bonn
- •
- Engineering, Fluid Mechanics, Mathematical Sciences, Steady state

In the present paper, we report on micropaleontological (dinocysts) and isotopic ( 18 O and 13 C in foraminifers) analyses performed in Holocene sediments from fifteen cores raised from the central and northwest North Atlantic. Sea-surface temperature (SST), sea-surface salinity (SSS), thus potential density, and sea-ice cover are reconstructed based on dinocyst assemblages. After proper calibration, oxygen isotope data on the mesopelagic foraminifer Neogloboquadrina pachyderma left coiled (Npl) are converted into potential density values deeper in the water column, thus allowing documentation of vertical density gradients and identification of intervals favourable for winter convection to occur with formation of intermediate Labrador Sea Water (LSW). The most important findings from this study include: (1) the existence of an early-mid Holocene thermal optimum with positive anomalies up to 6°C above present along the main SW-NE axis of the North Atlantic Current, but no significant SST maximum at most sites along eastern Canadian margins; (2) the evidence for larger than present amplitude of annual SSTs during the early Holocene, thus for a stronger seasonality; (3) minimum sea-ice cover from 11 500 to 6000 cal years BP, and a slight increase of seaice variability, and average seasonal duration of 0.5 to 1 month per year afterwards; (4) variable SSS during the entire Holocene, suggesting changes in the routing and rates of freshwater-meltwater discharges from the Arctic and eastern Canada; (5) the setting of conditions compatible with LSW production after 8 ka only, and likely a more steady production during the late Holocene; (6) an overall trend for a potential density increase of the Labrador Sea, throughout the Holocene, matching a decreasing trend eastward, thus suggesting a progressive enhancement of the western branch of the Atlantic Meridional Overturning with respect to its northeastern route; and (7) indication of maximum production and fast dispersal of LSW in the entire North Atlantic during recent times only, as suggested by linearly-converging δ 18 O-values of Npl from all sites, towards its modern relatively homogeneous composition (∼ 2.5/2.6‰). The overall picture of the Holocene North Atlantic arising from this study is that of a basin marked by a strong regionalism with large discrepancies in hydrographical trends and high frequency oscillations, at least partly controlled by freshwater-meltwater routes and rates of export from the Arctic.

- by Claude Hillaire-Marcel
- •
- Earth Sciences, Sea Ice, Sea surface temperature, High Frequency

Lab: Lucerne is divided by sills into four major and two minor basins. During winter, differential vertical mixing due to non-uniform wind exposure and different total dissolved solid concentrations in the main tributaries of the lake cause considerable interbasin density gradients. These gradients induce density-driven currents across the sills that contribute significantly to the deep water exchange in the basins and gradually reduce the density gradients during summer. Over a period of 2 yr (1990-1992), the spatial and temporal evolution of water density and water age (measured by the 3H-3He method) was investigated. Variation in water age between the different basins and the occasional occurrence of age inversions (older water overlying younger one) can be explained in terms of the observed density distribution and the exchange flows the density gradients cause. Water age provides an integral measure of the magnitude of deep-water renewal, which in some basins can be roughly separated into contributions from density-driven currents and wind-induced vertical mixing. By correlating water age with dissolved helium and oxygen concentrations, radiogenic helium fluxes and average oxygen depletion rates were determined for the main basins of the lake. The helium flux was between 1 and 2 x lOlo atoms m-2 s-l, indicating dynamic equilibrium with helium production in the underlying crust. Oxygen depletion rates per unit sediment area were roughly equal in all basins (between 220 and 290 g m-2 yr-l), whereas volumetric depletion rates varied from 1.8 g m-3 yr-r in the deepest basin to 10 g m-3 yr-L in the shallowest. Currents driven by horizontal density differences, in particular exchange currents over sills, have been extensively studied in the ocean, both theoretically and experimentally (e.g. Chao and Paluszkiewicz 1991; Farmer and Armi 1986). Well-known examples are the exchange between the Mediterranean Sea and the Atlantic Ocean through the Strait of Gibraltar (e.g. Tziperman 1987) or the overflow of Arctic-type waters through the Denmark Strait into the North Atlantic (e.g. Dickson et al. 1990). Density currents in lakes have attracted less attention. Small and simple-shaped lakes often have rather small and short-lived horizontal density gradients. However, density-driven flow may be important in larger lakes that have distinct topographic features such as shallow em