Stratified Brine Research Articles

Hydrothermal metalliferous sediments in Red Sea deeps: Formation, characterization and properties

Sediment accumulations within the Red Sea central deeps have unique genesis and properties. We piece together available information to understand their geological setting and formation history, and conduct an extensive sediment characterization study to assess their geotechnical properties in order to anticipate engineering/mining implications. The various sediment columns reflect slow-rate background sedimentation (biogenic and detrital particles – Valdivia deep) and hydrothermal metalliferous sediments that nucleate and grow within the overlying brine pools (primarily in the Atlantis II, as well as in the Wando deep, and to a lesser extent in Discovery deep). All sediments are fine-grained silt and clay-size particles; smaller particles tend to have higher specific gravity and define the metalliferous content. Hydrothermal sediments exhibit extreme properties when compared to sediments worldwide: they have uncharacteristically large maximum void ratio and compressibility, and their self-compaction is very different from background Red Sea sediments. Their unique self-compaction trends have a strong effect on remote acoustic characterization and sampling, and must be carefully accounted for during field studies and resource assessment. Three distinct properties of hydrothermal metalliferous sediments are relevant for separation and enrichment: high specific surface area, high specific gravity, and ferromagnetic signature. Small grains and low-density flocs have low terminal Stokes' velocities and their residency times may be extended in convective stratified brine pools; this observation affects the environmental analysis of mining operations and tailings disposal.

Onset of Multilayer Convection in a Partially Heated Circular Cavity

In this article, the time-dependent double-diffusive natural convection is investigated inside a circular cavity containing stably stratified brine solution. The container is heated partially through its sidewalls by a constant heat flux rate. By using Galerkin finite element method, numerical solutions to the Navier-Stokes equations under the Boussinesq flow assumptions have been systematically organized and developed. The evolutions of the stream function, temperature and concentration fields are presented in each evaluated case. Steep and sharp concentration variation in the interfaces together with wavy distribution of temperature within the layers is observed. At first a length scale, which is the vertical displacement of a heated element of the fluid, in a stably stratified solution under Neumann boundary condition is defined. Then a special form of Rayleigh number based on the length scale is obtained and named as universal Rayleigh number. A categorization based on single, two and multilayer formation is established. Temperature distribution along the cavity in the different cases shows a correlation between heat transfer coefficient and the rate of formation and degradation of the layers.

Origin and geochemistry of Miocene marine evaporites associated with red beds: Great Kavir Basin, Central Iran

AbstractDuring the Cenozoic numerous shallow epicontinental evaporite basins formed due to tectonic movements in the Northern Province of the Central Iran Tectonic Zone (the Great Kavir Basin). During the Miocene, due to sea‐level fluctuations, thick sequences of evaporites and carbonates accumulated in these basins that subsequently were overlain by continental red beds. Development of halite evaporites with substantial thickness in this area implies inflow of seawater along the narrow continental rift axis. The early ocean basin development was initiated in Early Eocene time and continued up to the Middle Miocene in the isolated failed rift arms. Competition between marine and non‐marine environments, at the edge of the encroaching sea, produced several sequences of both abrupt and gradual transition from continental wadi sediments to marginal marine evaporites in the studied area. These evaporites show well‐preserved textures indicative of relatively shallow‐brine pools. The high Br content of these evaporites indicates marine‐derived parent brines that were under the sporadic influence of freshening by meteoric water or replenishing seawater. However, the association of hopper and cornet textures denotes stratified brine that filled a relatively large pool and prevented rapid variations in the Br profile. Unstable basin conditions that triggered modification of parent brine chemistry prevailed in this basin and caused variable distribution patterns for different elements in the chloride units. The presence of sylvite and the absence of Mg‐sulphate/chlorides in the paragenetic sequence indicate SO4−depleted parent brine in the studied sequence.Petrographic examinations along with geochemical analyses on these potash‐bearing halites reveal parental brines which were a mixture of seawater and CaCl2‐rich brines. The source of CaCl2‐rich brines is ascribed to the presence of local rift systems in the Great Kavir Basin up to the end of the Early Miocene. Copyright © 2007 John Wiley & Sons, Ltd.

The anhydrite saturation index of the ponded brines and sediment pore waters of the Red Sea deeps

We compiled the available chemical analyses of the ponded brines and of the sediment interstitial waters for the Atlantis II, Discovery, Suakin and Valdivia deeps in the Red Sea central valley, along with the DSDP data for interstitial waters of several cores in the rift axis (Sites 225–228 of Leg 23). Recent advances in the calculation of the thermodynamic properties of concentrated solutions at high temperatures and pressures allow the study of the equilibrium conditions between anhydrite and halite, and the brines. We first tested the influence of various parameters as temperature, pressure and solution composition on the calculated anhydrite saturation index in order to provide a clear criterion for equilibrium, which is assumed when the saturation index lies within 0.9 and 1.1. We found that at 62°C and 200 bar, the pressure effect on the saturation indices is of the same magnitude as that of temperature. Our calculations point to the overall undersaturation of the free brines with respect to halite. The calculated halite saturation state of the pore waters at the four DSDP sites is consistent with the mineralogical description of the sediments in which halite is not reported, except at Site 225 where the calculated undersaturation is in contradiction with the reported presence of halite at the base of the core. From the hypothesis of equilibrium between anhydrite and the pore waters in the DSDP cores, we have estimated values of the geothermal gradients off the rift axis which fall within the highest values obtained by extrapolation of direct temperature measurements. This suggests that, at the latitude of the Atlantis II and Suakin deeps, the high heat fluxes measured along the axial trough extend off the rift axis. Our calculations show that the Upper Convective Layer (UCL) of the Atlantis II hydrothermal system has always been undersaturated over the studied period (1965–1985), which is consistent with the absence of anhydrite in sediment underlying this brine. The calcium and sulfate contents of the Lower Convective Layer (LCL) show a parallel evolution: they decrease when temperature increases. Although this behaviour suggests control of brine chemistry by equilibrium with anhydrite, our calculations show that anhydrite has reached saturation in the lower brine only in 1966 and 1976. The general regime leading to the loss of calcium and sulfate outside these periods is tentatively attributed to diffusion. Calculations show that, throughout the whole stratified brine column (from the sediment to normal Red Sea water), there is no chemical potential gradient for calcium sulfate whereas the chemical potential of sodium chloride regularly decreases. We tentatively advocate the coupling of calcium sulfate diffusion with the large sodium chloride gradient to account for the loss in Ca and SO 4. Finally, the pore water chemistry of the Atlantis II and Discovery Deep sediments record two parallel evolutions from anhydrite undersaturation in 1966 to saturation (or a state close to) in 1976. These data are consistent with a periodical brine overspill from Atlantis II into Discovery.

Bifurcation phenomena in confined thermosolutal convection with lateral heating: Commencement of the double-diffusive region

As has recently been reported by Tsitverblit and Kit [Phys. Fluids A 5, 1062 (1993)], a vertical rectangular enclosure containing stably stratified brine and differentially heated from its side walls is characterized by complex steady bifurcation phenomena. In the present work, the structure of steady solutions in the enclosure has been studied in detail for several values of the salinity Rayleigh number, RaS, fixed near the commencement of the double-diffusive region. It was found that when the thermal Rayleigh number, RaT, is either very small or sufficiently large, the steady solution is unique while in an intermediate region of this parameter, there exists a great variety of the multiple steady flows, being the result of nondegenerate hysteresis points and isolas of asymmetric solutions forming as RaS is increased. In particular, at the maximal value of RaS considered there have been observed symmetric and asymmetric one-, two-, three-, four-, and five-cell flows. Despite the multiplicity of the flow patterns, a critical interval of the buoyancy ratio has been distinguished, above and below which the generic characteristics of the steady solutions were found to resemble the respective features of the ‘‘successive’’ and ‘‘simultaneous’’ regimes of layer formation whose existence was established in previous studies. Although the set of the steady solutions has been found to contain no linearly stable multicell flows, the perturbation was so long retained in the close proximity of the unstable steady solutions that such flows could be easily observable in the experiment. In spite of the appreciably different range of the Rayleigh numbers, the physically meaningful parameters suggested in previous studies were found to be represented in the present results.

The multiplicity of steady flows in confined double-diffusive convection with lateral heating

In a number of studies concerning the lateral heating of a vertical rectangular enclosure containing a stably stratified brine solution, diverse flow regimes have been observed for different values of the buoyancy ratio. In this Brief Communication, certain intervals of the buoyancy ratio have been revealed to possess multiple steady flows. The presence of the multiple solutions was demonstrated to be underlain by complex bifurcation phenomena including numerous limit points and the symmetry-breaking bifurcation. The results appear to be insightful for the understanding of various findings of previous works.

Optical visualization of internal gravity waves in stratified fluid

The refractive index changes associated with flows in salt water systems allow such flows to be visualized by means of optical methods, e.g. schlieren and interferometry. Experiments that have been conducted by our group with internal gravity waves in stratified brine are reviewed. The experiments encompass visualization and quantitative measurement of internal gravity waves generated by a body oscillating around a fixed position and a test body moving with constant speed vertically through the stratified brine. It is also shown that the velocity field of an internal gravity wave can be measured by means of particle image velocimetry. References to respective wave theories are made.

The phase configuration of trapped internal waves from a body moving in a thermocline

Some two-dimensional internal wave systems are generated experimentally by a cylinder moving with constant velocity in a thermocline modelled with stratified brine. The phase configuration of the waves is shown to compare quite well with ray theory, and the phase changes which result from energy reflections are well defined in some of the schlieren photographs.

Viscous effects in some internal waves

Experiments are described in which a horizontal circular cylinder is moved vertically and also horizontally at constant velocity normal to its axis in density stratified brine which has a constant buoyancy frequency, N. A six mirror Mach Zehnder interferometer is used to measure the density distributions within the far field wave systems. The Reynolds numbers based on the cylinder diameter, D, are between 1 and 10 and the frequency parameter, DN/U, where U is the body velocity, is of order one.

Viscous effects in lee waves

Abstract A horizontal cylinder is traversed horizontally normal to its longitudinal axis in density stratified brine, and the lee waves which are produced over a range of model velocities are viewed with a six mirror Mach Zehnder interferometer which allows the density distribution to be evaluated. At the lowest model velocities, viscous effects limit the number of visible waves to no more than three. The viscous internal wave theory of Chang and Stevenson is extended and used to calculate lee waves over a ridge. Although the linear theory does not include the no-slip condition at the lower boundary and does not include the upstream wake, which is clearly visible in the experiments, the way in which the waves attenuate away from the model is reproduced quite well by the theory. The viscous results will be compared with the standard inviscid calculations.

An experimental investigation of turbulent stratified shearing flow

Some experiments are described in which steady-state shearing flows are developed in stratified brine solutions contained in a cyclically continuous tank of rectangular cross-section. Over the range of overall Richardson numbers studied, the results suggest that whenever turbulent layers are present on either side of a region of fluid with a gravitationally stable density gradient, they cause erosion of this region to occur. The erosion leads to the formation of two homogeneous layers separated by a thin layer of strong density and velocity gradients. The gradient Richardson number, computed by using the velocity and density gradients in this transition layer, tends to have a value of order one.If we define an overall Richardson number Ri* by averaging the velocity and density gradients over the entire depth of fluid in the tank, we find that the non- dimensional buoyancy flux, Q, is functionally related to Ri* by an equation of the form Q = C1(Ri*)−1 where C1 is a constant, approximately, and Ri* ranges in value between one and thirty.To check the effect of a large variation of the molecular diffusivity coefficient on flow conditions, we ran a limited number of experiments with thermally stratified fluid. Over a restricted range, 1·0 < Ri* < 5·0, velocity profiles very similar to those measured in the brine-stratified experiments at like values of Ri* were obtained. This suggests that the coefficient of molecular diffusion is not an important parameter in either type of experiment.Other experiments, made in the same apparatus, describe the entrainment by a turbulent, homogeneous layer of an initially quiescent layer of fluid with a linear density gradient. The depth of the turbulent layer, D, increases with time, t, according to the relation. \[ D^3\propto t. \] This result is consistent with that found by Kato & Phillips (1969), although the turbulent layer in the present experiment is generated in a different manner.

The Rayleigh problem for a slightly diffusive density-stratified fluid

A doubly-infinite sloping flat plate, initially at rest in a slightly diffusive viscous density-stratified fluid, starts to move impulsively with a constant velocity along the line of greatest slope. The resulting flow is found to be an unsteady motion superimposed on a steady diffusion-induced flow, which is present throughout. Laplace transform methods give solutions which are valid either in an essentially non-diffusive outer layer or in a diffusive inner layer. The impulsive start sets up oscillations in the outer layer. These gradually die out, and a steady diffusive flow develops.A glass plate was towed vertically through stratified brine, into which aluminium particles were introduced. The flow velocities deduced from the particle motions confirmed the theoretical predictions.

Experiments on the Stability of Stratified Shear Flows

Some experiments are described in which a shear flow is developed in a stratified brine solution contained in a long, closed tube of rectangular cross section. The fluid motion is produced by tilting the tube from an initial horizontal position, and the flow thus generated is found to be uniformly accelerating and easily expressed in terms of the initial density profile in the tube until disrupted by instability. (The details of the method are given in an earlier paper.) The particular case of a diffuse interface between two homogeneous layers of different densities that move with equal and opposite speeds is discussed in some detail, and the transition to turbulence is described. The transition is characterized by the formation of waves that develop into rolls of a very beautiful form. Measurements at the onset of the wave motion are compared with theoretical predictions.

The effect of horizontal gradients on thermohaline convection

When the vertical boundaries of a container of stratified brine solution are heated, cells are observed to form in the brine with a layered structure. An explanation of this phenomenon has been given in terms of the successive growth of cells from the top and bottom of the container. An alternative explanation is given here which is based on an examination of the stability of the fluid layer in the neighbourhood of the vertical boundaries, a layer in which there exist horizontal gradients in both temperature and salinity, and the explanation is subjected to comparison with a carefully controlled laboratory experiment. The theoretical description is fairly general and it seems possible that the effects of horizontal gradients of salinity and temperature, which approximately compensate each other so as to leave no horizontal density gradient, may be important in the ocean, as suggested by Stern (1967).