Dense Water Layer Research Articles

Modelling secondary circulations in stratified bi-directional flows: A potential mechanism for flux transfer from lower to upper fluid layers

Results are presented for new laboratory-scale numerical simulations of bi-directional stratified flows generated within an idealized trapezoidal sill-channel topography, focussing on the lateral (or secondary) flow structure across the channel both in non-rotating and rotating frames of reference. The simulations utilise the non-hydrostatic Bergen Ocean Model (BOM), a three-dimensional general ocean circulation model. The results from the BOM simulations are verified by large-scale experimental data obtained in the LEGI Coriolis rotating platform in Grenoble, within which velocity and density fields for bi-directional stratified flows were measured through particle image velocimetry (PIV) and micro-conductivity density probes, respectively. The BOM simulations reproduce the main dynamic flow patterns and structure of the large-scale exchange flows generated through the trapezoidal sill-channel, with the lower layer saline intrusion flux shown to reduce, due to partial blockage, as the upper freshwater flow is increased (i.e. net barotropic forcing). Non-rotational BOM simulations show upward (and downward) flow along the centreline of the trapezoidal sill-channel in the upper (and lower) layers, with symmetrical counter-rotating circulation cells forming both above and below the density interface. By contrast, equivalent BOM simulations within a rotating frame of reference show the secondary flow circulation in the lower dense water layer to be dominated by Ekman dynamics, while two co-rotating cells are formed in the upper freshwater layer above the inclined density interface. In these rotational cases, significant upward and downward velocities (i.e. strong upwelling and downwelling) are generated adjacent to the inclined sill-channel side walls (in comparison to non-rotating cases). This secondary flow effect may be especially relevant for exchange flows in estuaries, fjords, deep-ocean channels and sea straits, where the channel width exceeds the Rossby radius of deformation and, thus, Earth rotation effects become important dynamically. In particular, any potential vertical transport pathway associated with this upwelling and downwelling at the inclined channel side slopes may have strong managerial implications in coastal areas, whereby tracers (e.g. contaminants, salinity, reduced DO waters, fine sediments) can be transferred from deep waters to the near surface.

The effect of ion shape and concentration on tunable wetting of water droplets on graphite surfaces

We study the wetting behaviors of aqueous NaCl and NaNO3 electrolyte nano-droplets on graphite surfaces by molecular dynamics simulations. Despite the same ion charge and similar Stokes radii, while the contact angle increases with NaCl concentration up to 3 M, the contact angle decreases with NaNO3 concentration up to 3 M. We show that while the liquid/vapor surface tension increases with both NaCl and NaNO3 concentrations, the solid/liquid interfacial tension varies with NaCl and NaNO3 concentrations oppositely. The Cl− ions are depleted from the first dense water layer at the graphite/water interface, leading to an increasing interfacial tension and an increasing contact angle with ion concentration. Beyond 3 M NaCl, the ionic double layer at the interface counteracts the depletion of ions at the immediate graphite/water interface. Therefore, the contact angle saturates beyond around 3 M NaCl. By contrast, the planar NO3− ions are adsorbed in the dense water layer at the immediate graphite/water interfaces, conform to the interface, and form excessive hydrogen bonds with water at the interface. Consequently, the interfacial tension is decreased with increasing NaNO3 concentrations, leading to a decreasing contact angle up to around 3 M NaNO3. At NaNO3 concentrations beyond 3 M, the interfacial tension no longer decreases, likely due to the saturation of NO3− in the dense region; therefore, the contact angle slightly increases with NaNO3 concentration due to the increasing liquid/vapor surface tension. Our research sheds light on the mechanisms of the effect of ion shape and concentration on droplet wetting on solids.

HRMS Detector for the New HILIC CBD Method Development in Hemp Seed Oil.

The cannabis analysis has gained new importance worldwide due to the rapid expansion of the hemp global market. Many reverse phase high performance liquid chromatography (HPLC) methods have been developed to analyze cannabidiol (CBD) and its analogues due to the nice fit with their log P values at around 6. However, when CBD was blended with hemp seed oil in consumer, cosmetic, and food industries, the high content of triacylglycerides (TAGs) from hemp seed oil were retained and accumulated inside C18 columns with the common mobile phases and caused a column pressure increase and ghost peaks after continuous sample injections. Coupled with the chemical profile from high resolution mass spectrometry (HRMS) detection, a novel hydrophilic interaction liquid chromatography (HILIC) CBD quantitation method was developed, specifically for CBD blended hemp seed oil materials, which can overcome the matrix buildup in reverse phase columns. The zwitterionic (ZIC)-cHILIC column containing a dense water layer on the stationary phase surface provided a stable partitioning separation mechanism to separate the CBD from TAGs in hemp seed oil. This water layer favors the sustaining NH4F buffer ions, which can maximize the salting-out action and help reduce the adsorptive interaction between TAGs and stationary phase sulfobetaine materials. The high percentage of acetonitrile (99%) contributed to method sensitivity and reduced instrument maintenance time. The method was developed and validated for the first time. It has been successfully applied to quantify CBD content in hemp seed oil samples, thus demonstrating it to be a useful tool for both quality control and safety assurance in CBD hemp seed oil raw materials and related products.

Ten Facets, One Force Field: The GAL19 Force Field for Water-Noble Metal Interfaces.

Understanding the structure of the water/metal interfaces plays an important role in many areas ranging from surface chemistry to environmental processes. The size, required phase-space sampling, and the slow diffusion of molecules at the water/metal interfaces motivate the development of accurate force fields. We develop and parametrize GAL19, a novel force field, to describe the interaction of water with two facets (111 and 100) of five metals (Pt, Pd, Au, Ag, Cu). To increase transferability compared to its predecessor GAL17, the water-metal interaction is described as a sum of pairwise terms. The interaction energy has three contributions: (i) physisorption is described via a Tang and Toennies potential, (ii) chemisorption and surface corrugation rely on an attractive Gaussian term, and (iii) the angular dependence is explicitly included as a truncated Fourier series. Thirteen parameters are used for each metal surface and were fitted on 250 water adsorption energies computed at the PBE+dDsC level. The performance of GAL19 was evaluated on a set of more than 600 DFT adsorption energies for each surface, leading to an average root-mean-square deviation of only 1 kcal/mol, correctly reproducing the adsorption trends: strong on Pt and Pd but weaker on Ag, Au, and Cu. This force field was then used to simulate the water/metal interface for all ten surfaces for 1 ns. Structural analyses reveal similar tendencies for all surfaces: a first, dense water layer that is mostly adsorbed on the metal top sites and a second layer up to around 6 Å, which is less structured. On Pt and Pd, the first layer is strongly organized with water lying flat on the surface. The pairwise additive functional form allows one to simulate the water adsorption on alloys, which is demonstrated at the example of Ag/Cu and Au/Pt alloys. The water/Ag-Cu interface is predicted to be disordered with water mostly adsorbed on Cu which should exacerbate the Ag reactivity. On the contrary, incorporating Pt into Au materials leads to a structuring of the water interface. Our promising results make GAL19 an ideal candidate to get representative sampling of complex metal/water interfaces as a first step toward accurate estimation of free energies of reactions in solution at the metal interface.

Collisional transport model for intense bed load

Abstract In an open channel with a mobile bed, intense transport of bed load is associated with high-concentrated sediment-laden flow over a plane surface of the eroded bed due to high bed shear. Typically, the flow exhibits a layered internal structure in which virtually all sediment grains are transported through a collisional layer above the bed. Our investigation focuses on steady uniform turbulent open-channel flow with a developed collisional transport layer and combines modelling and experiment to relate integral quantities, as the discharge of solids, discharge of mixture, and flow depth with the longitudinal slope of the bed and the internal structure of the flow above the bed. A transport model is presented which considers flow with the internal structure described by linear vertical distributions of granular velocity and concentration across the collisional layer. The model employs constitutive relations based on the classical kinetic theory of granular flows selected by our previous experimental testing as appropriate for the flow and transport conditions under consideration. For given slope and depth of the flow, the model predicts the total discharge and the discharge of sediment. The model also predicts the layered structure of the flow, giving the thickness of the dense layer, collisional layer, and water layer. Model predictions are compared with results of intense bed-load experiment carried out for lightweight sediment in our laboratory tilting flume.

Non-hydrostatic effects in the Dead Sea

Abstract The Dead Sea is the saltiest and lowest terminal lake in the world. For at least three decades, the Dead Sea's water level has been dropping by more than 1 m per year, due to excessive use of the water that previously flowed into it. The Dead Sea constitutes a unique environment and is important from economic, environmental, and touristic points of view. The winter deep convection of the Dead Sea and its deep and narrow basin suggest that non-hydrostatic effects may significantly affect its circulation. Despite these factors, the expected non-hydrostatic effects on the circulation of the Dead Sea have not been investigated. Here we perform high resolution (100 m) ocean general circulation model (the MITgcm) simulations of the Dead Sea and show that the non-hydrostatic results are very different from the hydrostatic ones. Specifically, we show that the winter non-hydrostatic simulations resulted in a layer of dense water overlaying slightly lighter water during the several last hours of the night; this convection process involved plumes of heavier sinking water and the entrainment of the plumes. We also studied the effect of the wind stress's diurnal variability and found it to be important, especially during the summer when the wind's variability drastically increased the surface kinetic energy; however, it did not alter the depth density profile. The results presented here may be important for the Dead Sea's potash industry and for the planned Red Sea-Dead Sea canal that aims to stop and, possibly, to increase the level of the Dead Sea using the Red Sea's water.

An axisymmetric non-hydrostatic model for double-diffusive water systems

Abstract. The three-dimensional (3-D) modelling of water systems involving double-diffusive processes is challenging due to the large computation times required to solve the flow and transport of constituents. In 3-D systems that approach axisymmetry around a central location, computation times can be reduced by applying a 2-D axisymmetric model set-up. This article applies the Reynolds-averaged Navier–Stokes equations described in cylindrical coordinates and integrates them to guarantee mass and momentum conservation. The discretized equations are presented in a way that a Cartesian finite-volume model can be easily extended to the developed framework, which is demonstrated by the implementation into a non-hydrostatic free-surface flow model. This model employs temperature- and salinity-dependent densities, molecular diffusivities, and kinematic viscosity. One quantitative case study, based on an analytical solution derived for the radial expansion of a dense water layer, and two qualitative case studies demonstrate a good behaviour of the model for seepage inflows with contrasting salinities and temperatures. Four case studies with respect to double-diffusive processes in a stratified water body demonstrate that turbulent flows are not yet correctly modelled near the interfaces and that an advanced turbulence model is required.

Variability of the Antarctic Slope Current System in the Northwestern Weddell Sea

AbstractThe dense water outflow from the Antarctic continental shelf is closely associated with the strength and position of the Antarctic Slope Front. This study explores the short-term and spatial variability of the Antarctic Slope Front system and the mechanisms that regulate cross-slope exchange using highly temporally and spatially resolved measurements from three ocean gliders deployed in 2012. The 22 sections along the eastern Antarctic Peninsula and west of the South Orkney Islands are grouped regionally and composited by isobaths. There is consistency in the front position around the Powell Basin, varying mostly between the 500- and 800-m isobaths. In most of the study area the flow is bottom intensified. The along-slope transport of the Antarctic Slope Current (upper 1000 m) varies between 0.2 and 5.9 Sv (1 Sv ≡ 106 m3 s−1) and does not exhibit a regional pattern. The magnitude of the velocity field shows substantial variability, up to twice its mean value. Higher eddy kinetic energy (0.003 m2 s−2) is observed in sections with dense water, possibly because of baroclinic instabilities in the bottom layer. Distributions of potential vorticity show an increase toward the shelf along isopycnals and also in the dense water layer. Glider sections located west of the South Orkney Islands indicate a northward direction of the flow associated with the Weddell Front, which differs from previous estimates of the mean circulation. This study provides some of the first observational confirmation of the high-frequency variability associated with an active eddy field that has been suggested by recent numerical simulations in this region.

Effect of hydrophobic groups on the adsorption conformation of modified polycarboxylate superplasticizer investigated by molecular dynamics simulation

All-atom molecular dynamics (MD) simulations were used to study the adsorption conformations of hydrophobically-modified comb-shaped polycarboxylate ether-based (PCE) superplasticizer molecules on a model surface of dicalcium silicate (C2S) in vacuum and in an explicit solution, respectively. Three different hydrophobic modifying groups, namely, the ethyl group, the n-butyl group and the phenyl group, decorated to the backbone, were examined. Comparing the hydrophobically-modified PCEs to the unmodified one, differences were found in the binding energy, the adsorption conformation and the water density at the interface. The interaction between PCE molecules and C2S was weakened in a solution with explicit solvents than that obtained from vacuum-based simulations. The presence of hydrophobic groups lowered the polymer-surface binding energy, decreased the radius of gyration (Rg) of the adsorbed polymer, increased the peak position in the heavy-atom density profiles in the direction perpendicular to the surface, and also caused the adsorbed conformations to be more globular in shape. The parallel and perpendicular components (relative to the surface plane) of the geometric sizes of the adsorbed polymers were calculated, and the results showed that the presence of hydrophobically modifying groups decreased the in-plane radius while increased the adsorption layer thickness compared to the unmodified control. The presence of PCEs perturbed the dense water layer above the C2S surface and lowered the water density. Perturbations to the interfacial water density were found to correlate nicely with the adsorbed conformations of PCEs.

Desorption of water from hydrophilic MCM-41 mesopores: positron annihilation, FTIR and MD simulation studies

The desorption mechanism of water from the hydrophilic mesopores of MCM-41 was studied using positron annihilation lifetime spectroscopy (PALS) and attenuated total reflection Fourier transform infrared spectroscopy supplemented with molecular dynamics (MD) simulation. PALS results indicated that water molecules do not undergo sequential evaporation in a simple layer-by-layer manner during desorption from MCM-41 mesopores. The results suggested that the water column inside the uniform cylindrical mesopore become stretched during desorption and induces cavitation (as seen in the case of ink-bottle type pores) inside it, keeping a dense water layer at the hydrophilic pore wall, as well as a water plug at both the open ends of the cylindrical pore, until the water was reduced to a certain volume fraction where the pore catastrophically empties. Before being emptied, the water molecules formed clusters inside the mesopores. The formation of molecular clusters below a certain level of hydration was corroborated by the MD simulation study. The results are discussed.

Effect of interfacial hydrogen bonds on the structure and dynamics of confined water

Reactive molecular dynamics (MD) is used to simulate the equilibrium process of water confined between two fully hydroxylated α-quartz (001) surfaces with separation distances from 7 to 20 Å. Effect of different patterns of interfacial hydrogen bonds on the structure and dynamics of confined water is investigated. Density profiles, radial distribution functions, number of interfacial hydrogen bonds, and mean square displacements are calculated. The α-quartz (001) surface is cut from an α-quartz crystal at a certain depth to construct a surface with geminal silanols after being fully hydroxylated. The silanol groups on the surface are treated in two different ways in the MD simulations. One of the silanol groups are treated as to be fixed, and the other one is treated as no constraint for the movement of surface silanols. Our results show that different patterns of hydrogen bonds are formed at the interface between SiO2 surface and water. For the fixed silanol surface there is one type of strong hydrogen bonds interacting between the oxygen atoms of water and the hydrogen atoms of surface silanols, leading to the dipole moment of water molecules pointing out from the surface. For the movable silanol surface there are two types of strong hydrogen bonds formed at the interface. One is between the oxygen atoms of water and the hydrogen atoms of surface silanols, and the other is between the oxygen atoms of surface silanols and the hydrogen atoms of water. The number of hydrogen bonds of the first type is much less than those of the second type, leading to the dipole moment of water molecules pointing to the surface. Moreover, the total number of interfacial hydrogen bonds formed on the fixed silanol surfaces is larger than that on the movable silanol surfaces. The density profiles of the confined water indicate the formation of a strong layering of water in the vicinity of the fixed silanol surface, and the water layer is also more ordered with an ice-like structure, as compared with a dense water layer with a liquid-like structure in the case of movable silanol surfaces. Thus the mean square displacements of confined water show that, as compared with interfacial hydrogen bonds formed on the fixed silanol surfaces, the weaker and the lesser interfacial hydrogen bonds formed on the movable silanol surfaces may be responsible for more intense movement of confined water between the movable silanol surfaces. Our simulation suggests that the different pattern of interfacial hydrogen bonds could signifiantly affect the structure and dynamic behaviors of the confined water between two fully hydroxylated silica surfaces.

Dynamics of Cross-Isobath Dense Water Transport Induced by Slope Topography

Abstract Dynamics of cross-isobath downslope transport of a dense water mass induced by small-scale topographic variation is investigated based on a high-resolution numerical experiment with realistic settings, a simplified analytical model for water particle advection, and idealized sensitivity experiments. The existence of a submarine ridge induces two different processes for cross-isobath downslope transport of dense water: a strong but narrow and thin downslope current at the east side of the ridge and cyclonic eddies with dense water cores to the west of the ridge. The former downslope current is produced in response to the rapid increase of slope angle near the ridge. The latter eddies are formed by stretching of the dense water layer near the crest, where isobath curvature is so high that offshore centrifugal force overcomes the coastward Coriolis force. From a simple analysis on the equation of motion for a fluid particle placed on a slope with curved isobaths, a general criterion that describes whether a density current follows or crosses isobaths is derived, which is supported by idealized sensitivity experiments. The location where cross-isobath transport of dense water takes place is determined by relative magnitude between spatial derivatives of isobath curvature, planetary vorticity, and slope angle. Based on these arguments, a parameterization is proposed to represent the effect of unresolved small-scale topography in coarse-resolution models.

A dense water outflow from the Ross Sea, Antarctica: Mixing and the contribution of tides

We use hydrographic, current, and microstructure measurements, and tide-forced ocean models, to estimate benthic and interfacial mixing impacting the evolution of a bottom-trapped outflow of dense shelf water from the Drygalski Trough in the northwestern Ross Sea. During summer 2003 an energetic outflow was observed from the outer shelf (∼ 500 m isobath) to the ∼ 1600 m isobath on the continental slope. Outflow thickness was as great as ∼ 200 m, and mean speeds were ∼ 0.6 m s − 1 relative to background currents exceeding ∼ 1 m s − 1 that were primarily tidal in origin. No outflow was detected on the slope in winter 2004, although a thin layer of dense shelf water was present on the outer shelf. When the outflow was well-developed, the estimated benthic stress was of order one Pascal and the bulk Froude number over the upper slope exceeded one. Diapycnal scalar diffusivity ( K z ) values in the transition region at the top of the outflow, estimated from Thorpe-scale analysis of potential density and measurements of microscale temperature gradient from sensors attached to the CTD rosette, were of order 10 − 3 −10 − 2 m 2 s − 1 . For two cases where the upper outflow boundary was particularly sharply defined, entrainment rate w e was estimated from K z and bulk outflow parameters to be ∼ 10 − 3 m s − 1 (∼ 100 m day − 1 ). A tide-forced, three-dimensional primitive equation ocean model with Mellor-Yamada level 2.5 turbulence closure scheme for diapycnal mixing yields results consistent with a significant tidal role in mixing associated with benthic stress and shear within the stratified ocean interior.

A theoretical, two‐layer, reduced‐gravity model for descending dense water flow on continental shelves/slopes

A theoretical, two‐layer, reduced‐gravity model for descending dense water flow on continental shelves/slopes has been developed to investigate the dynamics of bottom dense water plumes. The model is nonsteady state and includes vertical viscosity, the Coriolis force, and bottom friction. An integral solution rather than a perfect analytical expression is derived and, thus, the Simpson's 1/3 rule to approximate the integral is applied. At the very bottom, the dense water plume moves about 45° to the right (left) in the Northern (Southern) Hemisphere, looking downslope. From the bottom, the velocity vector rotates anticyclonically upward, indicating a bottom Ekman spiral that mimics the atmospheric Ekman boundary layer. The dense water within the bottom Ekman layer obeys a three‐force balance, while the dense water above the bottom Ekman layer is governed by a two‐force balance, which is a geostrophic flow with superimposed cycloidal inertial oscillations oriented from about 25° to 140° to the right (left) of the downslope direction in the Northern (Southern) Hemisphere. The transport within the bottom Ekman layer is directed about 60–70° to the right (left) of the downslope direction in the Northern (Southern) Hemisphere, forming an offshore (cross‐isobath) transport in the absence of eddy flux and wind‐forcing. The ratio of offshore transport to alongshore transport within the bottom Ekman layer is about 0.19 (19%), while the ratio above the bottom Ekman layer (i.e., geostrophic layer of the dense water) is only 3% (negligible compared to its alongshore transport), which, however, is equivalent in magnitude to its counterpart in the bottom Ekman layer if O(DE/h) ∼ 0.1 (where DE is the bottom Ekman layer thickness and h is the dense water layer thickness). In other words, the bottom Ekman layer and the geostrophic (dense) layer contribute equivalent dense water offshore (each contributes 50%). The magnitude of the descending dense water velocity depends linearly on reduced gravity and sin(θ) (slope angle).

Impact of multiple submarine channels on the descent of dense water at high latitudes

A three‐dimensional numerical hydrodynamic model is applied to examine the impact of multiple submarine channels (<10 km across, <100 m deep), common to most continental margins of the ocean, on the descent of dense water at high latitudes. The model consists of an ocean bottom layer of constant height that follows variable bottom topography under constant vertical grid spacing. An idealized continental slope of constant bottom slope is considered, including parallel channels that run perpendicular to main isobaths. The ocean is initially homogeneous and at rest. Forcing is due to a layer of dense water prescribed along the upslope boundary. When the channel aspect ratio (ratio of width to depth) exceeds the main bottom slope, dense water is carried downslope by narrow channel plumes centered along the channel axes. Owing to a small internal Rossby radius less than the channel width the plume dynamics are governed by a geostrophic balance across the channels. Interaction of adjacent channel plumes leads to complex bottom‐parallel circulations. The net downslope density flux resulting from these circulations exceeds that of viscous (ageostrophic) flow of dense water developing without channels. When the channel aspect ratio is less than the main bottom slope, the descent of dense water is dominated by viscous flow. Narrow geostrophic circulation patterns along channels, superimposed on the mean flow, however, induce advective entrainment of lighter ambient water across the leading density front of descending water. As a result of this, the net downslope density flux is reduced as compared to that without channels. Sensitivity studies reveal that the channel‐modified dynamics are independent of the magnitude of the eddy viscosity.

Experimental turbidity currents entering density‐stratified water: analogues for turbidites in Mediterranean hypersaline basins

ABSTRACTExperimental turbidity currents entering two‐layer density‐stratified water behave differently from similar currents flowing over the same topography into non‐stratified water. Experiments were designed as analogues for flows entering Mediterranean hypersaline pools. In both the hypersaline pools and the experiments, the water density changes abruptly across a pycnocline. Turbidity currents generated on a platform at the level of the pycnocline behaved in one of three ways as they flowed from the platform into deeper stratified water. (1) When the bulk density of the current was less than the dense water layer, the current spread at the pycnocline. The head of the current advanced rapidly when it lost contact with the bed. Grains settling out of the current fell through the dense water layer forming an extensive deposit. In nature this behaviour will lead to ‘turbidites’ with sharp but non‐erosive bases, strongly developed grading and no traction features. (2) When the bulk density of the current was greater than the dense water layer, the current continued as an underflow, plunging into the deeper water. Sedimentation lowered the bulk density of the current and the low‐density interstitial fluid caused the head to loft. Low‐density interstitial fluid convected from the body of the current, lofting particles into the water column. These particles were hydraulically sorted during upward transport and subsequent settling to the floor. The resulting turbidites had a more limited extent than the deposits of either non‐lofting underflows or interflows. By inference from the experiments, natural deposits of this type may have local (proximal) erosion and traction features at the base and strongly graded tops. (3) In some of the currents with high bulk density, the rising turbid water reached the pycnocline and spread at that level as a secondary interflow. The tail of the turbidity current, which was less dense than the head and body of the current, flowed above the pycnocline adding momentum to the secondary interflow. The thin non‐erosive graded deposit from the secondary interflow may extend beyond the deposits of the primary underflow. In all three cases (but more pronounced in cases 2 and 3) the interaction of the current with the pycnocline displaced that surface and generated a wave that was reflected back and forth from each end of the pool. The waves remobilized sediment on the ramp.

Bottomwater formation due to hydrothermal activity in Frolikha Bay, Lake Baikal, eastern Siberia

Hydrothermal water enters Frolikha Bay, a well-known site of high geothermal heat flux in the northern part of Lake Baikal, at 400 m depth. On the basis of CTD profiles, the hydrothermal water is identified as forming an anomalous bottom layer with a higher temperature (>0.15 °C) and salinity (>2.5 mg · kg −1) than the overlying water. Due to the entrainment of lake water, a distinct dense water layer up to 40 m thick, stabilised by its slightly higher salinity, becomes established close to the bottom of the bay. The density current thus generated flows out of the bay towards the deeper parts of the basin. Since helium isotope analysis shows that the geochemical characteristics of the hydrothermal water are similar to those of water from nearby hot springs on land, the bottomwater of Frolikha Bay is easily interpreted in terms of the mixing of ordinary fresh water from the lake and hydrothermal water carrying isotopically heavy He from the continental crust. Because of its high crustal He content, a similar hydrothermal component may even be identified in the open water of the northern basin.

Thermal Stratification in Lakes

EARLY in this century, Thienemann1 and Naumann2 described the physical conditions in deep freshwater lakes in the temperate zone. They found a yearly cycle of events which depended on the thermal stratification of the water. Their findings, and the conclusions they drew from them, can be summarized briefly, as follows. In winter and early spring, the temperature and density of the water mass is almost uniform from the surface of the lake to the bottom, so that the viscosity of the water is the only hindrance to mixing by wind. In late spring and early summer this state of affairs changes because the surface layers become warmed up by increased radiation from the sun and conduction from a warmer atmosphere, and by the inflow of warmer rivers. The result of this warming is that, in summer, a warm upper layer of less dense water, the epilimnion, comes to lie over a cold deeper water mass, the hypolimnion. The two are separated by a region of very steep vertical temperature gradient, the thermocline, with the result that mixing by wind is impossible.