Buoyancy Contrast Research Articles

2D numerical experiments on a plume-fed asthenosphere: Necessary preconditions and implications for geoid and dynamic topography

We explore the conditions necessary for mantle flow to include a plume-fed asthenosphere (PFA) as a key structure within its large-scale flow pattern. Using 2D finite element-based experiments, we examine temperature-dependent rheological effects of ridge accretion, plate cooling, and numerically well-resolved ∼10–30 km-thick asthenosphere dragdown by subducting slabs. We find that an average plume flux ∼1.2 times big as the average slab flux is needed to maintain a persistent PFA. These numerical experiments also demonstrate that, instead of generating dynamic topography on the sea floor, flow-induced dynamic relief due to sub-asthenospheric density anomalies will preferentially form at the buoyancy contrast associated with the base of a buoyant asthenosphere. This mode of dynamic internal relief may contribute significantly to near-surface density anomalies that are associated with Earth's low-order geoid, and local relief at the base of the asthenosphere near plumes, ridges, and trenches that can be imaged in seismic experiments.

Transient Injection of Flow: How Torn and Bent Slabs Induce Unusual Mantle Circulation Patterns Near a Flat Slab

AbstractTorn and bent slabs are usually associated with flat‐slab subduction where the descending plate develops a horizontal geometry beneath the overlying continent. How such slab dynamics modify the surrounding mantle flow and the overriding plate remains enigmatic. Here, we conduct three‐dimensional subduction numerical experiments to investigate the flat slab to steep‐angle slab transition region and examine the impact of slab geometry changes on mantle flows. The results show that the along‐strike change due to flattening a segment of slab induces oblique flow toward the mantle corner at the transition region where flat slab bends to steep‐subducting slab. Slab tears can occur due to the buoyancy contrast between an oceanic ridge and the surrounding dense oceanic crust and/or presence of weak zones. A vertical tear at the side edge of a flat slab causes toroidal flow around the steep‐angle slab where sub‐slab mantle floods rapidly through the tear. Flow through the tear spreads to all directions including upwelling toward the continental base that may trigger slab and partial melting and thus affect arc magmatism. A horizontal tear that occurs ahead of the flat portion where slab resumes its steep‐angle results in enhanced plate‐motion parallel flow. Notably, the tear‐induced flow behaves as a rapid injection through the slab breach with the peak velocity up to an order of magnitude higher than plate motion, lasting for 1–2 million years after initially tearing. This rapid pulse flow might be recorded in surface tectonics as distinct transient events of topography or metamorphism.

Initiation of Subduction Along Oceanic Transform Faults: Insights From Three-Dimensional Analog Modeling Experiments

Three-dimensional analogue experiments are employed to explore how self-sustaining subduction may initiate along an oceanic transform fault. The models include a realistic spatial distribution of plate, thickness, strength and buoyancy of the lithosphere near an oceanic transform fault characteristic of the spreading rate. Convergence is imposed across the transform fault and strain in the model lithosphere is quantified using a surface Particle Imaging Velocimetry system. A force sensor is employed to defined when a self-sustaining subduction regime is generated. Cylindrical experiments reveal that subduction polarity is controlled by the buoyancy gradient and the strengths of the plates. With no inclined weak zones, imposed orthogonal compression results in the nucleation of a new fault in the weakest plate leading to the young and positively buoyant plate subducting. However, with an inclined weak zone,the buoyancy contrast controls subduction polarity with the most negatively buoyant plate subducting and a self-sustaining subduction regime obtained after $\sim$300\,km of imposed shortening. This situation is obtained when including an inverted triangular weak zone on top of the transform fault associated with the serpentinization of the crust and mantle. In non-cylindrical experiments, taking into account the change along strike of plate strength and buoyancy, the capacity of the transform fault to generate a self-sustaining subduction regime is greatly reduced. Subduction initiates simultaneously with opposite polarity at the two extremities of the transform segment and, at depth, a lithospheric tear is produced that separates the two subducting slabs. In the center of the transform fault, the lack of buoyancy or strength contrast between the two plates leads to multiple thrusts with variable polarities, overlapping each other, and each accommodating too little shortening to become the new plate boundary. This indicates that additional mechanical work is required in the center of the transform fault which prevents the establishment of a self-sustaining subduction regime.

The compressibility of CaCO3–Li2CO3–Na2CO3–K2CO3 liquids: application to natrocarbonatite and CO2-bearing nephelinite liquids from Oldoinyo Lengai

To constrain the compressibility of natrocarbonate liquids, sound-speed measurements were made on 11 liquids in the CaCO3–Li2CO3–Na2CO3–K2CO3 quaternary system from 808 to 1323 K at 1 bar with a frequency-sweep acoustic interferometer. CaCO3 concentrations range from 15 to 50 mol% in four of the experimental liquids. The sound-speed data for all liquids were converted to isothermal compressibility (β T ), which were fit to an ideal mixing model with respect to composition; the average residual is 1.2 %. Fitted values (±1σ) of the partial molar compressibility (10−2 GPa−1) at 1100 K were derived for CaCO3 (5.36 ± 0.13), Li2CO3 (8.09 ± 0.06), Na2CO3 (10.62 ± 0.07), and K2CO3 (14.09 ± 0.06); these values translate to bulk modulus values of 18.7, 12.4, 9.4, and 7.1 GPa, respectively, reflecting the relatively large compressibility of carbonate liquids. The data are additionally used to estimate the partial molar volume and compressibility of the CO2 component dissolved as carbonate in nephelinite liquids; the density of this dissolved component at 1423 K and 1 GPa ranges from 1.62, 1.71 to 1.83 g/cm3 when it is complexed with K+, Na+, and Ca2+, respectively, and is estimated to be ~2.05 and 2.14 g/cm3 when complexed with Fe2+ and Mg2+, respectively. The results from this study can be applied to natrocarbonate liquids, such as those erupted from Oldoinyo Lengai volcano in Africa, and indicate a melt density of ~2.19 g/cm3 at 1150 °C and 1 GPa, which is ~18 % less dense than the average melt density (~2.67 g/cm3) calculated for associated Mg-poor nephelinite liquids at the same conditions and volatile-free. However, the dissolution of 10.1 wt% H2O and 8.7 wt% CO2 in the average nephelinite melt (based on volatile contents reported in the literature for these magmas) reduces its density to ~2.14 g/cm3 at 1150 °C and 1 GPa, eliminating the buoyancy contrast with the natrocarbonate melt. In turn, it is highly likely that the natrocarbonatite melts contained significant amounts of dissolved H2O and, as an example, the addition of 10 wt% H2O lowers the melt density by ~14 % to ~1.88 g/cm3, and therefore re-establishes a large density contrast with the volatile-rich nephelinite melts.

Development and Application of Eco-friendly Oil Stimulant on Oil Spill Control Training

We developed mimetic materials for oil spill drills of the Korean Coast Guard. Two selection criteria were adopted: physical properties and eco-friendliness. To make the drill effective, a mimetic substance candidate must have similar physical properties to crude oil, in terms of buoyancy, transportability, and significant color contrast. To minimize the possible impact on the marine environment in general and marine industry in particular, it must also be eco-friendly and non-toxic, as a major portion tends to be evaded from cleaning/capturing devices. Considering these, therefore, we selected natural oils extracted from marine biomass and presented results obtained from lab evaluations and field tests.

Craton stability and longevity: The roles of composition-dependent rheology and buoyancy

Survival of thick cratonic roots in a vigorously convecting mantle system for billions of years has long been studied by the geodynamical community. High strength of the cratonic root is generally considered to be the most important factor, but the role of lithospheric mantle depletion and dehydration in this strengthening is still debated. Geodynamical models often argue for a significant strength or buoyancy contrast between cratonic and non-cratonic mantle lithosphere, induced by mantle depletion and dehydration. But recent laboratory experiments argue for only a modest effect of dehydration strengthening. Can we reconcile laboratory experiments and geodynamical models?We perform and discuss new numerical models to investigate craton stability and longevity with different composition-dependent rheology and buoyancy. Our results show that highly viscous and possibly buoyant cratonic root is essential to stabilise a geometry in which thick cratonic lithosphere and thinner non-cratonic lithosphere coexist for billions of years. Using non-Newtonian rheology, a modest strengthening factor of Δη=3 can protect compositionally buoyant cratonic roots from erosion by mantle convection for over billions of years. A larger strengthening factor (Δη=10) can maintain long term craton stability even with little or no intrinsic buoyancy. Such composition-dependent rheology is comparable to the laboratory experiments. This implies that a strict isopycnic state of cratonic lithosphere may not be necessary for the preservation of a cratonic root, provided a sufficient level of compositional strengthening is present.

On ventilation of a heated room through a single doorway

Laboratory experiments are presented to examine the transient filling of a room of buoyant fluid when a doorway connects the room to a large reservoir of dense fluid. These experiments confirm that the filling time of the room scales as ( A/ wH)( H/ g′) 1/2 where H is the doorway height, w the doorway width, A the floor area of the room, and g′ the buoyancy contrast between the interior and exterior of the room. A model of the transient exchange flow is presented and shown to be in accordance with the experimental results. The more complex regime in which a source of buoyancy is also supplied to the room is then examined. It is found that a steady two-layer stratification becomes established, with the height of the interface, h i, dependent only on the aspect ratio of the doorway and the door height, h i= Hf 1( w/ H), while the buoyancy contrast between the two layers, g′, depends on both the buoyancy flux supplied to the room and the doorway aspect ratio g′= B 2/3 H −5/3 f 2( w/ H). The results imply that, in typical buildings, exchange flows require 5– 100 min to fill a room once a door has been opened. Also, when a heated room is connected to a cold exterior by a doorway, a layer of air 5–15°C warmer than the exterior may be maintained above a cold layer near the floor. The depth of this cold layer depends on the aspect ratio of the doorway, but may typically extend over one-half the height of the door. In steady state, all the heat supplied to the room is lost through the doorway.

Initiation of Subduction Zones as a Consequence of Lateral Compositional Buoyancy Contrast within the Lithosphere: a Petrological Perspective

Tonga and Mariana fore-arc peridotites, inferred to represent their respective sub-arc mantle lithospheres, are compositionally highly depleted (low Fe/Mg) and thus physically buoyant relative to abyssal peridotites representing normal oceanic lithosphere (high Fe/Mg) formed at ocean ridges. The observation that the depletion of these fore-arc lithospheres is unrelated to, and pre-dates, the inception of present-day western Pacific subduction zones demonstrates the pre-existence of compositional buoyancy contrast at the sites of these subduction zones. These observations allow us to suggest that lateral compositional buoyancy contrast within the oceanic lithosphere creates the favoured and necessary condition for subduction initiation. Edges of buoyant oceanic plateaux, for example, mark a compositional buoyancy contrast within the oceanic lithosphere. These edges under deviatoric compression (e.g. ridge push) could develop reverse faults with combined forces in excess of the oceanic lithosphere strength, allowing the dense normal oceanic lithosphere to sink into the asthenosphere beneath the buoyant overriding oceanic plateaux, i.e. the initiation of subduction zones. We term this concept the ‘oceanic plateau model’. This model explains many other observations and offers testable hypotheses on important geodynamic problems on a global scale. These include (1) the origin of the 43Ma bend along the Hawaii‐ Emperor Seamount Chain in the Pacific, (2) mechanisms of ophiolite emplacement, (3) continental accretion, etc. Subduction initiation is not unique to oceanic plateaux, but the plateau model well illustrates the importance of the compositional buoyancy contrast within the lithosphere for subduction initiation. Most portions of passive continental margins, such as in the Atlantic where large compositional buoyancy contrast exists, are the loci of future subduction zones.

The Association of Significant Tornadoes with a Baroclinic Boundary on 2 June 1995

On 2 June 1995, the large-scale environment of eastern New Mexico and western Texas was generally favorable for the occurrence of supercells because of the presence of strong deep shear and storm-relative helicity, as well as sufficient convective available potential energy (CAPE). Indeed, many supercells occurred, but the only storms to produce tornadoes were those supercells that crossed, or developed and persisted on the immediate cool side of a particular outflow boundary generated by earlier convection. Surface conditions, vertical vorticity, and horizontal vorticity near this boundary are documented using conventional and special observations from the VORTEX field program. It is shown that the boundary was locally rich in horizontal vorticity, had somewhat enhanced vertical vorticity, and enhanced CAPE. Theoretical arguments indicate that the observed horizontal vorticity (around 1 × 10−2 s−1), largely parallel to the boundary, can be readily produced with the type of buoyancy contrast observed. It is hypothesized that such local enhancement of horizontal vorticity often is required for the occurrence of significant (e.g., F2 or stronger) tornadoes, even in large-scale environments that appear conducive to tornado occurrence without the aid of local influences.

A numerical model for interdecadal variability of sea ice cover in the Greenland-Iceland-Norwegian Sea

A coupled ocean-sea ice-atmosphere model is used to study interdecadal variability (∼40 years) of sea ice depth and concentration in the Greenland-Iceland-Norwegian Sea. This oceanic region is represented by a meridionally aligned channel on a β-plane with open zonal boundaries at 60 °N and 80 °N. The model consists of a one and a half layer reduced gravity ocean model, a thermodynamic/dynamic sea ice model and an energy balance model of the atmosphere. The coupled model is driven by prescribed surface wind stress, fluxes of heat, salt and ice at inflow points on the northern and southern open zonal boundaries and annual distribution of solar radiation. It is shown that the coupled model supports unforced modes of interdecadal oscillation resulting from a form of hydraulic control which regulates the total fluid volume in the oceanic active layer. The mechanism for the oscillations relies on the presence of three key features: (1) a region of intense oceanic entrainment located in the eastern part of the domain, (2) a vigorous southward flowing western boundary current, representing the East Greenland Current (EGC), which supports most of the meridional transport across the domain, and (3) a marked buoyancy contrast between the relatively salty domain interior and the much fresher western boundary region. During an oscillation excess water is pumped into the domain via entrainment, thereby creating an active layer depth anomaly, which then propagates westward via long baroclinic Rossby waves until it reaches the EGC where it is subsequently drained out of the domain across the southern open zonal boundary. As the depth anomaly traverses the basin, an anomalous geostrophic circulation is established in which cold fresh Arctic water enters the domain interior, and this eventually promotes enhanced thermodynamic sea ice growth. Consequently, the interdecadal oscillations of the coupled model are characterised by pulse periods, typically spanning 20 years, during which there is an abnormally large winter sea ice cover, separated by interpulse periods, lasting another 20 years, during which the winter sea ice extent is nearly uniform and significantly smaller than in a pulse maximum. The duration of the interpulse periods is dictated by the time it takes for the Rossby waves to traverse the basin. In addition to the interdecadal oscillation solution, the coupled ocean-sea ice-atmosphere model is found to also have a stable cyclostationary state, with no interannual variability. Stochastic forcing, in the form of randomly specified interannual anomalies of salinity (of maximum amplitude 0.1 ppt) or ice inflow (of maximum amplitude 0.1 Sv) at the northern open zonal boundary, in both cases is capable of driving the model from the cyclostationary state solution to the interdecadal variability one.

The formation of a haline shelf front in wintertime in an ice-covered arctic sea

During some winters, the water overlying Mackenzie shelf in the southeastern Beaufort Sea becomes quite saline (33–35) and at freezing temperature throughout. Although this water is found at the surface, its density is that of waters within the halocline of the arctic basin, and ventilation of the halocline occurs. The formation of sea ice is a necessary, but not a sufficient condition for the production of this water. Observations from the winter of 1980–1981 are presented which illustrate that a two-stage preconditioning of shelf waters is also required: first, surface waters of low salinity must be driven from the shelf by strong westerly winds; then, more saline water must upwell onto the shelf in response to prolonged easterly winds. A haline front forms on the outer shelf separating cold, saline shelf waters from slightly warmer, less saline slope waters. The characteristics of this front are controlled by the input of negative buoyancy from ice growth over the shelf, and by turbulent entrainment and mixing driven by under-ice convection. A simple model to predict the position, dimension and buoyancy contrast of this front is presented. The cross-isobath circulation is examined and found to be relatively ineffective at flushing dense waters from the shelf over a winter. Thus shelf waters accumulate almost all the salt expelled from growing sea ice over the same time. The magnitude of the contribution by this shelf to renewal of waters in the arctic halocline is estimated to average 0.04 × 10 6m 3s −1 over a period of years. Although small relative to the overall rate of renewal (1 × 10 6m 3s −1), this contribution is in proportion to the fraction of the arctic shelf area which this region represents.

The influence of high‐latitude surface forcing on the global thermohaline circulation

The basin‐averaged, latitude‐depth ocean model of Wright and Stocker (1992) is used to simulate the deep circulation of the world ocean. Under present‐day surface forcing, sinking occurs in the North Atlantic and the southern ocean, and realistic temperature and salinity structures are obtained in the Atlantic, Pacific, and Indian oceans. “Color” tracers and radiocarbon are used to identify the composition of the deepwater masses and the associated renewal time scales. While broad agreement with observations is found in all basins, the water masses in the southern ocean are too young. The global thermohaline circulation and the composition of the deepwater masses are sensitive to the buoyancy contrast between the southern ocean and the North Atlantic. This contrast can be modified by changing relaxation values of temperature and salinity at the northern and southern high latitudes. If the model is forced with the zonal averages of the observed surface salinity, North Atlantic Deep Water is the dominant deep ocean water mass, and hardly any Antarctic Bottom Water flows into the Atlantic. Choosing instead the observed salinities of the newly formed deep water as the restoring values, the model realistically simulates the penetration of Antarctic Bottom Water into the different ocean basins. This has a global effect through reducing both strength and depth of North Atlantic Deep Water formation. If higher surface salinity values are applied in the southern ocean, a steady state is obtained whose tracer distributions and overturning are consistent with reconstructions of the deep circulation during the last glacial maximum. The two states are stable also under mixed boundary conditions and transitions are possible by smoothly varying the surface freshwater flux of one state to that of the other. These experiments suggest the importance of modified high‐latitude forcing in glacial‐to‐interglacial transitions.

Significance of anisotropy of magnetic susceptibility fabrics in Proterozoic mafic dykes, Hopedale Block, Labrador

Anisotropy of magnetic susceptibility fabrics have been studied in a suite of mafic early Proterozoic (Kikkertavak) dykes and two mid-Proterozoic (Kokkorvik) sills in the Hopedale Block of Labrador. Two sets of en-echelon dyke arrays from the Kikkertavak Swarm are considered to represent two linear dyke intrusions at depth. Three main fabrics were revealed in the Kikkertavak dykes. In the first, the maximum susceptibility axis is sub-horizontal or gently inclined and parallel to the dyke walls and is interpreted as the primary magma flow direction. A second fabric affects only dykes along strike from a much younger dyke and is interpreted as the result of local stress anisotropy. The third fabric with significantly lower susceptibility is only locally developed in the southeast, near the mid-Proterozoic Kanairiktok Shear Zone and is also interpreted as the result of local superimposed tectonic stress. The Kokkorvik sheets preserve a single flow-induced fabric, sub-parallel to the measured crack propagation lineation, indicating that the magma flow was parallel to the crack propagation direction. The gently inclined magma flow direction inferred for the Kikkertavak dykes is believed to reflect later-stage magmatic emplacement when consolidation had already commenced. At this stage, the buoyancy contrast between magma and host rock had been reduced to the point when the initially vertical magma flow was no longer sustainable and was replaced by lateral flow.

Flexure of the continental lithosphere beneath Apennine and Carpathian foredeep basins

The preservation of foredeep basins after an associated mountain belt has been greatly reduced by erosion suggests that an additional load must act on the subducted slab's lithosphere independently of topography. This hypothesis is presently tested for the case of young orogenic belts, through the construction of transects through the Apennine and outer Carpathian thrust belts of the Mediterranean region. A simple elastic model is used to show that the topographic loads present along the transects are not sufficient to cause the observed plate deflection, suggesting that an additional load or downward force is acting on the plate to generate the plate deflection observed. It is proposed that this extra load is due either to the negative buoyancy contrast between the subducted slab and the asthenosphere, or to the crustal structure's modification by backarc rifling in the hinterland.

The Influence of Wind Stress and River Runoff on a Shelf-Sea Front

Runoff from land gives rise to the formation of a front-jet system separating fresher coastal waters from more saline waters offshore, over some continental shelves, notably in the Mid-Atlantic Bight in winter. Three first-order characteristics of such fronts are the anchor depth H, where the front intersects the bottom, the stretch l, i.e., the distance between its surface and bottom intersections, and the buoyancy contrast b across the front. The three characteristics may be determined from the cross-front property transfer rate, the along-front momentum balance, and the turbulence-energy dissipation rate. Key independent variables are the horizontal buoyancy transport Bx, the friction velocity u*, and direction θ of the wind, and the Coriolis parameters f. Simple approximate formulae connecting dependent to independent variables arewhere m, λ and μ are constants of order 300, 1 and 0.1 respectively. The entrainment rate across the front is μu2*f−1 cosθ, which is small compared to Ekman transport on some continental shelves, but possibly not on all.

The Nanticoke shoreline diffusion experiment, June 1978—II. Internal boundary layer structure

Some observations of the structure of the internal boundary layer as a response to contrasts in shoreline temperature and buoyancy flux are described. The experiment, conducted in the spring of 1978 on the north shore of Lake Erie, near the town of Nanticoke, consisted of the spatial deployment of acoustic sounders, free rising and tethered balloons, surface tower and flux measurements, as well as associated chemistry and diffusion measurements. In this paper, the structure of the internal boundary layer is examined both descriptively and analytically in terms of a similarity format. It is shown that two internal boundary layers can be detected—one to adjust the bulk buoyancy contrast of the (cold) onshore flow to the warm land, the other to adjust the flow for the upward heat flux into the onshore flow during the day. An equilibrium level is achieved in the first case when the buoyant mixing completes the erosion of the local buoyant surplus near the ground thereby stopping the production of buoyant turbulence and hence the entrainment growth of the layer. The second type of layer, which apparently has not been described in the literature, appears to be more significant in defining the location and magnitude of maximum ground level concentrations in a daytime shoreline fumigation situation. Some simple empirical formulations are offered for the entrainment growth of each type of internal boundary layer.