708 research outputs found
Cascading water underneath Wilkes Land, East Antarctic ice sheet, observed using altimetry and digital elevation models
We describe a major subglacial lake drainage close to the ice divide in
Wilkes Land, East Antarctica, and the subsequent cascading of water
underneath the ice sheet toward the coast. To analyse the event, we combined
altimetry data from several sources and subglacial topography. We estimated
the total volume of water that drained from Lake Cook<sub>E2</sub> by differencing
digital elevation models (DEM) derived from ASTER and SPOT5 stereo imagery
acquired in January 2006 and February 2012. At 5.2 ± 1.5 km<sup>3</sup>, this
is the largest single subglacial drainage event reported so far in
Antarctica. Elevation differences between ICESat laser altimetry spanning
2003â2009 and the SPOT5 DEM indicate that the discharge started in November
2006 and lasted approximately 2 years. A 13 m uplift of the surface,
corresponding to a refilling of about 0.6 ± 0.3 km<sup>3</sup>, was observed
between the end of the discharge in October 2008 and February 2012. Using
the 35-day temporal resolution of Envisat radar altimetry, we monitored the
subsequent filling and drainage of connected subglacial lakes located
downstream of Cook<sub>E2</sub>. The total volume of water traveling within the
theoretical 500-km-long flow paths computed with the BEDMAP2 data set is
similar to the volume that drained from Lake Cook<sub>E2</sub>, and our
observations suggest that most of the water released from Lake Cook<sub>E2</sub>
did not reach the coast but remained trapped underneath the ice sheet. Our
study illustrates how combining multiple remote sensing techniques allows
monitoring of the timing and magnitude of subglacial water flow beneath the
East Antarctic ice sheet
Propagation of social representations
Based on a minimal formalism of social representations as a set of associated cognems, a simple model of propagation of representations is presented. Assuming that subjects share the constitutive cognems, the model proposes that mere focused attention on the set of cognems in the field of common conscience may replicate the pattern of representation from context into subjects, or, from subject to subject, through actualization by language, where cognems are represented by verbal signs. Limits of the model are discussed, and evolutionist perspectives are presented with the support of field data
Dynamic topography of passive continental margins and their hinterlands since the Cretaceous
Even though it is well accepted that the Earth\u27s surface topography has been affected by mantle-convection induced dynamic topography, its magnitude and time-dependence remain controversial. The dynamic influence to topographic change along continental margins is particularly difficult to unravel, because their stratigraphic record is dominated by tectonic subsidence caused by rifting. We follow a three-fold approach to estimate dynamic topographic change along passive margins based on a set of seven global mantle convection models. We first demonstrate that a geodynamic forward model that includes adiabatic and viscous heating in addition to internal heating from radiogenic sources, and a mantle viscosity profile with a gradual increase in viscosity below the mantle transition zone, provides a greatly improved match to the spectral range of residual topography end-members as compared with previous models at very long wavelengths (spherical degrees 2-3). We then combine global sea level estimates with predicted surface dynamic topography to evaluate the match between predicted continental flooding patterns and published paleo-coastlines by comparing predicted versus geologically reconstructed land fractions and spatial overlaps of flooded regions for individual continents since 140 Ma. Modelled versus geologically reconstructed land fractions match within 10% for most models, and the spatial overlaps of inundated regions are mostly between 85% and 100% for the Cenozoic, dropping to about 75-100% in the Cretaceous. Regions that have been strongly affected by mantle plumes are generally not captured well in our models, as plumes are suppressed in most of them, and our models with dynamically evolving plumes do not replicate the location and timing of observed plume products. We categorise the evolution of modelled dynamic topography in both continental interiors and along passive margins using cluster analysis to investigate how clusters of similar dynamic topography time series are distributed spatially. A subdivision of four clusters is found to best reveal end-members of dynamic topography evolution along passive margins and their hinterlands, differentiating topographic stability, long-term pronounced subsidence, initial stability over a dynamic high followed by moderate subsidence and regions that are relatively proximal to subduction zones with varied dynamic topography histories. Along passive continental margins the most commonly observed process is a gradual motion from dynamic highs towards lows during the fragmentation of Pangea, reflecting the location of many passive margins now over slabs sinking in the lower mantle. Our best-fit model results in up to 500 (± 150) m of total dynamic subsidence of continental interiors while along passive margins the maximum predicted dynamic topographic change over 140 million years is about 350 (± 150) m of subsidence. Models with plumes exhibit clusters of transient passive margin uplift of about 200 ± 200 m, but are mainly characterised by long-term subsidence of up to 400 m. The good overall match between predicted dynamic topography to geologically mapped paleo-coastlines makes a convincing case that mantle-driven topographic change is a critical component of relative sea level change, and indeed the main driving force for generating the observed geometries and timings of large-scale continental inundation through time
Circum-Arctic mantle structure and long-wavelength topography since the Jurassic
The circum-Arctic is one of the most tectonically complex regions of the world, shaped by a history of ocean basin opening and closure since the Early Jurassic. The region is characterized by contemporaneous large-scale Cenozoic exhumation extending from Alaska to the Atlantic, but its driving force is unknown. We show that the mantle flow associated with subducted slabs of the South Anuyi, Mongol-Okhotsk, and Panthalassa oceans have imparted long-wavelength deflection on overriding plates. We identify the Jurassic-Cretaceous South Anuyi slab under present-day Greenland in seismic tomography and numerical mantle flow models. Under North America, we propose the âFarallonâ slab results from Andean-style ocean-continent convergence around ~30°N and from a combination of ocean-continent and intraoceanic subduction north of 50°N. We compute circum-Arctic dynamic topography through time from subduction-driven convection models and find that slabs have imparted on average <1â16âm/Myr of dynamic subsidence across the region from at least 170âMa to ~50âMa. With the exception of Siberia, the main phase of circum-Arctic dynamic subsidence has been followed either by slowed subsidence or by uplift of <1â6âm/Myr on average to present day. Comparing these results to geological inferences suggest that subduction-driven dynamic topography can account for rapid Middle to Late Jurassic subsidence in the Slave Craton and North Slope (respectively, <15 and 21âm/Myr, between 170 and 130âMa) and for dynamic subsidence (<7âm/Myr, ~170â50âMa) followed by dynamic uplift (<6âm/Myr since 50âMa) of the Barents Sea region. Combining detailed kinematic reconstructions with geodynamic modeling and key geological observations constitutes a powerful tool to investigate the origin of vertical motion in remote regions
Special Libraries, May 1916
Volume 7, Issue 5https://scholarworks.sjsu.edu/sla_sl_1916/1004/thumbnail.jp
Parallel flow in Hele-Shaw cells with ferrofluids
Parallel flow in a Hele-Shaw cell occurs when two immiscible liquids flow
with relative velocity parallel to the interface between them. The interface is
unstable due to a Kelvin-Helmholtz type of instability in which fluid flow
couples with inertial effects to cause an initial small perturbation to grow.
Large amplitude disturbances form stable solitons. We consider the effects of
applied magnetic fields when one of the two fluids is a ferrofluid. The
dispersion relation governing mode growth is modified so that the magnetic
field can destabilize the interface even in the absence of inertial effects.
However, the magnetic field does not affect the speed of wave propagation for a
given wavenumber. We note that the magnetic field creates an effective
interaction between the solitons.Comment: 12 pages, Revtex, 2 figures, revised version (minor changes
Elongation of confined ferrofluid droplets under applied fields
Ferrofluids are strongly paramagnetic liquids. We study the behavior of
ferrofluid droplets confined between two parallel plates with a weak applied
field parallel to the plates. The droplets elongate under the applied field to
reduce their demagnetizing energy and reach an equilibrium shape where the
magnetic forces balance against the surface tension. This elongation varies
logarithmically with aspect ratio of droplet thickness to its original radius,
in contrast to the behavior of unconfined droplets. Experimental studies of a
ferrofluid/water/surfactant emulsion confirm this prediction.Comment: 12 pages, 7 figures. Submitted to Phys. Rev.
Rotating Hele-Shaw cells with ferrofluids
We investigate the flow of two immiscible, viscous fluids in a rotating
Hele-Shaw cell, when one of the fluids is a ferrofluid and an external magnetic
field is applied. The interplay between centrifugal and magnetic forces in
determining the instability of the fluid-fluid interface is analyzed. The
linear stability analysis of the problem shows that a non-uniform, azimuthal
magnetic field, applied tangential to the cell, tends to stabilize the
interface. We verify that maximum growth rate selection of initial patterns is
influenced by the applied field, which tends to decrease the number of
interface ripples. We contrast these results with the situation in which a
uniform magnetic field is applied normally to the plane defined by the rotating
Hele-Shaw cell.Comment: 12 pages, 3 ps figures, RevTe
Phase transitions in a ferrofluid at magnetic field induced microphase separation
In the presence of a magnetic field applied perpendicular to a thin sample
layer, a suspension of magnetic colloidal particles (ferrofluid) can form
spatially modulated phases with a characteristic length determined by the
competition between dipolar forces and short-range forces opposing density
variations. We introduce models for thin-film ferrofluids in which
magnetization and particle density are viewed as independent variables and in
which the non-magnetic properties of the colloidal particles are described
either by a lattice-gas entropy or by the Carnahan-Starling free energy. Our
description is particularly well suited to the low-particle density regions
studied in many experiments. Within mean-field theory, we find isotropic,
hexagonal and stripe phases, separated in general by first-order phase
boundaries.Comment: 12 pages, RevTex, to appear in PR
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