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  • 1
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    Cordilleran Ice Sheet mass loss preceded climate reversals near the Pleistocene Termination (2017)
    American Association for the Advancement of Science (AAAS)
    In: Science
    Details
    Publication Date: 2017-11-10
    Description: The Cordilleran Ice Sheet (CIS) once covered an area comparable to that of Greenland. Previous geologic evidence and numerical models indicate that the ice sheet covered much of westernmost Canada as late as 12.5 thousand years ago (ka). New data indicate that substantial areas throughout westernmost Canada were ice free prior to 12.5 ka and some as early as 14.0 ka, with implications for climate dynamics and the timing of meltwater discharge to the Pacific and Arctic oceans. Early Bølling-Allerød warmth halved the mass of the CIS in as little as 500 years, causing 2.5 to 3.0 meters of sea-level rise. Dozens of cirque and valley glaciers, along with the southern margin of the CIS, advanced into recently deglaciated regions during the Bølling-Allerød and Younger Dryas.
    Keywords: Atmospheric Science
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Geosciences , Computer Science , Medicine , Natural Sciences in General , Physics
    Published by American Association for the Advancement of Science (AAAS)
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  • 2
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    Statistics of bedload transport over steep slopes: Separation of time scales and collective motion (2013)
    Wiley
    Details
    Publication Date: 2013-01-18
    Description: [1]  Steep slope streams show large fluctuations of sediment discharge across several time scales. These fluctuations may be inherent to the internal dynamics of the sediment transport process. A probabilistic framework thus seems appropriate to analyze such a process. In this paper, we present an experimental study of bedload transport over a steep slope flume for small to moderate Shields numbers. The sampling technique allows the acquisition of high-resolution time series of the solid discharge. The resolved time scales range from 10 − 1 s up to 10 5 s. We show that two distinct time scales can be observed in the probability density function for the waiting time between moving particles. We make the point that the separation of time scales is related to collective dynamics. Proper statistics of a Markov process including collective entrainment are derived. The separation of time scales is recovered theoretically for low entrainment rates.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 3
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    Stochastic interpretation of the advection-diffusion equation and its relevance to bed load transport (2015)
    Wiley
    Details
    Publication Date: 2015-11-07
    Description: The advection-diffusion equation is one of the most widespread equations in physics. It arises quite often in the context of sediment transport, e.g., for describing time and space variations in the particle activity (the solid volume of particles in motion per unit streambed area). Phenomenological laws are usually sufficient to derive this equation and interpret its terms. Stochastic models can also be used to derive it, with the significant advantage that they provide information on the statistical properties of particle activity. These models are quite useful when sediment transport exhibits large fluctuations (typically at low transport rates), making the measurement of mean values difficult. Among these stochastic models, the most common approach consists of random walk models. For instance, they have been used to model the random displacement of tracers in rivers. Here we explore an alternative approach, which involves monitoring the evolution of the number of particles moving within an array of cells of finite length. Birth-death Markov processes are well-suited to this objective. While the topic has been explored in detail for diffusion-reaction systems, the treatment of advection has received no attention. We therefore look into the possibility of deriving the advection-diffusion equation (with a source term) within the framework of birth-death Markov processes. We show that in the continuum limit (when the cell size becomes vanishingly small), we can derive an advection-diffusion equation for particle activity. Yet, while this derivation is formally valid in the continuum limit, it runs into difficulty in practical applications involving cells or meshes of finite length. Indeed, within our stochastic framework, particle advection produces nonlocal effects, which are more or less significant depending on the cell size and particle velocity. Albeit nonlocal, these effects look like (local) diffusion and add to the intrinsic particle diffusion (dispersal due to velocity fluctuations), with the important consequence that local measurements depend on both the intrinsic properties of particle displacement and the dimensions of the measurement system.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 4
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    ERF115 controls root quiescent center cell division and stem cell replenishment (2013)
    American Association for the Advancement of Science (AAAS)
    Details
    Publication Date: 2013-10-26
    Description: The quiescent center (QC) plays an essential role during root development by creating a microenvironment that preserves the stem cell fate of its surrounding cells. Despite being surrounded by highly mitotic active cells, QC cells self-renew at a low proliferation rate. Here, we identified the ERF115 transcription factor as a rate-limiting factor of QC cell division, acting as a transcriptional activator of the phytosulfokine PSK5 peptide hormone. ERF115 marks QC cell division but is restrained through proteolysis by the APC/C(CCS52A2) ubiquitin ligase, whereas QC proliferation is driven by brassinosteroid-dependent ERF115 expression. Together, these two antagonistic mechanisms delimit ERF115 activity, which is called upon when surrounding stem cells are damaged, revealing a cell cycle regulatory mechanism accounting for stem cell niche longevity.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Heyman, Jefri -- Cools, Toon -- Vandenbussche, Filip -- Heyndrickx, Ken S -- Van Leene, Jelle -- Vercauteren, Ilse -- Vanderauwera, Sandy -- Vandepoele, Klaas -- De Jaeger, Geert -- Van Der Straeten, Dominique -- De Veylder, Lieven -- New York, N.Y. -- Science. 2013 Nov 15;342(6160):860-3. doi: 10.1126/science.1240667. Epub 2013 Oct 24.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Department of Plant Systems Biology, VIB, B-9052 Gent, Belgium.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/24158907" target="_blank"〉PubMed〈/a〉
    Keywords: Anaphase-Promoting Complex-Cyclosome/metabolism ; Arabidopsis/*cytology/*growth & development ; Arabidopsis Proteins/genetics/*metabolism ; Cell Cycle/genetics/physiology ; Cell Cycle Proteins/metabolism ; Cell Division/genetics/*physiology ; Mitosis/genetics/physiology ; Peptide Hormones/genetics/metabolism ; Plant Roots/*cytology/*growth & development ; Proteolysis ; Signal Transduction ; Stem Cell Niche ; Stem Cells/*physiology ; Transcription Factors/genetics/*metabolism
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Computer Science , Medicine , Natural Sciences in General , Physics
    Published by American Association for the Advancement of Science (AAAS)
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  • 5
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    Inner gorges cut by subglacial meltwater during Fennoscandian ice sheet decay (2014)
    Springer Nature
    Details
    Publication Date: 2014-05-11
    Description: Article Whether the origins of inner gorges were fluvial or subglacial has been debated for decades. Here, Jansen et al . present new evidence, in the form of a suite of cosmogenic nuclide exposure ages and a deglaciation map, which suggests a subglacial meltwater origin for inner gorges in northern Sweden. Nature Communications doi: 10.1038/ncomms4815 Authors: J.D. Jansen, A.T. Codilean, A.P. Stroeven, D. Fabel, C. Hättestrand, J. Kleman, J.M. Harbor, J. Heyman, P.W. Kubik, S. Xu
    Electronic ISSN: 2041-1723
    Topics: Biology , Chemistry and Pharmacology , Natural Sciences in General , Physics
    Published by Springer Nature
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  • 6
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    Spatial correlations in bed load transport: Evidence, importance, and modelling. (2014)
    Wiley
    Details
    Publication Date: 2014-07-23
    Description: This article examines the spatial dynamics of bed load particles in water. We focus particularly on the fluctuations of particle activity, which is defined as the number of moving particles per unit bed length. Based on a stochastic model recently proposed by Ancey and Heyman [2014], we derive the second moment of particle activity analytically; that is the spatial correlation functions of particle activity. From these expressions, we show that large moving particle clusters can develop spatially. Also, we provide evidence that fluctuations of particle activity are scale-dependent. Two characteristic lengths emerge from the model: a saturation length ℓ sat describing the length needed for a perturbation in particle activity to relax to the homogeneous solution, and a correlation length ℓ c describing the typical size of moving particle clusters. A dimensionless Péclet number can also be defined according to the transport model. Three different experimental data sets are used to test the theoretical results. We show that the stochastic model describes spatial patterns of particle activity well at all scales. In particular, we show that ℓ c and ℓ sat may be relatively large compared to typical scales encountered in bed load experiments (grain diameter, water depth, bed form wavelength, flume length…) suggesting that the spatial fluctuations of particle activity have a non-negligible impact on the average transport process.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 7
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    Entrainment, motion and deposition of coarse particles transported by water over a sloping mobile bed (2016)
    Wiley
    Details
    Publication Date: 2016-09-30
    Description: In gravel-bed rivers, bedload transport exhibits considerable variability in time and space. Recently, stochastic bedload transport theories have been developed to address the mechanisms and effects of bedload transport fluctuations. Stochastic models involve parameters such as particle diffusivity, entrainment and deposition rates. The lack of hard information on how these parameters vary with flow conditions is a clear impediment to their application to real-world scenarios. In this paper, we determined the closure equations for the above parameters from laboratory experiments. We focused on shallow supercritical flow on a sloping mobile bed in straight channels, a setting that was representative of flow conditions in mountain rivers. Experiments were run at low sediment transport rates under steady nonuniform flow conditions (i.e., the water discharge was kept constant, but bedforms developed and migrated upstream, making flow nonuniform). Using image processing, we reconstructed particle paths to deduce the particle velocity and its probability distribution, particle diffusivity, and rates of deposition and entrainment. We found that on average, particle acceleration, velocity and deposition rate were responsive to local flow conditions, whereas entrainment rate depended strongly on local bed activity. Particle diffusivity varied linearly with the depth-averaged flow velocity. The empirical probability distribution of particle velocity was well approximated by a Gaussian distribution when all particle positions were considered together. In contrast, the particles located in close vicinity to the bed had exponentially distributed velocities. Our experimental results provide closure equations for stochastic or deterministic bedload transport models.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 8
    Electronic Resource
    Electronic Resource
    Far-infrared pump-probe measurements of the intersubband lifetime in an AlGaAs/GaAs coupled-quantum well (1996)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 68 (1996), S. 3019-3021 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We report pump-and-probe measurements of the electron intersubband lifetime (T1) in an AlGaAs/GaAs heterostructure using a picosecond pulsed far-infrared laser. The subband spacing (11 meV) is less than the optical-phonon energy. Time-resolved measurements yield intersubband lifetimes ranging from T1=1.1±0.2 ns to T1=0.4±0.1 ns depending on measurement conditions. Results are in agreement with previous lifetime measurements on the same sample using continuous excitation at intensities ≤1 W/cm2. The steady-state measurements yielded shorter lifetimes at high excitation intensities, possibly due to carrier heating leading to intersubband scattering by optical phonon emission. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.116683
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  • 9
    Electronic Resource
    Electronic Resource
    Mid-infrared electroluminescence in GaAs/AlGaAs structures (1997)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 71 (1997), S. 2892-2894 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Design, growth, and operation of an unipolar light emitting diode based on the material system GaAs/AlGaAs is reported. We present mid-infrared transmission, photocurrent, and electroluminescence measurements on a quantum cascade structure with intersubband transition energies greater than the optical phonon energy. Electroluminescence powers up to a few nanowatts at 6.9 μm have been measured. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.119329
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  • 10
    Electronic Resource
    Electronic Resource
    Probing terahertz electron dynamics in semiconductor nanostructures with the UC Santa Barbara FELs
    Amsterdam : Elsevier
    Nuclear Instruments and Methods in Physics Research Section A: 341 (1994), S. 169-173 
    Details
    ISSN: 0168-9002
    Source: Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics: Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1016/0168-9002(94)90341-7
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