44 research outputs found
Interplay between spatially explicit sediment sourcing, hierarchical river-network structure, and in-channel bed material sediment transport and storage dynamics
Understanding how sediment moves along source to sink pathways through watershedsâfrom hillslopes to channels and in and out of floodplainsâis a fundamental problem in geomorphology. We contribute to advancing this understanding by modeling the transport and in-channel storage dynamics of bed material sediment on a river network over a 600ĂŠyear time period. Specifically, we present spatiotemporal changes in bed sediment thickness along an entire river network to elucidate how river networks organize and process sediment supply. We apply our model to sand transport in the agricultural Greater Blue Earth River Basin in Minnesota. By casting the arrival of sediment to links of the network as a Poisson process, we derive analytically (under supply-limited conditions) the time-averaged probability distribution function of bed sediment thickness for each link of the river network for any spatial distribution of inputs. Under transport-limited conditions, the analytical assumptions of the Poisson arrival process are violated (due to in-channel storage dynamics) where we find large fluctuations and periodicity in the time series of bed sediment thickness. The time series of bed sediment thickness is the result of dynamics on a network in propagating, altering, and amalgamating sediment inputs in sometimes unexpected ways. One key insight gleaned from the model is that there can be a small fraction of reaches with relatively low-transport capacity within a nonequilibrium river network acting as ñbottlenecksĂź that control sediment to downstream reaches, whereby fluctuations in bed elevation can dissociate from signals in sediment supply. ©2017. American Geophysical Union. All Rights Reserved
Controls on Subglacial Rock Friction: Experiments With Debris in Temperate Ice
Glacier sliding has major environmental consequences, but friction caused by debris in the basal ice of glaciers is seldom considered in sliding models. To include such friction, divergent hypotheses for clastâbed contact forces require testing. In experiments we rotate an ice ring (outside diameter = 0.9 m), with and without isolated till clasts, over a smooth rock bed. Ice is kept at its pressureâmelting temperature, and meltwater drains along a film at the bed to atmospheric pressure at its edges. The ice pressure or bedânormal component of ice velocity is controlled, while bed shear stress is measured. Results with debrisâfree ice indicate friction coefficients \u3c 0.01. Shear stresses caused by clasts in ice are independent of ice pressure. This independence indicates that with increases in ice pressure the water pressure in cavities observed beneath clasts increases commensurately to allow drainage of cavities into the melt film, leaving clastâbed contact forces unaffected. Shear stresses, instead, are proportional to bedânormal ice velocity. Cavities and the absence of regelation ice indicate that, unlike model formulations, regelation past clasts does not control contact forces. Alternatively, heat from the bed melts ice above clasts, creating pressure gradients in adjacent meltwater films that cause contact forces to depend on bedânormal ice velocity. This model can account for observations if rock friction predicated on Hertzian clastâbed contacts is assumed. Including debrisâbed friction in glacier sliding models will require coupling the ice velocity field near the bed to contact forces rather than imposing a pressureâbased friction rule
Sensory Stimulation-Dependent Plasticity in the Cerebellar Cortex of Alert Mice
In vitro studies have supported the occurrence of cerebellar long-term depression (LTD), an interaction between the parallel fibers and Purkinje cells (PCs) that requires the combined activation of the parallel and climbing fibers. To demonstrate the existence of LTD in alert animals, we investigated the plasticity of local field potentials (LFPs) evoked by electrical stimulation of the whisker pad. The recorded LFP showed two major negative waves corresponding to trigeminal (broken into the N2 and N3 components) and cortical responses. PC unitary extracellular recording showed that N2 and N3 occurred concurrently with PC evoked simple spikes, followed by an evoked complex spike. Polarity inversion of the N3 component at the PC level and N3 amplitude reduction after electrical stimulation of the parallel fiber volley applied on the surface of the cerebellum 2 ms earlier strongly suggest that N3 was related to the parallel fiberâPC synapse activity. LFP measurements elicited by single whisker pad stimulus were performed before and after trains of electrical stimuli given at a frequency of 8 Hz for 10 min. We demonstrated that during this later situation, the stimulation of the PC by parallel and climbing fibers was reinforced. After 8-Hz stimulation, we observed long-term modifications (lasting at least 30 min) characterized by a specific decrease of the N3 amplitude accompanied by an increase of the N2 and N3 latency peaks. These plastic modifications indicated the existence of cerebellar LTD in alert animals involving both timing and synaptic modulations. These results corroborate the idea that LTD may underlie basic physiological functions related to calcium-dependent synaptic plasticity in the cerebellum
Seas under ice: stability of liquid-water oceans within icy worlds
The present-day existence of internal oceans under the outer ice shell of several icy satellites of
the Solar System has been recently proposed. The presence of antifreeze substances decreasing iceâs
melting point (and tidal heating in Europaâs case) has been generally believed to allow the stability of such
oceans; limited cooling of the water (ice plus liquid) layer, due to stability against convection or to
stagnant lid convection in the icy shell, have been also considered. Here we propose that even pure liquidwater
oceans could survive today within several icy worlds, and we consider some factors affecting thermal
modeling in these bodies. So, the existence of such oceans would be a natural consequence of the physical
properties of water ice, independently from the addition of antifreeze substances or any other special
conditions. The inclusion of these substances would contribute to expand the conditions for water to stay
liquid and to increase oceanâs volume