Highly variable friction and slip observed at Antarctic ice stream bed

The slip of glaciers over the underlying bed is the dominant mechanism governing the migration of ice from land into the oceans, with accelerating slip contributing to sea-level rise. Yet glacier slip remains poorly understood, and observational constraints are sparse. Here we use passive seismic observations to measure both frictional shear stress and slip at the bed of the Rutford Ice Stream in Antarctica using 100,000 repetitive stick-slip icequakes. We find that basal shear stresses and slip rates vary from 104 to 107 Pa and 0.2 to 1.5 m per day, respectively. Friction and slip vary temporally over the order of hours, and spatially over 10s of metres, due to corresponding variations in effective normal stress and ice–bed interface material. Our findings suggest that the bed is substantially more complex than currently assumed in ice stream models and that basal effective normal stresses may be significantly higher than previously thought. Our observations can provide constraints on the basal boundary conditions for ice-dynamics models. This is critical for constraining the primary contribution of ice mass loss in Antarctica and hence for reducing uncertainty in sea-level rise projections

Elastic p-3He and n-3H scattering with two- and three-body forces

We report on a microscopic calculation of n-3H and p-3He scattering employing the Argonne v_{18} and v_8' nucleon-nucleon potentials with and without additional three-nucleon force. An R-matrix analysis of the p-3He and n-3H scattering data is presented. Comparisons are made for the phase shifts and a selection of measurements in both scattering systems. Differences between our calculation and the R-matrix results or the experimental data can be attributed to only two partial waves (3P0 and 3P2). We find the effect of the Urbana IX and the Texas-Los Alamos three-nucleon forces on the phase shifts to be negligible.Comment: submitted to Phys. Rev.

A mathematical framework for critical transitions: normal forms, variance and applications

Critical transitions occur in a wide variety of applications including mathematical biology, climate change, human physiology and economics. Therefore it is highly desirable to find early-warning signs. We show that it is possible to classify critical transitions by using bifurcation theory and normal forms in the singular limit. Based on this elementary classification, we analyze stochastic fluctuations and calculate scaling laws of the variance of stochastic sample paths near critical transitions for fast subsystem bifurcations up to codimension two. The theory is applied to several models: the Stommel-Cessi box model for the thermohaline circulation from geoscience, an epidemic-spreading model on an adaptive network, an activator-inhibitor switch from systems biology, a predator-prey system from ecology and to the Euler buckling problem from classical mechanics. For the Stommel-Cessi model we compare different detrending techniques to calculate early-warning signs. In the epidemics model we show that link densities could be better variables for prediction than population densities. The activator-inhibitor switch demonstrates effects in three time-scale systems and points out that excitable cells and molecular units have information for subthreshold prediction. In the predator-prey model explosive population growth near a codimension two bifurcation is investigated and we show that early-warnings from normal forms can be misleading in this context. In the biomechanical model we demonstrate that early-warning signs for buckling depend crucially on the control strategy near the instability which illustrates the effect of multiplicative noise.Comment: minor corrections to previous versio

Modern theories of low-energy astrophysical reactions

We summarize recent ab initio studies of low-energy electroweak reactions of astrophysical interest, relevant for both big bang nucleosynthesis and solar neutrino production. The calculational methods include direct integration for np radiative and pp weak capture, correlated hyperspherical harmonics for reactions of A=3,4 nuclei, and variational Monte Carlo for A=6,7 nuclei. Realistic nucleon-nucleon and three-nucleon interactions and consistent current operators are used as input.Comment: 29 pages, 4 figure

FIRN DENSIFICATION BY GRAIN-BOUNDARY SLIDING : A FIRST MODEL

La densification de névés très poreux à température constante se produit d'abord par glissement Newtonien sur les joints de grains, mais le nombre de coordinance croît avec la densité et limite le glissement ultérieur. Pour une densité relative d'environ 0,6 le nombre de coordinance est de l'ordre de 6 et le glissement n'est plus alors le mécanisme principal de densification ; la diminution de la vitesse de densification conduit à l'observation d'un point critique dans les profils densité/profondeur. Un modèle simple pour la densification par glissement aux joints conduit à une bonne correspondance avec les profils observés. La viscosité ainsi obtenue donne une énergie d'activation égale à celle de la diffusion des joints de grains.Densification in highly porous, isothermal firn occurs primarily by Newtonian sliding on grain boundaries, but the coordination number increases with density and restricts further sliding. At a relative density of about 0.6 the coordination number approaches 6 and sliding ceases to be a primary mechanism of densification ; the resulting decrease in densification rate causes the critical point in depth-density profiles. A simple model for densification by boundary sliding yields a good fit to observed profiles. The viscosities so obtained give an activation energy equal to that for grain-boundary diffusion

RHEOLOGY OF GLACIER ICE

The constitutive relation for glacier ice remains an issue in glaciology. This is evidenced by the recent appearance of several articles in the literature that report on interpretations of existing data and which draw conclusions ranging from newtonian viscous to power law creep for polycrystalline ice. In this paper we describe the results of a new analysis based on the height of bottom crevasses found in floating ice shelves. The analysis relates the effective stress in the glacier to the height of the crevasse. The power of this approach is that the computed stress takes into account all factors influencing the deformation of the ice shelf including ice rises and shear along the boundaries of the ice shelf. By comparing calculated stresses to measured surface strain rates, we are able to estimate the exponent in the flow law and the flow law constant. We find that strain rate increases as the third power of the deviatoric stress with a constant of proportionality equal to 2.3 x 10-25

GRAIN GROWTH IN UNSTRAINED GLACIAL ICE

We use theories for grain growth driven by surface tension and curvature of grain boundaries to explain published data on grain growth in cold (⩽-10°C) glacial ice that is not deforming rapidly. Among other results, we propose that small grain sizes in Wisconsinan ice are caused by large concentrations of soluble impurities. Major observations that we seek to explain are Gow, (1), (2) ; Gow and Williamson, (3) ; Duval and Lorius, (4) ; and Alley et al., (5) (i) in most post-Wisconsinan ice and firn, the average cross-sectional area of grains increases linearly with time. The rate of increase is nearly the same in firn and ice, although it may be slightly less in ice ; (ii) Grain sizes are smaller in ice rich in volcanic tephra than in adjacent, clean ice ; (iii) Bubbles, which form on grain boundaries at the firn-ice transition, separate from grain boundaries in a discrete depth interval below the transition ; and (iv) Grain size decreases downward across the firn-ice transition. When grain growth is driven by grain-boundary curvature and surface tension, grain-growth theory (reviewed by Higgins, (6)) predicts that the average cross-sectional area of grains in pure, fully consolidated materials will increase linearly with time. Bubbles, inert second-phase particles (microparticles), and dissolved impurities can slow grain growth and cause deviation from this linear area-time relation. For typical glacial ice, including Wisconsinan ice, we calculate that microparticles are too sparse to affect grain growth significantly, a conclusion reached previously by Duval and Lorius (4). For tephra-rich layers such as those in the Byrd core, however, we calculate that microparticles slow grain growth significantly, in accord with observations (Gow and Williamson, (3)) ; impurity drag and strain probably are important in the Byrd tephra layers also. Model calculations show that compression of bubbles deeper than the firn-ice transition reduces their mobility below that of grain boundaries so that bubble-boundary separation should occur ; such separation is observed (Gow and Williamson, (3). Observed bubble-boundary separation requires about 10 % of the driving force for grain growth and should reduce growth rates by about 10 %. Such a change would be largely masked by observational error, but may be observed. The downward decrease in grain size across the Holocene-Wisconsinan boundary correlates with a downward increase in soluble impurities, especially NaCl (Petit et al., (7)). The correlation has the form expected if the grain-size decrease is caused by the impurity increase. Furthermore, the interaction energy between grain boundaries and Nacl required to cause the grain-size decrease is of the same order as our best estimate of this interaction energy. We thus propose that the small grain sizes in Wisconsinan ice are caused by high concentrations of NaCl and possibly other soluble impurities