Jérôme Weiss

Eight glacial cycles from an Antarctic ice core

The Antarctic Vostok ice core provided compelling evidence of the nature of climate, and of climate feedbacks, over the past 420,000 years. Marine records suggest that the amplitude of climate variability was smaller before that time, but such records are often poorly resolved. Moreover, it is not possible to infer the abundance of greenhouse gases in the atmosphere from marine records. Here we report the recovery of a deep ice core from Dome C, Antarctica, that provides a climate record for the past 740,000 years. For the four most recent glacial cycles, the data agree well with the record from Vostok. The earlier period, between 740,000 and 430,000 years ago, was characterized by less pronounced warmth in interglacial periods in Antarctica, but a higher proportion of each cycle was spent in the warm mode. The transition from glacial to interglacial conditions about 430,000 years ago (Termination V) resembles the transition into the present interglacial period in terms of the magnitude of change in temperatures and greenhouse gases, but there are significant differences in the patterns of change. The interglacial stage following Termination V was exceptionally long—28,000 years compared to, for example, the 12,000 years recorded so far in the present interglacial period. Given the similarities between this earlier warm period and today, our results may imply that without human intervention, a climate similar to the present one would extend well into the future.The Antarctic Vostok ice core provided compelling evidence of the nature of climate, and of climate feedbacks, over the past 420,000 years. Marine records suggest that the amplitude of climate variability was smaller before that time, but such records are often poorly resolved. Moreover, it is not possible to infer the abundance of greenhouse gases in the atmosphere from marine records. Here we report the recovery of a deep ice core from Dome C, Antarctica, that provides a climate record for the past 740,000 years. For the four most recent glacial cycles, the data agree well with the record from Vostok. The earlier period, between 740,000 and 430,000 years ago, was characterized by less pronounced warmth in interglacial periods in Antarctica, but a higher proportion of each cycle was spent in the warm mode. The transition from glacial to interglacial conditions about 430,000 years ago (Termination V) resembles the transition into the present interglacial period in terms of the magnitude of change in temperatures and greenhouse gases, but there are significant differences in the patterns of change. The interglacial stage following Termination V was exceptionally long—28,000 years compared to, for example, the 12,000 years recorded so far in the present interglacial period. Given the similarities between this earlier warm period and today, our results may imply that without human intervention, a climate similar to the present one would extend well into the future.

Intermittent dislocation flow in viscoplastic deformation.

The viscoplastic deformation (creep) of crystalline materials under constant stress involves the motion of a large number of interacting dislocations. Analytical methods and sophisticated ‘dislocation dynamics’ simulations have proved very effective in the study of dislocation patterning, and have led to macroscopic constitutive laws of plastic deformation. Yet, a statistical analysis of the dynamics of an assembly of interacting dislocations has not hitherto been performed. Here we report acoustic emission measurements on stressed ice single crystals, the results of which indicate that dislocations move in a scale-free intermittent fashion. This result is confirmed by numerical simulations of a model of interacting dislocations that successfully reproduces the main features of the experiment. We find that dislocations generate a slowly evolving configuration landscape which coexists with rapid collective rearrangements. These rearrangements involve a comparatively small fraction of the dislocations and lead to an intermittent behaviour of the net plastic response. This basic dynamical picture appears to be a generic feature in the deformation of many other materials. Moreover, it should provide a framework for discussing fundamental aspects of plasticity that goes beyond standard mean-field approaches that see plastic deformation as a smooth laminar flow.

Positive trend in the mean speed and deformation rate of Arctic sea ice, 1979–2007

Using buoy data from the International Arctic Buoy Program, we found that the sea ice mean speed has substantially increased over the last 29 years (+17% per decade for winter and +8.5% for summer). A strong seasonal dependence of the mean speed is also revealed, with a maximum in October and a minimum in April. The sea ice mean strain rate also increased significantly over the period (+51% per decade for winter and +52% for summer). We check that these increases in both sea ice mean speed and deformation rate are unlikely to be consequences of a stronger atmospheric forcing. Instead, they suggest that sea ice kinematics play a fundamental role in the albedo feedback loop and sea ice decline: increasing deformation means stronger fracturing, hence more lead opening, and therefore a decreasing albedo. This accelerates sea ice thinning in summer and delays refreezing in early winter, therefore decreasing the mechanical strength of the cover and allowing even more fracturing, larger drifting speed and deformation, and possibly a faster export of sea ice through the Fram Strait. The September minimum sea ice extent of 2007 might be a good illustration of this interplay between sea ice deformation and sea ice shrinking, as we found that for both winter 2007 and summer 2007 exceptionally large deformation rates affected the Arctic sea ice cover.

A new modeling framework for sea-ice mechanics based on elasto-brittle rheology

Abstract We present a new modeling framework for sea-ice mechanics based on elasto-brittle (EB) behavior. the EB framework considers sea ice as a continuous elastic plate encountering progressive damage, simulating the opening of cracks and leads. As a result of long-range elastic interactions, the stress relaxation following a damage event can induce an avalanche of damage. Damage propagates in narrow linear features, resulting in a very heterogeneous strain field. Idealized simulations of the Arctic sea-ice cover are analyzed in terms of ice strain rates and contrasted to observations and simulations performed with the classical viscous–plastic (VP) rheology. the statistical and scaling properties of ice strain rates are used as the evaluation metric. We show that EB simulations give a good representation of the shear faulting mechanism that accommodates most sea-ice deformation. the distributions of strain rates and the scaling laws of ice deformation are well captured by the EB framework, which is not the case for VP simulations. These results suggest that the properties of ice deformation emerge from elasto-brittle ice-mechanical behavior and motivate the implementation of the EB framework in a global sea-ice model.

Fracture and fragmentation of ice : a fractal analysis of scale invariance

Abstract The fracture and fragmentation processes of ice are reviewed using fractal concepts. Numerous evidences for the scale invariance of fracture and fragmentation patterns in ice are given, including fracture networks at small (laboratory) and large (geophysical) scales, the distribution of fragment sizes in crushed ice or the distribution of sea ice floe sizes, or self-affine fracture surfaces. These observations strongly argue for the scale invariance of fracture and fragmentation processes in ice. This implies that the fracture mechanisms and the physical parameters revealed at the laboratory scale are still relevant at large scale. However, apparent scale effects can be observed for some parameters if the fractal geometry is ignored or neglected. Scale invariance also implies that the homogenization procedures used in the damage mechanics of ice have to be taken with caution.

SCALING OF FRACTURE AND FAULTING OF ICE ON EARTH

The scaling properties of fracture and faulting of ice on Earth are reviewed.Numerous evidences for the scaling of fracture and faulting of ice are given,including self-affine fracture surfaces, fractal fracture networks at small(laboratory) and large (geophysical) scales, power law distributions of fracturelengths or of fragment sizes within fault gouges. These scaling laws are discussedin terms of the underlying mechanics. Scaling of the observables associated withfracture and faulting argues for the scale invariance of the fracture and faultingprocesses and indicates that small scales cannot be arbitrarily disconnected fromlarge scales. Consequently, quantitative links between scales cannot be performedthrough classical homogenization procedures. Scaling can also induce scale effectson different mechanical parameters such as fracture energy, strength or stiffness.Although scaling is ubiquitous for the fracture of ice on Earth, important exceptionsexist such as the nucleation of microcracks or the crevassing of glaciers. Theseexceptions are stressed and discussed.

Coulombic faulting from the grain scale to the geophysical scale: lessons from ice

Coulombic faulting, a concept formulated more than two centuries ago, still remains pertinent in describing the brittle compressive failure of various materials, including rocks and ice. Many questions remain, however, about the physical processes underlying this macroscopic phenomenology. This paper reviews the progress made in these directions during the past few years through the study of ice and its mechanical behaviour in both the laboratory and the field. Fault triggering is associated with the formation of specific features called comb-cracks and involves frictional sliding at the micro(grain)-scale. Similar mechanisms are observed at geophysical scales within the sea ice cover. This scale-independent physics is expressed by the same Coulombic phenomenology from laboratory to geophysical scales, with a very similar internal friction coefficient (μ ≈ 0.8). On the other hand, the cohesion strongly decreases with increasing spatial scale, reflecting the role of stress concentrators on fault initiation. Strong similarities also exist between ice and other brittle materials such as rocks and minerals and between faulting of the sea ice cover and Earths crust, arguing for the ubiquitous nature of the underlying physics.

Complexity in dislocation dynamics: experiments

We present a statistical analysis of the acoustic emissions induced by dislocation motion during the creep of ice single crystals. The recorded acoustic waves provide an indirect measure of the inelastic energy dissipated during dislocation motion. Compression and torsion creep experiments indicate that viscoplastic deformation, even in the steady-state (secondary creep), is a complex and inhomogeneous process characterized by avalanches in the motion of dislocations. The distribution of avalanche sizes, identified with the acoustic wave amplitude (or the acoustic wave energy), is found to follow a power law with a cutoff at large amplitudes which depends on the creep stage (primary, secondary, tertiary). These results suggest that viscoplastic deformation in ice and possibly in other materials could be described in the framework of non-equilibrium critical phenomena.

Statistical analysis of dislocation dynamics during viscoplastic deformation from acoustic emission

We present experimental data of acoustic emission (AE) induced by dislocation motion during “pure” viscoplastic (ductile) deformation of singlecrystals and polycrystals of ice which provide opportunity to revisit collective dislocation dynamics as a critical phenomenon, as recently proposed for brittle fracturing. The data were recorded during compression and torsion creep experiments. AE statistics of power law type were systematically obtained under different experimental conditions. Among the possible candidates for such a system with threshold dynamics exhibiting power law statistics, critical points, disordered first-order transitions, and self-organized criticality should be considered. The revisitation of dislocation dynamics as a critical phenomenon allows rationalization of collective effects as well as of the heterogeneity and complexity of viscoplastic deformation of crystalline materials. Such critical behavior implies that dislocation avalanches and strain localizations are unpredictible, in a deterministic sense, in space, time, and energy domains and that large plastic instabilities account for most of the viscoplastic deformation.

Fracturing of ice under compression creep as revealed by a multifractal analysis

Fracturing of freshwater granular ice up to failure under uniaxial compression creep was investigated from series of interrupted creep tests and from a multifractal analysis of the corresponding fracture patterns. At the early stages of damage corresponding to primary and secondary creep, the fracturing process is dominated by the nucleation of microcracks from stress concentrations within the material (unlike rocks, artificial freshwater ice does not contains starter flaws). Because of the crack nucleation mechanisms, the microstructure of the material (e.g., the nonfractal grain size distribution) strongly influences the organization of fracturing which is therefore nonfractal. As fracturing proceeds during tertiary creep, a hierarchical (fractal) organization of the fracturing emerges progressively over a wider scale range. At failure, this fractal organization is fully developed without detectable lower or upper bound, and the role of the initial microstructure has completely disappeared. Similarly, cracks are preferentially oriented along the compression axis at the early stages of damage, but this anisotropy vanishes as failure is approached. The simultaneity between the onset of tertiary creep and the emergence of fractal organization suggests that the acceleration of the deformation during tertiary creep is due to the cataclasis of a material which becomes granular. An important consequence of the fractal organization of fracturing is that homogenization procedures, as well as damage mechanics, developed to study the behavior of damaged materials, cannot be used to describe tertiary creep and failure.