E. Le Meur

Dynamical processes involved in the retreat of marine ice sheets

Marine ice sheets with mechanics described by the shallow-ice approximation by definition do not couple mechanically with the shelf. Such ice sheets are known to have neutral equilibria. We consider the implications of this for their dynamics and in particular for mechanisms which promote marine ice-sheet retreat. The removal of ice-shelf buttressing leading to enhanced flow in grounded ice is discounted as a significant influence on mechanical grounds. Sea-level rise leading to reduced effective pressures under ice streams is shown to be a feasible mechanism for producing postglacial West Antarctic ice-sheet retreat but is inconsistent with borehole evidence. Warming thins the ice sheet by reducing the average viscosity but does not lead to grounding-line retreat. Internal oscillations either specified or generated via a MacAyeal-Payne thermal mechanism promote migration. This is a noise-induced drift phenomenon stemming from the neutral equilibrium property of marine ice sheets. This migration occurs at quite slow rates, but these are sufficiently large to have possibly played a role in the dynamics of the West Antarctic ice sheet after the glacial maximum. Numerical experiments suggest that it is generally true that while significant changes in thickness can be caused by spatially uniform changes, spatial variability coupled with dynamical variability is needed to cause margin movement.

Coupling of ice‐shelf melting and buttressing is a key process in ice‐sheets dynamics

[1]xa0Increase in ice-shelf melting is generally presumed to have triggered recent coastal ice-sheet thinning. Using a full-Stokes finite element model which includes a proper description of the grounding line dynamics, we investigate the impact of melting below ice shelves. We argue that the influence of ice-shelf melting on the ice-sheet dynamics induces a complex response, and the first naive view that melting inevitably leads to loss of grounded ice is erroneous. We demonstrate that melting acts directly on the magnitude of the buttressing force by modifying both the area experiencing lateral resistance and the ice-shelf velocity, indicating that the decrease of back stress imposed by the ice-shelf is the prevailing cause of inland dynamical thinning. We further show that feedback from melting and buttressing forces can lead to nontrivial results, as an increase in the average melt rate may lead to inland ice thickening and grounding line advance.

Dynamic behaviour analysis of glacier de Saint Sorlin, France, from 40 years of observations, 1957-97

Mass-balance and dynamic measurements carried out on glacier de Saint Sorlin since 1957 provide a good opportunity to study the dynamics of this glacier. Ice-flow analysis shows that dynamic changes have been important over the last 40 years and that these changes are not consistent with the concepts usually used in glacier modelling. Present velocities are larger than the 1960 velocities, although the thickness decreased everywhere (10-30 m in the ablation zone). A simple numerical ice-flow model which does not include longitudinal stress gradients has been used to investigate these phenomena. This model allows us to infer the sliding velocity from observed surface and calculated deformation velocities. We conclude that: (1) the sliding velocity cannot be described by Weertman analysis or empirical relations which link the sliding to the thickness and surface slope; (2) the inferred sliding velocity is uniform over at least half of the glacier; and (3) there is no clear link between the sliding process and the quantity of water coming from surface ablation. Furthermore, it may not be reasonable to calibrate model flow parameters from geometry changes because the surface geometry is relatively insensitive to velocity changes over some decades.

Outburst flood hazard for glacier-dammed Lac de Rochemelon, France

Supraglacial Lac de Rochemelon was formed 50 years ago behind an ice dam and grew steadily until 2004. In October 2004, the volume of the lake reached 650 000 m3, bringing its surface within 0.2 m of the top of the ice dam. To eliminate the threat to towns located below in the event of an overflow, the lake was drained artificially in October 2004 and during the summer of 2005. Once the volume had been reduced to 250 000 m3 by siphoning, a channel was dug with explosives and the remaining water overflowed naturally. This offered a very good opportunity to investigate the breaching of an ice dam accompanied by thermal erosion of the drainage channel. Extensive field measurements were carried out during drainage. Analysis of the energy dissipated in the channel reveals that only half of the available energy was used for breach erosion. A numerical model was used to simulate the evolution of a number of variables during drainage and to study the sensitivity of discharge and ice erosion to different parameters, revealing a high sensitivity to water temperature. Model simulations indicate that natural drainage of this lake at the beginning of October 2004 would have led to a peak discharge of <6 m3 s-1.

Origin of the outburst flood from Glacier de Tete Rousse in 1892 (Mont Blanc area, France)

Extensive field measurements and historical data have been used to re-analyse the cause of the outburst flood from Glacier de Tete Rousse that devastated the village of Saint-Gervais-Le Fayet, French Alps in 1892, causing 175 fatalities. The origin of this disaster was the rupture of an intraglacial cavity in Glacier de Tete Rousse that released 200 000 m 3 of water and ice. All previous studies have concluded that the intraglacial cavity was formed from a crevasse that was filled and enlarged by meltwater. The re-analysis presented here suggests that the reservoir of the upper cavity did not originate as an enlarging crevasse. The origin of the meltwater reservoir was more likely a supraglacial lake formed before 1878 during a period of negative mass balance. Following a period of positive mass balance after 1878, the lake was hidden until the outburst flood of 1892. This means that such hazards may be detected by checking regularly for the formation of a lake on the surface of the glacier before it is hidden.

Coupled marine-ice-sheet/Earth dynamics using a dynamically consistent ice-sheet model and a self-gravitating viscous Earth model

We use a self-gravitating viscoelastic model of the Earth and a dynamically consistent marine ice-sheet model to study the relationships between marine ice-sheet dynamics, relative sea level, basal topography and bedrock dynamics. Our main conclusion is that sea-level change and lithospheric coupling are likely to have played limited roles in the postglacial retreat of marine ice sheets. The postglacial rise in sea level would only have caused at the most around 100 km of grounding-line retreat for an ice sheet of similar dimensions to the West Antarctic ice sheet, compared with the several hundred km of retreat which has occurred in the Ross Sea. There is no evidence that reverse slopes lead to instability. Incorporating coupling with lithospheric dynamics does not produce markedly different effects. The implication of these studies is that marine ice-sheet retreat is the result of physical mechanisms other than lithospheric coupling and sea-level rise.

Effects of a viscoelastic lithosphere on the isostatic bedrock response

Abstract The lithosphere responds to loading by elastic flexure which is followed by viscous relaxation, the amount of which depends on the stress duration. This study compares results of an Earth model in which the lithosphere is modelled as a purely elastic layer and as a more general viscoelastic solid overlain by a rigid crust. It shows the emergence of a noticeable difference in the short wavelength (1×102–5×102 km) component of the bedrock deformation after loading durations of the order of 105–106 yr assuming a lower lithosphere viscosity of the order of 1023–1024 Pa.s. In particular, for the long-term loading hypothesised to be imposed by the East Antarctic ice sheet, we find that a viscoelastic lithosphere yields a more local deformation pattern to which ice sheet dynamics are highly sensitive. It confirms that modelling of the Antarctic long-term evolution would benefit from a fully coupled ice/bedrock approach in which the lithosphere would be represented by a viscoelastic solid.