Emmanuel Le Meur

A comparison of different ways of dealing with isostasy: examples from modelling the Antarctic ice sheet during the last glacial cycle

The bedrock isostatic response exerts a strong control on ice sheet dynamics and is therefore always taken into account in ice sheet models. This paperreviews the various methods normally used in the ice-sheet modeling community to deal with the bedrock response and compares these with a moresophisticated full Earth model. Each of these bedrock treatments, five in total, are coupled with a three-dimensional thermomechanical ice sheet model underthe same forcing conditions to simulate the Antarctic ice sheet during the last glacial cycle. The outputs of the simulations are compared on the basis of thetime-dependent behavior for the total ice volume and the mean bedrock elevation during the cycle, and of the present rate of uplift over Antarctica. Thiscomparison confirms the necessity of accounting for the elastic bending of the lithosphere in order to yield realistic bedrock patterns. It furthermoredemonstrates the deficiencies inherent to the diffusion equation in modeling the complex deformation within the mantle. Nevertheless, when characteristicparameters are varied within their range of uncertainty, differences within one single method are often of the same order as those between the variousmethods. This overview finally tries to point out the main advantages and drawbacks of each of these methods and to determine which one is mostappropriate depending on the specific modeling requirements.

Full Stokes modeling of marine ice sheets: influence of the grid size

Abstract Using the finite-element code Elmer, we show that the full Stokes modeling of the ice-sheet/ice-shelf transition we propose can give consistent predictions of grounding-line migration. Like other marine ice-sheet models our approach is highly sensitive to the chosen mesh resolution. However, with a grid size down to <5 km in the vicinity of the grounding line, predictions start to be robust because: (1) whatever the grid size (<5 km) the steady-state grounding-line position is sensibly the same (6 km standard deviation), and (2) with a grid-size refinement in the vicinity of the grounding line (200 m), the steady-state solution is independent of the applied perturbation in fluidity, provided this perturbation remains monotonic.

Acoustic impedance and basal shear stress beneath four Antarctic ice streams

Abstract The acoustic impedance of the subglacial material beneath 7.2 km profiles on four ice streams in Antarctica has been measured using a seismic technique. The ice streams span a wide range of dynamic conditions with flow rates of 35–464 m a–1. The acoustic impedance indicates that poorly lithified or dilated sedimentary material is ubiquitous beneath these ice streams. Meanacoustic impedance across each profile correlates well with basal shear stress and the slipperiness of the bed, indicating that acoustic impedance is a good diagnostic not only for the porosity of the subglacial material, but also for its dynamic state (deforming or non-deforming). Beneath two of the ice streams, lodged (non-deforming) and dilated (deforming) sediment coexist but their distribution is not obviously controlled by basal topography or ice thickness. Their distribution may be controlled by complex material properties or the deformation history. Beneath Rutford Ice Stream, lodged and dilated sediment coexist and are distributed in broad bands several kilometres wide, whileon Talutis Inlet there is considerable variability over much shorter distances; this may reflect differences in the mechanism of drainage beneath the ice streams. The material beneath the slow-moving Carlson Inlet is probably lodged but unlithified sediment; this is consistent with the hypothesis that Carlson Inlet was once a fast-flowing ice stream but is now in a stagnant phase, which could possibly be revivedby raised basal water content. The entire bed beneath fast-flowing Evans Ice Stream is dilated sediment.

Improvement of a 2-D SIA ice-flow model : application to Glacier de Saint-Sorlin, France

A number of improvements have been made to an existing two-dimensional ice-flow model applied to an alpine glacier. Analysis of the results of the existing model revealed several shortcomings. The first concerns the lack of mass conservation of the applied alternating-direction-implicit (ADI) scheme. A semi-implicit (SI) scheme is therefore proposed and the effects on mass conservation assessed by a comparison with the ADI scheme. The comparison is first carried out with a simple theoretical glacier for which the improvement is significant. Concerning the real case of Glacier de Saint-Sorlin, France, the initial deviation in mass conservation was much less pronounced such that the new scheme, although improving mass conservation, does not significantly change the modelled dynamics. However, other shortcomings that have a more profound impact on the modelling of glacier behaviour have been identified. The ice thickness may become negative over some gridpoints, leading to an inconsistency. The problem is partly resolved by incorporating extra checks on critical gridpoints at the glacier border. Finally, with the help of ice particle tracking, unrealistic ice settlement above the bergschrund has been identified as the main reason for spurious dynamic effects and has been corrected.

Asuma : un raid scientifique pour documenter la zone côtière de l'Antarctique

Le bilan de masse de surface des grandes calottes, c’est-à-dire le bilan comptable entre apports (précipitation, dépôt de neige par le vent, givre) et pertes (fonte, sublimation, érosion de la neige par le vent) de masse d’eau en surface des calottes, réagit en permanence aux variations du climat. Selon les estimations actuelles, l’augmentation de l’accumulation de neige en surface de l’Antarctique prévue pour la f in du XXIe siècle (15 % environ) représentera une compensation de l’élévation du niveau des mers d’environ 5 cm (voire 15 cm d’ici à 2200). Cette évolution prend en compte les conséquences de l’augmentation de l’humidité atmosphérique en réponse au réchauffement climatique, mais prend mal en considération les changements potentiels de circulation atmosphérique au-dessus de l’océan Austral et le long des côtes qui bordent l’Antarctique. Pourtant, en raison des variations attendues du gradient de pression entre moyennes et hautes latitudes, des déplacements du rail des dépressions sont prévus dans l’hémisphère Sud au cours du prochain siècle. C’est d’ailleurs déjà le cas, et des effets devraient déjà se faire sentir sur le bilan de masse de surface de l’Antarctique de l’Est. Certes, le bilan de masse de surface de cette partie du continent ne semble pas avoir connu de tendance notable au cours des dernières décennies, mais cette conclusion est facilement remise en doute en raison du manque de données de terrain de long terme, tout particulièrement dans la zone côtière de cette partie du continent. Le 1er décembre 2016, le raid scientif ique Asuma (improving the Accuracy of the Surface Mass balance of Antarctica) quittait la base de Cap Prud’homme en Antarctique, à quelques kilomètres de la base française de Dumont-d’Urville, en direction du centre du continent (figure 1). À bord de quatre tracteurs à chenilles et d’une dameuse, cinq scientifiques de l’Institut des géosciences de l’environnement (IGE) étaient épaulés par trois mécaniciens de l’Institut polaire PaulÉmile-Victor (Ipev) et un médecin. Pendant un peu plus d’un mois, ils ont parcouru 1 371 km sur la calotte polaire pour contribuer à améliorer la connaissance du continent antarctique et mieux évaluer les variations spatiotemporelles de son bilan de masse de surface et relier ces variations à d’éventuels changements de circulation dans la région.