Impact of debris cover on glacier ablation and atmosphere - glacier feedbacks in the Karakoram

This work was partly carried out under the Collaborative Adaptation Research Initiative in Africa and Asia (CARIAA) with financial support from the UK Government’s Department for International Development and the International Development Research Centre, Ottawa, Canada.The Karakoram range of the Hindu-Kush Himalaya is characterized by both extensive glaciation and a widespread prevalence of surficial debris cover on the glaciers. Surface debris exerts a strong control on glacier surface-energy and mass fluxes and, by modifying surface boundary conditions, has the potential to alter atmosphere– glacier feedbacks. To date, the influence of debris on Karakoram glaciers has only been directly assessed by a small number of glaciological measurements over short periods. Here, we include supraglacial debris in a high-resolution, interactively coupled atmosphere–glacier modeling system. To investigate glaciological and meteorological changes that arise due to the presence of debris, we perform two simulations using the coupled model from 1 May to 1 October 2004: one that treats all glacier surfaces as debris-free and one that introduces a simplified specification for the debris thickness. The basin-averaged impact of debris is a reduction in ablation of 14 %, although the difference exceeds 5mw:e: on the lowest-altitude glacier tongues. The relatively modest reduction in basin-mean mass loss results in part from non-negligible sub-debris melt rates under thicker covers and from compensating increases in melt under thinner debris, and may help to explain the lack of distinct differences in recent elevation changes between clean and debriscovered ice. The presence of debris also strongly alters the surface boundary condition and thus heat exchanges with the atmosphere; near-surface meteorological fields at lower elevations and their vertical gradients; and the atmospheric boundary layer development. These findings are relevant for glacio-hydrological studies on debris-covered glaciers and contribute towards an improved understanding of glacier behavior in the Karakoram

The Topographic evolution of the African continent, constraints from coupling deep mantle, climate and surface processes models

International audienceThe African continent is characterized by an anomalous topography made of long (1000 km) wavelength features that cannot be easily explained by variations in crustal and lithospheric thickness and for which we possess relatively few constrains on the timing of surface uplift and subsidence. We have attempted to use the sedimentary record from the marginal basins surrounding the continent to constrain the timing and amplitude of the various phases of vertical movement responsible for this anomalous topography, in the hope of gaining more insight on the mechanism(s) responsible for its formation. By its nature (amplitude, timing and dimensions) the anomalous topography seems to be linked to dynamical processes originating in the underlying mantle. However, the sedimentary record must be deconvolved of the effects of long-term, continental-scale climatic variations before it can be used to provide constraints on the topographic evolution. To this effect, we have combined numerical models of the past climate constrained by geology with a large-scale surface processes model for erosion and sediment transport (TopoSed; Simoes et al., 2010) in which the long-term tectonic uplift and subsidence is retrodicted by a global mantle convection model (Moucha et al., 2011). We focused on the topographic evolution of the late Cenozoic African continent and quantified the relative contributions of climate, rock erodibility, mantle rheology, and present-day mantle heterogeneity in terms of the modeled sediment supply to the margins and compare this with the observed sedimentary fluxes inferred from the geological record

The Topographic evolution of the African continent, constraints from coupling deep mantle, climate and surface processes models

International audienceThe African continent is characterized by an anomalous topography made of long (1000 km) wavelength features that cannot be easily explained by variations in crustal and lithospheric thickness and for which we possess relatively few constrains on the timing of surface uplift and subsidence. We have attempted to use the sedimentary record from the marginal basins surrounding the continent to constrain the timing and amplitude of the various phases of vertical movement responsible for this anomalous topography, in the hope of gaining more insight on the mechanism(s) responsible for its formation. By its nature (amplitude, timing and dimensions) the anomalous topography seems to be linked to dynamical processes originating in the underlying mantle. However, the sedimentary record must be deconvolved of the effects of long-term, continental-scale climatic variations before it can be used to provide constraints on the topographic evolution. To this effect, we have combined numerical models of the past climate constrained by geology with a large-scale surface processes model for erosion and sediment transport (TopoSed; Simoes et al., 2010) in which the long-term tectonic uplift and subsidence is retrodicted by a global mantle convection model (Moucha et al., 2011). We focused on the topographic evolution of the late Cenozoic African continent and quantified the relative contributions of climate, rock erodibility, mantle rheology, and present-day mantle heterogeneity in terms of the modeled sediment supply to the margins and compare this with the observed sedimentary fluxes inferred from the geological record