Nunataks as barriers to ice flow: implications for palaeo ice sheet reconstructions

Abstract

Abstract. Numerical models predict that discharge from the polar ice sheets will become the largest contributor to sea-level rise over the coming centuries. However, the predicted amount of ice discharge and associated thinning depends on how well ice sheet models reproduce glaciological processes, such as ice flow in regions of large topographic relief, where ice flows around bedrock summits (i.e. nunataks) and through outlet glaciers. The ability of ice sheet models to capture long-term ice loss is best tested by comparing model simulations against geological data. A benchmark for such models is ice surface elevation change, which has been constrained empirically at nunataks and along margins of outlet glaciers using cosmogenic exposure dating. However, the usefulness of this approach in quantifying ice sheet thinning relies on how well such records represent changes in regional ice surface elevation. Here we examine how ice surface elevations respond to the presence of strong topographic relief that acts as an obstacle by modelling ice flow around and between idealised nunataks during periods of imposed ice sheet thinning. We find that, for realistic Antarctic conditions, a single nunatak can exert an impact on ice thickness over 20 km away from its summit, with its most prominent effect being a local increase (decrease) of the ice surface elevation of hundreds of metres upstream (downstream) of the obstacle. A direct consequence of this differential surface response for cosmogenic exposure dating is a delay in the time of bedrock exposure upstream relative to downstream of a nunatak summit. A nunatak elongated transversely to ice flow is able to increase ice retention and therefore impose steeper ice surface gradients, while efficient ice drainage through outlet glaciers produces gentler gradients. Such differences, however, are not typically captured by continent-wide ice sheet models due to their coarse grid resolutions. Their inability to capture site-specific surface elevation changes appears to be a key reason for the observed mismatches between the timing of ice-free conditions from cosmogenic exposure dating and model simulations. We conclude that a model grid refinement over complex topography and information about sample position relative to ice flow near the nunatak are necessary to improve data–model comparisons of ice surface elevation and therefore the ability of models to simulate ice discharge in regions of large topographic relief.

Regional sea-level highstand triggered Holocene ice sheet thinning across coastal Dronning Maud Land, East Antarctica

“…Although there is an excellent match between the Holocene deglaciation history in Grjotfjellet and Rabben, we note that local ice sheet history should not be directly interpreted as reflecting regional changes. The ice sheet response at a specific site could be exaggerated, reduced, or even reversed by effects of local bedrock topography above and below the ice sheet 22 . To examine these effects at our sites, we performed high-resolution (with grid-size down to ~500 m) ice sheet model experiments for the entire Jutulstraumen catchment across western and central DML regions (Fig.…”

Antarctic ice stream thickening under Pliocene warmth

“…These models are usually run at low spatial resolutions to achieve long simulation times, and consequently cannot represent regional patterns of ice dynamics, especially over areas of steep subglacial topography [34,35]. This inability can at least partially explain why such models do not capture the timing and magnitude of ice thinning recorded by geological constraints [36,37]. At the same time, such constraints, which are discrete in space, are extrapolated over a large area using assumptions based on the modern ice sheet geometry, and potentially miss important regional variations.…”

“…Our results demonstrate that caution is needed when using AIS responses to past warm climates as analogues for the future, because subglacial topography changes significantly on geological time scales. Furthermore, considering the importance of subglacial topography for ice sheet response [35,36,46], there is a clear need to use topographic reconstructions of the target geological period, and at a spatial resolution that captures the complex-terrain influence when modelling ice streams. It is important to keep in mind that the geophysical relief approach [58] applied to reconstruct the Pliocene topography [59] produces a smooth palaeo topography that does not capture some local features, such as the preserved fluvial valleys beneath Jutulstraumen [33].…”

“…As in atmosphere and ocean models, the artificial smoothing of topography due to low spatial resolution prevents coarse-resolution ice sheet models from properly capturing regional topographically controlled dynamics [34][35][36]. In the case of ice sheet models, the inclusion of overdeepened subglacial topographies is detrimental for a proper assessment of whether marine ice sheet instability might occur in a retreating ice stream [27].…”

“…All simulations were performed using the ice-flow model Úa [38], which has been used previously for palaeo-time scale simulations [36,50]. We model the Jutulstraumen catchment, as defined in Ref.…”