Abstract
Abstract. Chinese scientists will start to drill a deep ice core at Kunlun station near Dome A in the near future. Recent work has predicted that Dome A is a location where ice older than 1 million years can be found. We model flow, temperature and the age of the ice by applying a three-dimensional, thermomechanically coupled full-Stokes model to a 70 × 70 km2 domain around Kunlun station, using isotropic non-linear rheology and different prescribed anisotropic ice fabrics that vary the evolution from isotropic to single maximum at 1/3 or 2/3 depths. The variation in fabric is about as important as the uncertainties in geothermal heat flux in determining the vertical advection which in consequence controls both the basal temperature and the age profile. We find strongly variable basal ages across the domain since the ice varies greatly in thickness, and any basal melting effectively removes very old ice in the deepest parts of the subglacial valleys. Comparison with dated radar isochrones in the upper one third of the ice sheet cannot sufficiently constrain the age of the deeper ice, with uncertainties as large as 500 000 years in the basal age. We also assess basal age and thermal state sensitivities to geothermal heat flux and surface conditions. Despite expectations of modest changes in surface height over a glacial cycle at Dome A, even small variations in the evolution of surface conditions cause large variation in basal conditions, which is consistent with basal accretion features seen in radar surveys.
Highlights
The search for ice older than 1.5 million years is an active and key question for the Quaternary science community (e.g. Severinghaus, 2010; Van Liefferinge and Pattyn, 2013)
Temperature and the age of the ice by applying a three-dimensional, thermomechanically coupled full-Stokes model to a 70 × 70 km2 domain around Kunlun station, using isotropic non-linear rheology and different prescribed anisotropic ice fabrics that vary the evolution from isotropic to single maximum at 1/3 or 2/3 depths
The age of the ice depends on the vertical velocity profile in the ice, which depends on the basal melt rate, governed entirely in our model by the geothermal heat flux, surface temperature and the ice fabric
Summary
The search for ice older than 1.5 million years is an active and key question for the Quaternary science community (e.g. Severinghaus, 2010; Van Liefferinge and Pattyn, 2013). In stark contrast with the nearly flat surface topography, radar mapping suggests that the Gamburtsev Mountains are very rugged alpine-style terrain (Bell et al, 2011; Fig. 1) This produces complex ice flow which, in some places, may cause mixing of basal ice layers or localized basal melting and refreezing. Ice is a strongly anisotropic medium, and deep ice cores typically show that the initially random orientation of ice grains alters as the ice is buried and subjected to strain, leading to preferential orientation of the crystal c axis (Shoji and Langway, 1985; Wang et al, 2003) This anisotropic ice may be an order of magnitude “softer” against deformation in certain directions than ice with random fabric, and has an important influence on the age of ice in the lower 1/3 of the ice thickness (Martín and Gudmundsson, 2012; Seddik et al, 2011). At Dome F, which is perhaps the closest analogue to the Dome A region, a single maximum fabric dominated the bottom 1/3 of the ice core (Seddik et al, 2011)
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