How old is the ice beneath Dome A, Antarctica?
Chinese scientists will start to drill a deep ice core at Kunlun station near Dome A in the near future with the expectation of finding very old ice. Uncertainties are introduced by a complicated bedrock topography underneath the in parts more than three kilometre thick ice masses, imposed by the deep valleys of the Gamburtsev mountains. This, and the fact that we resolve ice flow around a dome, makes the deployment of a full Stokes model necessary. Further impact on the basal distribution of the age of the ice is given by the certainly existing ice fabric and the consequently anisotropic viscosity of the ice. Based on findings from other drill sites, we impose different type of prescribed fabrics and use an ortho-normal model for non-linear anisotropic ice flow. The second highly influential parameter determining the basal age of the ice is the geothermal heatflux at the bottom and – with somewhat lower impact - the averaged surface temperature. By variations of those three parameters (fabric distribution, geothermal heat flux and mean surface temperature) on a fixed domain (which excluded the study of changing accumulation and surface elevation), the authors found that large uncertainties of about 500ka for the age at 95% depth underneath Kunlun station based on best fits to existing measured dates in the upper
first third of the ice core exists. In other words, the model is able to fit the existing age/depth horizon with a non-unique set of parameter combinations. A second finding is, that basal conditions are very sensitive to those parameters and gradients of spatial distribution of the age close to the bottom are extraordinary high. Despite the challenges this imposes for this drill site, this can turn out to be an asset, as high resolution ice cores (at places with significant basal melt) as well as very old basal ice could be accessible at the very same spot, just a few hundred metres apart.
The picture shows the distribution of the vertical velocity (left column) and the age of the ice (right column) over depth underneath Kunlun station for several combination of the above discussed model parameters. Measured dates are marked with black dots.
Sun, B., Moore, J. C., Zwinger, T., Zhao, L., Steinhage, D., Tang, X., Zhang, D., Cui, X., and Martín, C., 2014: How old is the ice beneath Dome A, Antarctica?, The Cryosphere, 8, 1121-1128, doi:10.5194/tc-8-1121-2014 [link to paper]
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that has been tuned using observations (velocity, temperature distribution) including Benn's calving criterion. The model was forced with different melting scenarios at its face, different surface melt distributions and - in a simplistic way – also ice mélange in front of the terminus. According to the findings with this model, it is rather the changing surface mass balance than the expected increase in ocean induced melting or the disappearing ice mélange that have the highest impact on accelerating the downwasting of a Greenlandic tide-water glacier such as Helheim.
