Master thesis defence by Lisbeth Nielsen – Niels Bohr Institute - University of Copenhagen

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Master thesis defence by Lisbeth Nielsen

Modelling the Internal Layering of the Greenland Ice Sheet - Investigating a method for modelling proxy profiles in large scale ice sheet models

The tracing of the depositional characteristics of the ice found in deep layers in the polar ice sheets offers the possibility to model the stratigraphy of the ice and thereby reconstruct profiles of different proxy variables for the modelled ice sheet. The modelling of these additional variables allows high resolution proxy records obtained from ice core studies to be used as constraints for evaluating the performance of numerical ice sheet models in reconstructing the present state of the ice sheet (Clarke et al., 2005).

Following the work of Cuffey and Marshall (2002) and Lhomme et al. (2005), a tracer model
was developed and used to successfully calculate the fields of depositional characteristics within the ice sheet, using outputs from the three dimensional, thermo-mechanical ice sheet model PISM (Bueler and Brown, 2009; the PISM Authors, 2011). In order to assess the potential of adding a tracer scheme to PISM, the tracer model was tested for three different model experiments of the Greenland ice sheet; one steady state ice sheet and two time depended experiments covering the ice sheet evolution of the last 20 kyr.

From comparison of the modelled stratigraphy at the NGRIP site to a high resolution, counted age-depth scale, it was found that the modelled stratigraphy gave reasonable results for the upper parts of the profile, corresponding to the last 10 kyr. Below this depth, all modelled profiles developed considerable errors and for one experiment a consistent error for the entire profile was found. Additional analysis showed similar trends in larger areas of the interior of the ice sheet in the central and northern parts. These results suggested that the precipitation forcing used in the model experiments might not be able to reconstruct the large scale characteristics of the internal layering of the ice sheet.

In addition, the results of the depositional history of the ice sheet was used to reconstruct the isotopic records of 18O for four ice core sites. The comparison of the modelled and measured profiles on a time scale indicated that the present spatial distribution of 18O combined with modelled upstream effects and a uniform climatic variation resulted in reasonable isotopic profiles for the interior sites of NEEM and GRIP for the last 10 kyr, with a good agreement for the NEEM profile for most of the period reaching back to 15 kyr. The results for the Dye 3 site was similar to those of GRIP, with reasonable reconstructions for the most of the Holocene and larger deviations at the transition of the glacial-interglacial and prior to this. The modelled results of the Camp Century site deviated considerable from the measured profile for all experiments.

Christine Hvidberg, Ass. Professor, Centre for Ice and Climate
Bo Vinther, Ass. Professor, Centre for Ice and Climate