Master thesis defense – Niels Bohr Institute - University of Copenhagen

Niels Bohr Institute > Calendar > 2009 > Master thesis defense

Master thesis defense

Friday, February 6th, Sebastian Bjerregaard Simonsen defends his master thesis. The title of the thesis is "A New Ice Core Paleothermometer for Holocene and Transition; Diffusion Processes Quantified by Spectral Methods".

Supervisor: Sigfús J. Johnsen
Censor: Andreas P. Ahlstrøm

Since the early 1950's stable water isotopes have been considered a temperature proxy with application in paleoclimatic reconstruction of permanently snow covered areas. In this thesis a new method of reconstructing the paleoclimatic information preserved in the isotopic records from Arctic ice cores is tested. The method is based on the definition of the differential diffusion length. An investigation of the processes affecting the differential diffusion length is undertaken, covering the densification of firn, ice flow modelling and isotopic diffusivity in the firn and ice matrix.

Three numerical power spectra density estimation methods have been used to retrieve the differential diffusion length in 22 sections of ice from two Greenlandic ice cores, GRIP and NGRIP. None of the methods are found superior, but both Burg's algorithm and the Autocorrelation methods show promising behaviors.

The modelled and measured differential diffusion lengths form an estimate of the surface temperature at the origin of the ice core sections. The investigation of the differential diffusion length in deep sections from the NGRIP ice core shows that the temperature reconstruction based on the differential diffusion length must be limited to ice younger than about 40 kyr for the two cores. This reduces the number of ice core sections available to 12. The low number of sections limits the interpretation of the tested temperature reconstruction. However, the estimated surface temperatures are within uncertainties of paleotemperatures records derived from combined bore hole temperature and isotope profiles from the GRIP and NGRIP ice cores.