Master thesis defence by Yongbiao Weng – Niels Bohr Institute - University of Copenhagen

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Master thesis defence by Yongbiao Weng

Continuous Flow and Discrete Measurements of the Water Isotopes in an Firn Core from Greenland

This thesis presents discrete and continuous flow measurements of the water isotopes in a firn core from Northeast Greenland Ice Stream (NEGIS). Discrete samples are cut with a 5 cm resolution, and the continuous flow is obtained with an approximate 5 mm resolution by continuous melting the firn core. Samples are measured with Infra-Red Cavity Ring-Down spectroscopy. Techniques of each type of measurement are described, including the different calibration procedures leading to the final results. For the data processing of the continuous flow measurements, semi-automatic methods of extracting the useful isotope signal and generating the depth scale are required and developed.
The results from continuous flow measurements have been compared to the discrete
samples. Comparison shows an overall agreement and an comparable precision level of
both measurements, which indicates a good reliability of the continuous flow measurements. Given that the continuous measurements have both high measuring speed and high resolution, this approach has great potential. The results also indicate a reliable long term stability of the analytical system, considering only two times of calibration have been performed for the whole continuous measurements of 28 days. A small remaining deviation compared to the discrete measurements may still due to slight instrumental drifts though.
The high resolution obtained by the continuous flow approach can be useful for reconstructions of climate change in the past. However, the diffusion imposed both in the snow and firn layers of the ice sheet and during the measurements limits the available resolution. In the last part of the thesis, the study of a firn diffusion model developed by Johnsen (1977, 2000) is carried out to address the diffusion process in the NEGIS firn core.

Bo M. Vinther and Trevor J. Popp, Centre for Ice and Climate