NGRIP isotopic gas measurments from 10 to 38 kyr and temperature reconstruction for the last glacial

Kindler, Philippe (2013). NGRIP isotopic gas measurments from 10 to 38 kyr and temperature reconstruction for the last glacial (Unpublished). (Dissertation, Universität Bern, Philosophisch–naturwissenschaftliche Fakultät, Physikalisches Institut, Abteilung für Klima– und Umweltphysik)

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During the last four to five decades, scientists became aware that the last glacial period featured a series of abrupt climate changes, especially on the Northern Hemisphere and in Greenland. The unique archive of ice cores from Greenland where climatic information is registered and stored layer by layer offers a valuable access for researchers to access the glacial climate and its variability.
This thesis is a contribution to this research. Based on isotopic gas analysis of air enclosed in ice from the Greenland ice sheet, a temperature reconstruction for the NGRIP (North Greenland Ice Core Project) site, central Greenland, from 10 to 120 kyr b2k covering the whole last glacial cycle, has been derived.
First, an historical introduction to the discovery of these rapid climatic variations, now called Dansgaard-Oeschger (DO) events, is given in Chap. 1. Up to now, scientists still struggle to find mechanisms which can explain this observed variability. However, it is clear, that all parts of the climate system are involved. As the temperature reconstruction presented in this thesis is based on isotopic variations of air created in the firn, where the snow is gradually transformed to ice, a brief summary of these processes will be given as well.
The measurement system, which was used to measure the isotopic composition of air from ice-bubbles is explained in Chap. 2. The system is not yet perfect and some problems were faced during the measurements. This was mostly due to elevated dust contents in the ice during the coldest parts of the last glacial. Despite a lot of effort to overcome these problems, no significant amelioration could be achieved. The 600 measured data points consisting of isotopic measurements of nitrogen and oxygen can be seen in Fig. 2.12, p. 58. They cover the period from 10 to 38 kyr b2k.
The NGRIP temperature reconstruction based on variations in the nitrogen isotope 15N is presented in Chap. 3 and can be seen in Fig. 3.2, p. 70. In order to do a reconstruction for the first time over the whole glacial period, previously measured data were taken into account. In agreement with previous studies, we find temperature amplitudes at DO events ranging from +5 ◦C to 16.5 ◦C and a need to reduce the assumed accumulation rates in order to be able to model the observed 15N data. By comparing the temperature-δ18Oice-relationship (isotopic composition of the precipitation regarding temperature) to orbital parameters, we found for the first time an imprint of obliquity.
Chapter 4 shows a comparison of two different models used for climate reconstructions. It becomes clear, that the two different models produce very similar results which are a confirmation for model based temperature reconstructions. However, it is also shown that in periods of relatively stable climate the models can be unconstrained and that two different climate scenarios can lead to similar modelled gas isotopes in the bubbles.
As the greenhouse gas record of CH4 is now complete for NGRIP as well, a comparison between NGRIP temperature and CH4 over the whole last glacial period is discussed in Chap. 5. Special attention was given to the temporal behaviour at DO event onsets between temperature and CH4. It was found, that methane in general lags temperature by roughly 60 yr.
Publications where data are used which were measured in the frame of this thesis are given in Chap. 6. In the first publication, 15N measurements over DO 8 from this work were used to assess spatial gradients of temperature and accumulation rates in Greenland. In the second publication, oxygen and nitrogen elemental ratio measurements are used to obtain a better understanding of the processes in the clathrate zone.

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