26 June 2020

Congratulations to Jesper Liisberg

Jesper Liisberg
Jesper Liisberg with Head of Assessment Committee and Supervisor. From left: Thomas Blunier, Jesper Liisberg and Anders Svensson.

Congratulations to Jesper Liisberg at Physics of Ice, Climate and Earth who successfully defended his PhD thesis Friday 26 June 2020, and obtained the degree of Doctor of Philosophy.

Title and abstract of PhD thesis:
Title: Abrupt climate change and the nitrogen cycle


For studying the climate of the past hundred thousand years, the long-lived ice-sheets located in Greenland and Antarctica offer one of the most versatile types of climatic archives. Especially of interest is the air captured in the ice, allowing for measurement of paleo atmosphere. The analysis of gaseous isotopes is a powerful method for determining the changes taking place during past climatic transitions. Whether it be physical changes like the firn column depth as can be determined from measuring the enrichment of 15N in N2, or the changes in biological output as can be determined by the position of 15N in N2O, isotope analysis is offering a way to assess this. Isotope analysis has conventionally been done by isotopic ratio mass spectrometry, but thanks to the advances in laser and detection technology, analysis by laser spectroscopy is becoming possible. The first part of this thesis outlines the development of an experimental setup allowing for improved measurement of gaseous isotopes in ice cores. This was accomplished by combining the existing methods of continuous flow analysis giving high temporal resolution with the measurements of nitrogen and argon isotopes by isotope ratio mass spectrometry. The measurement of argon isotopes was made possible by a novel method for continuous oxygen removal using a perovskite membrane. This system was put to the test on ice from the bottom of Dye 3, one of the first deep cores, collected form Greenland in the beginning of the 1980’s. Due to the uncommonly shallow firn column of Dye 3 compared to other cores, the isotopic increases were bigger and more sudden compared to other cores. It was able to detect the relative changes in δ15N with a 50 per meg uncertainty, and was used together with the δ40Ar to quantify the driving fractionation effects in the firn during climate transitions. Additionally, the trends in total air content were detectable and related to the climatic variation. While the system and calibration scheme ultimately failed at binding the isotope ratios to an absolute scale, the concept of the system was proven and offers a powerful addition to the CFA gas measurements.
The second part of this work is a paper outlining an inter-lab collaboration undertaken at Empa in Switzerland. Here the performance and limitations of ambient measurements of N2O and its isotopes were compared between different instruments using laser spectroscopy. Effects from trace gas and matrix composition were determined, and the non-linear relationship with N2O abundance was described. An extensive guideline was developed for how to produce reproducible measurements of N2O isotopes using these instruments, giving comparable precision to IRMS.
The last addition to this work details a project investigating a novel way of removing methane from a sample stream for the purpose of scrubbing gas for analysis. The motivation for this was the observed interference from methane on N2O measurements found in the inter-lab comparison. This was done by photolyzing chlorine gas and let it speed up the oxidation path of methane, similar to the process in the atmosphere. A removal efficiency of 98% was achieved, and the effect of varying parameters was determined via a combination of experiments and modelling. Additional experiments are needed to determine if the achieved removal was sufficient to correct for the methane interference.

Prof. Thomas Blunier, Physics of Ice, Climate and Earth, Niels Bohr Institute, University of Copenhagen

Assessment of Committee
Assoc. Prof. Anders Svensson, Physics of Ice, Climate and Earth, Niels Bohr Institute, University of Copenhagen
Prof. Barth F. Smets, DTU ENVIRONMENT, Department of Environmental Engineering
Dr. Thomas Keith Bauska, British Antarctic Survey, Cambridge