PhD defence by Mads D. Ellehøj – Niels Bohr Institute - University of Copenhagen

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PhD defence by Mads D. Ellehøj

Ice-vapor equilibrium fractionation factor: Experimental investigations and possible impacts on the understanding of the hydrological cycles on Earth and Mars


Supervisor:
Sigfus J. Johnsen
External Supervisor: Morten Bo Madsen  

Abstract
Since it controls the magnitude of the isotope fractionation during
phase change, the ice-vapor equilibrium fractionation factor is a core
component in isotope models e.g. used to interpret ice core data for
palaeoclimate studies. In this work we present an experimental setup
with the purpose of investigating the temperature dependency of the
fractionation factor and to improve the temperature range of earlier
work with the use of both conventional isotope ratio mass spectrometry
and a Picarro cavity ringdown spectrometer.

The results of the experiments show fractionation factors for deuterium
and oxygen, with a temperature dependency well in accordance with theory
for equilibrium fractionation for temperatures between 0C and -40C.
Compared to previous work, a significantly larger fractionation factor
for deuterium is obtained while for oxygen, the fractionation factor is
larger for temperatures below -20C and slightly lower for temperatures
above this.

A case study of condensation in the martian atmosphere show that the
differences between Mars temperature (-70C) extrapolated fractionation
factors from earlier work and this work not surprisingly cause
significant changes in D/H ratios, and better knowledge of the
fractionation at martian temperatures is needed for proper understanding
of the processes in the martian water cycle.

Furthermore, investigations with a Rayleigh distillation model show that
the differences between earlier work and this work cause significant
changes in both magnitude and shape of an annual deuterium excess signal
with offsets and variations similar in magnitude to natural variations.
This emphasizes the importance of the fractionation factor for accurate
studies of the processes in the hydrological cycle and underlines the
significance of the differences between the results of this work and
earlier work. The results of this work emphasize the need for better
characterization of the basic parameters in the models used to
investigate present and past climates, and should be considered in
future evaluations of the ice-vapor processes in the hydrological cycle.

If interested please contact Mads D. Ellehøj for copy of the thesis.
E-mail: ellehoj @ gfy.ku.dk