PhD defence by Vasileios Gkinis – Niels Bohr Institute - University of Copenhagen

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PhD defence by Vasileios Gkinis

High resolution water isotope data from ice cores

We here present a new technique for the measurement of water isotope ratios
in an online fashion using Cavity Ring Down spectroscopy in the Near Infra Red region.
The technique reaches a level of precision comparable or better to what can be
achieved with traditional Mass Spectrometry achieving very low instrumental drifts.
Attached to an ice core melter the system can offer water isotope measurements
of unprecedented resolution. We demonstrate the feasibility of the technique
by deploying the system on the Greenland ice sheet during the NEEM ice core
project. The online nature of the data calls for a different approach regarding
data analysis. We outline the methods we use in order to put the water isotope time
series on a depth scale. Based on a step response analysis we are able to derive transfer functions that describe  the attenuation of signals with different wavelengths.
Allan variance analysis is also used to assess the precision and long term
stability of the analytical system. Results are compared to discrete samples
and an overall agreement is observed.

The high resolution obtained with this technique can be useful for temperature
reconstructions based on the study of the isotopic diffusion in firn.
In this work we describe the process of  water isotope firn diffusion and
derive expressions for the diffusion length. Based on a diffusivity
parametrization and a series of different temperature history scenarios we are able
to model the diffusion length history. Spectral estimation techniques applied on the
time series can yield the diffusion length value. We show how this is practically
done and exemplify it  by using  available  water isotope high resolution data from
Dome C.
We compare the model and data outcome and based on that we infer past isotope
slopes for the last 90 kyr for the Dome C site. Our results indicate a variable isotope
slope during this period. The temperature history inferred from the estimated isotope
slope shows a glacial-- interglacial step of approximately 9 K and warming events
during Marine Isotope Stages 3 and 4 that reach Holocene levels in terms of temperature.

If interested please send an e-mail to v.gkinis for a copy of the thesis.