7 July 2022

Congratulations to Giulia Sinnl

Giulia Sinnl presenting her PhD thesis
Giulia Sinnl presenting her PhD thesis. Supervisor Sune Olander Rasmussen and co-supervisors Raimund Muscheler & Mai Winstrup standing beside her.

Thesis Title: "Improved ice-core chronologies and synchronization of climate records for a better understanding of the global climate"

Thesis Abstract:

Greenlandic ice cores contain a wealth of information thanks to the perpetual fall of snow and the chemical signals recorded in the deposited layers. The study of ice cores thus relies on the accurate time scale of these layers. Other layered archives, such as tree rings or speleothems, form by different mechanisms and are to be dated independently from ice cores.

In this thesis, I address some problems in the previously established Greenland Ice Core Chronology (GICC05). I propose a new timescale of Greenlandic ice cores covering the most recent 3800 years, which is called GICC21. The method for constructing GICC21 is based on annual layer counting between multiple ice cores. I verified the good accuracy of the new time scale against some single-ice-core time scales and the tree-ring time scale.

With the aid of the GICC21 timescale and the newly measured data from the EastGRIP and NEEM ice cores, I present a study of post-volcanic climate on the Greenlandic ice sheet. I find that, after strong tropical eruptions, the climate in Greenland is cooled down for the subsequent decade, after which the regional climate returns to its pre-eruptive state. Furthermore, I find frequent co-occurrence of warm decades in the North American continent with surface melt on the ice sheet.

In the period between 20 and 25 thousand years ago, I have analysed the concentration of the cosmogenic radionuclide Beryllium-10 in the ice from NorthGRIP (Greenland) and WAIS (Antarctica). I have compared these datasets to the concentration of cosmogenic radionuclide Carbon-14 in a Chinese speleothem. The three climatic archives are connected by the solar activity that modulates the production of these atoms, by which they can be synchronized. I find that the GICC05 timescale was dated about 400 years too young compared to the speleothem. Moreover, I add the new result that the Antarctic timescale is also too young, by about 200 years.

These time scale discrepancies may be caused by less snowfall in the coldest periods of the last glacial, the Heinrich Stadials. Thanks to the synchronization of the three time scales, I was able reconstruct the sequence of climatic events in the period between 20 and 25 thousand years ago, examining the timing of two northern-hemispheric Dansgaard Oeschger events relative to the Antarctic Isotope Maximum 2.

Chair of committee: Thomas Blunier

Censors: Online on Zoom: Margareta Hansson and Eric Wolff

Supervisor: Sune Olander Rasmussen and co-supervisors: Raimund Muscheler & Mai Winstrup