PhD-Ordovician cyclostratigraphy

PhD-Project (FWO PhD fellowship)

“Astronomical cycle identification - new methodological approaches and application to high-resolution Ordovician stratigraphy” (link)

Matthias Sinnesael

Supervisors:

- Prof. dr. P. Claeys, Vrije Universiteit Brussel, (VUB, Belgium)

- Prof. dr. T. Vandenbroucke, (UGent, Belgium)

The state of the Earth’s climate has changed in the past. These changes in climate often occurred on long time scales – thousands to even many millions of years. This is in contract with the current-day fast global warming which is caused by the human-induced increase of certain gases in the atmosphere (CO2 or methane) which reinforce the greenhouse effect. Therefore, documenting the climate of the past is crucial to understand future climate changes and to better calibrate climate models.

We chose to study the Ordovician period (~485-444 million years ago) as it is particularly intriguing and contains crucial information on the processes behind rapid and brutal climate variability. The Ordovician period was characterized by CO2 concentrations several times higher than today and the virtual lack of vegetation on the continents. The largest fraction of developed life was still living in the oceans. Most of the land masses were located south of the equator and a gigantic ocean occupied the Northern Hemisphere. Another fascinating feature of the Ordovician is the evolution of warm conditions towards an intense period of glaciations.

We focused on the role of astronomical cycles as a climate forcing during the Ordovician. The orbit and orientation of the Earth relative to the Sun vary continuously in a very regular way. This variation changes climate as it controls the distribution of solar energy on the Earth’s surface through space and time. We used, and developed, different techniques to identify such cycles in the rock record. Because we studied very old rocks, we had to pay special care to how we approached this challenge. As a result, large parts of this thesis focus on methodological questions. How can we measure these astronomical cycles and test if they were really there?

This thesis explores ways of measuring the geochemical composition of rocks, how we can analyze those measured signals and what the reproducibility of these approaches is. We demonstrate that astronomically driven variations in solar irradiation are indeed traceable in Ordovician rocks. These findings result in a better insight into the causes, timing and duration of the Ordovician climatic events. Moreover, because of the very regular nature of theses cycles, we can construct very precise geological time scales which are unique so far back in time.