Documenting the climate of the past is crucial to understand future climate changes and to better calibrate climate models. The period selected for this paleoenvironmental study is particularly intriguing and contains crucial information on the processes behind rapid and brutal climate variability. The Ordovician period (450 Ma) was characterized by CO2 concentrations up to 10 times higher than today and the virtual lack of vegetation on the continents. Most of the land masses were located south of the equator and a gigantic ocean occupied the Northern Hemisphere. Another fascinating feature is the interruption of the warm conditions by short, but probably intense, pulses of glaciations. This specific setting offers the potential to study extreme climatic conditions, rapid transitions from warm to cold climate, and the functioning of a climate system without the CO2 sink effect provided by continental vegetation. Using different techniques for time-series analysis applied to available geochemical and geophysical data-series, climate cycles can be identified in the rock record. This project also involves the application of a climate model to the boundary conditions inferred for this specific period in the Earth's history. This methodology results in a better insight into the causes, timing and duration of the Ordovician-Silurian climatic events and sheds light on the role of astronomically driven variations in solar irradiation.