The Eocene-Oligocene boundary or transition event is also known as an extinction event – see https://en.wikipedia.org/wiki/Eocene_Oligocene_extinction_event … which has a condensed version of the subject. There are of course a lot of research papers on the event that can be accessed on the Net. It is linked with extinctions of some species and more commonly with change in floral and fauna assemblies. It represents a period of rapid evolution, with animals locking into new niche locations as a result of the disappearance of some species. Most of the affected organisms appear to be marine or aquatic in nature, we are told, and explanations for the event include meteors impacting earth. Two suspected locations, are 1] the Chesapeake Bay crater, and 2] the Popigai impact site in Siberia.
A leading model of climate cooling at this time predicts a decrease in atmospheric carbon dioxide – which is thought to have slowly declined during the Eocene. This idea is derived from the mantra that adding co2 to the atmosphere causes warming. At the moment there is no scientific study that has shown this happens and is at best, a hypothesis awaiting confirmation. It is also true that some geologists take it onboard as gospel but large numbers of geologists do not. The author of the Wiki piece obviously thinks it is a fact, suggesting there may have been a revision of the original article by non-geologists.
The cooling event is thought to be responsible for the formation of the Antarctic ice sheet over the eastern Antarctic continent. It is thought it occurred in two pulses – at 33.5 and 34 million years ago. These dates are derived from geological investigation of sedimentary layers, by the uniformitarian method. In reality the timescale may have been somewhat shorter – although half a million years is short in geological terms.
The Grande Coupure [great break] is French. It saw a turnover in mammal fauna at 33.5 million years ago, marking the end of Eocene assemblages. The Grande Coupure is marked by widespread extinctions and changes amongst isolated populations. A comparable turnover in Asia is known as the Mongolian Remodelling. Climate change associated with polar glaciation, and a drop in sea levels, have been blamed.