The Paleocene-Eocene Thermal Maximum (PETM) is a well-studied warming event near the Paleocene-Eocene boundary.  
It is characterized by its extreme warming rate: from onset to recovery the event lasted ca. 100 kyrs, and the global temperature is thought to have increased by ~6°C in barely 20 kyrs.  
This event is of particular interest not only for its $\delta^{18}O$ but also a sharp negative excursion in the $\delta^{13}C$, implying a strong input of $^{13}C$-depleted carbon into the system.  

![Isotopic curves of Paleogene climate, showing abrupt peak at PETM][1]

<sub>Simplified and redrawn after Zachos et al. 2001</sub>

One frequently invoked hypothesis to explain this input is the leakage of methane from clathrates trapped in seafloor sediments.  
Being a paleontologist and not a geochemist, I struggle to understand the actual chemical mechanism involved.
**I was thus wondering if someone could explain the mechanisms allowing the destabilization of methane clathrates at the same time on a global scale.**  
If it's just linked to a temperature threshold, how come it wasn't reach prior to that (warmer temperature were reached during the Cretaceous)?


  [1]: https://i.sstatic.net/UmI5m.png