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.
The most commonly accepted 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 destabilisation 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)?

_{Simplified and redrawn after Zachos et al. 2001}