3. Hydrothermal Vents, Sills and the Great Methane Escape

Everyone knows hydrothermal vents. They’re underwater fissures that spew out, amongst other things, superheated water. In the deep sea they play host to whole ecosystems that are completely independent of light - perhaps even the only ecosystem. 

The world's hottest hydrothermal vents - Two Boats and Sister's Peak.
Here water reaches up to 464°C

But, as with most things in life, they’re only good some of the time. Huge hydrothermal vent and sill complexes have been studied in the Vøring and Møre basins in the North Atlantic covering an area of over 80 000km². This is the area here - between Greenland and Norway under the Norwegian Sea. 

Associated with these hydrothermal vent complexes are a series of sills. These horizontal intrusions run for hundreds of kilometers, so likely formed very quickly. The sills, which also fueled the hydrothermal vents with hot magma, intruded into organic-rich Cretaceous and Paleocene mudstones. 

A Mudstone

The contact aureoles, a region where the hot magma of the sill has metamorphosed the surrounding rock, has been shown to have an organic carbon content with a δ¹³C of up to -50‰ which is very isotopically depleted indeed. It’s almost like it’s methane - and it is! There’s alot of it too! 1m³ of magma can release 3.6kg of CO₂ whereas the same volume but intruded through organic sediments can produce anywhere between 25 and 100kg of CO₂. So, it makes a pretty big difference.

Igneous intrusions across the whole of the North Atlantic Volcanic Province have been calculated to have produced enough methane through this process to create the -2.5‰ excursion seen across the early parts of the PETM.

Previously, I referred to methane as a source for the carbon excursion observed over the PETM - now it’s also a cause.