These methane-emitting sea vents date back 56 million years

As this happened, large volumes of magma were released through ruptures in the Earth's crust, forming large igneous provinces (LIPs) worldwide. 

This volcanism caused global warming of at least five degrees Celsius. It led to a mass extinction event known as the Paleocene-Eocene Thermal Maximum (PETM). 

"At the Paleocene-Eocene boundary, some of the most powerful volcanic eruptions in Earth's history took place over a period of more than a million years," said coauthor Christian Berndt in a press release. 

Geologists had previously struggled to explain why this volcanism caused warming, contrary to the typical cooling effects of most modern volcanic eruptions due to aerosols released into the stratosphere.

In this latest study, researchers drilled and sampled one such LIP between Greenland and Europe, which is submerged several kilometers below the ocean surface. 

Five of the 20 boreholes were drilled directly into one of the thousands of hydrothermal vents. The cores obtained can be read by scientists like a diary of the Earth's history. 

These provided compelling evidence of the vent's activity just before the PETM and the rapid filling of the resulting crater as global warming began. 

Methane-releasing hydrothermal vents

Surprisingly, the data revealed that the vent was active at a very shallow water depth, possibly less than 100 meters.

This discovery has profound implications for understanding the impact on the climate:

"Most of the methane that enters the water column from active deep-sea hydrothermal vents today is quickly converted into carbon dioxide, a much less potent greenhouse gas," explained Berndt.

"Since the vent we studied is located in the middle of the rift valley, where the water depth should be greatest, we assume that other vents were also in shallow water or even above sea level."

He added that this would have allowed significant amounts of greenhouse gases to enter the atmosphere.

Furthermore, the study has implications for our understanding of modern climate change. The cores indicate that the Earth's system eventually regulated itself, gradually cooling over millennia. 

That said, this timescale is not relevant to the urgency of today's climate crisis.

The complete study was published in Nature Geoscience on August 3 and can be found here.

Study abstract:

The Palaeocene–Eocene Thermal Maximum (PETM) was a global warming event of 5–6 °C around 56 million years ago caused by input of carbon into the ocean and atmosphere. Hydrothermal venting of greenhouse gases produced in contact aureoles surrounding magmatic intrusions in the North Atlantic Igneous Province have been proposed to play a key role in the PETM carbon-cycle perturbation, but the precise timing, magnitude and climatic impact of such venting remains uncertain. Here we present seismic data and the results of a five-borehole transect sampling the crater of a hydrothermal vent complex in the Northeast Atlantic. Stable carbon isotope stratigraphy and dinoflagellate cyst biostratigraphy reveal a negative carbon isotope excursion coincident with the appearance of the index taxon Apectodinium augustum in the vent crater, firmly tying the infill to the PETM. The shape of the crater and stratified sediments suggests large-scale explosive gas release during the initial phase of vent formation followed by rapid, but largely undisturbed, diatomite-rich infill. Moreover, we show that these vents erupted in very shallow water across the North Atlantic Igneous Province, such that volatile emissions would have entered the atmosphere almost directly without oxidation to CO2 and at the onset of the PETM.