Until about 56 Ma ago North America and Europe were connected: one of the last relics of the Pangaea supercontinent. Oxygen isotopes and magnesium/calcium ratios in the tests of both surface- and bottom-dwelling foraminifera suggest that around that time global mean surface temperature increased by about 5 to 6°C within 10 to 20 thousand years. The rate of global warming was comparable to that currently being induced by human activities. The Palaeocene-Eocene thermal maximum (PETM) is seen by climatologists as a dreadful warning of times to come in the not so distant future. The PETM event marks the most dramatic biological changes since the mass extinction at the Cretaceous-Palaeogene boundary 10 million years earlier. They included the rapid expansions of mammals and land plants and major extinction of deep-water foraminifera. The PETM also coincided with an equally profound excursion in the δ¹³C of carbon-rich strata of that age, whose extreme negative value marks the release of a huge mass of previously buried organic carbon into the atmosphere. It was probably methane, much more potent at delaying heat loss to space than carbon dioxide – methane has more than 80 times the warming effect of carbon dioxide. Since CH₄ is soon oxidised to CO₂ and H₂O estimates of atmospheric greenhouse gas levels are generally expressed in terms of CO₂. The PETM release was equivalent to about 4.4 x 10¹³metrictons over 50 ka; on average 0.24 gigatons per year compared with 0.51 Gt from energy-related sources in 2022.

During the Palaeocene, areas around the present North Atlantic were subject to basaltic continental volcanism before the rifting that opened the North Atlantic from 62 to 58 Ma. Magmatism, dominated by intrusions, began again at the Palaeocene-Eocene boundary from 56 to 54 Ma, linked to the start of continental rifting. Both episodes suggest a rising mantle plume. Once the rift had truly opened volcanism became restricted to the mid Atlantic ridge and a mantle plume remains active beneath Iceland. After geoscientists became aware of the PETM and its coincidence with North Atlantic igneous activity many palaeoclimatologists suggested methane release from organic-rich sediments heated by intrusion of basaltic sills below the opening seaway (but see 2022 post on alternative hypotheses). As with so many extreme geological events, choosing a most-likely scenario depends ultimately on tangible evidence. A convincing sign has been demonstrated dramatically in a recent study by a multinational team of geophysicists, oceanographers, geochemists, palaeontologists and sedimentologists (Berndt, C. and 35 others 2023. Shallow-water hydrothermal venting linked to the Palaeocene–Eocene Thermal Maximum. Nature Geoscience, v. 16, p. 803–809; DOI: 10.1038/s41561-023-01246-8).

The breakthrough by Berndt et al. stemmed from a detailed 3-D seismic survey off the coast of Norway. It revealed an unconformity at the P-E boundary beneath which were clear signs of hundreds of large pockmarks, up to 80 m deep. Seismic reflection from older sediments beneath the unconformity showed the distinctive presence of intrusive sills of igneous rocks. The consortium drilled 20 boreholes into the seabed beneath the survey area. Five of them penetrated crater-like features to yield cores through the sediments that had filled them. The fills were muds, which were interleaved beds of volcanic ash in the sequences marking the P-E boundary suggesting an igneous influence. Organic remains in the muds established the depositional timing of several distinct layers and also gave clues to their depositional conditions. Those spanning the 50 ka of the PETM were dominated by plant debris, pollen and spores, together with abundant marine diatoms that live in very shallow water. Laminations in the muds dip radially inwards towards the deeper parts of some craters to define funnel-like structures. In others the sediments have been domed upwards. The sediments and their structures closely resemble those in blow-out craters formed during petroleum drilling accidents and in onshore maar volcanoes produced by sudden explosive eruptions on land. The pockmarks formed suddenly, to be filled by mobilised mud and volcanic ash.

The evidence points to explosive vents formed by massive degassing of deeper sediments induced by igneous intrusions. Such systems are common around active ocean-floor rifts: ‘black-‘ and ‘white smokers’, but those off Norway formed in shallow water. That has an important bearing on their potency during the PETM. Deep hydrothermal systems may emit methane, but it is oxidised to CO₂ in seawater. Those very close to the surface vent their gas almost directly into the atmosphere before such oxidation can consume methane. Intrusive sills also underlie the eastern continental margin of Greenland, so such explosive hydrothermal vents may have been widespread during the initial rifting of the North Atlantic’.