The long road to testing the hydrothermal venting explanation for the Paleocene Eocene Thermal Maximum

IODP Expedition 396 to the Norwegian margin provides insight into the timing and emplacement setting of hydrothermal vents that were active during the PETM during the emplacement of the North Atlantic Igneous Province (NAIP) and how they may have influenced the hyperthermal event.
The long road to testing the hydrothermal venting explanation for the Paleocene Eocene Thermal Maximum

In the late 1990s and early 2000s, there was a concerted effort to understand the formation, timing, and significance of large igneous provinces or LIPs. Soon it became clear that these periods of short-lived but enormously voluminous volcanism coincided with large changes of the earth system and that the boundaries of most geological epochs are concurrent with the emplacement of a large igneous province. As the geological epochs are characterized by their particular set of environmental conditions and corresponding biology that is preserved in the fossil record, it was becoming obvious that the emplacement of large igneous provinces was probably affecting these conditions severely. However, the reasons remained enigmatic, as most modern volcanic eruptions cause cooling by putting aerosols into the stratosphere. The geochemical records for large igneous provinces, however, suggest that climate warmed during their emplacement.

Pioneering studies of the Karoo large igneous province in South Africa revealed an abundance of hydrothermal vents associated with magmatic intrusions into the sedimentary basin. This observation in conjunction with a reinterpretation of seismic data from the North Atlantic Igneous Provinces led Henrik Svensen and co-workers in 2004 to hypothesize that similar intrusions into the sedimentary basins had occurred during the NAIP. Their work indicated how these intrusions may have released large amounts of carbon in form of the greenhouse gases carbon dioxide and methane. These gases reached the atmosphere via hydrothermal venting, and ultimately led to transient warming events such as the PETM, the most rapid period of warming of the Cenozoic.

As soon as this hypothesis and its corroborating data was published in Nature, we set out to test the hypothesis by drilling the ancient vent systems around the North Atlantic to obtain precise information on their functioning, timing, and the type of vent fluids that they have released. Our International Ocean Drilling proposal was well received, but never got scheduled as it required riser drilling which is only possible on commercial drill rigs or the Japanese vessel Chikyu, which never ventured into the North Atlantic.

It would take another 15 years before modern three-dimensional seismic data revealed vent systems that were within reach of riser-less drilling, and in 2018 we resubmitted a revised version of the drilling proposal to the International Ocean Discovery Program as it scheduled the future path of the drilling vessel JOIDES Resolution that would bring it back to the North Atlantic. The proposal was approved quickly and in 2021 – seventeen years after the submission of the first proposal – the expedition finally took place.

In these seventeen years, science had moved on, and most stratigraphic studies found corroborating evidence for the venting hypothesis. An important step forward was made by analysing drill chippings from an industry borehole on the Norwegian Margin that penetrated a hydrothermal vent. Even though chippings – the small pieces of rock that are washed out by drilling fluids – are notoriously difficult to put into stratigraphic context they showed that the borehole penetrated strata of the Paleocene Eocene Thermal Maximum which represents the warming period during the emplacement of the North Atlantic Igneous Province. They also showed that microfossils had a thermal overprint consistent with hydrothermal activity. This was entirely what was expected if the hypothesis was true, but the uncertain stratigraphic position of the material with respect to the hydrothermal vent left many other explanations open.

Drillship JOIDES Resolution during Expedition 396 in the North Atlantic. Photo: Peter Betlem.

During the drilling campaign in 2021, five boreholes were drilled into one of the thousands of hydrothermal vents on the Vøring Margin off Norway and a further five were drilled through sedimentary successions deposited during the time when the North Atlantic Igneous Province was emplaced about 55 Ma ago. The results were compelling. In the paper we show that the vent was active just before the Paleocene Eocene Thermal Maximum, that it filled quickly and that the infill records the carbon isotope signature associated with climate warming during the Paleocene Eocene Thermal Maximum. This means that at least this vent was active precisely at the right time, with the rapid formation and quick burial consistent with a sudden expulsion of hydrothermal fluids.

Quite unexpectedly, the data also show that the vent was active in very shallow water depth of probably less than 100 m, even though it is close to the centre of the rift axis where the water depth should be greatest. This has wide-reaching consequences for the potential climate impact of the hydrothermal fluids, as most methane that enters the water column in modern deep-water settings is quickly turned into climatologically much less potent carbon dioxide by oxidation. The fact that even this vent in the centre of the rift was at shallow water depth suggests that most of the other vents were also in shallow water or even above sea level which would have allowed much larger amounts of greenhouse gases to enter the atmosphere to cause the observed climate change.

With future riser drilling the very deep parts of the hydrothermal vents should be sampled. Here, we would expect to find the geochemical signatures that would allow us to constrain the type and volume of emitted gas. In the meantime, the expedition participants are working on the core material from the surrounding areas to test if this holds further clues on the type of emitted fluids, and on a range of further topics. Also, marine scientific expeditions are being scheduled to study the volumes and types of fluids at modern vent systems that may serve as an analogue to the geological setting in the North Atlantic 54 million years ago. This work is the latest in a series of groundbreaking discoveries made as a result of ocean drilling as a result of the IODP program.

(Credit for the poster image: Sandra Herrmann, IODP JRSO)

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