This is the 3rd part in a 3-part series on the PETM expanded from an article I originally wrote for Quanta Magazine and features quotes from interviews that appeared in that piece. Click here for Part 2.
Animation of igneous sills progressively intruding sediments during the onset of the PETM. Red: sills emitting carbon. Blue ellipses: location of hot mantle blob. From Jones et al Nat Commun 2019 CC BY 4.0
In 2017 Marcus Gutjahr of the University of Southampton, UK, with colleagues, published a new estimate of the carbon that drove the PETM. Their conclusion was that a “very large release of mostly volcanic carbon” drove the event. Their estimate of around 10 trillion tons of carbon (as methane and CO2) was about two to three times the amount of prior estimates. Gutjahr et al based their estimate on boron isotopes as a proxy for ocean pH. They showed that ocean pH stayed low (more acidic) for around 50,000 years, which is very hard to achieve because ocean carbonate chemistry works to neutralize ocean acidification on timescales of around 10,000 years. To overcome that negative feedback and keep oceans acidic for 50,000 years you need sustained high carbon emissions.
The volcanic portion of that carbon - as distinct from the carbon baked from sediments by sills - had very little carbon-12, and so it does not show-up in the shift of carbon isotopes recorded in sediments, even as it suppresses ocean pH. It was emitted in addition to the organic carbon baked from the sediments. This also allows for the possibility that warming from volcanic carbon began before the sill-baked organic carbon isotope signal.
"two independent constraints showing something similar, which is quite powerful!"
The Birmingham team’s calculated emissions agrees with those by Gutjahr et al, despite being calculated from “the ground up,” i.e. from the mantle plume spreading rate through the baking rate of sills. This match from two independent calculations lends confidence that they’re close to reality:
“This is the first method that’s completely independent of those proxies, just based on the geological structures and the modeling of that plume to predict a carbon release rate,” said Sarah Greene from the Birmingham team. “The fact that it overlaps quite nicely is two independent constraints showing something similar, which is quite powerful!”
Sill-baked carbon emissions. Red: carbon emissions from sill intrusions, based on 100 model runs with different sill intrusion rates -red cloud = median, red solid lines = 25% and 75%, red dotted lines = 10% and 90% magma supply rate from the Iceland Plume. Blue line and shading – independent estimates of carbon emissions based on ocean pH (Gutjahr et al 2017). Green: Independent estimated emissions based on deep sea carbonate dissolution. From Jones et al 2019 Fig 8 a, Nat Commun 2019 CC BY 4.0
The work is a vindication for Henrik Svensen, who was not involved in the Birmingham study: “The arrival of the plume is nicely quantified. It’s a very interesting study taking a novel approach to understand how a Large Igneous Province may have triggered one of the most intense climatic changes we know from deep time,” said Henrik Svensen. “[it] supports that volcanic-sedimentary interactions generated the gases - not gas hydrates as many researchers have proposed.”
"methane clathrate involvement is not needed"
Lee Kump of Penn State, who has argued for clathrates responding to a volcanic trigger said the study is “… compelling evidence in support of the NAIP as the trigger for, and main mechanism of, carbon emission during the PETM: methane clathrate involvement is not needed.” But: “the resulting warming likely triggered destabilization of whatever methane clathrates existed.”
James Zachos of University of California, Santa Cruz, and a long-time proponent of the clathrate hypothesis told me: “The short answer is yes, that NAIP is the trigger and main source of carbon.” But he still sees a supporting role for clathrates: “feedbacks such as hydrate dissociation act as an accelerant.”
“In my view, the volcanism could certainly have triggered the event,” said Appy Sluijs of Utrecht University. “This study shows it is possible that CO2 release was fast but hey, it’s a model and you can put lipstick on a model but it’s still a model!” He went on to argue for “multiple positive feedbacks” to amplify the warming.
"you can put lipstick on a model but it’s still a model"
Contrarily, the new study by Jones et al, and the earlier Gutjahr et al study, both suggest such feedbacks were smaller than the volcanic and sill-baked carbon release:
“There’s been a lot of thought that the PETM, and other similar events, there’s an initial pulse of carbon and the rest comes from runaway positive feedbacks,” said Greene. “The feedbacks are small relative to the initial forcing from the volcanism.”
Richard Zeebe of the University of Hawaii remains unconvinced, pointing out that the PETM and later warm episodes (“hyperthermals”) in the Eocene coincide with times when Earth’s orbit around the sun would deliver extra solar warmth:
“As with all subsequent hyperthermals, no other trigger than orbital forcing is necessary. The PETM is part of a long series of hyperthermals and invoking a special trigger for one (say volcanism for the PETM) but not for all others seems illogical.”
But, as I outlined in part 1, others argue that the PETM was by far the largest, was much more abrupt, and may have been out of sync with orbital warmth, indicating a different cause from the subsequent hyperthermals that are indeed controlled by Earth’s orbit like most sediments throughout geological time, without clathrates needed to explain them.
"invoking a special trigger for one (say volcanism for the PETM) but not for all others seems illogical"
By constraining the onset and trigger of the PETM, scientists can focus on quantifying the climate feedbacks involved, which are important in estimating Earth’s long-term sensitivity (Earth System Sensitivity) to today’s carbon emissions, bearing in mind the world was a warmer planet than today and continental configuration was different.
But Sluijs’ point is important – the scientific inquiry will continue.
These new model results will be tested in the coming years, and issues like the ratio of organic carbon to volcanic carbon will continue to be examined critically. Other areas of focus are likely to include the contribution of eroded soil to the carbon isotope record, and fine-tuning of the relative contributions of orbits, igneous activity, and weathering in the fluctuating Eocene hot climate.
On the mantle part of the process, IODP expedition 395 is scheduled to drill those v-shaped ridges near Iceland this year (although due to COVID-19 that’s uncertain), which should shed more light on the mantle pulses, and precise dates on the British part of the NAIP previewed at the 2019 AGU Fall Meeting, and on volcanic ashes in Denmark previewed at the EGU 2020 meeting, will help tie down the pulsed nature and timing of the eruptions. Morgan Jones of the University of Oslo highlighted several upcoming NAIP-PETM papers in his EGU 2020 presentation.
So, watch this space…
No, the Iceland Plume isn’t responsible for global warming today Some readers may wonder about any role the Iceland Plume might have in warming today: effectively none. A recent survey of CO2 from Iceland’s volcanoes sums to about 0.03% of human emissions. The Iceland Plume is still active today, but there’s no giant pulse of mantle intruding into sediments and the seabed over the plume is mostly barren basalt whereas at the PETM the magma intruded a thick basin full of oil-rich sediments. The widening Atlantic Ocean has since put those a safe distance from Iceland: “Since then we’ve opened up an ocean, and we just don’t have that same interaction,” said Dunkley Jones, a coauthor of the new study. CO2 emitted today from all volcanoes and other magmatically active regions, including mid-ocean ridges, is estimated to be 280 to 360 million (with an M) tons per year, a tiny fraction of human emissions at around 35 Billion (with a B) tons per year.
Even though the PETM emissions over 50,000 years were far more than we are likely to emit, “If we can understand how that’s happened in the closest analog that we’ve got in the past hundred million years, then we’re going to be in a better shape to think about what’s going to happen in our lifetimes,” said Jones, the lead author of of the new study.
It’s a bit of a good-news-bad-news situation.
On the good news front, both the Gutjahr et al and Jones et al research suggests that any amplifying feedback from an extra carbon reservoir (like clathrates or permafrost) was small if it existed at all in the PETM world. This echoes recent research that showed that methane clathrates didn’t make a large contribution to climate warming at the end of the last ice age, and are unlikely to do so in the near future. It suggests other carbon cycle feedbacks, and not a mysterious extra reservoir of carbon, explain how Earth responded to large perturbations in the carbon cycle over the last 60 million years.
On the bad news front, We're not living in the Paleocene or Eocene - permafrost and clathrates do, for sure, exist in today's world. But, crucially, the PETM’s several-thousand-year onset timeframe gave Earth’s oceans and weathering processes time to work against it, avoiding more extreme warming. By burning fossil fuels, ironically some form the very same source rocks as fed the PETM, we are recreating it, but much faster. This fast rate doesn’t give Earth’s system sufficient time to neutralize emissions through dilution in the deep ocean (about 1,000 years to mix), dissolving of ocean carbonate (about 5,000 to 10,000 years to respond), and weathering of silicate rock on land (about 100,000 years to cut in).
“It’s over an order of magnitude faster, what we are doing today, compared to the peak of the PETM,” said Greene.
Posted by howardlee on Monday, 18 May, 2020
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