Geology Tackles an Earlier Time That It Got Hot

Model stages: 1, progressive thermo-mechanical weakening of the sub-continental lithospheric mantle (SCLM) by the Iceland plume; 2, disruption of the ruptured metasomatized SCLM zone by incipient asthenospheric upwelling; 3, delamination of carbonated fragments of the deep SCLM during lithospheric stretching, which generate the enduring enriched components (‘EM1’) still present in the Icelandic mantle today

About 56 million years ago, the Earth experienced something called the Paleocene-Eocene Thermal Maximum (PETM). The Earth’s atmosphere warmed by about 6.5° C, and sea surface temperatures by about 5° C. There was serious ocean acidification, which is associated with very high CO2 levels in the atmosphere. It’s too long after the Dinosaur Killer’s cataclysmic arrival 9 million years earlier for that catastrophe to have played a role. After the PETM, it took about 170,000 years for the planet to cool back down to pre-PETM levels.

Geologists have suspected that the contemporaneous opening of the northern Atlantic Ocean by rifting might be implicated. But even assuming that volcanism was implicated, ordinary volcanism would not have produced enough atmospheric carbon to meet the estimated 1 – 1.5 x 104 gigatons of carbon required to cause the PETM.1 Even more confusingly, the isotope ratios for Carbon-12 and Carbon-13 were wrong for ordinary volcanic outgassing.

So where did all that PETM carbon come from?

Thomas Gernon and his co-authors, in a paper published in July 2022, may have puzzled out the source. In “Transient mobilization of subcrustal carbon coincident with Paleocene-Eocene Thermal Maximum,” (the title just trips off the tongue, doesn’t it?), the authors show strong evidence that a combination of the earlier creation of the supercontinent Pangaea, the initiation of the Iceland mantle plume and the rifting that created the North Atlantic Ocean created the mechanism by which those vast amounts of carbon were released. As they re-construct the PEMT, it happened like this.

When older tectonic plates merged to create the supercontinent of Pangaea, a lot of ocean floor, rich in carbonates, got shoved under the topmost layer of the Earth, the continental crust. That created a deep layer of topmost mantle that was unusually rich in carbonate rocks. Under the heat and pressure at those depths, the carbon in that mantle was kept and held in the semi-molten mantle.

When the Iceland mantle plume – the “hotspot” currently underlying Iceland – started or the mid-Atlantic rift started, geology is unsure which came first, that volcanism forced the magma up through all that buried carbonaceous rock, creating immense amounts of greenhouse gases. Freed from the heavy pressure of the continental crust, the carbonate “burned,” creating vast amounts of planet-warming CO2. When the Mid-Atlantic Ridge formed, still more of that buried carbonate was emitted as CO2.

(a) carbonatitic xenolith with an envelope of metasomatized peridotite and (b) carbonatitic xenolith exhibiting a sharp boundary with the host basalt. (c) Carbonatitic xenolith containing abundant irregular blebs/cavities on the scale of micrometers to millimetres.

In support of that theory, Gernon and his co-authors point to carbonate-enriched xenoliths embedded in Icelandic basalt laid down 52 million years ago. Xenoliths are inclusions in formerly molten rock carried up intact from under the earth’s crust. They also point to the ratios of the carbon isotopes in those xenoliths, which are more typical of ocean bottom calcium carbonate and magnesium carbonate than long-buried carbon in ordinary basalt. There are voluminous lava flows, part of the North Atlantic Igneous Province, on the easterly coast of modern Greenland that date to 52 million years ago.

Gernon’s group created computer simulations to model all of these effects. They conclude:

Our data-driven models of the PETM demonstrably support the proposal that large-scale lithospheric melting can induce global warming, if the tectonic setting is primed to facilitate intensive volcanic CO2 degassing. Our study highlights the critical role that solid Earth degassing plays in driving abrupt shifts in climate, and in promoting fundamental reorganization of Earth’s surface environment and biosphere.

Germon et al., Transient mobilization of subcrustal carbon coincident with Paleocene-Eocene Thermal Maximum

It’s a very nice piece of geologic detective work.

1 A gigaton is a billion tons. 104 gigatons is a billion, billion tones of CO2. That, gentle reader, is a whole lot of carbon dioxide.

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