How to Freeze a Holobiont: The Great Proterozoic Catastrophe

 

Image created by Dall-E

One of the great features of climate science is that you always learn new things. It is the story of how our planet evolved, changed, and arrived where we are, silly naked apes as we are. One of the most fascinating moments of this long history is the "Snowball Earth" period. Some 700 million years ago, in the Sturtian period, at the end of the Proterozoic Eon, the whole planet got frozen, covered by an ice sheet. Yes, Earth was all white, ice-covered, and nicely frozen. The great holobiont that some call "Gaia" nearly died. It may have survived, barely, because some small areas, maybe volcanic hot springs, remained ice-free.

Why this event? An ongoing research effort is aimed at unraveling the story. The latest results show that the great freezing was preceded by the "Franklin Large Igneous Province" (Franklin LIP). It was another spectacular event but for the opposite reasons. It involved a gigantic volcanic eruption that spread molten lava over a huge region that went from the regions now called Alaska and Greenland. It lasted about one million years. Earth's history is never boring!

Now, some recent data show that the Franklin LIP took place immediately before the great Snowball Glaciation. And the proposal is that the LIP caused the snowball. 

Great, but one moment. There have been several Large Igneous Provinces, LIPs, in the later history of Earth. In most cases, the result was warming, not cooling. The "great dying" of the end of the Permian, about 250 million years ago (at the "Permian-Triassic" boundary) was caused by a giant LIP in the region called Siberia today. The demise of the dinosaurs (apart from birds) was caused by another huge LIP taking place in the Deccan region in what is now India (an asteroid may have helped, but the LIP was probably the main cause). Smaller LIPs also caused havoc in the ecosystem. 

The basic idea is that these huge eruptions inject large amounts of CO2 into the atmosphere, generating a rapid and catastrophic warming that, in turn, causes a dieoff in the ecosystem. So, what did the Franklin LIP have that is different from the more recent LIPs? Is this a new demonstration that climate science is all wrong, actually a hoax created by the appliance industry to force us to cook on induction stoves?

Well, no. If you know the basic mechanisms that control CO2 concentrations and consequently Earth's temperature, the story makes plenty of sense. The Franklin LIP was not different (not so much, at least) from the later LIPs. But the world in which these eruptions took place was completely different. 

Let's start from the beginning: How do LIPs emit CO2? It is because the volcanic lava contains carbonates, compounds that can decompose, producing CO2 and oxides. As for all chemical reactions, the equilibrium between CO2 release and uptake depends on temperature. At the high temperatures of molten lava, carbonates tend to decompose, but, when the lava cools down, the reverse reaction occurs. Now, CO2 reacts with silicates to form carbonates. If the mass of lava is truly huge, as it was in the Franklin LIP, then it is no surprise that the CO2 drawdown is massive. If the temperature is hot enough, but not too hot, the drawdown is also rapid, at least on the geological time scale. Starved of CO2, the atmosphere doesn't act any longer as a blanket to keep the Earth warm. Bang! It is snowball Earth. 

Now, there comes the fundamental point: at the time of the Franklin LIP, the interplay between CO2 release and uptake took place in a relatively simple world: There was no life on land; it only appeared a couple of hundred million years later. So, since the LIP took place mainly on land, it didn't interact with the land biosphere. Then, fast forward 400 million years in the future, and you are now seeing the huge, gigantic, enormous, appalling lava landscape that we call the Siberian LIP. Same thing as before, but with a profound difference. By then, life had spread on land and had plenty of time to create huge reservoirs of carbon that resulted from the decomposition of living matter. This carbon we call "coal." In addition, there were also reservoirs of kerogen (solid hydrocarbons dispersed in the soil) and those hydrocarbons that the naked apes living today on Earth call "fossil fuels." 

Imagine the lava spewing out from the Siberian LIP going through this mass of fossil carbon, and you see that there was a factor that didn't exist with the Franklin LIP: coal was burned by the hot lava and turned into CO2. The same thing happened some 190 million years later with the Deccan LIP. Huge amounts of carbon were turned into CO2 and the resulting warming wrecked the whole ecosystem. Bang! Another mass extinction (the poor dinosaurs were boiled alive). That was the rule with all the Phanerozoic LIPs; their correlation with mass extinctions is reasonably well established, although the details can be complicated. You can find more information in this paper by Ernst and Youbi.

And there we stand: the story of life on Earth is an incredible adventure that sees the geosphere, the atmosphere, and the biosphere interacting with each other, usually resulting in a certain degree of stability but sometimes leading to great upheavals. It is what's happening now, with a large tribe of naked monkeys having taken control of the biosphere and playing the role of a Large Igneous Province by burning the huge reserves of fossil hydrocarbons built over the Phanerozoic Eon. For sure, this new "artificial LIP" will not lead to cooling but to warming. A huge and rapid warming. Gaia will probably survive it, but the monkeys... well, it all to be seen

A simple discussion of the recent results on the Franklin LIP  can be found on Anton Petrov's blog.

How Gaia survived Snowball Earth is described in this post by Ugo Bardi https://theproudholobionts.blogspot.com/2022/12/how-gaia-saved-earth-from-cold-death.html

The paper on the Franklin LIP by Dufour et al. that stimulated this post can be found at https://www.sciencedirect.com/science/article/pii/S0012821X23002728

The link to the paper by Ernst and Youbi on mass extinctions is: https://www.sciencedirect.com/science/article/pii/S0031018217302857

If you cannot access these papers, ask me for a copy

How Gaia Saved the Earth from a Cold Death

 

The Goddess Gaia in the form of the winter deity Khione, daughter of Boreas, the North Wind, and the Athenian princess Oreithyia (image by "Nobody-Important"). 

Earth is a fragile planet and it might freeze to a snowball if not taken care of. So far, the Goddess has done a good job at that but, at least a couple of times during the past few billion years, the Earth actually froze. Might that happen again? It seems that we were close to that just a few tens of thousands of years ago. Now, the problem doesn't exist anymore, with humans pumping zillions of tons of greenhouse gases into the atmosphere. And, who knows? Humans could be the tool used by the Goddess to avoid another "snowball Earth." But now we may have too much of a good thing and the Earth risks boiling. Hopefully, Gaia can take care of that, too.   

It is always amazing to realize how complex is the system that we call the "Ecosphere". And how the system's complexity keeps its parameters within the limits needed for life to exist and prosper. It is the concept of "Gaia" as it was proposed by James Lovelock and Lynn Margulis. The ecosystem is in homeostasis and tends to maintain relatively constant parameters by means of a tangle of internal feedbacks, as all complex adaptive systems ("CAS") do. 

But homeostasis doesn't mean perfect stability. The system's parameters may oscillate - even wildly - before the internal feedbacks can bring them back to the "good" values. Sometimes the system gets close to its limits and it may well be that, at times in its long history, the ecosystem risked going over the edge and then Gaia could "die." This seems to be a common destiny for extrasolar planets, as recently argued by Chopra and Linewaver.

A recent paper by Galbraith and Eggleston on Nature starts from these concepts, noting how the concentration of CO2 in the atmosphere never went below ca. 190 ppm during the past 800,000 years. That happened in correspondence with the lowest temperatures ever observed during that period: the planet was going through a harsh ice age.

This figure from a recent paper by Galbraith and Eggleston on Nature shows an interesting fact: the concentration of CO2 in the atmosphere never went below ca. 190 ppm over the past million years or so. Possibly, it touched the danger limit for the ecosystem to survive. For lower concentrations, plants wouldn't have been able to perform photosynthesis and the biosphere would have largely disappeared.

About these ice ages, there is an interesting point related to the system's feedback. The more ice there is, the more reflective the planet's surface becomes (more exactly, the planetary albedo increases). But, the more reflective the planet's surface is, the cooler the planet becomes. So, we have an enhancing feedback that may transform the whole planet into a single, frozen ball: "snowball earth". It has happened, although possibly not completely, at least twice in the history of Earth. It was during the period we call, appropriately "Cryogenian," from 720 to 635 million years ago. It was not a real "snowball" -- not all of Earth was covered in ice. But what was not under the ice was a frozen desert. To give you some idea of the fascination of this subject, here is an excerpt from the abstract of a paper by Hoffmann et al. on "Science"

"....the small thermal inertia of a globally frozensurface reverses the annual mean tropical atmospheric circulation, producing an equatorial desert and net snow and frost accumulation elsewhere. Oceanic ice thickens, forming a sea glacier that flows gravitationally toward the equator, sustained by the hydrologic cycle and by basal freezing and melting. Tropical ice sheets flow faster as CO2rises but lose mass and become sensitive to orbital changes. Equatorial dust accumulation engenders supraglacial oligotrophic meltwater ecosystems, favorable for cyanobacteria and certain eukaryotes. Meltwater flushing through cracks enables organic burial and submarine deposition of airborne volcanic ash. The sub-glacial ocean is turbulent and well mixed, in response to geothermal heating and heat loss through the icecover, increasing with latitude. Terminal carbonate deposits, unique to Cryogenian glaciations, are products of intense weathering and ocean stratification. "

Can you imagine the Earth in these conditions? A wasteland of dry deserts and ice sheets. At that time, there were no multicellular creatures and life may have survived in hot pockets, maybe volcanic lakes, where it was still possible to find liquid water. 

We may have been dangerously close to a new snowball Earth episode during the past million years or so. Not a trifling matter because today the ecosphere is much more complex than it was at the time of the Cryogenian. A new snowball Earth would likely cause all vertebrate lifeforms to go extinct. It is not just a question of being too cold: the limit of concentration of CO2 that permits plants to perform photosynthesis at a reasonable rate is considered to be around 150 ppm, at least for the most common kind of plants. Under that value, all multicellular plants die, and with them all animal life. Only single-celled creatures could eke out a precarious existence in those conditions. 

But something prevented the ice sheets to expand all the way to envelop the whole Earth and, at the same time, prevented the CO2 concentration to go below 190 ppm. What was that? Several hypotheses are possible. Galbraith and Eggleston favor a biological one, saying that:

In terrestrial ecosystems, carbon fixation by plants is limited by low ambient CO2 (ref. 31). On this basis, ref. 12 proposed that CO2-limitation had significantly reduced plant-mediated silicate weathering during low-CO2 intervals of the past 24 million years, thereby enforcing a lower bound on the ocean–atmosphere carbon inventory over >10^5 yr timescales. Subsequent experiments have been consistent with this ‘carbon starvation’ mechanism, showing reduced weathering by tree-root-associated fungi under low CO2 (ref. 32). Although the feedback on silicate weathering would appear too slow to play a role on the 104 yr timescale of glacial CO2 minima 30, it may be possible that strongly reduced weathering rates lowered ocean alkalinity (thereby decreasing CO2 solubility) on a millennial timescale. Alternatively, reduced photosynthesis rates during the LGM (last glacial maximum) would have slowed the accumulation of terrestrial biomass14, consistent with estimates for lower terrestrial primary production rates33. By slowing the accumulation of carbon in vegetation and soils, this would have provided a stabilizing feedback via an increase of the ocean–atmosphere carbon pool.

Complicated stuff, right? But, basically, the idea is that CO2 is slowly drawn down from the atmosphere by a reaction with rocks (silicates), forming carbonates. This reaction is called "weathering" and it is favored by plants, whose roots provide a good environment for it to take place. Fewer plants, less CO2 drawdown. At the same time, a smaller global biomass means that the quantity of CO2 stored in it becomes lower and this extra carbon most likely ends up in the atmosphere as CO2. So, there are two feedbacks embedded in the system that tend to stabilize its temperature. But, as you may understand from the text by Galbraith and Eggleston, it is even more complicated than this! In any case, these stabilizing geobiological feedbacks oppose the ice/albedo feedback and tend to slow down the glaciation before the two sides of the ice sheet touch each other at the equator. 

But suppose that the Earth really became the snowball that some studies claim to have observed: how did it recover? If it is frozen, it is frozen. Maybe not completely dead, but poor Gaia was reduced to a minor sprite inhabiting hot springs. How could Earth return to the lush ecosphere we are used to?

There is an explanation: it is because volcanoes do not care whether the Earth's surface is frozen or not. They continue pumping CO2 and other greenhouse gases into the atmosphere. Again from Hofmann et al. 

“If a global glaciation were to occur, the rate of silicate weathering should fall very nearly to zero (due to the cessation of nor-mal processes of precipitation, erosion, and runoff), and carbon dioxideshould accumulate in the atmosphere at whatever rate it is releasedfrom volcanoes. Even the present rate of release would yield 1 bar ofcarbon dioxide in only 20 million years. The resultant large green houseeffect should melt the ice cover in a geologically short period of time”[(69), p. 9781]. Because Snowball Earth surface temperatures are below the freezing point of water everywhere, due to high planetary albedo,there is no rain to scrub CO2(insoluble in snow) from the atmosphere."

Note one subtle detail: if temperatures were to go below the freezing point of CO2 (-78 C) even in small regions at the poles, that would form a nearly infinite CO2 sink. And that would be "snowball forever" -- maybe it would have made the Goddess Khione happy, but it didn't happen. Possibly, that was too cold even for a Winter Goddess!

In any case, it seems that CO2 was pumped into the atmosphere by volcanoes, maybe it was the work of the volcanic form of Gaia, the goddess Pele, known for her habit of taking lava showers. 

When the CO2 concentration arrived at levels hundreds of times those of the present-day atmosphere, the result was a cataclysmic rapid collapse of the glaciers and a rise in temperatures. Not only the Earth's ecosystem was saved from a cold death, but it rebounded spectacularly: it was now the time of the "metazoa," the formal term indicate animals. There came the Cenozoic, in which we are still living, with its incredible variety of lifeforms when plants and animals colonized the continental lands. 

You see how the job of Gaia is not so simple. it involves a delicate balance of many factors. Some tend to stabilize the system, while others tend to destabilize it. During the past 15 million years or so, cooling factors took the lead and slowly pushed Earth to lower and lower CO2 concentrations and, with that, lower temperatures.

 Image from Wikipedia Commons. The x scale is in million years from the present. Note the rapid cooling of the past million years or so.

We do not know exactly what caused the cooling, there are several theories. But one thing is sure, Gaia started feeling that it was too cold for her, even in her form of Khiome, goddess of ice. She could die and, this time, perhaps for good. 

(Image source)

So, it became imperative for Gaia to mobilize some of the geosphere carbon and push it into the atmosphere in the form of a greenhouse gas that would warm the Earth back to comfortable temperatures. The Goddess Pele was too slow for that, maybe she is now a little tired after blowing CO2 into the atmosphere for four billion years. So, maybe Gaia thought of a more creative solution. 

Why not use those clever monkeys which had just evolved in Earth's savannas to dig carbon out of Earth's crust, combine it with oxygen, and then pump it back into the atmosphere?  It worked: in just a few hundred years, the monkeys managed to bring back the CO2 concentration to the levels that were typical of Earth as it was a few tens of millions of years ago. 

It may be that, now, Gaia faces the opposite problem: those monkeys have pumped so much CO2 into the atmosphere that now we risk pushing the planet on the opposite side of a climate collapse, to a "hothouse Earth" that might kill the biosphere. Something like that happened with the great extinctions at the end of the Permian and the Cretaceous. Alas, life is difficult, but Gaia can cope. Does that mean getting rid of those pesky carbon-burning monkeys? Maybe. After all, Gaia is a Goddess, she ought to know what she is doing and she has no qualms when it is time to do what's to be done. She can find ways. 

(Image Source)