This is part two of a three-part series on Earth's climate through time. You can find part one here and part three here.
The Neoproterozoic Era: setting the stage
Let’s fast forward to almost 800 million years ago. The Neoproterozoic era is in full swing on Earth. For the previous billion years, Earth’s climate has been fairly warm and stable — a period in the planet’s history that is sometimes referred to as the “boring billion years”. The land is collected in the supercontinent Rodinia, which is mainly located in low latitudes. There is already multicellular life on Earth, but still very primitive — filamentous algae, microscopic fungi, protozoa, and the similar.
The onset of snowball Earth
And suddenly a disaster happened. For some reason, global cooling occurred on Earth, which led to the fact that the planet was completely covered with ice. At the height of glaciation, the average temperature of the land surface near the equator was -25 degrees Celsius, and the minimum temperature in the area of the West African protocontinent (which was then at 60 degrees south latitude) dropped to as much as -110 degrees (these are, of course, modeling results, since it is not possible to reconstruct temperature using conventional paleogeographic methods — for example, from the geochemical composition of sedimentary rocks). The average temperature of the planet was about −50 degrees — somewhat similar to modern Mars.
The thickness of ice on land in those years could reach five kilometers. But not all of it was completely covered with ice — in such extremely cold conditions, the air is too dry and does not carry enough moisture for the formation of glaciers in areas remote from the ocean. The areas of land not covered with ice turned into a cold, dry wasteland. The ocean was not frozen to the bottom — its bottom layers were maintained in liquid form by geothermal heat, similar to what happens now with subglacial lakes in Antarctica, but it was covered by a kilometer-long sheet of ice. However, there could also be areas with relatively thin ice, which allowed a small amount of light to reach the water, supporting photosynthetic organisms.
All this looks like a disaster movie script, but, according to modern science, this is exactly how it all happened. This moment in the history of the Earth is called “Snowball Earth”. This term was coined by Joseph Lynn Kirschvink in 1992.
Supercontinent Rodinia. ed. Ramstein G. et al. / Paleoclimatology, 2021
Life's struggle and survival in extreme cold
Life on Earth has died out almost completely; only small oases have survived in some ecological niches. At the bottom of the oceans near hot springs, chemoautotrophic microbes (those that do not need sunlight) survived, psychrophilic (cold-loving) microbes survived, and some types of cyanobacteria and algae living in snow and porous rocks survived. If sea ice allowed some part of the light to pass through, then some photosynthetic marine organisms could also survive.
Strictly speaking, the Earth turned into such a snowball several times. There are two major periods of glaciation:
Sturt 717–659 million years ago;
Marinoan 650–635 million years ago.
They were separated by a warm period. During both of these episodes, the Earth was completely covered in ice.
Unraveling the mystery of global glaciation
Why do we think that everything was exactly like this? How could the idea of global glaciation even occur to scientists?
This story began in the 1960s. Then geologists noticed that glacial deposits are found in Neoproterozoic rocks almost everywhere, in all parts of Rodinia, which was located near the equator. That is, at that time there were glaciers in tropical latitudes and at sea level. You can still find ice on the equator, but to do this you need to climb to a height of 5000 meters and above.
These results baffled scientists. At the same time, immediately above these glacial deposits, there are layers of limestone rocks that could only form in warm water. These rocks, overlying glacial deposits, are called “crown carbonates”, in English cap carbonates. It looked as if the transition from a glacial to a tropical climate had happened in an instant.
Rock outcrop in the Skeleton Coast region, Namibia. The white lines at the bottom of the photo highlight a layer of glacial deposits, above which there is a thick layer of “crown carbonates”. Hoffmann P.F. et al. / Scientific American, 2000
Finally, to complete this puzzle, banded iron ores were found in Neoproterozoic sediments, which could only form in the absence (or very low concentration) of oxygen in the ocean and atmosphere. Such ores are characteristic of rocks about 2.2 billion years old. Before this, the amount of oxygen in the atmosphere was small, and large amounts of ferrous iron were dissolved in the ocean. But then oxygen appeared — and iron, in the form of a water-insoluble oxide, precipitated. The same thing happens when you open a bottle of iron-containing mineral water: as long as the bottle is tightly closed, the water remains clear; but if you open it and let it stand for a while, the iron will react with oxygen from the air and soon settle to the bottom as flakes of rust.
But 700 million years ago. The amount of oxygen in the atmosphere, although less than what it is now, was already very large, and there could be no accumulation of divalent iron in the ocean. Where did these ferruginous rocks come from then?
To explain the widespread distribution of glacial deposits in the Neoproterozoic, British geologist Brian Harland was one of the first to propose the global glaciation hypothesis in 1964. It explained a lot, but there were no answers to the main questions: How did the Earth get into such a situation, and, more importantly, how was it possible to get out of this bind?
Climatologists tried to answer the first question. Using a relatively simple climate model, he showed the important role of Earth’s albedo — the reflectivity of the surface — in climate change. If, as a result of the initial cooling, snow, and ice appear on the planet, which is white, that is, they reflect most of the solar radiation, then positive feedback is activated: more snow and ice — more albedo — the Earth reflects more solar energy — it becomes colder — even more snow and ice. According to Budyko’s calculations, if glaciers reach 30 degrees north or south latitude, the process gets out of control and glaciers cover the entire planet.
By the way, the reverse mechanism also works: the warmer it is, the less snow and ice, the less albedo, the warmer it is. This is one of the reasons why the Arctic is now warming much faster than the rest of the planet.
Thus, climate scientists have shown that making the Earth completely covered with ice is not so difficult in principle. However according to their calculations, the climate of the ice-covered Earth is very stable, so it remained completely unclear how our planet was able to get out of this ice trap.
The Great Earth Thermostat: Escaping the Ice Age
The answer came in 1981 when American researcher James Walker and his co-authors published an article in which they explained the mechanism of thermoregulation of our planet. This mechanism (which can be called the Great Earth Thermostat) is based on feedback in the carbon cycle. The Earth’s carbon cycle is, generally speaking, very complex. There are many sources and sinks of carbon, but on the scale of hundreds of thousands and millions of years, two flows become of paramount importance:
the constant release of CO2 into the atmosphere through volcanoes;
the natural burial of atmospheric CO2 through weathering processes, during which this gas binds to rocks (in particular, silicates), enters the ocean along with surface waters, and is buried in the form of carbonates on the seabed. Part of the carbon is also removed biogenically: marine microorganisms consume carbon dissolved in water and, dying, bury it at the bottom of the sea.
The mechanism of the “earth thermostat”. CO2 enters the atmosphere from the bowels of the earth through volcanoes and is removed from the atmosphere due to weathering when reacting with silicates. ed. Ramstein G. et al. / Paleoclimatology, 2021
Both of these fluxes are extremely small: for example, the modern CO2 flux from volcanoes is two orders of magnitude less than anthropogenic carbon emissions and three orders of magnitude less than carbon fluxes from the atmosphere to vegetation and back. Moreover, under stable climates (again, on scales of hundreds of thousands of years), these two flows roughly balance each other. If the climate changes, then this thing begins to happen.
In a relatively warm era, the rate of weathering increases due to an increase in temperature and river flow, CO2 begins to be rapidly removed from the atmosphere, the greenhouse effect weakens, and the Earth cools. Conversely, in colder eras, the rate of weathering decreases, but as volcanoes continue to supply carbon dioxide, it accumulates in the atmosphere, increasing the greenhouse effect and warming the planet.
Nota bene: If you now think that this thermostat will help us cope with modern global warming, I have to disappoint you: as said above, this mechanism works very slowly. We don’t have time to wait that long, so we’ll have to deal with CO2 emissions ourselves.
But let’s go back to the Neoproterozoic. So, the Earth is completely covered with ice, there is no liquid water, and the weathering processes have almost completely stopped. But volcanoes continue to steadily deliver CO2 into the atmosphere, increasing its concentration year after year. At a certain level of CO2, the greenhouse effect becomes so powerful that it can raise temperatures to the melting point of ice. According to calculations, to escape from the ice trap, the partial pressure of CO2 would have to rise to 0.29 bar (which is about 1000 times higher than pre-industrial levels). If volcanoes at that time supplied CO2 at the same rate as now (on the order of 0.3–0.4 billion tons per year), then it could take about 8 million years to accumulate the required amount. This is a minimum estimate, which is obtained with the assumption that there were no carbon dioxide sinks at all. If we take them into account, then it could take volcanoes several tens of millions of years to heat the planet — which is precisely consistent with the duration of the Sturt and Marinoan glaciations.
Once the temperature near the equator reaches the melting point, the ocean begins to become ice-free. White ice is replaced by dark water, which begins to absorb solar energy. Evaporation increases — water vapor begins to further increase warming since it is a greenhouse gas. There is an avalanche-like instantaneous (by geological standards, of course) restructuring of the climate system from the “freezer” mode to the “greenhouse” mode. At the same time, weathering processes are sharply activated, and active leaching of CO2 from the atmosphere into the ocean begins, which is buried there in the form of thick layers of “crown carbonates.” And this freezer-greenhouse cycle was completed at least twice!
Now let’s see how the formation of ferruginous sedimentary rocks fits into this scheme. At the height of the glaciation, when the ocean was completely covered with ice and isolated from the atmosphere, it lacked oxygen. Ferrous iron, coming from underwater hydrothermal vents, accumulated in the ocean — and when the sea ice broke up, it precipitated as oxides. An interesting mechanism has recently been proposed to explain the banded texture of these ferruginous deposits: it may be related to the Milankovitch cycles mentioned above. During the cold phases of the cycles, conditions could develop that favor the accumulation of iron, and during the warm phases, the ocean could be partially freed from ice, leading to the deposition of iron on the seabed.
A failure in the mechanism of this thermostat can also explain the beginning of global glaciation in the Neoproterozoic. According to modern ideas, the trigger for these events was tectonic processes. About 800 million years ago, Rodinia began to move: the processes of formation of faults in the earth’s crust (rifting) and at the same time the growth of mountains (orogenesis) began, accompanied by the outpouring of basalts, which contain large amounts of silicates. Further, since Rodinia broke up into several small continents, the access of moisture to land became easier. Taken together, all this has led to a sharp acceleration in the removal of CO2 from the atmosphere through weathering. CO2 concentrations dropped from 1,800 to 250 ppm (parts per million), after which the Earth cooled so much that glaciation began. Then the feedback mechanism described above came into force, intensifying this cooling.
Also, do not forget that the luminosity of the Sun in that era was 6 percent less than today, that is, all other things being equal, the Earth was colder.
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