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Icing of the earth. The earth is slushy snow. Surviving life during ice ages

The beginning of "Snowball Earth"

CaSiO 3 + CO 2 + H 2 O → Ca 2+ + SiO 2 + HCO 3 -

When the Earth cools (due to natural climatic fluctuations and changes in solar radiation), the rate of chemical reactions decreases and this type of weathering slows down. As a result, less carbon dioxide is extracted from the atmosphere. The increase in the concentration of carbon dioxide, which is a greenhouse gas, leads to the opposite effect - the Earth is warming up. This negative feedback limits the cooling power. At the time of cryogeny, all continents were in the tropics near the equator, which made this containment process less effective, since a high rate of weathering persisted on land even during the cooling of the Earth. This allowed glaciers to move far away from the polar regions. When the glacier moved close enough to the equator, positive feedback through an increase in reflectivity (albedo) led to further cooling until the Earth was completely frozen over.

During the ice age

The global temperature dropped so low that it was as cold at the equator as it is in modern Antarctica. This low temperature was maintained by ice, whose high albedo caused most of the incoming solar radiation to be reflected back into space. This effect was exacerbated by the small amount of clouds caused by the freezing of the water vapor.

The end of the ice age

The level of carbon dioxide required to defrost the Earth is estimated to be 350 times the current level, approximately 13% of the atmosphere. Since the Earth was almost completely covered with ice, carbon dioxide could not be removed from the atmosphere by the weathering of silicate rocks. Over millions of years, CO 2 and methane accumulated, mostly erupted by volcanoes, sufficient for the greenhouse effect that melted the surface ice in the tropics before the formation of a belt of ice-free water and land; this belt will be darker than ice, and therefore will absorb more solar energy, triggering "positive feedback".

On continents, melting glaciers will expose large amounts of glacial deposits, which will begin to erode and erode.

As a result of this, rainfall, rich in nutrients such as phosphorus, coupled with an abundance of CO 2, will cause an explosive growth in populations of cyanobacteria. This will lead to a relatively rapid reoxygenation of the atmosphere, which may be associated with the emergence of the Ediacaran biota and the subsequent "Cambrian explosion" - a high concentration of oxygen allowed the development of multicellular forms. This positive feedback loop melted the ice in a geologically short time, perhaps less than 1000 years; the accumulation of oxygen in the atmosphere and the decrease in the content of CO 2 continued for several subsequent millennia.

The water dissolved the residual CO 2 from the atmosphere, forming carbonic acid, which precipitated in the form of acid rain. This, by enhancing the weathering of outcropped silicate and carbonate rocks (including easily weathered glacial deposits), released large amounts of calcium, which, when washed into the ocean, formed clearly textured carbonate sediments. Similar abiotic "crowning carbonates" (eng. "Cap carbonates"), which can be found at the summit of glacial ales, first suggested the idea of a snowball Earth.

Perhaps the level of carbon dioxide dropped so much that the Earth froze again; this cycle could be repeated until the drift of the continents led to their movement to more polar latitudes.

Arguments in favor of the hypothesis

Glacial deposits at low latitudes

Sedimentary rocks deposited by the glacier have specific traits to identify them. Long before the hypothesis appeared Snowball earth many deposits of the Neoproterozoic have been identified as glacial. However, many of the precipitation features commonly associated with a glacier may have other origins. The evidence includes:

erratic boulders (stones that have fallen into sediment), which may be caused by glaciers or other causes;
layering (annual sedimentation in periglacial lakes);
glacial striation (formed when rock debris caught up by a glacier scratches the underlying rock): such striation is sometimes caused by mudflows.

Paleomagnetism

During the formation of rocks, magnetic domains in ferromagnetic minerals present in the rock line up in accordance with the lines of force of the Earth's magnetic field. Accurately measuring this direction allows you to estimate the latitude (but not longitude) where the rock was formed. Paleomagnetic data suggests that many of the Neoproterozoic glacial sediments were formed within 10 degrees of the equator. Paleomagnetic data, together with evidence from precipitation (such as erratic boulders), suggest that glaciers reached sea level in tropical latitudes. It is unclear whether this speaks of global glaciation or the existence of local, possibly land-limited, glaciers.

Carbon isotope ratio: no photosynthesis

There are two stable carbon isotopes in seawater: carbon-12 (C-12) and the rare carbon-13 (C-13), which makes up about 1.109% of all carbon atoms. The lighter C-12 is mainly involved in biochemical processes (photosynthesis, for example). Thus, oceanic photosynthetics, both protists and algae, are somewhat depleted in C-13 relative to the primary volcanic sources of terrestrial carbon. Therefore, in an ocean with photosynthetic life, the C-12 / C-13 ratio will be higher in organic residues and lower in the surrounding water. Organic component lithified sediments remain a little forever, but are measurably depleted in carbon-13. During the presumed global glaciation, the variations in C-13 concentration were rapid and extreme relative to the observed normal variations. This is consistent with a significant cooling that killed most or almost all of the photosynthetics in the ocean. The main issue associated with this idea is to determine the simultaneity of variations in the ratio of carbon isotopes, for which there is no geochronological confirmation.

Ferrous-silicon formations

A stone with ferruginous-silicon formations, 2.1 billion years old

Ferrous-silicon formations are sedimentary rock consisting of layers of iron oxide and iron-poor flint. In the presence of oxygen, iron rusts and becomes insoluble in water. Ferruginous-silicon formations are usually very old and their deposition is often associated with the oxidation of the earth's atmosphere during the Paleoproterozoic, when dissolved iron in the ocean came into contact with oxygen released by photosynthetics and precipitated as an oxide. The layers were formed at the interface between the oxygen-free and oxygen-containing atmospheres. Since the modern atmosphere is rich in oxygen (approximately 21% by volume), it is impossible to accumulate enough iron oxide to deposit a siliceous ferruginous formation. The only massive ferruginous-silicon formations deposited after the Paleoproterozoic are associated with cryogenic glacial deposits. For such iron-rich rocks to form, an oxygen-free ocean is needed where large amounts of dissolved iron (as iron (II) oxide) can accumulate before the oxidant precipitates as iron (III) oxide. For the ocean to become anoxic, it is necessary to restrict gas exchange with the oxygen atmosphere. Proponents of the hypothesis believe that the reappearance of ferrous-silicon formations is the result of limited oxygen levels in the ice-bound ocean.

"Crowning carbonates"

Above, Neoproterozoic glacial deposits usually turn into chemically precipitated limestones and dolomites with a thickness of meters to tens of meters. These "crown carbonates" are sometimes found in a sequence of sediments lacking other carbonates, suggesting that their formation is the result of profound changes in ocean chemistry.

These "crown carbonates" have an unusual chemical composition and a strange sedimentary structure, often interpreted as large deposits. The formation of such sedimentary rocks could have occurred with large increases in alkalinity due to high rates of weathering during the extreme greenhouse effect following global glaciation.

Surviving life during ice ages

The grandiose glaciation should have suppressed plant life on Earth and, therefore, led to a significant decrease in the concentration or even the complete disappearance of oxygen, which allowed the formation of unoxidized iron-rich rocks. Skeptics argue that such glaciation should have led to the complete disappearance of life, which did not happen. The supporters of the hypothesis answer them that life could survive in the following ways.

Oases of anaerobic and anoxiphilic life, fueled by the energy of deep-sea fluids, survived deep in the oceans and crust - but photosynthesis was not possible there.
In the open ocean, far from the supercontinent Rodinia or its fragments after its disintegration, small areas of open water could remain that survived with access to light and carbon dioxide for photosynthetics, which provided small amounts of oxygen sufficient to support some oxyphilic organisms. This option is also possible if the ocean is completely frozen, but small areas of ice were thin enough to let light through.
On nunataks in the tropics, where during the day the tropical sun or volcanic heat warmed up the rocks, protected from the cold wind, and formed temporary melt water bodies that froze after sunset.
Eggs, spores and dormant stages frozen in ice could survive the most severe phases of glaciation.
Under a layer of ice, in chemolithotrophic ecosystems, theoretically expected in the beds of modern glaciers, alpine and arctic permafrost. This is especially likely in areas of volcanism or geothermal activity.
In pools of liquid water inside and under a layer of ice, like Lake Vostok in Antarctica. According to the theory, these ecosystems are similar to microbial communities living in the permanently frozen lakes of the Antarctic dry valleys.

Russian paleontologist Mikhail Fedonkin, however, pointing out that modern data (both paleontological and molecular biological) suggest that most groups of eukaryotic organisms appeared before the Neoproterozoic glaciation, considers this evidence against "extreme paleoclimatic models in the form of the Snowball Earth hypothesis", without denying the role of cooling in the eukaryotization of the biosphere.

Evolution of life

Criticism of the hypothesis

Simulation results

Based on climate simulations, Dick Peltier of the University of Toronto concluded that large ocean areas should have remained ice-free, arguing that the “strong” version of the hypothesis is implausible for reasons of energy balance and global circulation models.

Non-glacial origin of diamictites

Sedimentary rock diamictite, usually interpreted as glacial deposition, has also been interpreted as mudflow (Eyles and Januszczak, 2004).

High slope hypothesis

One of the competing hypotheses explaining the presence of ice on the equatorial continents is the high inclination of the earth's axis, about 60 °, which placed the earth's land in high "latitudes". A weaker version of the hypothesis assumes only the migration of the Earth's magnetic field to this tilt, since the reading of the paleomagnetic data, which speaks of low-latitude glaciations, is based on the proximity of the magnetic and geographic poles (there is some data that allow us to think this way). In either of these two situations, glaciation will be limited to a relatively small area, as it is now, and radical changes in the Earth's climate will not be needed.

Inertial true displacement of the poles

Another alternative explanation of the obtained data is the concept of inertial true displacement of the poles. Proposed by Kirshvink and others in July 1997, this concept suggests that landmasses may have moved much faster than previously thought under the influence of the physical laws governing the distribution of mass around the planet as a whole. If the continents are too far from the equator, the entire lithosphere can move to bring them back at speeds hundreds of times faster than normal tectonic movements. It should look as if the magnetic pole was moving, while in fact the continents were realigning relative to it. This idea has been challenged by Torsvik (1998), Meert (1999) and Torsvik and Rehnstorm (2001), who showed that the range of pole displacement proposed by Kirshvink (1997) is insufficient to support the hypothesis. Thus, while the geophysical mechanism for the true movement of the poles is credible, the same cannot be said for the idea that such an event happened in the Cambrian.

If such a rapid movement has taken place, it must be responsible for the existence of such features of glaciation in time intervals close to the equatorial location of the continents. The inertial true displacement of the poles was also associated with the Cambrian explosion, since animals had to adapt to the rapidly changing environment... However, recent evidence no longer supports the existence of such a rapid movement in Cambrian time.

Causes of global glaciation

It is incredible that only one factor started the global glaciation. On the contrary, several factors must have coincided.

Atmosphere composition

Low levels of greenhouse gases such as carbon dioxide, methane and water vapor are required for global glaciation to begin.

Distribution of continents

The concentration of continents near the tropics is necessary for the start of global glaciation. Large quantity rainfall in the tropics leads to increased river runoff, which stores more carbonates, removing carbon dioxide from the atmosphere. The polar continents, due to low evaporation, are too dry for so much carbon deposition. The gradual increase in the proportion of the isotope carbon-13 relative to carbon-12 in sediments preceding the Varangian glaciation indicates that this is a slow, gradual process.

History of theory

1952: Australia

1998: Namibia

Interest in the Snowball Earth hypothesis increased significantly after Paul F. Hoffman, a professor of geology at Harvard University and his co-authors, published an article in Science applying Kirshvink's ideas to a sequence of Neoproterozoic sediments in Namibia.

2007: Oman: glacial-interglacial cyclicity

A group of authors, based on the chemistry of cryogenic sedimentary rocks in Oman, described active hydrological cycles and changes in climate that brought the Earth out of a completely icy state. Using the ratio of mobile cations to those remaining in the soil during chemical weathering (chemical alteration index), they concluded that the intensity of chemical weathering changed cyclically, increasing during interglacials and decreasing during cold and dry glaciations.

State of the art (April 2007)

Currently, the debate around the hypothesis continues under the auspices of the International Geoscience Program - Project 512 "Neoproterozoic Ice Age".

Other prospective global glaciations

Paleoproterozoic glaciation

The Snowball Earth hypothesis has been used to explain glacial deposits in Canada's Huron supergroup, although paleomagnetic evidence for low latitude glaciers is controversial. The glacial sediments of the South African MacGyenne Formation are slightly younger than the Huronian glacial deposits (approximately 2.25 billion years old) and formed in tropical latitudes. It was assumed that the increase in the concentration of free oxygen during this part of the Paleoproterozoic removed methane from the atmosphere, oxidizing it. Since the Sun at that time was much weaker than it is today, it was methane, as a strong greenhouse gas, that could keep the Earth's surface from freezing. In the absence of the methane greenhouse effect, temperatures dropped and global glaciation could occur.

Carboniferous glaciation (early suppositions)

Notes (edit)

For a short, simplified description, see the book by Tjeerd van Andel New Views on an Old Planet: A History of Global Change(Cambridge University Press) (1985, second edition 1994).
Hyde, W.T .; Crowley, T.J., Baum, S.K., Peltier, W.R. (2000). "Neoproterozoic" snowball Earth "simulations with a coupled climate / ice-sheet model" (PDF). Nature 405 (6785): 425-9. DOI: 10.1038 / 35013005. PMID 10839531. Retrieved on 2007-05-05.
Hoffman, P.F. (1999). "The break-up of Rodinia, birth of Gondwana, true polar wander and the snowball Earth." Journal of African Earth Sciences 28 (1): 17-33. Retrieved 2007-04-29.
D.A.D. Evans (2000). "Stratigraphic, geochronological, and paleomagnetic constraints upon the Neoproterozoic climatic paradox." American Journal of Science 300 (5): 347 – 433.
Young, G.M. (1995-02-01). “Are Neoproterozoic glacial deposits preserved on the margins of Laurentia related to the fragmentation of two supercontinents? ". Geology 23 (2): 153-156. Retrieved 2007-04-27.
D.H. Rothman; J.M. Hayes; R.E. Summons (2003). Dynamics of the Neoproterozoic carbon cycle. PNAS 100 (14): 124 – 129.
Kirschvink Joseph Late Proterozoic low-latitude global glaciation: the Snowball Earth // The Proterozoic Biosphere: A Multidisciplinary Study / J. W. Schopf; C. Klein. - Cambridge University Press, 1992.
M.J. Kennedy (1996). "Stratigraphy, sedimentology, and isotopic geochemistry of Australian Neoproterozoic postglacial camp dolostones: deglaciation, d13C excursions and carbonate precipitation." Journal of Sedimentary Research 66 (6): 1050 – 1064.
Spencer, A.M. (1971). Late Pre-Cambrian glaciation in Scotland. Mem. Geol. Soc. Lond. 6 .
P. F. Hoffman; D. P. Schrag (2002). "The snowball Earth hypothesis: testing the limits of global change." Terra Nova 14 : 129 – 155.
Fedonkin, M.A. (2006). "Two annals of life: the experience of comparison (paleobiology and genomics about the early stages of the evolution of the biosphere)". Sat. Art., dedicated. To the 70th anniversary of Academician N.P. Yushkin: "Problems of Geology and Mineralogy": 331-350.
Fedonkin, M.A. (2003). "The origin of the Metazoa in the light of the Proterozoic fossil record." Paleontological Research 7 (1).
Peltier W.R. Climate dynamics in deep time: modeling the “snowball bifurcation” and assessing the plausibility of its occurrence // The Extreme Proterozoic: Geology, Geochemistry, and Climate / Jenkins, GS, McMenamin, MAS, McKey, CP, & Sohl, L. ( - American Geophysical union, 2004. - P. 107-124.
Schrag, D. P .; Berner, R. A., Hoffman, P. F., Halverson, G. P. (2002). "On the initiation of a snowball Earth". Geochem. Geophys. Geosyst 3 (10.1029). Retrieved 2007-02-28.
A. R. Alderman; C. E. Tilley (1960). Douglas Mawson, 1882-1958. Biographical Memoirs of Fellows of the Royal Society 5 : 119 – 127.
W. B. Harland (1964). "Critical evidence for a great infra-Cambrian glaciation." International Journal of Earth Sciences 54 (1): 45 – 61.
M.I. Budyko (1969). "Effect of solar radiation variation on climate of Earth". Tellus 21 (5): 611 – 1969.
P. F. Hoffman, A. J. Kaufman; G. P. Halverson; D. P. Schrag (1998). "A Neoproterozoic Snowball Earth". Science 281 : 1342 – 1346.
R. Rieu; P.A. Allen; M. Plotze; T. Pettke (2007). "Climatic cycles during a Neoproterozoic" snowball "glacial epoch". Geology 35 (5): 299–302.
http://www.igcp512.com/
Williams G.E .; Schmidt P.W. (1997). Paleomagnetism of the Paleoproterozoic Gowganda and Lorrain formations, Ontario: low paleolatitude for Huronian glaciation. EPSL 153 (3): 157-169.
Robert E. Kopp, Joseph L. Kirschvink, Isaac A. Hilburn, and Cody Z. Nash (2005). "The Paleoproterozoic snowball Earth: A climate disaster triggered by the evolution of." PNAS 102 (32): 11131-11136.
Evans, D. A., Beukes, N. J. & Kirschvink, J. L. (1997) Nature 386, 262-266.

Literature

Arnaud, E. and Eyles, C.H. 2002. Catastrophic mass failure of a Neoproterozoic glacially-influenced continental margin, the Great Breccia, Port Askaig Formation, Scotland. Sedimentary Geology 151: 313-333.
Arnaud, E. and Eyles, C. H. 2002. Glacial influence on Neoproterozoic sedimentation: The Smalfjord Formation, northern Norway, Sedimentology, 49: 765-788.
Eyles, N., and Januszczak, N. (2004). "Zipper-rift": a tectonic model for Neoproterozoic glaciations during the break up of Rodinia after 750 Ma. Earth Science Reviews 65, 1-73.
Fedonkin, M.A. 2003. The origin of the Metazoa in the light of the Proterozoic fossil record. Paleontological Research, 7: 9-41
Gabrielle Walker, 2003, Snowball earth, Bloomsbury Publishing, ISBN 0-7475-6433-7
Jenkins, Gregory, et al, 2004, The Extreme Proterozoic: Geology, Geochemistry, and Climate AGU Geophysical Monograph Series Volume 146, ISBN 0-87590-411-4
Kaufman, A.J .; Knoll, A.H., Narbonne, G.M. (1997). Isotopes, ice ages, and terminal Proterozoic earth history (National Acad Sciences).... Includes data on the effect of global glaciation on life.
Kirschvink, Joseph L., Robert L. Ripperdan, and David A. Evans, "Evidence for a Large-Scale Reorganization of Early Cambrian Continental Masses by Inertial Interchange True Polar Wander." Science 25 July 1997: 541-545.
Roberts, J.D., 1971. Late Precambrian glaciation: an anti-greenhouse effect? Nature, 234, 216-217.
Roberts, J. D., 1976. Late Precambrian dolomites, Vendian glaciation, and the synchroneity of Vendian glaciation, J. Geology, 84, 47-63.
Meert, J.G. and Torsvik, T.H. (2004) Paleomagnetic Constraints on Neoproterozoic 'Snowball Earth' Continental Reconstructions, AGU Monograph Extreme Climates.
Meert, J.G. 1999. A paleomagnetic analysis of Cambrian true polar wander, Earth Planet. Sci. Lett., 168, 131-144.
Sankaran, A.V., 2003. Neoproterozoic ‘snowball earth’ and the ‘cap’ carbonate controversy. Current Science, vol. 84, no. 7. (includes multiple references within, online at

Back in the middle of the 20th century, geologists began to find evidence indicating that in the past, our planet could survive a worldwide glaciation. Over the years, this theory has found more and more confirmation and is now known as "Snowball Earth". According to its main provisions, in the interval between 630 and 850 million years ago, the Earth was for some time almost completely covered with ice, which at that time even reached the equator - this is evidenced by sedimentary deposits and paleomagnetic data. In total, geologists count two peaks of glaciation, which occurred 710 and 640 million and each of which lasted 10 million years.

The trigger of the glaciation was the removal of CO2 from the atmosphere, which led to a cooling and the beginning of the ice age. When the ice reached the tropics, a feedback mechanism was launched: as you know, snow and ice reflect from 55% to 80% of the incident sunlight, while for the oceans this figure is 12%, and for land from 10% to 40 %. The more part of the Earth's surface was covered with ice, the more light was reflected into space, which accordingly increased the speed of glaciation.

Like many other large-scale concepts of this kind, "Snowball Earth" has its critics. In addition, the theory itself exists in two versions: strong and weak. Strong suggests that ice completely covered the entire Earth, including the surface of the oceans, forming a layer almost a kilometer thick. The weak option is based on the fact that at least in the equator region there should have been ice-free areas of water - otherwise how then did life on our planet manage to survive this event? Especially given the fact that there is no evidence that some kind of mass extinction of species occurred during this period. In addition, the question arises of how then the Earth managed to get out of such an extreme ice age with a global freeze. As an option, the gradual accumulation of greenhouse gases in the atmosphere due to volcanic activity was called. When the amount of CO2 in the atmosphere reached 13%, this led to the end of the glaciation. However, the geological records do not contain evidence that there was so much CO2 in the earth's atmosphere at that time.

And so, a group of scientists from Columbia University climate of the "Snowball Earth" era. Modern climatic models were taken as a basis, which were then adapted to the realities of that period, including the fact that the Sun was then 6% weaker than it is now, and all land at the time of the start of the cooling was part of the supercontinent Rodinia. According to the simulation results, even if the average temperature of the Earth was 12 degrees below zero, about half of the water surface would remain free of ice - currents like the Gulf Stream would prevent the oceans from freezing completely. So, if this model is correct, instead of "Earth - Snowball" we had "Earth - Slushy Snowball".

The group is currently continuing to refine its model, trying to assess the possible impact of other factors on the climate of the "Snowball Earth" era - for example, the fact that at that time the length of the day was 21.9 hours. If the conclusions obtained are correct, then they can be useful not only for geologists but also for astrobiologists, as they can increase the boundaries of the habitable zone. The habitable zone is the region of space around the star where liquid water can exist on the surface of the planets. Usually it is calculated only based on the distance of the planet to the star. However, as the model "Earth - slushy snow" shows, the process of freezing the planet is very complex and depends on many factors. Even if the average temperature on the planet is much below freezing, open bodies of water can still exist on it - at least in theory.

The biological properties of molecular oxygen (O 2) are at least twofold. Oxygen is a powerful oxidizing agent, with the help of which you can get a lot of useful energy, and at the same time, a strong poison that freely passes through cell membranes and destroying cells if handled carelessly. It is sometimes said that oxygen is a double-edged sword ( Current biology, 2009, 19, 14, R567 – R574). All organisms dealing with oxygen must also have special enzyme systems that quench its chemical effects. Those who do not have such enzyme systems are doomed to be strict anaerobes, surviving only in an oxygen-free environment. On modern Earth, these are some bacteria and archaea.

Almost all oxygen on Earth is of biogenic origin, that is, it is released by living beings (of course, we are now talking about free oxygen, and not about oxygen atoms that make up other molecules). The main source of O 2 is oxygenic photosynthesis; there are simply no other known reactions capable of producing it in comparable quantities. From the school biology course, we know that photosynthesis is the synthesis of glucose C 6 H 12 O 6 from carbon dioxide CO 2 and water H 2 O, which occurs with the help of light energy. The main "actor" here is carbon dioxide, which is recovered by water; oxygen in this reaction is nothing but a by-product, a waste. It is less widely known that photosynthesis may not produce oxygen if, instead of water, some other substance is used as a reducing agent, for example, hydrogen sulfide H 2 S, free hydrogen H 2, or some iron compounds; this kind of photosynthesis is called oxygen-free; there are several different variants of it.

Anoxic photosynthesis almost certainly appeared much earlier than oxygen. Therefore, in the first billion years of life (and most likely longer), although photosynthesis went on, it did not cause any saturation of the Earth's atmosphere with oxygen. The oxygen content in the atmosphere at that time was no more than 0.001% of the modern one - simply put, this means that it was not really there.

Everything changed when blue-green algae, or cyanobacteria, entered the scene. Subsequently, these creatures became the ancestors of plastids, photosynthesizing organelles of eukaryotic cells (recall that organisms with cell nuclei are called eukaryotes, in contrast to prokaryotes, which have nuclear-free cells). Cyanobacteria are a very ancient evolutionary branch. By the standards of earthly history, they are surprisingly unchanged. For example, the blue-green algae oscillatoria ( Oscillatoria) has fossil relatives that lived 800 million years ago, and they are practically indistinguishable from modern oscillators (Ecology of Cyanobacteria II. Their Diversity in Space and Time, Springer, 2012, 15–36). Thus, the oscillator is an impressive example of a living fossil. But the very first cyanobacteria appeared much earlier than her - this is confirmed by paleontological data.

At first, cyanobacteria were not numerous, because the oxygen photosynthesis mastered by them did not give any serious advantages in comparison with anoxic, which was possessed by other groups of microbes. But the chemical environment of these microbes gradually changed. The moment came when there was simply no longer enough “raw material” for oxygen-free photosynthesis. And then the hour of cyanobacteria struck.

Oxygen photosynthesis has one big advantage - a completely unlimited supply of the initial reducing agent (water) and one big drawback - the high toxicity of the by-product (oxygen). Unsurprisingly, this type of exchange was not "popular" at first. But with the slightest shortage of substrates other than water, the owners of oxygenic photosynthesis should immediately gain a competitive advantage, which is what happened. After that, an epoch of about a billion years began, during which the appearance of the Earth was determined primarily by cyanobacteria. Recently, it has even been suggested to be unofficially named after them “cyanozoic” (M. Barbieri, Code Biology. A New Science of Life, Springer, 2015, 75–91).

It was because of cyanobacteria that the oxygen revolution began 2.4 billion years ago, it is also the oxygen catastrophe, or the Great Oxidative Event ( Great Oxidation Event, GOE). Strictly speaking, this event was neither instantaneous nor absolutely unique ( Nature, 2014, 506, 7488, 307-315). Short bursts of oxygen concentration, "oxygen blows", have happened before, this has been recorded paleontologically. Yet something new happened 2.4 billion years ago. For a short time by the standards of Earth's history (a few tens of millions of years), the concentration of oxygen in the atmosphere increased by about a thousand times and remained at this level; it never dropped to its former insignificant values. The biosphere has irreversibly become oxygenated.

For the vast majority of ancient prokaryotes, this level of oxygen was deadly. Unsurprisingly, the first result of the oxygen revolution was mass extinction. The survivors were mainly those who managed to create enzymes protecting from oxygen, and sometimes also thick cell walls in addition (including the cyanobacteria themselves had to do this). There is reason to believe that in the first 100-200 million years of the "new oxygen world" oxygen was only poison for living organisms and nothing more. But then the situation changed. The biota's response to the oxygen challenge was the emergence of bacteria, which included oxygen in a chain of reactions that decompose glucose, and thus began to use it for energy.

It immediately turned out that oxygen oxidation of glucose (respiration) is energetically much more efficient than anoxic (fermentation). It gives several times more free energy per glucose molecule than any arbitrarily complicated version of oxygen-free exchange. At the same time, the initial stages of glucose breakdown among the users of respiration and fermentation remained common: oxygen oxidation served only as a superstructure over the already existing ancient biochemical mechanism, which itself did not need oxygen.

The group of microbes that have mastered the risky but efficient production of energy with oxygen is called proteobacteria. According to the now generally accepted theory, it was from them that the respiratory organelles of eukaryotic cells - mitochondria - originated.

According to genetic data, the closest modern relative of mitochondria is the purple helical alpha-proteobacteria Rhodospirillum rubrum (Molecular Biology and Evolution, 2004, 21, 9, 1643-1660). Rhodospirillum has respiration, fermentation, and oxygen-free photosynthesis, in which hydrogen sulfide is used instead of water, and can switch between these three types of exchange, depending on external conditions. Undoubtedly, such a symbiont - that is, in this case, an internal cohabitant - was very useful to the ancestor of eukaryotes.

Moreover, many modern scientists believe that the symbiosis of ancient archaea with proteobacteria - the ancestors of mitochondria - was the impetus for the very formation of the eukaryotic cell (Evgeny Kunin. Logic of the case. M .: Tsentrpoligraf, 2014). This hypothesis is called "early mitochondrial". She suggests that the division of the future eukaryotic cell into the cytoplasm and the nucleus occurred only after the introduction of the proteobacterial symbiont into it. The older "late mitochondrial" scenario, according to which the proteobacteria was simply swallowed by a ready-made eukaryotic cell (self-generated from an archaeal cell), now looks much less likely. In fact, both cells - both archaeal and proteobacterial - were seriously "reassembled" in the process of combining, giving rise to a kind of chimera with new properties. This chimera became a eukaryotic cell; the molecular components of archaeal and proteobacterial origin are strongly mixed in it, dividing functions among themselves (Paleontological Journal, 2005, 4, 3–18). Without proteobacteria, eukaryotes would not have arisen. This means that their appearance was a direct consequence of the oxygen revolution.

In light of the above, the words of two modern major scientists, a paleontologist and a geologist, almost do not look like an exaggeration: "Everyone agrees that the evolution of blue-green algae was the most significant biological event on our planet (even more significant than the development of eukaryotic cells and the emergence of multicellular organisms)" (Peter Ward, Joe Kirshvink. A new history of the origin of life on Earth. St. Petersburg: Publishing House "Peter", 2016). Indeed, the familiar world of animals and plants would not exist now if it were not for cyanobacteria and the crisis caused by them.

Eras of life

The entire history of the Earth is divided into four huge intervals, called eons (this is higher than the era). The names of the aeons are as follows: katarchean, or viper (4.6–4.0 billion years ago), archaea (4.0–2.5 billion years ago), Proterozoic (2.5–0.54 billion years ago) and Phanerozoic (started 0.54 billion years ago and continues now). This division will constantly help us, it is really convenient. Let's make a reservation that in almost all such cases, remembering is not time boundaries, but the sequence of eras and related events: this is much more important. An exception can be made only for two or three fundamental dates like the age of the Earth.

Katarchei is the so-called pre-geological era, from which no "normal" rocks, located in layers, remained. Classical geological and paleontological methods based precisely on the comparison of successive layers do not work there. The objects remaining from the catarchea are mostly small grains of zircon, the very ones in which presumably biogenic carbon has recently been found. Very little is known about Catarchean life (if any).

In the Archean, the Earth belongs to prokaryotes - bacteria and archaea (just no confusion, the coincidence of the roots in the name of the geological era "archaea" and the group of microbes "archaea" is actually accidental). The border of the Archean and Proterozoic period falls approximately at the time of one of the strong "oxygen breaths" preceding the oxygen revolution. The oxygen revolution itself took place at the beginning of the Proterozoic.

The Proterozoic is the era of oxygen and eukaryotes. An interesting paradox is associated with the dating of the origin of eukaryotes. The point is that more or less reliably definable multicellular eukaryotes appear in the fossil record much earlier than equally reliably definable unicellular ones. Filamentous algae Grypania spiralis, which is usually considered a eukaryote, appeared 2.1 billion years ago ( Australasian Journal of Palaeontology, 2016, doi: 10.1080 / 03115518.2016.1127725). In fairness, it must be said that the main argument for the eukaryotic nature of influenza is its large size - all other signs do not give confidence that this is not a giant cyanobacterium ( Palaeontology, 2015, 58, 1, 5–17). But the fact is that this find is not the only one. The oldest known eukaryote is now considered a mushroom-like organism. Diskagma buttonii 2.2 billion years old ( Precambrian Research, 2013, 235, 71–87). And then there are mysterious large spiral-shaped creatures - most likely algae, the remains of which are at least 2.1 billion years old, like that of influenza ( Nature, 2010, 466, 7302, 100–104). But the earliest unicellular organisms, unambiguously identified as eukaryotes, are only 1.6 billion years old ( , 2006, 361, 1470, 1023-1038). This, of course, does not mean that multicellular eukaryotes really appeared before unicellular ones - this assumption contradicts all available molecular data. Single-celled ones are simply worse preserved, and they have fewer signs by which one can determine the organism.

Nevertheless, very important conclusions follow from such dating. Recall that the date of the oxygen revolution is 2.4 billion years ago. Therefore, we know that only 200 million years after it, not just eukaryotes, but multicellular eukaryotes appear in the fossil record. This means that the first stages of the evolution of eukaryotes were passed by the standards of global history very quickly. Of course, it took a eukaryotic cell time to form a symbiosis with the ancestors of mitochondria, create a nucleus, and complicate the cytoskeleton - the intracellular system of supporting structures. But when these processes ended, it was possible to create the first multicellular organisms almost immediately. This did not require any additional adaptations at the level of the cage. Any eukaryotic cell already has a complete set of molecular elements necessary to build a multicellular body (at least relatively simple) from such cells. Of course, all these elements are no less useful for the life of a single cell, otherwise they simply would not have arisen. The common ancestor of eukaryotes, without a doubt, was unicellular, and many of its descendants never needed multicellularity. We know examples of modern unicellular eukaryotes - amoeba, euglena, ciliates - thanks to school textbooks, but in fact there are many more of them.

The oxygen revolution had another important effect on the composition of the atmosphere. The Archean atmosphere was rich in nitrogen (as it is now), as well as carbon dioxide and methane (much more than now). Carbon dioxide and methane absorb infrared radiation very well and thereby retain heat in the Earth's atmosphere, preventing it from going into space. This is called the greenhouse effect. Moreover, it is believed that the greenhouse effect from methane is at least 20-30 times stronger than from carbon dioxide. And in Archean times, there was about 1000 times more methane in the Earth's atmosphere than it is now, and this provided a rather warm climate.

Here astronomy also intervenes. According to the generally accepted theory of stellar evolution, the luminosity of the Sun is slowly but steadily increasing. In the Archean, it was only 70–80% of the modern one - it is understandable why the greenhouse effect was important for keeping the planet warm. But after the oxygen revolution, the atmosphere became oxidizing and almost all of the methane (CH 4) turned into carbon dioxide (CO 2), which is much less efficient as a greenhouse gas. This caused the catastrophic Huronian glaciation, which lasted about 100 million years and at some points covered the entire Earth: traces of glaciers were found on land areas that were then only a few degrees of latitude from the equator ( , 2005, 102, 32, 11131-11136). The peak of the Huronian glaciation came 2.3 billion years ago. Fortunately, glaciation could not stop the tectonic activity of the earth's mantle; volcanoes continued to emit carbon dioxide into the atmosphere, and over time it accumulated enough to restore the greenhouse effect and melt the ice.

However, the main climatic tests were still ahead.

The end of the boring billion

The turbulent events of the beginning of the Proterozoic were followed by the so-called "boring billion years" ( Boring billion). At this time, there were no glaciations, no abrupt changes in the composition of the atmosphere, no biospheric upheavals. Eukaryotic algae lived in the oceans, gradually releasing oxygen. Their world was diverse and complex in its own way. For example, from the era of the "boring billion" known multicellular red and yellow-green algae, surprisingly similar to their modern relatives ( Philosophical Transactions of the Royal Society B, 2006, 361, 1470, 1023–1038). Mushrooms also appear at this time ( Paleobiology, 2005, 31, 1, 165-182). But multicellular animals are absent in the vastness of the "boring billion years". Let's be careful: at the moment, no one can say with complete confidence that there were no multicellular animals then, but all the data on this topic is, at best, very controversial ( Precambrian Research, 2013, 235, 71–87).

What's the matter here? The thought suggests itself that multicellularity as such is much more compatible with the way of life of a plant than of an animal. Any plant cell is enclosed in a rigid cell wall, and there is no doubt that this greatly facilitates regulation mutual arrangement cells in a complex body. On the contrary, animal cells lack a cell wall, their shape is unstable, and even constantly changes during acts of phagocytosis, that is, the absorption of food particles. Collecting a whole organism from such cells is a difficult task. If no multicellular animals appeared at all, and representatives of plants or fungi became biologists, they, most likely, after studying this problem, would come to the conclusion that the combination of multicellularity with the absence of a cell wall is simply impossible. In any case, this explains why multicellularity has arisen many times in different groups of algae, but only once - in animals.

There is also another idea. In 1959, the Canadian zoologist John Ralph Nursell linked the sudden (as was then believed) appearance of animals in the fossil record with an increase in the concentration of oxygen in the atmosphere ( Nature, 1959, 183, 4669, 1170-1172). Animals, as a rule, have active mobility, which requires so much energy that they cannot do without oxygen breathing. And you need a lot of oxygen. And in the era of the "boring billion", the O 2 content in the atmosphere almost certainly did not reach 10% of the current level - the minimum often considered necessary to support animal life. True, this suspiciously round figure is most likely overstated ( Proceedings of the National Academy of Sciences USA, 2014, 111, 11, 4168–4172). Such reservations, however, do not prevent us from admitting that Nersell's old idea at least does not contradict modern data: the supposed beginning of the evolution of multicellular animals is very approximate, but coincides in time with a new increase in the concentration of atmospheric oxygen at the end of the Proterozoic ( Annual Review of Ecology, Evolution, and Systematics, 2015, 46, 215–235). It simply could not but be a factor that facilitated the appearance of animals: after all, the more oxygen, the better. Do not just consider the oxygen factor to be strictly the only one. Let us remember that even at a time when there was as much oxygen as needed, no repeated attempts to create multicellularity of the animal type were noted. This experiment has succeeded nature only once.

The cozy era of "boring billion years" could have lasted a long time if geography had not intervened in biology. Dramatic events, the hero of which became the planet itself, attracted the attention of scientists for half a century, but only 15 years ago, information about them was able to be combined into a more or less integral picture. Let's take a quick look at this picture, starting from the beginning, as it should be.

In 1964, the English geologist Brian Harland published an article in which he stated that absolutely on all continents there are traces of ancient glaciation dating back to the same time - the Late Proterozoic. It was in the early 60s that geologists learned to determine the past position of continents using data on the magnetization of rocks. Harland collected this data and saw that there was only one way to explain it: assuming that the Late Proterozoic glaciation covered all the latitudes of the Earth at once, that is, it was planetary. Any other hypothesis looked even less plausible (for example, one would have to assume an incredibly fast movement of the poles, so that all the lands in turn were covered by the polar cap). As Sherlock Holmes said during his quest for Jonathan Small, "Throw away the impossible, what remains will be the answer, no matter how incredible it seems." This is exactly what Harland did. The detailed article written by him with a co-author does not pretend to be any sensation - it simply honestly sets out the facts and conclusions ( Scientific american, 1964, 211, 2, 28–36). And yet the hypothesis of a planetary glaciation was too bold for most scientists.

Literally in the same years, the famous geophysicist, Leningrader Mikhail Ivanovich Budyko took up the theory of glaciers. He drew attention to the fact that glaciation can self-develop. The ice cover has a high reflectivity (albedo); therefore, the larger the total glacier area, the greater the proportion of solar radiation reflected back into space, taking away heat with it. And the less heat the Earth receives, the colder it becomes, and the ice cover area as a result grows, increasing the albedo even more. It turns out that glaciation is a process with positive feedback, that is, it is capable of strengthening itself. And in this case, there must be some critical level glaciation, after which it will grow until ice waves from the North and South Poles collapse at the equator, completely enclosing the planet in the ice cover and lowering its temperature by several tens of degrees. Budyko showed mathematically that such a development of events is possible ( Tellus, 1969, 21, 5, 611-619). But he had no idea that it happened several times in the history of the Earth! Because at that time Budyko and Harland had not read each other yet.

Snowball Earth

Now the glaciation that Harland discovered is usually called the era of "Snowball Earth" ( Snowball earth). Apparently, it really was planetary. And its main reason is considered to be a sharp weakening of the greenhouse effect due to a drop in the concentration of carbon dioxide (which became the main greenhouse gas after oxygen "ate" almost all the methane). Photosynthesis and respiration probably have nothing to do with it. If the biota of the Earth arranged the oxygen revolution for itself, now it has become a victim of an external factor, completely non-biological in nature.

The fact is that the turnover of carbon dioxide is much less dependent on living beings than the turnover of oxygen. The main source of atmospheric CO 2 on Earth is still volcanic eruptions, and the main sink is a process called chemical weathering. Carbon dioxide interacts with rocks, destroying them, and itself turns into carbonates (HCO 3 - or CO 3 2− ions). The latter dissolve well in water, but they no longer enter the atmosphere. And the result is an extremely simple dependence. If the intensity of the volcanoes exceeds the intensity of chemical weathering, the atmospheric concentration of CO 2 increases. If on the contrary, it falls.

At the end of the "boring billion", 800 million years ago, almost all of the earth's land was part of the only supercontinent called Rodinia. According to one well-known geologist, giant supercontinents, like large empires in the social history of the Earth, have always been unstable (VE Khain, MG Lomize. Geotectonics with the basics of geodynamics. M: Izd-vo MGU, 1995). Therefore, it is not surprising that Rodinia began to split. At the edges of the faults, erupted basalt solidified, which immediately became the object of chemical weathering. There was no soil then, and the products of weathering were easily carried into the ocean. Rodinia eventually split into seven or eight small — about the size of Australia — continents that drifted apart. The consumption of CO 2 for the weathering of basalt led to a drop in its level in the atmosphere.

Volcanism, which inevitably accompanied the disintegration of the supercontinent, could compensate for this, if not for one accidental circumstance. Due to some quirks of continental drift, both Rodinia and its fragments were located at the equator, in a warm belt, where chemical weathering proceeded especially rapidly. Mathematical models show that it is for this reason that the concentration of CO 2 dropped below the threshold beyond which glaciation begins ( Nature, 2004, 428, 6980, 303-306). And when it started, it was too late to slow down the weathering.

It must be admitted that the position of the continents in the Late Proterozoic was as unfortunate (from the point of view of the planet's inhabitants) as possible. Continental drift is driven by flows of material in the earth's mantle, the dynamics of which is, in fact, unknown. But we know that in this case, these streams have collected all of the earth's land in a single continent, located exactly at the equator and elongated in latitude. If it were at one of the poles or was extended from north to south, the onset of glaciation would have closed some of the rocks from weathering and thereby stopped the escape of carbon dioxide from the atmosphere - then the process could have slowed down. We are witnessing just such a situation now, when there are ice sheets of Antarctica and Greenland ( Scientific american, 1999, 9, 38). And at the end of the Proterozoic, almost all large areas of land were close to the equator - and were exposed until the moment when the northern and southern ice sheets closed. The earth has become an ice ball.

In fact, there were at least three episodes of Snowball Earth. The first of them was related to the Huronian glaciation (which, as we remember, was not due to carbon dioxide, but due to methane). Then, for more than a billion years, there was no glaciation at all. And then two more planetary glaciations, separated by a small interruption, followed, one of which lasted about 60 million years, the other - about 15 million years. They were discovered by Brian Harland. The geological period covering these glaciations is called cryogeny (it is part of the Proterozoic).

Little is known about the living nature of cryogeny. The climate then on the whole Earth was, by today's standards, Antarctic. Most of the oceans were covered with a kilometer-long layer of ice, so the rate of photosynthesis could not be high. Light, unexpectedly becoming a valuable resource, only entered the ocean in places, through cracks, openings, or small patches of thin ice. It is surprising that some multicellular organisms have managed to survive cryogeny without changing at all, such as red algae. Even now they are adapted to use very weak light penetrating to such a depth where no other photosynthetic creatures live anymore (Yu. T. Dyakov. Introduction to algology and mycology. M .: Publishing house of Moscow State University, 2000). The unicellular plankton has not gone anywhere either. The oxygen content in the cryogenic ocean has dropped dramatically, so life at its bottom was most likely mostly anaerobic, but the details of this are still hidden from us.

The endings of the episodes of "Snowball Earth" are also dramatic in their own way. During the planetary glaciations, all processes associated with the absorption of large volumes of carbon dioxide were literally frozen. Meanwhile, volcanoes (whose work no one stopped) threw out and emitted CO 2 into the atmosphere, gradually bringing its concentration to enormous values. At some point, the ice sheet could no longer resist the greenhouse effect, and then an avalanche-like process of heating the planet began. In literally several thousand years - that is, geologically in an instant - all the ice melted, the released water flooded a significant part of the land with shallow marginal seas, and the temperature the earth's surface, judging by the calculations, jumped to 50 ° С ( Engineering and Science, 2005, 4, 10–20). And only after this did the Earth begin a gradual return to the "normal" non-glacial state. During cryogenesis, this entire cycle was passed at least twice.

Researchers from China and the United States have analyzed the content of various magnesium isotopes in rocks from southern China dating back 635 million years. The content of various magnesium isotopes indicated that these rocks at that time were undergoing severe erosion under the influence of carbonic acid. The discovery confirms the long-developed hypothesis that the Snowball Earth melted when acid rains began to fall over it en masse. Corresponding published in Proceedings of the National Academy of Sciences.

Scientists have examined a piece of rock that was part of a mountain peak 635 million years ago. It protruded above the planetary glacier that covered the Earth at that time, and was exposed to direct contact with rains containing carbonic acid. This changed the ratio of magnesium isotopes in the glacier. As the researchers note, their find shows that it was the huge concentration of carbon dioxide in the air that led to the thawing of the Earth. If it was enough for showers with carbonic acid, then the greenhouse effect reached a level unthinkable by today's standards.

In addition, the new work points to the source of the carbonate "cap" - a layer of carbonate deposits overlying the layers of global glaciation. Carbonic acid was a corrosive chemical medium through which carbonates were formed from rocks. With the melting water, they flowed into the oceans, where they became the basis for a sharp increase in the content of calcium compounds. An excess of this substance played a large role in the formation of the Cambrian fauna. The then multicellular creatures often used calcium to "build" external hard defects.

Our planet's climate is governed in the long term by the carbon cycle. If it is too hot on it, carbon dioxide from the air is actively absorbed by rocks. With a low content of carbon dioxide in the air, the greenhouse effect weakens - and the Earth cools again. If it gets cold, the rate of chemical reactions slows down and carbon dioxide is less absorbed by rocks, accumulating in the atmosphere. From this comes global warming, and the climate is still returning to normal. 650 million years ago, this natural thermostat failed for reasons not yet clear.

Once there was so little carbon dioxide that a global glaciation was established on the planet: all water and land were covered with ice, even at the equator. This state in geology is referred to as snowball earth. According to the logic of the carbon cycle, volcanic eruptions that replenish atmospheric carbon dioxide, over time, should have raised its concentration to enormous values, because rocks and sea water from under the ice could not bind the key greenhouse gas. Over time, its share in the air rose so much that the greenhouse effect overpowered the cooling of the Earth due to the reflection of sunlight by ice.

The hypothesis had a serious flaw: it was very difficult to test it. In theory, a high concentration of carbon dioxide in the air should lead to the spontaneous formation of carbonic acid and its precipitation with water in the form of acid rain. However, earlier all attempts to find direct chemical traces of such rains were unsuccessful. The fact is that they walked when the planet was completely covered with ice and it was very difficult to get to the rocks.

7.10.11 Some researchers believe that twice or three times in the history of our planet there was a period, conventionally designated "Earth-snowball", when ice almost completely covered the surface of the Earth. The last time this happened was about 635 million years ago. Then, for a number of reasons, the greenhouse effect occurred, and the planet thawed.

However, an international team of scientists questioned the surge in atmospheric carbon dioxide concentrations at the time. According to new data, the greenhouse effect was not strong enough to melt thick ice. Consequently, the Earth did not turn into a big snowball.

The main evidence in favor of the hypothesis is glacial deposits, which were 635 million years ago in the equator. Above them is a layer of "cap carbonates", which are believed to have formed when the glaciers melted or shortly thereafter, that is, when there was an abundance of carbon dioxide in the atmosphere.

It is believed that the "Snowball Earth" period ended when the level of carbon dioxide in the atmosphere rose. Volcanic activity could have been the cause. The factors that, under normal conditions, remove carbon dioxide from the atmosphere have been blocked by ice. In addition, the cold weather prevented the weathered rocks from absorbing carbon dioxide to form bicarbonates. All this led to the accumulation of greenhouse gas in the atmosphere.

The researchers decided to find out how much carbon dioxide was in the atmosphere at that time. To do this, they analyzed the chemical composition of the Brazilian rocks of that time and the fossilized organic matter inside them. The specialists were interested in the isotope ratio.

Both rocks and organic matter (mainly algae) extract carbon from carbon dioxide dissolved in the ocean. A decrease in gas concentration leads to the fact that the algae begin to lean on the heavier isotope. On the other hand, the ratio of carbon isotopes in carbonate rocks does not change regardless of the concentration of carbon dioxide.

Comparison of indicators of stone and organics showed that the concentration of carbon dioxide in the atmosphere was much lower than previous estimates. It was said to be 90 thousand parts per million, and the new analysis claims it was less than 3,200 parts per million. It is possible that the concentration was approaching today's (about 400 ppm).

Red-brown, iron-rich glacial deposits in the Ogilvy Mountains (Yukon Territory, Canada). They formed 716.2 million years ago, when the planet may have been almost completely covered in ice. (Photo by Francis Macdonald.)

“And since there was no high concentration of carbon dioxide in the atmosphere, it means that there could not be Snowball Earth, otherwise the Earth would have been frozen to this day,” summarizes the study's author Magali Ader from the Geophysical Institute in Paris (France).

She, however, warns that many ambiguities remain. It is possible, for example, that the rocks were not dated correctly. There is also a possibility that the greenhouse effect was caused not by carbon dioxide, but by methane ...