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We know that the earth now looks like a blue water polo from outer space. But have you ever thought about such questions? Is it always this blue and woof? In winter, it often snows in some places, while snow is seldom seen in some places. So is it possible that at some point, every corner of the earth will be covered with snow and ice? With these questions, let's go into the Precambrian Neoproterozoic and take a look at the story of the earth's "snow quilt".

1. The discovery of "snow quilt" Neoproterozoic is a very interesting geological era in the process of earth evolution. During this period, the earth was like a teenager, full of vitality, bacteria and algae flourished, and animal fossils began to appear; like women in love, the climate was constantly changing, hot and cold, and had a restless heart. the plates are unstable and the continents are divided. About 750 million years ago, the entire Rodinia supercontinent split in two (figure 1), opening the ancient ocean; North America moved south toward the snow-covered South Pole. Most of the northern half of Rodinia rotates counterclockwise and crosses the frigid North Pole northward. In the following two hundred million years (750 million to 550 million years ago), intermittent large-scale glacier events occurred on the earth. "snow quilt" is a phenomenon in this period.

Fig. 1 schematic diagram of the breakup of the Rodinia supercontinent (from the network) in the course of studying the Neoproterozoic, scientists have noticed many strange geological phenomena:

Many glacial sediments contain carbonate detritus or are directly covered by carbonate rocks, like a cap, called capped carbonate rock. In geoscience, it is inconceivable that glacial deposits represent a cold climate, while carbonate rocks indicate a warm environment.

Banded iron formation appeared in some glacial units (BIF, a special marine iron-rich sedimentary rock, almost only occurred in Precambrian, with characteristic Si-rich-iron interlaced zone, figure 2) banded iron formation was completed 1.8 billion years ago, and the global banded iron formation deposition event reappeared a billion years later.

The carbon isotope has a strong negative drift, which is very significant compared with the previous 1.2 billion years. (the carbon isotope value is δ 13C = (13C: 12C) sample / (13C: 12C) reference standard)-1, which can be roughly regarded as the relative ratio of 13C to 12C. Most scholars believe that the negative drift of carbon isotopes is a process in which carbon isotopes drift rapidly to negative values and gradually return) this problem will be discussed in detail later.

Paleomagnetic evidence also shows that during at least two Neoproterozoic ice ages, the ice almost spread to the equator. This leads us to guess that if the equator freezes, the whole world must have become a glacier. At this time, the earth is undoubtedly equivalent to a layer of snow quilt. Then a new question arises: how did the "snow quilt" come into being? How did it disappear? Why can't you see it now?

Figure 2 banded iron construction (from the network) 2. The formation and disappearance of "snow quilts" to explain these problems, in 1992, Kirschvinck proposed the snowball Earth hypothesis, which was later developed by Hoffman. They believe that during the Neoproterozoic ice age, all the oceans froze and the earth became a huge snowball, which is what we call a "snow quilt".

The reason for the formation of "snow quilt" is a feedback effect of albedo of ice and snow: ice and snow reflect a large amount of sunlight-- the global temperature becomes lower-- the coverage area of ice and snow increases-- the reflection speed of sunlight increases-- and the global surface temperature decreases sharply. Scientists call it out of control of the albedo feedback effect. At the same time, the breakup of the Rodinia supercontinent and the expansion of the continental margin accelerated rock weathering and consumed a lot of carbon dioxide, which, as a gas of Greenhouse Effect, will also promote global snow cover. When the earth was frozen, there was a lack of gas exchange between the ocean and the atmosphere, leaving the ocean in an anoxic environment, which could explain the banded iron construction caused by lack of oxygen in the ocean during this period.

Now that the earth has become a snowball, how did the earth take off the snowball hat? This is the regulating function of the earth system. At this time, the global temperature is as low as-50 ℃, the whole ocean is completely frozen, the surface water-air exchange is seriously hindered, and the surface weathering and carbon sequestration of marine organisms are stagnant, resulting in the accumulation of a large number of CO particles produced by volcanic eruptions in the atmosphere. When the atmospheric CO concentration reaches about 350 times the current concentration, the strong Greenhouse Effect will cause the global ice sheet to disintegrate instantly, eventually cause the glacier to melt, and the "snow quilt" will disappear. Such a mechanism enables global snow cover and melting to be completed in a relatively short period of time. You can imagine what a magical scene it would be if you were standing in outer space and watching the earth.

Fig. 3 the earth is gradually covered with ice and snow (Baidu encyclopedia) 3. The discovery of "snow quilt" after the disappearance of "snow quilt", a series of stories are happening quietly. Our earth removes the "snow quilt" and covers it with a "carbonate" hat, the so-called capped carbonate rock, and the carbon isotope in this layer of carbonate is significantly negative. Of course, there is also the prosperity of life. You know, the "snow quilt" looks good but does not keep warm. At that time, the creatures fell asleep like hibernation, waiting for the opportunity to come. The disappearance of the "snow quilt" is like a key to rebirth. Some animals and plants can't wait to get on the stage of life after the snow quilt disappears. As for the reasons behind these stories, let's follow Hoffman to find out.

In 1998, Hoffman studied a carbonate platform of the Otavi formation south of the Congo Craton in northern Namibia. The main strata studied include two separate glacial units of the Sturtian era: the Chuos and the Ghaub formation. (Sturtian: about 760-700Ma, a period of glaciation, which consists of four global glacial events: Sturtian, Varangian, Smalfjord, Mortesnes). Under the two units are thick carbonate strata with high carbon isotope values, and the two units are covered by unique cap carbonate rocks. The difference between the upper and lower parts records the negative drift of carbon isotopes.

Fig. 4 Otavi formation stratigraphic map Carbonate rock and organic carbon are the two major reservoirs of carbon in nature, and the rest are sea water, atmosphere and biosphere. The δ 13C in sea water and carbonate rocks is about 0 ‰, and that in atmosphere is about-7 ‰, and the organic carbon is the lowest. At the beginning of the ice age, the carbon isotope dropped sharply, with a minimum of-6 ‰, and did not rise to 0 ‰ until 480 meters above the capped carbonate rock. The whole negative δ 13C drift occupies about 500 meters of the carbonate profile of the platform.

The variation trend of δ 13C in Otavi group was consistent with that of snowball hypothesis. Hoffman explained the carbon isotope changes before and after the ice age and believed that the negative drift of carbon isotopes was due to the decrease of organic carbon burial flux. Because organisms give priority to the use of light carbon, organic carbon is mainly rich in 12C, while carbonate rocks are rich in 13C. The proportion of organic carbon to total carbon burial is about 0.5 before glacial deposition and about 0 after glacial deposition. This is also consistent with the Snowball Earth hypothesis, according to the "Snowball Earth" hypothesis, the ice will block the sun, ocean photosynthesis will be seriously reduced over millions of years, and the amount of organic carbon buried will be reduced, causing δ 13C negative drift.

So how long did it take for the snow quilt to disappear?

According to Caldeira and Kasting's estimates, modern carbon dioxide input from volcanoes requires an atmosphere of about 0.12bar (bar, a commonly used unit of pressure, abbreviated to bar) to overcome snowball albedo and cause melting. This estimate means that in the absence of atmospheric-ocean gas exchange, the ice age will last for about 4 million years at the modern rate of carbon dioxide released by onshore volcanic activity. If there is a partial gas exchange through the cracks between the sea ice, the time will be extended. In the Neoproterozoic, due to the low solar luminosity and the decrease of carbonate pelagic deposition, the snowball glaciation lasted longer, which would also reduce the release rate of volcanic CO rocks at the margin of the converging plate. If the maximum duration is 30 million years, then the duration of isotope drift of the Otavi formation estimated by Hoffman is 9 million years, within this range.

Fig. 5 there is another problem with the negative drift of δ 13C in the Otavi formation. How is such a thick cap carbonate rock formed?

With regard to the formation of capped carbonate rocks, three factors need to be considered. The concentration and alkalinity of carbon and calcium. Due to mid-ocean ridge hydrothermal activity and basalt low-temperature alteration, the Ca / Mg ratio of seawater increases; the reduction of atmospheric CO pressure from 0.12bar to 0.001bar (from terminal snowball conditions to normal Neoproterozoic values) will provide about 1020 grams of carbon, so much carbon content will produce 8 × 105cubic kilometers of carbonate rocks, which, if flattened to the present continental crust, are five meters thick!

If there is no alkalinity input from the river, the carbonate rock will dissolve into the deep sea driven by the CO input of mid-ocean ridge volcanic activity. However, in the process of melting snow and ice, the warm climate, high CO content and strong hydrological cycle promote strong continental weathering, which increases the alkalinity of rivers, and a large amount of alkali is imported into the ocean. On the other hand, there is gas exchange between the ocean surface and the high concentration of CO. Sufficient calcium and CO lead to the formation of carbonate rocks (calcium carbonate), and excessive alkalinity also inhibits the dissolution of carbonate, which makes the seafloor carbonate supersaturated. As a result, it is not surprising to form thick carbonate strata!

3.2 Open the door to rebirth-after the Ediacaran life explosion "snow quilt" disappeared, the life that survived in the glacier-closed ocean began to "awaken", and the species and number of organisms increased by leaps and bounds in the extreme greenhouse environment. Many prokaryotes that dominated the Neoproterozoic biosphere developed vigorously, eukaryotes (including red algae, green algae, etc.) evolved in the late Neoproterozoic, and suspicious source microfossils changed frequently, thus a long-awaited life prosperity began.

It is necessary for us to explain how earth scientists find the evidence of the explosion of life from the earth.

Reference:

[1] Paul F. Hoffman,Alan J. Kaufman,Galen P. Halverson,Daniel P. Schrag. A Neoproterozoic Snowball Earth [J]. Science, 1998 (5381).

This article comes from the official account of Wechat: stone popular Science Studio (ID:Dr__Stone). Author: Ruoyu, Beautiful Editor: strange cc

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