The Atmosphere And The Global Ocean

There can be no discussion about the atmosphere without including the global ocean. The two are coupled; even small changes in the temperature or chemistry of the global ocean can produce enormous changes in the atmosphere.

The composition of Earth's present-day atmosphere is basically known. Essentially it is made up of two gases: 78 percent of its volume is nitrogen, and 21 percent is oxygen. The remaining 1 percent is made up of trace amounts of other gases. Yet even at this small volume, this 1 percent has a huge effect on the planet, for within this 1 percent are both the important greenhouse gas of carbon dioxide and water vapor (itself a gas). "Greenhouse gas" is the term now used to describe any gas at work to trap heat in the atmosphere, thus warming the planet. How long has our planet had this atmosphere?

The atmosphere of our planet is as old as Earth itself. The two originated at the same time somewhere around 4.6 billion years ago— a date that is almost one-third the age of the Universe itself. The planet was molten soon after formation, but rapid cooling set in and as temperatures dropped, the planet rapidly evolved. Once formed, the solid Earth and its gaseous atmosphere evolved in quite different ways, even though each influenced the other over time. Like all planets, Earth formed through accretion of particles in a solar or planetary nebula. The formation of Earth was but one part of the formation of the entire solar system. As our planet accreted, it began to differentiate, with the heavier elements sinking toward the center and the lighter elements staying near the surface. In this fashion the major structural elements of our planet, its dense inner core, middle mantle, and outer crust regions formed. This process led to rapid changes in the atmosphere of the forming earth as well. Enormous quantities of gas were trapped in the differentiating Earth and sequestered far beneath the surface of the planet. Over time this gas began to escape into the atmosphere and in so doing rapidly changed the composition of the planet's gaseous envelope. We have a clue about the nature of the gas still trapped within Earth by studying the gas composition of volcanoes. Present-day composition of volcano effluents are 50-60 percent water vapor, 24 percent carbon dioxide, 13 percent sulfur, and about 6 percent nitrogen, with traces of other gases, a composition that differs markedly from the current atmospheric composition.

Our world ocean (since it is interconnected, even though we give parts of it separate names) has also changed its chemistry, mainly by changes in salinity through time. Most scientists believe the oceans have gradually become saltier through time, although a smaller but vocal group advocates that the oceans have become less salty through time. (The amount of salt in the oceans has no effect on the atmosphere and thus plays no part in our story.) The most characteristic aspect of our planet is its envelope of liquid water, and it would seem reasonable to assume that the voluminous oceans of planet Earth were created as part of the natural evolution of the cooling planet. This may not be the case, however. While the outer planets and moons of our solar system, from Jupiter outward, are rich in water, astronomers modeling how solar systems form have discovered that water should be in short supply among the inner parts of the solar system. Because of this, it is now believed that an appreciable volume of Earth's surface water was brought here from the outer reaches of the solar system by comets impacting the planet early in its history. If this is the case, it indicates that much of our oceans and perhaps an appreciable portion of our atmosphere are exotic to Earth. Most of this delivery happened in the first 500 million years of Earth's history, and the rain of comets onto the planet during the period from 4.2 billion to 3.8 billion years ago, known as the Heavy Bombardment period, may have caused Earth's early oceans to be repeatedly vaporized into steam.

The composition of Earth's atmosphere early in its history is a controversial and heavily researched topic. While the amount of nitrogen may have been similar to that of today, there are abundant and diverse lines of evidence indicating that there was little or no oxygen available. Carbon dioxide, however, would have been present in much higher volumes than today and this carbon dioxide-rich atmosphere would have created hothouse-like conditions through a super greenhouse effect, with carbon dioxide partial pressures (measured as the actual amount of total gas pressure exerted by the atmosphere) 10,000 times higher than today.

There is abundant evidence that the present-day atmosphere is very different from that of the past. The most compelling lines are geologic. Today, the atmosphere contains so much oxygen that reduced metal species quickly oxidize: the familiar rusting of iron to a red color or the oxidation of copper to shades of green is evidence of this. In similar fashion, many metal-rich or organic-rich types of sediment quickly bind with atmospheric oxygen to produce oxidized minerals. Long ago in Earth's history, however, minerals formed that are no longer seen on the planet's surface. Before about 2.5 billion years ago the formation of "red beds," sedimentary beds rich in oxidized iron minerals such as hematite did not form. Instead, there was formation of "banded iron formations," composed of only partly oxidized iron species. Other rock types from this ancient time include uranium oxides and iron pyrites that cannot form in today's atmosphere. This evidence strongly suggests that prior to 2.2 billion years ago there was no free oxygen in the atmosphere and little oxygen dissolved in seawater.

Even though there must have been, at most, only a few percent of oxygen in the gases making up Earth's atmosphere as late as 2.2 billion years ago, soon after that the amount of oxygen began to climb rapidly. Where did all the oxygen come from? Some oxygen can be generated by photochemical reactions, where water high in the atmosphere is broken by sunlight into hydrogen and oxygen, but this process could account for only a small percentage of the oxygen rise. The most likely explanation is that most came from photosynthesis by single-celled bacteria. Life is known to have evolved on Earth by about 3.5 billion years ago, perhaps hundreds of millions of years earlier than that. Certainly, by 3.5 billion years ago, life had evolved to the point where cyanobacteria (informally and improperly known as blue-green algae) were widespread in the oceans.

The cyanobacteria were the first organisms to use carbon dioxide to produce free oxygen. They still exist and use carbon dioxide as a source of carbon for building cells. They cannot use it for energy. They also made nitrogen available to their protoplasm by developing specialized structures (Heterocysts) as locations for nitrogen fixation. The cyanobacteria were eventually co-opted by other, larger cells (the eu-karyotic cells that contained a distinct, membrane-bounded nucleus, in contrast to the smaller bacteria without a nucleus). This theory, known as the Endosymbiosis Theory, was proposed by biologist Lynn Margulis. Some members of the cyanobacteria became the modern chloroplast, the part of the plant cell in which photosynthesis is carried out. This transition to larger "plant" cells took place perhaps 2.7 billion years ago, and by 2.3 billion years ago a buildup of oxygen in the atmosphere was taking place.

The buildup of oxygen in Earth's atmosphere led to the formation of an ozone layer thick enough to shield life on the surface of the planet from the harmful effects of ultraviolet radiation. Ozone is another chemical form of oxygen. Because of its different bonds, it cannot be used to "burn" sugars but does screen out harmful radiation that would otherwise hurt organisms on Earth. The amount of oxygen depends in part on the amount of oxygen in the atmosphere. At times of low-oxygen, all the oceans will similarly have little oxygen in them. However, since the amount of oxygen that can dissolve into seawater is also affected by temperature, as shown in the previous chapter, warm oceans might have little oxygen in them everywhere but in a narrow surface zone, despite there being high-oxygen levels in the atmosphere. For instance, this condition exists in the modern Black Sea.

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