Earth’s atmosphere has changed through time. Compared to the Sun, whose composition is representative of the raw materials from which Earth and other planets in our solar system formed, Earth contains less of some volatile elements, such as nitrogen, argon, hydrogen, and helium. These elements were lost when the envelope of gases, or primary atmosphere, that surrounded early Earth, was stripped away by the solar wind or by meteorite impacts, or both. Little by little, the planet generated a new, secondary atmosphere by volcanic outgassing of volatile materials from its interior.
Volcanic outgassing continues to be the main process by which volatile materials are released from Earth—although it is now going on at a much slower rate. The main chemical constituent of volcanic gases (as much as 97 percent of volume) is water vapor, with varying amounts of nitrogen, carbon dioxide, and other gases. In fact, the total volume of volcanic gases released over the past 4 billion years or so is believed to account for the present composition of the atmosphere with one important exception: oxygen. Earth had virtually no oxygen in its atmosphere more than 4 billion years ago, but the atmosphere is now approximately 21 percent oxygen.
Traces of oxygen were probably generated in the early atmosphere by the breakdown of water molecules into oxygen and hydrogen by ultraviolet light (a process called photodissociation). Although this is an important process, it cannot begin to account for the present high levels of oxygen in the atmosphere. Almost all of the free oxygen now in the atmosphere originated through photosynthesis, the process whereby plants use light energy to induce carbon dioxide to react with water, producing carbohydrates and oxygen.
Oxygen is a very reactive chemical, so at first most of the free oxygen produced by photosynthesis was combined with iron in ocean water to form iron oxide-bearing minerals. The evidence of the gradual transition from oxygen-poor to oxygen-rich water is preserved in seafloor sediments. The minerals in seafloor sedimentary rocks that are more than about 2.5 billion years old contain reduced (oxygen-poor) iron compounds. In rocks that are less than 1.8 billion years old, oxidized (oxygen-rich) compounds predominate. The sediments that were precipitated during the transition contain alternating bands of red (oxidized iron) and black (reduced iron) minerals. These rocks are called banded-iron formations. Because ocean water is in constant contact with the atmosphere, and the two systems function together in a state of dynamic equilibrium, the transition from an oxygen-poor to an oxygen-rich atmosphere also must have occurred during this period.
Along with the buildup of molecular oxygen (O2) came an eventual increase in ozone (O3) levels in the atmosphere. Because ozone filters out harmful ultraviolet radiation,this made it possible for life to flourish in shallow water and finally on land. This critical state in the evolution of the atmosphere was replaced between 1100 and 542 million years ago. Interestingly, the fossil record shows an explosion of life forms 542 million years ago.
Oxygen has continued to play a key role in the evolution and form of life. Over the last 200 million years, the concentration of oxygen has risen from 10 percent to as much as 25 percent of the atmosphere, before setting (probably not permanently) at its current value of 21 percent. This increase has benefited mammals, which are voracious oxygen consumers. Not only do we require oxygen to fuel our high-energy, warm-blooded metabolism, our unique reproductive system demands even more. An expectant mother’s used (venous) blood must still have enough oxygen in it to diffuse through the placenta into her unborn child’s bloodstream. It would be very difficult for any mammal species to survive in an atmosphere of only 10 percent oxygen.
Geologists cannot yet be certain why the atmospheric oxygen levels increased, but they have a hypothesis. First, photosynthesis is only one part of the oxygen cycle. The cycle is completed by decomposition, in which organic carbon combines with oxygen and forms carbon dioxide. But if organic matter is buried as sediment before it fully decomposes, its carbon is no longer available to react with the free oxygen. Thus there will be a net accumulation of carbon in sediments and of oxygen in the atmosphere.
29. In paragraph 1, why does the author state that Earth has less nitrogen, argon, hydrogen, and helium that the Sun?
A. To argue that these elements were once part of an early atmosphere, which disappeared
B. To suggest that these elements were drawn into the Sun’s atmosphere
C. To provide evidence that Earth’s original atmosphere came primarily from meteorites
D. To support the claim that Earth’s atmosphere would have changed even more if it had contained more volatile elements
30. The word “constituent” in the passage is closet in meaning to
31. According toparagraph 2, the history of volcanic outgassing cannot explain which of the following?
A.The lack of oxygen in the atmosphere 4 billion years ago
B. The amount of water vapor in the atmosphere today
C. The proportions of nitrogen and carbon dioxide in the atmosphere today
D. The present abundance of oxygen in the atmosphere
32. Paragraph 3 suggests which of the following about the process of photodissociation?
A.It is more common today than it was in the early history of the atmosphere.
B. It is responsible for only a small amount of the oxygen in the atmosphere today.
C. It removes trace amounts of oxygen from the atmosphere.
D. It produces more free oxygen than photosynthesis does.
33. The word “gradual” in the passage is closet in meaning to
34. The word “predominate” in the passage is closet in meaning to
A.are in the majority
B. are present
C. are increasing
D. first appear
35. Which of the sentences below best expresses the essential information in the highlighted sentence in the passage? Incorrect choices change the meaning in important ways or leave out essential information.
A.Since the oceans and the atmosphere function together, the atmosphere must have become oxygen rich during this period.
B. Because ocean water is in constant contact with the atmosphere, the two systems maintain a dynamic equilibrium.
C. The transition to an oxygen-rich atmosphere could not have happened without constant contact with the oceans.
D. Much of the oxygen in the oceans must have been pulled out of the atmosphere during this period.
36. According to paragraph 4, what can be learned from the type of iron compounds in seafloor rocks?
A.How the process of photosynthesis has changed over time
B. The level of oxygen in the water at a certain time in history
C. How levels of iron in ocean water decreased over time
D.The overall mineral content of the ocean water
37. According to paragraph 3, banded-iron formations are found in what kind of rocks?
A.Those that are more than 2.5 billion years old
B. Those that do not contain oxidized compounds
C. Those that are from a transitional period in terms of oxygen richness
D. Those that are less than 1.8 billion years old
38. According to paragraph 5, which of the following happened sometime between 1100 and 542 million years ago?
A.Asudden explosion of life forms on land occurred together with a sharp decline of life in the water.
B. Ultraviolet radiation became more harmful to living organisms.
C. Molecular oxygen levels in the atmosphere stabilized, and ozone levels began to rise.
D. Ozone reduced ultraviolet radiation to a level acceptable for life on land.
39. The word “diffuse” in the passage is closet in meaning to
40. According to paragraph 6, which of the following is NOT true of the connection between mammals and oxygen?
A.Mammals are able to survive only because oxygen levels are relatively high.
B. Mammals first emerged when atmospheric oxygen levels reached 10 percent.
C. A mammal’s unborn child receives oxygen through the mother’s placenta.
D. Mammals use a lot of oxygen partly because they are warm-blooded.
Paragraph 5 Along with the buildup of molecular oxygen (O2) came an eventual increase in ozone (O3) levels in the atmosphere. ■Because ozone filters out harmful ultraviolet radiation, this made it possible for life to flourish in shallow water and finally on land. ■This critical state in the evolution of the atmosphere was replaced between 1100 and 542 million years ago. ■Interestingly, the fossil record shows an explosion of life forms 542 million years ago.■
41. Look at the four squares [■] that indicate where the following sentence could be added to the passage. The timing strongly suggests that atmospheric changes were responsible for this sudden increase in new life.
Where would the sentence best fit? Click on a square [■] to add the sentence to the passage.
42. Directions: An introductory sentence for a brief summary of the passage is provided below. Complete the summary by selecting the THREE answer choices that express the most important ideas in the passage. Some answer choices do not belong in the summary because they express ideas that are not presented in the passage or are minor ideas in the passage. This question is worth 2 points. Drag your choices to the spaces where they belong. To review the passage, click on View Text.
Internal and external forces on Earth’s atmosphere have changed its chemical composition over time. ● ● ●
1.Mammals could not have survived without an oxygen-rich atmosphere, and land-based life would not be possible without the ozone layer to filter solar radiation.
2.Although they are currently at about 21 percent, oxygen levels will probably not always remain this high.
3.The breakdown of organic matter removes free oxygen, but if this process is interrupted, extra oxygen may accumulate in the atmosphere.
4.Over the last 4 billion years, outgassing destroyed Earth’s primary atmosphere of volatile elements and replaced it with nonvolatile materials including carbon dioxide.
5.The small amount of oxygen in Earth’s early atmosphere was due to photo dissociation and, later, photosynthesis created free oxygen.
6.When oxygen levels in the ocean water reached a critical level about 542 million years ago, life emerged in the oceans, as shown by sedimentary rocks