TF阅读真题第883篇The Azolla Event

The Azolla Event

 

 

During the early Eocene period, about 56 million years ago, Earth’s atmosphere had high levels of carbon dioxide(CO2) which warmed the planet. But about 49 million years ago the climate began to cool, mostly as the result of activity in the Arctic. In the early Eocene the Arctic Ocean, unlike today, was almost completely surrounded by land, isolating it from the other oceans. One consequence of this isolation was that the Arctic Ocean was able to retain much of the freshwater that fell as rain and that entered along with the rivers that flowed into it from the nearby land. This freshwater accumulated on the ocean surface as a light, slightly salty layer floating on denser, saltier water a few meters below. Because rainfall was even more intense than it is today, and because there was less disruption by waves from other oceans, this layer of surface water occasionally became completely fresh. Those were the conditions that allowed rapid growth across the Arctic of Azolla, a freshwater fern (a plant) normally incapable of living in the sea but able to thrive on this very odd, almost salt-free ocean.

Azolla is a fascinating plant. Unlike the land-loving ferns most of us are familiar with, Azolla grows as free-floating mats on the surfaces of lakes and rivers. It can do this because of its mutually beneficial relationship with bacteria that are able to break apart the strong molecular bonds of atmospheric nitrogen to make the nitrate that plants need for growth. In other words, it obtains its food from the air. Azolla carries these useful companions around in specially formed growths that provide it with the nitrate fertilizer that a free-floating plant cannot get by the usual approach of sticking roots into soil. In return for this invaluable service, the plant provides its bacterial guests with food in the form of glucose, and this partnership allows Azolla to thrive in nutrient-poor waters where other aquatic plants struggle.

In the case of the Eocene Arctic Ocean with its very unusual freshwater surface layer, this ability occasionally allowed ocean-sized Azolla blooms to form in the polar summer. The entire Arctic Ocean became plant-green rather than the ice-white one might have expected. But in the long, dark, polar winter, these weeds died or possibly just became dormant (inactive), and much of the vigorous summer growth sank into the deep ocean. This could well have been the cause of the mid-Eocene cooling, because large-scale burial of plant matter in ocean sediments removes CO2 from the atmosphere. Levels of the greenhouse gas, CO2, would have fallen as Azolla remains accumulated at the seafloor, and the resulting temperature drop may have been large enough to begin the descent toward the icy world of the last 2.5 million years: a world of regular ice ages interspersed by relatively short but warmer interglacial phases such as the one we have been living through since about 9000 B.c.E. Many other factors played important roles in these changes to global climate, but Azolla blooms may have been the final trigger that set off a complex chain of events flipping the world from the warm climate of the early Eocene into the colder climate of more recent times.

This “Azolla event” was, geologically speaking, a brief episode lasting little more than a million years. Even then, Azolla blooms occurred only rarely and seem to have been associated with occasional drops in global sea level that further cut off the Arctic Ocean from the rest of the world. The world’s oil industry is currently very interested in exploiting the oil that can be found in the Arctic Ocean, and an Azolla layer, which is one of the principal sources for this oil, has been found everywhere that drills have penetrated marine sediments of the right age. This discovery of the remains of a freshwater plant across an entire saltwater ocean was more than a little surprising, but the evidence for ocean-scale blooms of this particular aquatic weed is strong. The sediments show too much decomposed Azolla spread across too wide an area for it to simply be the result of river runoff from the land. The sediments also show no other terrestrial (land) remains, such as wood fragments, that would be expected in land-derived deposits.

 

 

 

 

 

1

During the early Eocene period, about 56 million years ago, Earth’s atmosphere had high levels of carbon dioxide(CO2) which warmed the planet. But about 49 million years ago the climate began to cool, mostly as the result of activity in the Arctic. In the early Eocene the Arctic Ocean, unlike today, was almost completely surrounded by land, isolating it from the other oceans. One consequence of this isolation was that the Arctic Ocean was able to retain much of the freshwater that fell as rain and that entered along with the rivers that flowed into it from the nearby land. This freshwater accumulated on the ocean surface as a light, slightly salty layer floating on denser, saltier water a few meters below. Because rainfall was even more intense than it is today, and because there was less disruption by waves from other oceans, this layer of surface water occasionally became completely fresh. Those were the conditions that allowed rapid growth across the Arctic of Azolla, a freshwater fern (a plant) normally incapable of living in the sea but able to thrive on this very odd, almost salt-free ocean.

According to paragraph 1, which of the following contributed to the formation of a slightly salty surface layer in the Eocene Arctic Ocean?

Factual Information Questions事实信息题

AThe warm water temperature

BThe location of surrounding land areas

CThe rapid sinking of salt to the ocean bottom

DThe flow of salt water from the Arctic Ocean to other oceans

 

2

During the early Eocene period, about 56 million years ago, Earth’s atmosphere had high levels of carbon dioxide(CO2) which warmed the planet. But about 49 million years ago the climate began to cool, mostly as the result of activity in the Arctic. In the early Eocene the Arctic Ocean, unlike today, was almost completely surrounded by land, isolating it from the other oceans. One consequence of this isolation was that the Arctic Ocean was able to retain much of the freshwater that fell as rain and that entered along with the rivers that flowed into it from the nearby land. This freshwater accumulated on the ocean surface as a light, slightly salty layer floating on denser, saltier water a few meters below. Because rainfall was even more intense than it is today, and because there was less disruption by waves from other oceans, this layer of surface water occasionally became completely fresh. Those were the conditions that allowed rapid growth across the Arctic of Azolla, a freshwater fern (a plant) normally incapable of living in the sea but able to thrive on this very odd, almost salt-free ocean.

According to paragraph 1, the growth of Azolla in the Eocene Arctic Ocean was a result of

Factual Information Questions事实信息题

Athe decrease in atmospheric CO2 levels

Bthe presence of a layer of mostly freshwater in the ocean

CAzolla’s adaptation to salt water

Dthe increased density of the ocean water

 

3

Azolla is a fascinating plant. Unlike the land-loving ferns most of us are familiar with, Azolla grows as free-floating mats on the surfaces of lakes and rivers. It can do this because of its mutually beneficial relationship with bacteria that are able to break apart the strong molecular bonds of atmospheric nitrogen to make the nitrate that plants need for growth. In other words, it obtains its food from the air. Azolla carries these useful companions around in specially formed growths that provide it with the nitrate fertilizer that a free-floating plant cannot get by the usual approach of sticking roots into soil. In return for this invaluable service, the plant provides its bacterial guests with food in the form of glucose, and this partnership allows Azolla to thrive in nutrient-poor waters where other aquatic plants struggle.

Paragraph 2 suggests which of the following about nitrate?

Inference Questions推理题

AIt is absorbed into Azolla from the surface layer of lakes and rivers

BIts molecular bonds are stronger than those of atmospheric nitrogen.

CIt can be used by certain bacteria to produce atmospheric nitrogen.

DIt is as essential for the growth of aquatic plants as it is for the growth of land plants.

 

4

Azolla is a fascinating plant. Unlike the land-loving ferns most of us are familiar with, Azolla grows as free-floating mats on the surfaces of lakes and rivers. It can do this because of its mutually beneficial relationship with bacteria that are able to break apart the strong molecular bonds of atmospheric nitrogen to make the nitrate that plants need for growth. In other words, it obtains its food from the air. Azolla carries these useful companions around in specially formed growths that provide it with the nitrate fertilizer that a free-floating plant cannot get by the usual approach of sticking roots into soil. In return for this invaluable service, the plant provides its bacterial guests with food in the form of glucose, and this partnership allows Azolla to thrive in nutrient-poor waters where other aquatic plants struggle.

According to paragraph 2. which of the following is true about bacteria living in the growths on Azolla?

Factual Information Questions事实信息题

AThey obtain the nitrogen they need for growth from Azolla.

BThey use up most of the nutrients in the water, forcing Azolla to obtain nutrients from the air

CThey use Azolla as a source of glucose.

DThey release chemicals that limit the growth of aquatic plants that compete with Azolla

 

5

In the case of the Eocene Arctic Ocean with its very unusual freshwater surface layer, this ability occasionally allowed ocean-sized Azolla blooms to form in the polar summer. The entire Arctic Ocean became plant-green rather than the ice-white one might have expected. But in the long, dark, polar winter, these weeds died or possibly just became dormant (inactive), and much of the vigorous summer growth sank into the deep ocean. This could well have been the cause of the mid-Eocene cooling, because large-scale burial of plant matter in ocean sediments removes CO2 from the atmosphere. Levels of the greenhouse gas, CO2, would have fallen as Azolla remains accumulated at the seafloor, and the resulting temperature drop may have been large enough to begin the descent toward the icy world of the last 2.5 million years: a world of regular ice ages interspersed by relatively short but warmer interglacial phases such as the one we have been living through since about 9000 B.c.E. Many other factors played important roles in these changes to global climate, but Azolla blooms may have been the final trigger that set off a complex chain of events flipping the world from the warm climate of the early Eocene into the colder climate of more recent times.

Why does the author provide the information that “The entire Arctic Ocean became plant-green rather than the ice-white one might have expected.”?

Factual Information Questions事实信息题

ATo illustrate the big difference between animal life and plant life in the Eocene Arctic Ocean

BTo suggest that much about the Eocene Arctic Ocean is still no known

CTo emphasize the extent of Azolla blooms in the Eocene Arctic Ocean in summer

DTo support the idea that temperatures in the Arctic Ocean were much higher during the Eocene than today

 

6

In the case of the Eocene Arctic Ocean with its very unusual freshwater surface layer, this ability occasionally allowed ocean-sized Azolla blooms to form in the polar summer. The entire Arctic Ocean became plant-green rather than the ice-white one might have expected. But in the long, dark, polar winter, these weeds died or possibly just became dormant (inactive), and much of the vigorous summer growth sank into the deep ocean. This could well have been the cause of the mid-Eocene cooling, because large-scale burial of plant matter in ocean sediments removes CO2 from the atmosphere. Levels of the greenhouse gas, CO2, would have fallen as Azolla remains accumulated at the seafloor, and the resulting temperature drop may have been large enough to begin the descent toward the icy world of the last 2.5 million years: a world of regular ice ages interspersed by relatively short but warmer interglacial phases such as the one we have been living through since about 9000 B.c.E. Many other factors played important roles in these changes to global climate, but Azolla blooms may have been the final trigger that set off a complex chain of events flipping the world from the warm climate of the early Eocene into the colder climate of more recent times.

The word “vigorous” in the passage is closest in meaning to

Vocabulary Questions词汇题

Aprevious

Bstrong

Cbrief

Dold

 

7

In the case of the Eocene Arctic Ocean with its very unusual freshwater surface layer, this ability occasionally allowed ocean-sized Azolla blooms to form in the polar summer. The entire Arctic Ocean became plant-green rather than the ice-white one might have expected. But in the long, dark, polar winter, these weeds died or possibly just became dormant (inactive), and much of the vigorous summer growth sank into the deep ocean. This could well have been the cause of the mid-Eocene cooling, because large-scale burial of plant matter in ocean sediments removes CO2 from the atmosphere. Levels of the greenhouse gas, CO2, would have fallen as Azolla remains accumulated at the seafloor, and the resulting temperature drop may have been large enough to begin the descent toward the icy world of the last 2.5 million years: a world of regular ice ages interspersed by relatively short but warmer interglacial phases such as the one we have been living through since about 9000 B.c.E. Many other factors played important roles in these changes to global climate, but Azolla blooms may have been the final trigger that set off a complex chain of events flipping the world from the warm climate of the early Eocene into the colder climate of more recent times.

The word “relatively” in the passage is closest in meaning to

Vocabulary Questions词汇题

Aapparently

Bcomparatively

Cincreasingly

Dpredictably

 

8

This “Azolla event” was, geologically speaking, a brief episode lasting little more than a million years. Even then, Azolla blooms occurred only rarely and seem to have been associated with occasional drops in global sea level that further cut off the Arctic Ocean from the rest of the world. The world’s oil industry is currently very interested in exploiting the oil that can be found in the Arctic Ocean, and an Azolla layer, which is one of the principal sources for this oil, has been found everywhere that drills have penetrated marine sediments of the right age. This discovery of the remains of a freshwater plant across an entire saltwater ocean was more than a little surprising, but the evidence for ocean-scale blooms of this particular aquatic weed is strong. The sediments show too much decomposed Azolla spread across too wide an area for it to simply be the result of river runoff from the land. The sediments also show no other terrestrial (land) remains, such as wood fragments, that would be expected in land-derived deposits.

According to paragraph 4， which of the following statements is true about the world’s oil industry?

Factual Information Questions事实信息题

AThe industry has removed most of the oil formed from Azolla in the Arctic Ocean

BThe industry has located Azolla sediments across different regions of the Arctic.

CThe industry is prevented from drilling for oil in the Arctic Ocean because of a large layer of Azolla sediments

DThe industry’s drilling for oil contributed to the death of a large number of Azolla

 

9

During the early Eocene period, about 56 million years ago, Earth’s atmosphere had high levels of carbon dioxide(CO2) which warmed the planet. ⬛ But about 49 million years ago the climate began to cool, mostly as the result of activity in the Arctic.⬛  In the early Eocene the Arctic Ocean, unlike today, was almost completely surrounded by land, isolating it from the other oceans. ⬛ One consequence of this isolation was that the Arctic Ocean was able to retain much of the freshwater that fell as rain and that entered along with the rivers that flowed into it from the nearby land. ⬛ This freshwater accumulated on the ocean surface as a light, slightly salty layer floating on denser, saltier water a few meters below. Because rainfall was even more intense than it is today, and because there was less disruption by waves from other oceans, this layer of surface water occasionally became completely fresh. Those were the conditions that allowed rapid growth across the Arctic of Azolla, a freshwater fern (a plant) normally incapable of living in the sea but able to thrive on this very odd, almost salt-free ocean.

Look at the four squaresthat indicate where the following sentence could be added to the passage

Why did that region in particular play such an important role in this change?Insert Text Questions句子插入题

Where would the sentence best fit？Click on a square  sentence to the passage.

10

The growth of Azolla in the Arctic during the Eocene period had a substantial impact on Earth’s climate.

Prose Summary Questions概要小结题

Select 3 answers

AThe mid-Eocene period, an ice age that followed the interglacial period of the early Eocene, was a time when many freshwater plants went extinct

BDuring the Eocene, isolation, little disruption from waves from other oceans, and retention of freshwater created favorable conditions for the growth of the fern Azolla in the Arctic Ocean

CAs large amounts of Azolla remains built up on the seafloor, CO2 was removed from the atmosphere, leading to a drop in Earth’s temperature.

DThe high levels of CO2 in the atmosphere before the mid-Eocene cooling took place allowed Azolla and the bacteria living inside it to thrive.

EAzolla, aided by its mutually beneficial relationship with bacteria, occasionally thrived in the Arctic Ocean during Eocene summers.

FThe locations of decomposed Azolla in the oceans suggest that significant amounts of Azolla lived on land and reached the ocean by way of rivers