Extraterrestrial lmpact and Megafauna Extinctions
About 13,000 years ago, near the end of a period called the Pleistocene, North America was populated by megafauna (large animals), which included mammoths, giant ground sloths, and horses. Also present was a prehistoric group of people called the Clovis. Suddenly, and perhaps catastrophically, most of the megafauna disappeared, and the Clovis culture was no longer recognizable. The climate abruptly cooled, forming the cold period known as the Younger Dryas. The possible causes of the extinction of the megafauna and the termination of the Clovis culture have been a longstanding and controversial subject. The two major competing hypotheses to explain the extinction of the megafauna are overkill by human hunters and abrupt cooling (i.e., rapid climate change). Both of these hypotheses have shortcomings. Human overkill seems unlikely in light of the very large numbers of animals that went extinct, including many that the North American people at the time evidently did not regularly hunt. The magnitude of the abrupt cooling at the onset of the Younger Dryas was little different than what had often occurred over the previous 80,000 years, and none of the earlier events were associated with major extinctions. The conclusion is that the extinction of Pleistocene megafauna is unique during the latest Pleistocene period and was too abrupt and widely distributed to have resulted from human overkill or climatic cooling.
Natural materials, such as rocks and soil, settle on Earth’s surface and accumulate in layers, with older layers sitting beneath new layers. At many Clovis sites, archaeologists have identified what is known as a black mat, which is a thin layer of dark, organic-rich clays. The base of the black mat coincides with the beginning of Younger Dryas cooling, after which there is no evidence for remains of the megafauna or artifacts from the Clovis culture. Murray Springs in the United States is a well-known and well-studied Clovis site, where the youngest (most recent) mammoth bones and Clovis tools are directly in contact with the bottom of the black layer. At that site, it is apparent that termination of Pleistocene megafauna and Clovis culture was very sudden, coinciding with the deposition of the black mat.
New evidence suggests that there may have been an extraterrestrial (cosmic) impact 12,900 years ago that contributed to the Pleistocene megafauna extinctions, as well as to the disappearance of the Clovis culture. Evaluation of ten Clovis sites, selected because they were well documented and dated, support the hypothesis that there may have been a major extraterrestrial collision over North America 12,900 years ago, at the beginning of the Younger Dryas event. Hypothetically, the event consisted of one or more(perhaps many) low-density extraterrestrial objects(probably comets)that exploded in the atmosphere over much of North America. At that time, much of Canada was covered by the Laurentide Ice Sheet, which was partially destabilized by the proposed event. It is speculated that many objects exploded over the continent, including the area covered by the ice sheet, setting off heat flashes and shock waves and generating intense winds (hundreds of kilometers per hour) over North America. The event also would have created fireballs that destroyed forests, grasslands, and animals, while producing an abundance of charcoal ash, soot, and toxic fumes. Evidence of this is found as far west as Santa Rosa Island in the Santa Barbara Channel off the western coast of the United States.
Physical evidence for the Younger Dryas event includes the discovery of a variety of possible extraterrestrial markers including magnetic grains with iridium(a chemical element that is much more common in extraterrestrial objects than on Earth’s surface), charcoal and soot, and evidence for intense wildfires. An abundance of diamonds, including nanodiamonds (among them forms that are normally interpreted to be formed only by cosmic impact), microdiamonds, and shock diamonds (also thought to form from cosmic impact), have been discovered within the black mat but much less frequently above and below the mat. Although no craters (large circular marks that result from impact) of Younger Dryas age have yet been found near the suspected impact region, as the hypothesis is thoroughly tested, it is likely that more direct physical evidence of the cosmic impact that brought about the demise of the Pleistocene megafauna will be discovered.
题目：
1
Why does the author provide the information that “The magnitude of the abrupt cooling at the onset of the Younger Dryas was little different than what had often occurred over the previous 80,000 years”?
Rhetorical Purpose Questions修辞目的题
ATo suggest that some extinctions were caused by a series of cooling events, rather than by a single abrupt cooling
BTo indicate how far back in time cooling events occurred before the Younger Dryas period
CTo explain why cooling events before the Younger Dryas were not associated with megafauna extinctions
DTo challenge the claim that the extinctions that occurred about 13,000 years ago were caused by climate change
2
About 13,000 years ago, near the end of a period called the Pleistocene, North America was populated by megafauna (large animals), which included mammoths, giant ground sloths, and horses. Also present was a prehistoric group of people called the Clovis. Suddenly, and perhaps catastrophically, most of the megafauna disappeared, and the Clovis culture was no longer recognizable. The climate abruptly cooled, forming the cold period known as the Younger Dryas. The possible causes of the extinction of the megafauna and the termination of the Clovis culture have been a longstanding and controversial subject. The two major competing hypotheses to explain the extinction of the megafauna are overkill by human hunters and abrupt cooling (i.e., rapid climate change). Both of these hypotheses have shortcomings. Human overkill seems unlikely in light of the very large numbers of animals that went extinct, including many that the North American people at the time evidently did not regularly hunt. The magnitude of the abrupt cooling at the onset of the Younger Dryas was little different than what had often occurred over the previous 80,000 years, and none of the earlier events were associated with major extinctions. The conclusion is that the extinction of Pleistocene megafauna is unique during the latest Pleistocene period and was too abrupt and widely distributed to have resulted from human overkill or climatic cooling.
According to paragraph 1， which of the following weakens the hypothesis that the North American megafauna extinctions were caused by human hunters?
Factual Information Questions事实信息题
ASome animals went extinct before Clovis hunters were present in North America.
BSome of the animals that went extinct were too big to be successfully hunted by prehistoric humans.
CSome of the animals that went extinct lived in colder regions, where few humans were present.
DSome of the animals that went extinct were likely not a frequent target of human hunting.
3
About 13,000 years ago, near the end of a period called the Pleistocene, North America was populated by megafauna (large animals), which included mammoths, giant ground sloths, and horses. Also present was a prehistoric group of people called the Clovis. Suddenly, and perhaps catastrophically, most of the megafauna disappeared, and the Clovis culture was no longer recognizable. The climate abruptly cooled, forming the cold period known as the Younger Dryas. The possible causes of the extinction of the megafauna and the termination of the Clovis culture have been a longstanding and controversial subject. The two major competing hypotheses to explain the extinction of the megafauna are overkill by human hunters and abrupt cooling (i.e., rapid climate change). Both of these hypotheses have shortcomings. Human overkill seems unlikely in light of the very large numbers of animals that went extinct, including many that the North American people at the time evidently did not regularly hunt. The magnitude of the abrupt cooling at the onset of the Younger Dryas was little different than what had often occurred over the previous 80,000 years, and none of the earlier events were associated with major extinctions. The conclusion is that the extinction of Pleistocene megafauna is unique during the latest Pleistocene period and was too abrupt and widely distributed to have resulted from human overkill or climatic cooling.
According to paragraph 1， which of the following weakens the hypothesis that the North American megafauna extinctions were caused by human hunters?
Factual Information Questions事实信息题
ASome animals went extinct before Clovis hunters were present in North America.
BSome of the animals that went extinct were too big to be successfully hunted by prehistoric humans.
CSome of the animals that went extinct lived in colder regions, where few humans were present.
DSome of the animals that went extinct were likely not a frequent target of human hunting.
4
Natural materials, such as rocks and soil, settle on Earth’s surface and accumulate in layers, with older layers sitting beneath new layers. At many Clovis sites, archaeologists have identified what is known as a black mat, which is a thin layer of dark, organic-rich clays. The base of the black mat coincides with the beginning of Younger Dryas cooling, after which there is no evidence for remains of the megafauna or artifacts from the Clovis culture. Murray Springs in the United States is a well-known and well-studied Clovis site, where the youngest (most recent) mammoth bones and Clovis tools are directly in contact with the bottom of the black layer. At that site, it is apparent that termination of Pleistocene megafauna and Clovis culture was very sudden, coinciding with the deposition of the black mat.
According to paragraph 2， which of the following is an important finding about the black mat layer at many Clovis sites?
Factual Information Questions事实信息题
AIt suddenly disappears at the beginning of the Younger Dryas.
BIt is similar to other dark clay layers deposited during earlier periods.
CIt has remains of megafauna below it but not above it.
DIt contains the first appearance of Clovis artifacts at its base.
5
New evidence suggests that there may have been an extraterrestrial (cosmic) impact 12,900 years ago that contributed to the Pleistocene megafauna extinctions, as well as to the disappearance of the Clovis culture. Evaluation of ten Clovis sites, selected because they were well documented and dated, support the hypothesis that there may have been a major extraterrestrial collision over North America 12,900 years ago, at the beginning of the Younger Dryas event. Hypothetically, the event consisted of one or more(perhaps many) low-density extraterrestrial objects(probably comets)that exploded in the atmosphere over much of North America. At that time, much of Canada was covered by the Laurentide Ice Sheet, which was partially destabilized by the proposed event. It is speculated that many objects exploded over the continent, including the area covered by the ice sheet, setting off heat flashes and shock waves and generating intense winds (hundreds of kilometers per hour) over North America. The event also would have created fireballs that destroyed forests, grasslands, and animals, while producing an abundance of charcoal ash, soot, and toxic fumes. Evidence of this is found as far west as Santa Rosa Island in the Santa Barbara Channel off the western coast of the United States.
The word”speculated”in the passage is closest in meaning to
Vocabulary Questions词汇题
Aconfirmed
Bsupposed
Cunderstood
Dconcluded
6
New evidence suggests that there may have been an extraterrestrial (cosmic) impact 12,900 years ago that contributed to the Pleistocene megafauna extinctions, as well as to the disappearance of the Clovis culture. Evaluation of ten Clovis sites, selected because they were well documented and dated, support the hypothesis that there may have been a major extraterrestrial collision over North America 12,900 years ago, at the beginning of the Younger Dryas event. Hypothetically, the event consisted of one or more(perhaps many) low-density extraterrestrial objects(probably comets)that exploded in the atmosphere over much of North America. At that time, much of Canada was covered by the Laurentide Ice Sheet, which was partially destabilized by the proposed event. It is speculated that many objects exploded over the continent, including the area covered by the ice sheet, setting off heat flashes and shock waves and generating intense winds (hundreds of kilometers per hour) over North America. The event also would have created fireballs that destroyed forests, grasslands, and animals, while producing an abundance of charcoal ash, soot, and toxic fumes. Evidence of this is found as far west as Santa Rosa Island in the Santa Barbara Channel off the western coast of the United States.
According to paragraph 3， all of the following were likely consequences of a cosmic impact EXCEPT:
Negative Factual Information Questions否定事实信息题
AAn ice sheet in Canada became unstable.
BVery strong winds blew across the land.
CFireballs killed a number of living organisms.
DThe Santa Barbara Channel was formed.
7
Physical evidence for the Younger Dryas event includes the discovery of a variety of possible extraterrestrial markers including magnetic grains with iridium(a chemical element that is much more common in extraterrestrial objects than on Earth’s surface), charcoal and soot, and evidence for intense wildfires. An abundance of diamonds, including nanodiamonds (among them forms that are normally interpreted to be formed only by cosmic impact), microdiamonds, and shock diamonds (also thought to form from cosmic impact), have been discovered within the black mat but much less frequently above and below the mat. Although no craters (large circular marks that result from impact) of Younger Dryas age have yet been found near the suspected impact region, as the hypothesis is thoroughly tested, it is likely that more direct physical evidence of the cosmic impact that brought about the demise of the Pleistocene megafauna will be discovered.
The word”markers”in the passage is closest in meaning to
Vocabulary Questions词汇题
Aformations
Bparticles
Cindicators
Ddeposits
8
Physical evidence for the Younger Dryas event includes the discovery of a variety of possible extraterrestrial markers including magnetic grains with iridium(a chemical element that is much more common in extraterrestrial objects than on Earth’s surface), charcoal and soot, and evidence for intense wildfires. An abundance of diamonds, including nanodiamonds (among them forms that are normally interpreted to be formed only by cosmic impact), microdiamonds, and shock diamonds (also thought to form from cosmic impact), have been discovered within the black mat but much less frequently above and below the mat. Although no craters (large circular marks that result from impact) of Younger Dryas age have yet been found near the suspected impact region, as the hypothesis is thoroughly tested, it is likely that more direct physical evidence of the cosmic impact that brought about the demise of the Pleistocene megafauna will be discovered.
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.
Sentence Simplification Questions句子简化题
AMany diamonds, including those thought to form from cosmic impact, have been found more often inside the black mat than above the mat and below the mat.
BDiamonds within the black mat formed from cosmic impact, unlike those above the mat and below the mat.
CThree different types of diamonds have been discovered within the black mat as well as above the mat and below the mat.
DNanodiamonds and shock diamonds, which are thought to form from cosmic impact, are found much more frequently in the black mat than microdiamonds are.
9
Natural materials, such as rocks and soil, settle on Earth’s surface and accumulate in layers, with older layers sitting beneath new layers. [■] At many Clovis sites, archaeologists have identified what is known as a black mat, which is a thin layer of dark, organic-rich clays. [■] The base of the black mat coincides with the beginning of Younger Dryas cooling, after which there is no evidence for remains of the megafauna or artifacts from the Clovis culture. [■] Murray Springs in the United States is a well-known and well-studied Clovis site, where the youngest (most recent) mammoth bones and Clovis tools are directly in contact with the bottom of the black layer. [■] At that site, it is apparent that termination of Pleistocene megafauna and Clovis culture was very sudden, coinciding with the deposition of the black mat.
Look at the four squares [■] that indicate where the following sentence could be added to the passage
One layer has helped archaeologists identify the timing of the disappearance of the North American megafauna and of the Clovis culture.Insert Text Questions句子插入题
Where would the sentence best fit？Click on a square sentence to the passage.
10
Scientists have been trying to explain why the North American megafauna became extinct and Clovis culture ended 13,000 years ago.
Prose Summary Questions概要小结题
Select 3 answers
AAt first, scientists thought the extinctions were caused by the combination of a gradually cooling climate and an increase in human hunting, and various Clovis artifacts seemed to support this idea.
BThe extinctions were likely caused by a catastrophic collision with one or several extraterrestrial objects that exploded in the air over North America and created intense fires.
CScientists have identified the location of a likely cosmic event in a region of Canada once covered by the Laurentide Ice Sheet, where chemical elements in the soil provide evidence of impact.
DA great number of animals suddenly went extinct over a wide area, which shows that the extinctions were caused neither by the cold climate of the Younger Dryas nor by human hunting.
EFindings at Clovis sites suggest that the Clovis people spread over North America after the megafauna disappeared, although the culture changed so much that it was no longer fully recognizable.
FVarious materials in the dark soil layer deposited at the time of the extinctions provide indirect evidence of a cosmic impact, and direct evidence will probably be found as well.
