The Radiocarbon Dating of Prehistoric American Sites
Because all living things contain carbon, artifacts and organic remains from archaeological sites can often be dated by comparing the proportion of carbon-14 (radiocarbon) remaining in them(or in the location where they were found) to the proportion of non-radiocarbon (carbon-12 and carbon-13) in them, a method called radiocarbon dating. This technique is based on the fact that the amount of carbon-12 and carbon-13 stays constant in organisms, while the amount of carbon-14 declines at a steady rate once the organism dies. When an animal or plant dies, it stops exchanging gases with the atmosphere and its carbon-14 begins to decrease. Thus the amount of carbon-14 remaining indicates how long ago an organism died. Radiocarbon is the preferred method for dating the sites of the earliest Americans. Its range reaches back 50,000 years, which is the period of interest, and its reliability is well attested since variations in atmospheric carbon levels over time have been well studied and can readily be adjusted for in age calculations. Moreover, the technique can be applied to any material that incorporates radiocarbon, such as bone, wood, charcoal (burned wood), and even soils, the last by virtue of the organic acids that seep into them.
Yet not all of these materials, including charcoal and bone from which radiocarbon ages are most often obtained, give results that are equally reliable (if we run the sample multiple times, will we get the same result?) or valid (is the result correct?). An example of this situation occurred with the Folsom site, a Paleo-Indian culture of Central and North America dated to about 10,500-8,000 years ago, which is distinguished by the remains of bison, the site being a location for killing and processing these animals, possibly using stone points. In the 1990s, the bison kill was thought to have dated to 10,890 B.P., based on radiocarbon ages of charcoal found with the bones. Yet the subsequent radiocarbon dating of the bison bones themselves yielded an average age 400 years younger: 10,490 B.P. Why the discrepancy?
Charcoal has the virtue of being relatively impervious to contamination and thus normally yields a reliable age. But the age may not be valid since the charcoal may not have been formed at the same time that, for example, the bison were killed at the Folsom site. If the charcoal came from, say, a lightning-strike forest fire, it will have no bearing on the age of the kill. Even if the charcoal was from wood that stoked the fires over which the bison meat was cooked, it could still overestimate the age of the kill site. If the charcoal was from wood from the oldest part of a long-lived tree, it could be decades or even centuries older than other parts of the same tree and thus older than the kill site.
Conversely, radiocarbon dating of animal bones can more precisely pin down the age of a site, since animals live shorter lives than trees, though, of course, only bones present as a result of human activity (which is certainly the case of the Folsom bison) are of interest to archaeologists. But dating bone itself can be challenging, since it is made of multiple constituents, some of which are highly susceptible to contamination. The inorganic portion, for example, readily absorbs ancient carbon via groundwater or other contaminants. Before that was realized, multiple radiocarbon ages on the same skeleton often gave wildly divergent results-in one case, for example, the oldest and youngest radiocarbon ages on the same mammoth were nearly 3,000 years apart. In the 1980s, after laboratory advances enabled the extraction of protein and individual amino acids from organic bone collagen, substances less susceptible to contamination, it became possible to derive ages from bone that were more reliable. Hence, the conclusion that the bone amino acid obtained from the Folsom bison better estimated when that kill occurred.
But the handicap of radiocarbon dating, which will be felt if the first Americans prove to be far older than supposed, is that it does not provide reliable ages for any samples that might prove to be significantly more than 50,000 years old. For that, there are a number of other dating methods that can go beyond radiocarbon’s limits.
1
Because all living things contain carbon, artifacts and organic remains from archaeological sites can often be dated by comparing the proportion of carbon-14 (radiocarbon) remaining in them(or in the location where they were found) to the proportion of non-radiocarbon (carbon-12 and carbon-13) in them, a method called radiocarbon dating. This technique is based on the fact that the amount of carbon-12 and carbon-13 stays constant in organisms, while the amount of carbon-14 declines at a steady rate once the organism dies. When an animal or plant dies, it stops exchanging gases with the atmosphere and its carbon-14 begins to decrease. Thus the amount of carbon-14 remaining indicates how long ago an organism died. Radiocarbon is the preferred method for dating the sites of the earliest Americans. Its range reaches back 50,000 years, which is the period of interest, and its reliability is well attested since variations in atmospheric carbon levels over time have been well studied and can readily be adjusted for in age calculations. Moreover, the technique can be applied to any material that incorporates radiocarbon, such as bone, wood, charcoal (burned wood), and even soils, the last by virtue of the organic acids that seep into them.
Which of the following can be inferred from paragraph 1 about the amount of carbon-14 in organisms?
AIt is higher in living organisms than the amount of either carbon-12 or carbon-13.
BIt is higher when organisms are alive than it is long after they have died.
CIt declines more quickly in living organisms than in dead ones.
DIt is less variable in organisms than the amounts of carbon-12 and carbon-13 are.
2
Because all living things contain carbon, artifacts and organic remains from archaeological sites can often be dated by comparing the proportion of carbon-14 (radiocarbon) remaining in them(or in the location where they were found) to the proportion of non-radiocarbon (carbon-12 and carbon-13) in them, a method called radiocarbon dating. This technique is based on the fact that the amount of carbon-12 and carbon-13 stays constant in organisms, while the amount of carbon-14 declines at a steady rate once the organism dies. When an animal or plant dies, it stops exchanging gases with the atmosphere and its carbon-14 begins to decrease. Thus the amount of carbon-14 remaining indicates how long ago an organism died. Radiocarbon is the preferred method for dating the sites of the earliest Americans. Its range reaches back 50,000 years, which is the period of interest, and its reliability is well attested since variations in atmospheric carbon levels over time have been well studied and can readily be adjusted for in age calculations. Moreover, the technique can be applied to any material that incorporates radiocarbon, such as bone, wood, charcoal (burned wood), and even soils, the last by virtue of the organic acids that seep into them.
According to paragraph 1,radiocarbon dating is now the preferred method for determining the age of ancient biological specimens for all of the following reasons EXCEPT:
AIt can be used to date specimens that are tens of thousands of years old.
BIt is considered scientifically reliable.
CIt can be used to date all materials that have radiocarbon.
DIt can be used to determine whether organic acids have seeped into materials being tested.
3
Yet not all of these materials, including charcoal and bone from which radiocarbon ages are most often obtained, give results that are equally reliable (if we run the sample multiple times, will we get the same result?) or valid (is the result correct?). An example of this situation occurred with the Folsom site, a Paleo-Indian culture of Central and North America dated to about 10,500-8,000 years ago, which is distinguished by the remains of bison, the site being a location for killing and processing these animals, possibly using stone points. In the 1990s, the bison kill was thought to have dated to 10,890 B.P., based on radiocarbon ages of charcoal found with the bones. Yet the subsequent radiocarbon dating of the bison bones themselves yielded an average age 400 years younger: 10,490 B.P. Why the discrepancy?
The phrase “distinguished by” in the passage is closest in meaning to
Asurrounded by
Bstudied for
Ccovered with
Dnoted for
4
Yet not all of these materials, including charcoal and bone from which radiocarbon ages are most often obtained, give results that are equally reliable (if we run the sample multiple times, will we get the same result?) or valid (is the result correct?). An example of this situation occurred with the Folsom site, a Paleo-Indian culture of Central and North America dated to about 10,500-8,000 years ago, which is distinguished by the remains of bison, the site being a location for killing and processing these animals, possibly using stone points. In the 1990s, the bison kill was thought to have dated to 10,890 B.P., based on radiocarbon ages of charcoal found with the bones. Yet the subsequent radiocarbon dating of the bison bones themselves yielded an average age 400 years younger: 10,490 B.P. Why the discrepancy?
According to paragraph 2, the radiocarbon dating of charcoal suggested which of the following about the Folsom kill site?
APaleo-Indians used the site partly as a place for making stone points.
BPaleo-Indians were using the site earlier than radiocarbon dating based on bison bones indicated.
CPaleo-Indians were using the site near the end of the history of the Folsom culture.
DPaleo-Indians used the site for about 400 years.
5
Charcoal has the virtue of being relatively impervious to contamination and thus normally yields a reliable age. But the age may not be valid since the charcoal may not have been formed at the same time that, for example, the bison were killed at the Folsom site. If the charcoal came from, say, a lightning-strike forest fire, it will have no bearing on the age of the kill. Even if the charcoal was from wood that stoked the fires over which the bison meat was cooked, it could still overestimate the age of the kill site. If the charcoal was from wood from the oldest part of a long-lived tree, it could be decades or even centuries older than other parts of the same tree and thus older than the kill site.
The word “relatively” in the passage is closest in meaning to
Aparticularly
Bfairly
Csurprisingly
Dnaturally
6
Charcoal has the virtue of being relatively impervious to contamination and thus normally yields a reliable age. But the age may not be valid since the charcoal may not have been formed at the same time that, for example, the bison were killed at the Folsom site. If the charcoal came from, say, a lightning-strike forest fire, it will have no bearing on the age of the kill. Even if the charcoal was from wood that stoked the fires over which the bison meat was cooked, it could still overestimate the age of the kill site. If the charcoal was from wood from the oldest part of a long-lived tree, it could be decades or even centuries older than other parts of the same tree and thus older than the kill site.
In paragraph 3, why does the author discuss charcoal from different sources?
ATo present possible reasons why the charcoal tested may provide a wrong date for the Folsom kill
BTo support the claim that charcoal is relatively impervious to contamination
CTo emphasize the reliability of charcoal in radiocarbon dating
DTo explain why scientists used charcoal to help determine the age of the Folsom kill
7
Conversely, radiocarbon dating of animal bones can more precisely pin down the age of a site, since animals live shorter lives than trees, though, of course, only bones present as a result of human activity (which is certainly the case of the Folsom bison) are of interest to archaeologists. But dating bone itself can be challenging, since it is made of multiple constituents, some of which are highly susceptible to contamination. The inorganic portion, for example, readily absorbs ancient carbon via groundwater or other contaminants. Before that was realized, multiple radiocarbon ages on the same skeleton often gave wildly divergent results-in one case, for example, the oldest and youngest radiocarbon ages on the same mammoth were nearly 3,000 years apart. In the 1980s, after laboratory advances enabled the extraction of protein and individual amino acids from organic bone collagen, substances less susceptible to contamination, it became possible to derive ages from bone that were more reliable. Hence, the conclusion that the bone amino acid obtained from the Folsom bison better estimated when that kill occurred.
According to paragraph 4, why has radiocarbon dating of bone been inaccurate at times?
ASome parts of bones can be easily contaminated.
BSome parts of bones do not absorb radiocarbons.
CHuman activity can affect the substances in bones.
DSome bones came from animals that had no contact with humans.
8
But the handicap of radiocarbon dating, which will be felt if the first Americans prove to be far older than supposed, is that it does not provide reliable ages for any samples that might prove to be significantly more than 50,000 years old. For that, there are a number of other dating methods that can go beyond radiocarbon’s limits.
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.
AThe handicap of radiocarbon dating is that it cannot be used to determine whether samples of the first Americans are older than supposed.
BRadiocarbon dating is not reliable for samples significantly older than 50,000 years.
CRadiocarbon dating of samples more than 50,000 years old has been proved inaccurate.
DThe handicap of radiocarbon dating is that there is not a significant number of samples more than 50,000 years old.
9
Conversely, radiocarbon dating of animal bones can more precisely pin down the age of a site, since animals live shorter lives than trees, though, of course, only bones present as a result of human activity (which is certainly the case of the Folsom bison) are of interest to archaeologists. But dating bone itself can be challenging, since it is made of multiple constituents, some of which are highly susceptible to contamination. [■]The inorganic portion, for example, readily absorbs ancient carbon via groundwater or other contaminants. [■]Before that was realized, multiple radiocarbon ages on the same skeleton often gave wildly divergent results-in one case, for example, the oldest and youngest radiocarbon ages on the same mammoth were nearly 3,000 years apart.[■] In the 1980s, after laboratory advances enabled the extraction of protein and individual amino acids from organic bone collagen, substances less susceptible to contamination, it became possible to derive ages from bone that were more reliable. [■]Hence, the conclusion that the bone amino acid obtained from the Folsom bison better estimated when that kill occurred.
Look at the four squaresthat indicate where the following sentence could be added to the passage
Eventually, a way was found to obtain consistently reliable findings.
Where would the sentence best fit？Click on a square sentence to the passage.
10
Radiocarbon dating is commonly used to determine the age of prehistoric Native American sites.
ARadiocarbon dating can date specimens as old as 50,000 B.P., but it does not always provide equally reliable and valid results, as observed at the Folsom site.
BWhile charcoal is often useful for radiocarbon dating,the charcoal found at the Folsom site was too damaged by fire and contamination to be tested for its age.
CBone dating is reliable when researchers test components of collagen rather than components that are more likely to become contaminated.
DRadiocarbon dating is most reliable when the organic acids in a specimen can be compared to known radiocarbon levels in the atmosphere from a given time.
EBecause animals usually live shorter lives than trees do,researchers can often obtain more precise site ages by radiocarbon dating bones than by radiocarbon dating wood or charcoal.
FRemoving proteins and individual amino acids from a bone before radiocarbon dating helps ensure that the resulting radiocarbon date for the bone is accurate.
