Passage 1.At the 2007 American Geophysical Union’s.meeting in Acapulco, Mexico, some two dozen.scientists presented multiple studies arguing that a.comet or asteroid exploded above or on the northern.ice cap almost 13,000 years ago—showering debris.across the North American continent and causing.temperatures to plunge for the next millennium..The team argues that its idea explains multiple.observations: not only the climate cooling and the.disappearance of the Clovis hunters, but also the.near-simultaneous extinction of the continent’s large.mammals..Not all will be convinced. Several leading.hypotheses already explain each of these three events..A change in ocean circulation is generally thought to.have brought about the onset of the millennium-long.cooling, which is known as the Younger Dryas. This.cooling might, in turn, have caused the Clovis.hunters to disappear. And, if they had not previously.been killed by disease or hunted to extinction, the big.prehistoric beasts may also have been doomed by this.change in climate..The new evidence comes in the form of.geochemical analysis of sedimentary layers at 25.archaeological sites across North America—9 of.them Clovis. Certain features of the layers, say the.team, suggest that they contain debris formed by an.extraterrestrial impact. These include spherules of.glass and carbon, and amounts of the element.iridium said to be too high to have originated on.Earth. In addition, the rocks contain black layers of.carbonized material, which the team says are the.remains of wildfires that swept across the continent.after the impact..Passage 2.Proponents of the Younger Dryas impact.hypothesis have claimed various kinds of evidence.for the hypothesis, including deposits of the element.iridium (rare on Earth but abundant in meteorites),.microscopic diamonds (called nanodiamonds), and.magnetic particles in deposits at sites supposedly.dated to about 12,800 years ago. These claims were.sharply contested by some specialists in the relevant.fields, however, who either did not detect such.evidence or argued that the deposits had other causes.than a cosmic impact. For example, some say that.nanodiamonds are common in ordinary geological.formations, and that magnetic particles could come.from ordinary fires..Now comes what some researchers consider the.strongest attack yet on the Younger Dryas impact.hypothesis. In a paper published recently in the.Proceedings of the National Academy of Sciences,a.team led by David Meltzer, an archaeologist at.Southern Methodist University, Dallas, in Texas,.looks at the dating of 29 different sites in the.Americas, Europe, and the Middle East in which.impact advocates have reported evidence for a.cosmic collision. They include sites in which.sophisticated stone projectiles called Clovis points,.used by some of the earliest Americans to hunt.mammals beginning about 13,000 years ago, have.been found. The team argues that when the quality.and accuracy of the dating—which was based on.radiocarbon and other techniques—is examined.closely, only three of the 29 sites actually fall within.the time frame of the Younger Dryas onset, about 12,800 years ago; the rest were probably either earlier.or later by hundreds (and in one case, thousands) of.years..“The supposed Younger Dryas impact fails on.both theoretical and empirical grounds,” says.Meltzer, who adds that the popular appeal of the.hypothesis is probably due to the way that it provides.“simple explanations for complex problems.” Thus,.“giant chunks of space debris clobbering the planet.and wiping out life on Earth has undeniably broad.appeal,” Meltzer says, whereas “no one in Hollywood.makes movies” about more nuanced explanations,.such as Clovis points disappearing because early.Americans turned to other forms of stone tool.technology as the large mammals they were hunting.went extinct as a result of the changing climate or.hunting pressure..But impact proponents appear unmoved by the.new study. “We still stand fully behind the [impact.hypothesis], which is based on more than a.confluence of dates,” says Richard Firestone, a.nuclear chemist at the Lawrence Berkeley National.Laboratory in California. “Radiocarbon dating is a.perilous process,” he contends, adding that the.presence of Clovis artifacts and mammoth bones just.under the claimed iridium, nanodiamond, and.magnetic sphere deposits is a more reliable indicator.that an extraterrestrial event was responsible for their.disappearance.Q: Based on Passage 1, which hypothetical discovery would provide the most support for the impact hypothesis? Answer Choices: (A)An asteroid impact crater beneath the northern ice cap contains high levels of iridium and has been dated to well after the start of the Younger Dryas. (B)Glass and carbon spherules appear at multiple points in the geologic record but never in conjunction with iridium deposits. (C)Analysis of ice cores suggests that global temperatures started declining approximately 13,000 years before the onset of the Younger Dryas. (D)High levels of osmium, which is rare on Earth but relatively common in asteroids, are observed in the geologic record from approximately 13,000 years ago.
A: Among A through D, the answer is
(A)An asteroid impact crater beneath the northern ice cap contains high levels of iridium and has been dated to well after the start of the Younger Dryas.
(B)Glass and carbon spherules appear at multiple points in the geologic record but never in conjunction with iridium deposits.
(C)Analysis of ice cores suggests that global temperatures started declining approximately 13,000 years before the onset of the Younger Dryas.
(D)High levels of osmium, which is rare on Earth but relatively common in asteroids, are observed in the geologic record from approximately 13,000 years ago.
Answer: 3