In the final paragraph of the passage, the author is concerned

Recent studies of sediment in the North Atlantic’s deep waters reveal possible cyclical patterns in the history of Earth’s climate. The rock fragments in these sediments are too large to have been transported there by ocean currents; they must have reached their present locations by traveling in large icebergs that floated long distances from their point of origin before melting. Geologist Gerard Bond noticed that some of the sediment grains were stained with iron oxide, evidence that they originated in locales where glaciers had overrun outcrops of red sandstone. Bond’s detailed analysis of deep-water sediment cores showed changes in the mix of sediment sources over time: the proportion of these red-stained grains fluctuated back and forth from lows of 5 percent to highs of about 17 percent, and these fluctuations occurred in a nearly regular 1,500-year cycle.

Bond hypothesized that the alternating cycles might be evidence of changes in ocean-water circulation and therefore in Earth’s climate. He knew that the sources of the red-stained grains were generally closer to the North Pole than were the places yielding a high proportion of “clean” grains. At certain times, apparently, more icebergs from the Arctic Ocean in the far north were traveling south well into the North Atlantic before melting and shedding their sediment.

Ocean waters are constantly moving, and water temperature is both a cause and an effect of this movement. As water cools, it becomes denser and sinks to the ocean’s bottom. During some periods, the bottom layer of the world’s oceans comes from cold, dense water sinking in the far North Atlantic. This causes the warm surface waters of the Gulf Stream to be pulled northward. Bond realized that during such periods, the influx of these warm surface waters into northern regions could cause a large proportion of the
icebergs that bear red grains to melt before traveling very far into the North Atlantic. But sometimes the ocean’s dynamic changes, and waters from the Gulf Stream do not travel northward in this way. During these periods, surface waters in the North Atlantic would generally be colder, permitting icebergs bearing red-stained grains to travel farther south in the North Atlantic before melting and depositing their sediment.

The onset of the so-called Little Ice Age (1300-1860), which followed the Medieval Warm Period of the eighth through tenth centuries, may represent the most recent time that the ocean’s dynamic changed in this way. If ongoing climate-history studies support Bond’s hypothesis of 1,500-year cycles, scientists may establish a major natural rhythm in Earth’s temperatures that could then be extrapolated into the future. Because the midpoint of the Medieval Warm Period was about A.D. 850, an extension of Bond’s cycles would place the midpoint of the next warm interval in the twenty-fourth century.

In the final paragraph of the passage, the author is concerned primarily with

  1. answering a question about Earth’s climatic history
  2. pointing out a potential flaw in Bond’s hypothesis
  3. suggesting a new focus for the study of ocean sediments
  4. tracing the general history of Earth’s climate
  5. discussing possible implications of Bond’s hypothesis

An important thing to keep in mind about the Reading Comprehension section of the GRE as we use PowerPrep online to study is that it is just that—reading comprehension. In other words, as difficult as it may seem, and it can be pretty tricky, the test makers will always give us all the information we need in the passage to answer the question. For questions that come from monster passages, such as question 9 of the first Verbal section on practice test 1, the search can be difficult, but we still need to learn to find support for an answer from the passage.

Alright, there’s always one LONG passage on the GRE—fortunately only one, but it’s something we can’t really get around. Since the test will tell us that questions 8-11 (four whole questions) are based on this passage, it is worth our time to read the passage. We’ll need to read somewhat quickly—after all, four questions cannot possibly quiz us over every detail—but we need to be able to find information quickly. One way we can keep track of everything is to take very short notes on our scratch paper about each paragraph.

This question asks us about a “primary concern” of the author’s; roughly translated, the question wants to know the main idea of the last paragraph. If we’ve been keeping notes about each paragraph, now is the time to use them! If not, another useful trick is that we might read the beginnings and ends of each paragraph to get a sense of what they talk about and how they relate. Even though we’re asked about the last paragraph, we still need to quickly recap what each paragraph is about in order to have the necessary context in order to answer this question correctly.

The first paragraph, we may have noted, introduced a phenomenon that Bond studied: the red iron-oxide stains on some sediments carried to the North Atlantic by iceburgs. The second paragraph explained a hypothesis of Bond’s: that cycles evinced by these sediments proved a point about ocean current and climate cycles. The third paragraph expanded on this by explaining how ocean currents are controlled by temperature.

Finally, the last paragraph connects the “Little Ice Age” and the “Medieval Warm Period” to this 1,500 year cycle. It explains that if Bond’s hypothesis is correct, scientist can expect the middle of the next warm period to be in the twenty-fourth century.

Alright, the last paragraph seemed to broaden out the passage a little bit. Instead of discussing how Bond arrived at his conclusions, the last paragraph connects his hypothesis to the past and present. Let’s see if there’s an answer choice that reflects something similar.

  • answering a question about Earth’s climatic history

Well, this paragraph does mention something about Earth’s climatic history, but this answer ignores the connection to the future that seems to be a key idea in this paragraph. Also, the previous paragraphs didn’t pose any questions about Earth’s climatic history that needed to be answered: instead, the information in this paragraph is simply presented as additional evidence in-line with Bond’s observations. We can eliminate A

  • pointing out a potential flaw in Bond’s hypothesis

The paragraph provides information from history that seems to be in-line with Bond’s hypothesis, so it doesn’t really weaken or find faults with Bond’s hypothesis. We can eliminate B.

  • suggesting a new focus for the study of ocean sediments

What was the original focus of the “study of ocean sediments”? We know from the second paragraph that Bond’s hypothesis was always about suggesting how these ocean sediments might help explain climate cycles, and this last paragraph continues with the same focus. We can eliminate C because, while the last paragraph suggests new information and a new application of Bond’s research, it does not suggest a new focus.

  • tracing the general history of Earth’s climate

Hmm. Our paragraph did mention two historical climatic events, so this must be the answer, right? In the words of the great Admiral Ackbar, IT’S A TRAP! The test makers know that we might be feeling flustered and just want to pick an answer, and here’s one that sounds like it fits the general idea of the paragraph. The only problem is that the paragraph DOES NOT give us a complete general history of Earth’s climate—it only lists TWO events—nor does the paragraph focus solely on history—the future is alluded to, as well. We can eliminate D.

  • discussing possible implications of Bond’s hypothesis

Aha! This is the only answer that alludes to the fact that our last paragraph discusses how Bond’s hypothesis might be applied, which was the point of relating the hypothesis to the past and future. The passage specifically talks about what might be “extrapolated” from Bond’s hypothesis, which is another way of talking about its “implications.” E is a good answer!


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