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.
According to the passage, Bond hypothesized that which of the following circumstances would allow red-stained sediment grains to reach more southerly latitudes?
- Warm waters being pulled northward from the Gulf Stream
- Climatic conditions causing icebergs to melt relatively quickly
- Icebergs containing a higher proportion of iron oxide than usual
- The formation of more icebergs than usual in the far north
- The presence of cold surface waters in the North Atlantic
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 10 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 begins “according to the passage,” which is a hint that it is asking us about a specific detail in the passage. The only problem is that this passage is lousy with details! However, we can use keywords in the question to help us navigate. “Bond” would usually be a good keyword since its capitalized, but “Bond” is mentioned in every paragraph. There are a couple of places where “red-stained sediment” show up as well, BUT we might look for “red-stained grains” mentioned somewhere in the same area as something like “southerly latitudes,” which only happens at the end of the third paragraph! (Alternatively, if we’ve taken notes on the passage, we likely would have noted that this paragraph describes where icebergs containing the sediments travel). Let’s look in this part of the passage.
“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.”
Aha. From this part of the passage, we learn that when Gulf Stream waters do NOT travel northward, the surface waters in the North Atlantic are colder, which is why icebergs can travel further south (if we are confused, we can backtrack a little bit further in the paragraph where the opposite scenario is described, but really this information is enough for us to infer that when the reverse is true and Gulf waters DO travel north, the North Atlantic is warmer and not conducive to iceberg travel). We want to look for an answer that reflects this information from the passage.
- Warm waters being pulled northward from the Gulf Stream
This answer is the opposite of what the passage said, which is a common type of wrong answer on reading comprehension questions. We can eliminate A.
- Climatic conditions causing icebergs to melt relatively quickly
This answer is ALSO the opposite of what the passage said; the icebergs need time to travel further south before melting. We can eliminate B.
- Icebergs containing a higher proportion of iron oxide than usual
This answer discusses information from another paragraph which should be a red flag! This may sound logical since it does use information from the passage, but it isn’t the answer we found in the passage. We can eliminate C.
- The formation of more icebergs than usual in the far north
The passage never said anything about the formation of more icebergs. This answer is irrelevant, so we can eliminate D.
- The presence of cold surface waters in the North Atlantic
Aha! This answer matches information we found in the last couple of sentences of the third paragraph, which is where the mechanism behind red-stained grains travelling south was described. E is the correct answer.
As we can see, keywords are not enough to make an answer a good answer! Answers to questions about specific details will often include keywords from elsewhere in the passage OR state the opposite of what the passage said in order to use the same keywords in an answer choice. While we know answers will often be paraphrased, we want to make sure we’re careful to find answers that are the same idea as what was actually stated in the passage. E is correct.
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