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.
It can be inferred from the passage that in sediment cores from the North Atlantic’s deep waters, the portions that correspond to the Little Ice Age
- differ very little in composition from the portions that correspond to the Medieval Warm Period
- fluctuate significantly in composition between the portions corresponding to the 1300s and the portions corresponding to the 1700s
- would be likely to contain a proportion of red-stained grains closer to 17 percent than to 5 percent
- show a much higher proportion of red-stained grains in cores extracted from the far north of the North Atlantic than in cores extracted from further south
- were formed in part as a result of Gulf Stream waters having been pulled northward
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 11 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 specifically asks for something that can be “inferred” from the passage, meaning that the correct answer will not necessarily be spelled out for us in the passage. Still, our answer should be something that we can logically conclude from the information given, and in order to find what information is relevant, we’ll still need to rely on keywords in our question.
This question gives us some great keywords “the Little Ice Age,” which is something easy to skim for—we need information from the last paragraph. HOWEVER, the question also asks about “sediment cores” from the “deep waters of the North Atlantic,” and these keywords do NOT appear in the last paragraph. We can see that part of what makes this an inference question is that we’ll need to use information from across the passage in order to arrive at a conclusion about these sediments. Since we pretty quickly found information about the “Little Ice Age,” let’s see what we can find out about these keywords that might connect to something in the passage and work backwards.
“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. “
Ok, the first sentence of the last paragraph, the only sentence that specifically mentions our keywords, connects the Little Ice Age to the last time “the ocean’s dynamic changed in THIS way.” The phrase “this way” clearly refers back to something earlier in the passage. Was there a point in which the passage described a dynamic shift in the ocean—likely at the end of the last paragraph so that “this way” would logically connect back to something? YES INDEED! Ok, let’s look to see if there’s anything about sediment cores or the North Atlantic there.
“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.”
Ok, now we know that when the dynamic shifts, icebergs carrying “red-stained grains” can move further south in the North Atlantic because the waters are cooler, but where do “sediment cores” come into play with all of this? Well, “deep-water sediment cores” are mentioned only WAY back in the first paragraph. Let’s see if there’s a connection.
“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.”
Things are starting to come together! We’re told here that the “red-stains” come from iron oxide and the cores can show us a percentage of red-stained grains given a particular time—like say during the Little Ice Age. Still, what does this say about the North Atlantic and icebergs and all that? Well, we know that during the Little Ice Age, the sediments bearing these red-stained grains traveled further south in their icebergs, into the North Atlantic. Therefore, we can logically conclude that the “sediment cores” will contain a higher percentage of these stained grains during the time of the Little Ice Age. Let’s see if there’s an answer that reflects this.
- differ very little in composition from the portions that correspond to the Medieval Warm Period
The passage implies that the Medieval Warm Period would NOT have been a time when icebergs could travel south, which means that sediment cores from this time would NOT have as many stained grains and would likely vary in composition. We can eliminate A.
- fluctuate significantly in composition between the portions corresponding to the 1300s and the portions corresponding to the 1700s
These dates are roughly the period given for the Little Ice Age itself. We are not told that the ocean cycle would have changed back during this time—in fact the passage seems to imply the opposite—so we have no reason to believe the composition of the sediment cores throughout this time would change. Eliminate B.
- would be likely to contain a proportion of red-stained grains closer to 17 percent than to 5 percent
These numbers aren’t arbitrary! 17 percent tends to be the highest concentration of red-stained grains found in core samples, according to the passage. We know that during the Little Ice Age the icebergs containing these samples would have been able to travel into the North Atlantic, which would logically mean there would be a higher concentration of these grains. C seems to work, but let’s check the other answers to be sure.
- show a much higher proportion of red-stained grains in cores extracted from the far north of the North Atlantic than in cores extracted from further south
Hmm. This answer may seem logical, but actually we don’t know—if ALL of the icebergs were permitted to travel further south, then there actually might NOT be a higher concentration of the red-stained grains farther north. We do not have enough information to support D, so we can eliminate this answer.
- were formed in part as a result of Gulf Stream waters having been pulled northward
This answer uses irrelevant information from the passage (the passage did not say that waters form sediment cores) AND we know that during the Little Ice Age the Gulf Stream waters were NOT pulled North. We can eliminate E.
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