Source: Revised GRE PDF 1st Ed. Section 4: Verbal; #15 (p. 65)

7

It can be inferred from the passage

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 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

(A) differ very little in composition from the portions that correspond to the MedievalWarm Period
(B) fluctuate significantly in composition between the portions corresponding to the 1300s and the portions corresponding to the 1700s
(C) would be likely to contain a proportion of red-stained grains closer to 17 percent than to 5 percent
(D) 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
(E) were formed in part as a result of Gulf Stream waters having been pulled northward

6 Explanations

1

Ruichen Sun

Hi thanks for the video! Could you maybe say more about why B is not correct? If the composition fluctuates in 1500-year cycle, wouldn't it have fluctuated between1300s and 1700s? Thanks!

Jul 4, 2018 • Comment

Sam Kinsman, Magoosh Tutor

Hi Ruichen,

Happy to help!

First, notice that the passage says that the "fluctuations occurred in a nearly regular 1,500-year cycle." This most likely means that every 1,500 years, there was a switch. So the composition of the sediments is constant for 1,500 years, then switches, and is then constant again for 1,500 years, then switches, etc. So this makes it seem unlikely that, during the Little Age (which was only 560 years long), the sediment would have fluctuated.

Also, the passage says:

"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." So we know that during the onset (the beginning) of the Little Ice Age, the ocean's dynamic changed. We area being told that it changed once - and not that it fluctuated during the entire Little Ice Age. That's why B doesn't quite work.

I hope that helps! :)

Jul 6, 2018 • Reply

1

nirmal aditya priyan

i couldn't understand the little part of the explanation: it is at the little ice age how the icebergs travels far south?. please explain me this.
thank you.

Jun 2, 2018 • Comment

David Recine, Magoosh Tutor

The passage is talking about icebergs formed in the Northern Hemisphere. In the Northern hemisphere, icebergs eventually melt if they float southward, because the farther south one gets, the warmer the climate is.

However, during an ice age-- including the Little Ice Age that occurred in the 14th to 19th centuries, the whole world is colder. This means that the ice bergs in the Northern Hemisphere can float much father south without melting; a colder world means that a larger portion of southern areas have freezing temperatures.

Hope this helps, but let me know if you still have any doubts or questions.

Jun 4, 2018 • Reply

1

Aman Prasad

I am having difficulty understanding this question. What do the portions corresponding the Little Ice Age mean?
Also I have some confusion regarding North Atlantic. A sentence from the second paragraph reads " 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"

What does travelling south into the North Atlantic mean? I was not able to decipher what exactly North Atlantic is.

Please help me with the above,
Thanks!

Nov 13, 2017 • Comment

Cydney Seigerman, Magoosh Tutor

Hi Aman,

I'm happy to help :) First, here's what we learn about the Little Ice Age:
"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 oceans dynamic changed in this way."

The key is this last part, that during the Little Ice Age, the ocean's movements changed in the way described earlier in the passage: "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."

The passage states that this fluctuation was last observed during the Little Ice Age, so we can infer that (C) would be likely to contain a proportion of red-stained grains closer to 17 percent than to 5 percent.

Does that make sense?

To answer your other question, the North Atlantic refers to the northern part of the Atlantic Ocean. So, the sentence is saying that icebergs traveled south from the Arctic Ocean into a specific part of the Atlantic Ocean before melting.

I hope this clears up your doubts! If not, please let me know :)

Nov 19, 2017 • Reply

1

Xiao Lu

Thank you for the video, but answer C still seems totally wrong to me. During Little Ice Age, iceberg can travel to the south, meaning that it will not melt half way in the North Atlantic. In other words, fewer red-stained grains (~5%) will be deposited in the Atlantic North, and more grains (~17%) will be found in the south.

In the case of answer C, if 17% are found in Atlantic North, it means icebergs did not travel far into the south. This happens in the Medieval Warm Period. The observations will be consistent with answer D. Did I miss something?

Oct 27, 2016 • Comment

Adam

Hi Xiao,

The text states:

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.

So, if icebergs make it into the North Atlantic, they have already traveled quite far south from the Arctic. The passage never mentions icebergs traveling farther south than the North Atlantic.

Oct 28, 2016 • Reply

3

Arjun

Hi Chris,
I was able to deduce and understand the question. Also why c is the answer.
However, can you explain a bit, why D can not be the answer. My guess is probably it is true, but in warm periods. not in little Ice age.

Oct 9, 2015 • Comment

Adam

Hi Arjun,

Yes, the problem with (D) is that it would not correspond to the Little ice Age, but rather to warm periods, such as the Medieval Warm Period. If there are more red-stained grains in cores from the far north than from further south, this means that surface temperatures are higher, and therefore the sediment cores would not be able to have been deposited further south, because the water would be too warm -- they would have mostly melted in the north.

Nov 6, 2015 • Reply

7

Gravatar Chris Lele, Magoosh Tutor

Sep 26, 2012 • Comment

Lucio Chen

The video is not displaying anything.

Aug 29, 2016 • Reply

Cydney Seigerman, Magoosh Tutor

Hi Xiaocong,

The answer is a video hosted on YouTube. If your country blocks YouTube you won't be able to view the video :(

Aug 29, 2016 • Reply

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