Discovering the Ice Ages

In the middle of the nineteenth century, Louis Agassiz, one of the first scientists to study glaciers, immigrated to the United States from Switzerland and became a professor at Harvard University, where he continued his studies in geology and other sciences. For his research, Agassiz visited many places in the northern parts of Europe and North America, from the mountains of Scandinavia and New England to the rolling hills of the American Midwest. In all these diverse regions, Agassiz saw signs of glacial erosion and sedimentation. In flat plains country, he saw moraines (accumulations of earth and loose rock that form at the edges of glaciers) that reminded him of the terminal moraines found at the end of valley glaciers in the Alps. The heterogeneous material of the drift (sand, clay, and rocks deposited there) convinced him of its glacial origin.

The areas covered by this material were so vast that the ice that deposited it must have been a continental glacier larger than Greenland or Antarctica. Eventually, Agassiz and others convinced geologists and the general public that a great continental glaciation had extended the polar ice caps far into regions that now enjoy temperate climates. For the first time, people began to talk about ice ages. It was also apparent that the glaciation occurred in the relatively recent past because the drift was soft, like freshly deposited sediment. We now know the age of the glaciation accurately from radiometric dating of the carbon-14 in logs buried in the drift. The drift of the last glaciation was deposited during one of the most recent epochs of geologic time, the Pleistocene, which lasted from 1.8 million to 10,000 years ago. Along the east coast of the United States, the southernmost advance of this ice is recorded by the enormous sand and drift deposits of the terminal moraines that form Long Island and Cape Cod.

It soon became clear that there were multiple glacial ages during the Pleistocene, with warmer interglacial intervals between them. As geologists mapped glacial deposits in the late nineteenth century, they became aware that there were several layers of drift, the lower ones corresponding to earlier ice ages. Between the older layers of glacial material were well-developed soils containing fossils of warm-climate plants. These soils were evidence that the glaciers retreated as the climate warmed. By the early part of the twentieth century, scientists believed that four distinct glaciations had affected North America and Europe during the Pleistocene epoch.

This idea was modified in the late twentieth century, when geologists and oceanographers examining oceanic sediment found fossil evidence of warming and cooling of the oceans. Ocean sediments presented a much more complete geologic record of the Pleistocene than continental glacial deposits did. The fossils buried in Pleistocene and earlier ocean sediments were of foraminifera—small, single-celled marine organisms that secrete shells of calcium carbonate, or calcite. These shells differ in their proportion of ordinary oxygen (oxygen-16) calcite of a foraminifer's shell depends on the temperature of the water in which the organism lived. Different ratios in the shells preserved in various layers of sediment reveal the temperature changes in the oceans during the Pleistocene epoch.

Isotopic analysis of shells allowed geologists to measure another glacial effect. They could trace the growth and shrinkage of continental glaciers, even in parts of the ocean where there may have been no great change in temperature—around the equator, for example. The oxygen isotope ratio of the ocean changes as a great deal of water is withdrawn from it by evaporation and is precipitated as snow to form glacial ice. During glaciations, the lighter oxygen-16 has a greater tendency to evaporate from the ocean surface than the heavier oxygen-18 does. Thus, more of the heavy isotope is left behind in the ocean and absorbed by marine organisms. From this analysis of marine sediments, geologists have learned that there were many shorter, more regular cycles of glaciation and deglaciation than geologists had recognized from the glacial drift of the continents alone.

 

Paragraph 1: In the middle of the nineteenth century, Louis Agassiz, one of the first scientists to study glaciers, immigrated to the United States from Switzerland and became a professor at Harvard University, where he continued his studies in geology and other sciences. For his research, Agassiz visited many places in the northern parts of Europe and North America, from the mountains of Scandinavia and New England to the rolling hills of the American Midwest. In all these diverse regions, Agassiz saw signs of glacial erosion and sedimentation. In flat plains country, he saw moraines (accumulations of earth and loose rock that form at the edges of glaciers) that reminded him of the terminal moraines found at the end of valley glaciers in the Alps. The heterogeneous material of the drift (sand, clay, and rocks deposited there) convinced him of its glacial origin.

 

1.  The word “accumulations” in the passage is closest in meaning to

O    signs

O    pieces

O    types

O    deposits

 

1. The word “heterogeneous” in the passage is closest in meaning to

O    remaining

O    varied

O    familiar

O    layered

 

1.  According to paragraph 1, what persuaded Louis Agassiz that glaciation in the past had been widespread?

O    Geologic differences between mountain valleys and flat plains

O    The presence of similar glacial material in many different regions

O    Geologic research on mountain glaciers in the Alps

O    Evidence of regional differences in the drift caused by glacial erosion

 

Paragraph 2: The areas covered by this material were so vast that the ice that deposited it must have been a continental glacier larger than Greenland or Antarctica. Eventually, Agassiz and others convinced geologists and the general public that a great continental glaciation had extended the polar ice caps far into regions that now enjoy temperate climates. For the first time, people began to talk about ice ages. It was also apparent that the glaciation occurred in the relatively recent past because the drift was soft, like freshly deposited sediment. We now know the age of the glaciation accurately from radiometric dating of the carbon-14 in logs buried in the drift. The drift of the last glaciation was deposited during one of the most recent epochs of geologic time, the Pleistocene, which lasted from 1.8 million to 10,000 years ago. Along the east coast of the United States, the southernmost advance of this ice is recorded by the enormous sand and drift deposits of the terminal moraines that form Long Island and Cape Cod.

 

1.  The word “enjoy” in the passage is closest in meaning to

O    Experience

O    Resemble

O    Expect

O    Dominate

 

1.  It can be inferred from paragraph 2 that Agassiz and other geologists of his time were not able to determine

O    which geographic regions had been covered with ice sheets in the last ice age

O    the exact dates at which drifts had been deposited during the last ice age

O    the exact composition of the drifts laid during the last ice age

O    how far south along the east coast of the United States the ice had advanced during the last ice age

 

Paragraph 3: It soon became clear that there were multiple glacial ages during the Pleistocene, with warmer interglacial intervals between them. As geologists mapped glacial deposits in the late nineteenth century, they became aware that there were several layers of drift, the lower ones corresponding to earlier ice ages. Between the older layers of glacial material were well-developed soils containing fossils of warm-climate plants. These soils were evidence that the glaciers retreated as the climate warmed. By the early part of the twentieth century, scientists believed that four distinct glaciations had affected North America and Europe during the Pleistocene epoch.

 

1.  According to paragraph 3, what did geologists conclude as a result of finding well-developed soils containing warm-climate plant fossils between layers of glacial drift?

O    There had been only one warm period before the Pleistocene epoch.

O    There had been multiple periods of mild weather between ice ages.

O    Several glacial periods occurred after the Pleistocene epoch.

O    Some earlier epochs were warmer thant the Pleistocene.

 

Paragraph 4: This idea was modified in the late twentieth century, when geologists and oceanographers examining oceanic sediment found fossil evidence of warming and cooling of the oceans. Ocean sediments presented a much more complete geologic record of the Pleistocene than continental glacial deposits did. The fossils buried in Pleistocene and earlier ocean sediments were of foraminifera—small, single-celled marine organisms that secrete shells of calcium carbonate, or calcite. These shells differ in their proportion of ordinary oxygen (oxygen-16) and the heavy oxygen isotope (oxygen-18). The ratio of oxygen-16 to oxygen-18 found in the calcite of a foraminifer's shell depends on the temperature of the water in which the organism lived. Different ratios in the shells preserved in various layers of sediment reveal the temperature changes in the oceans during the Pleistocene epoch.

 

1.  According to paragraph 3 and 4, scientists modified their theory about the exact number of glaciations because of evidence obtained from

O    ocean sediments

O    interglacial soils

O    glacial deposits

O    air samples

 

1. The word “reveal” in the passage is closest in meaning to

O    result from

O    vary with

O    show

O    preserve

 

Paragraph 5: Isotopic analysis of shells allowed geologists to measure another glacial effect. They could trace the growth and shrinkage of continental glaciers, even in parts of the ocean where there may have been no great change in temperature—around the equator, for example. The oxygen isotope ratio of the ocean changes as a great deal of water is withdrawn from it by evaporation and is precipitated as snow to form glacial ice. During glaciations, the lighter oxygen-16 has a greater tendency to evaporate from the ocean surface than the heavier oxygen-18 does. Thus, more of the heavy isotope is left behind in the ocean and absorbed by marine organisms. From this analysis of marine sediments, geologists have learned that there were many shorter, more regular cycles of glaciation and deglaciation than geologists had recognized from the glacial drift of the continents alone.

 

1. According to paragraph 4, scientists use foraminifera shells to learn about Pleistocene ocean conditions by

O    measuring the amount of calcium carbonate present in the shells

O    determining the proportion of shell in each layer of sediment

O    comparing shells deposited during the Pleistocene with those buried earlier

O    calculating the relative quantity of two oxygen isotopes in the calcite

 

1. In can be inferred from paragraph 5 that foraminifera fossil shells containing calcite with high percentages of oxygen-16 were deposited at times when

O    polar ice extended as far as equatorial regions of land and sea

O    extensive glaciation was not occurring

O    there were no great increases in ocean temperature

O    there was heavy snowfall on continental glaciers

 

1.  In paragraph 5, why does the author include the information that the “oxygen isotope ratio of the ocean changes as a great deal of water is withdrawn from it by evaporation and is precipitated as snow to form glacial ice” ?

O    To explain how scientists were able to calculate how frequently the continental ice sheets expanded and contracted

O    To explain how scientists have determined that there was no great change in ocean temperatures at the equator during past glaciations

O    To provide evidence that oxygen-16 has a greater tendency to evaporate than does oxygen-18

O    To suggest that equatorial marine organisms absorb more heavy isotopes than do marine organisms

 

1.  According to the passage, when did scientists begin to realize that more than one ice age had occurred ?

O    In the mid nineteenth century

O    In the late nineteenth century

O    In the early twentieth century

O    In the late twentieth century

 

Paragraph 1: In the middle of the nineteenth century, Louis Agassiz, one of the first scientists to study glaciers, immigrated to the United States from Switzerland and became a professor at Harvard University, where he continued his studies in geology and other sciences. For his research, Agassiz visited many places in the northern parts of Europe and North America, from the mountains of Scandinavia and New England to the rolling hills of the American Midwest. ■ In all these diverse regions, Agassiz saw signs of glacial erosion and sedimentation. ■ In flat plains country, he saw moraines (accumulations of earth and loose rock that form at the edges of glaciers) that reminded him of the terminal moraines found at the end of valley glaciers in the Alps. ■ The heterogeneous material of the drift (sand, clay, and rocks deposited there) convinced him of its glacial origin. ■

1.  Look at the four squares [■] that indicate where the following sentence could be added to the passage.

In his view, there could be no other explanation for the composition of such drift.

Where would the sentence best fit ?

 

1.  Directions: An introductory sentence for a brief summary of the passage is provided below. Complete the summary by selecting the THREE answer choices that express the most important ideas in the passage. Some sentences do not belong in the summary because they express ideas that are not presented in the passage or are minor ideas in the passage. This question is worth 2 points.

 

Louis Agassiz was the first to note signs of glacial erosion and sedimentation in diverse regions of Europe and North America.

O     

O     

O     

Answer Choices

O    Evidence of a pattern of glacier-like deposits eventually convinced most geologists that an enormous continental glacier had extended into the temperate zone.

O    Glacial research showed that many layers of ice were deposited, with each new period of glaciation extending farther south than the one before.

O    Isotopic analysis of marine sediments showed that periods of glaciation and deglaciation were more frequent, shorter, and more cyclic than previously thought.

O    Nineteenth-century geologists came to accept the idea that the areas covered by polar ice had reached as far as the equator, a far larger area than Agassiz had thought.

O    Nineteenth-century geologists studying the layers of drift concluded that during the Pleistocene epoch, several glaciations had occurred with war periods between them.

O    Research involving foraminifera fossil shells show that ocean temperatures in the Northern Hemisphere varied greatly during the most extensive periods of glaciation.

参考答案： 

1.  D

2.  B

3.  B

4.  A

5.  B

6.  B

7.  A

8.  C

9.  D

10.  B

11.  A

12.  B

13.  4th square

14.  Evidence of a ...

    Nineteenth-century geologists

    Isotopic analysis of...

 

参考译文： 

发现冰河时代 

在十九世纪中期，第一个研究冰川的科学家路易斯·阿加西从瑞士移民到美国成为了哈佛大学的一位教授，在那里他开始研究地质及相关科学。从他的研究看，他访问了欧洲和美国北部的很多地方，从斯堪的纳维亚，新英格兰翻山越岭到达了美国中西部。在所有这些不同的地区里阿加西看到了冰川融化和沉降的痕迹。在平原国家他看见了冰碛石（冰川边缘松散的土壤和岩石的聚集体），这些东西让他想起了在阿尔卑斯山谷冰川里发现的终碛石。这些不均匀的漂流物（沙子，泥土和沉积的石头）使他对冰川起源产生兴趣。

 

这些碛石覆盖的区域是如此之大以至于那些使它们沉积下来的的冰川几乎是比格林兰或者南极还要大的大型冰川。最终阿加西和他的支持者说服了地质学家和公众相信大型冰川已经从极地冰盖延伸到那些现在气温温和的地区。人们第一次开始讨论冰河时代这样一类问题。很明显冰川作用就发生在最近，因为漂流运动很温和，只有新形成的沉积物。我们知道冰川年龄的准确判断依靠冰块中的碳十四的同位素鉴定。上次冰川的漂流沉积作用发生在最近的几个地质代之一——更新世，从180万年到1万年前。沿着美国东海岸，最南边的冰川运动被来自长岛和科德角的大量的沙子和终磧石沉积物所记录下来。

 

很快我们就知道了在更新代有多个冰川代，这中间有几个间冰期和间歇期存在。在十九世纪地质学家对这些冰川沉降进行定位的时候，他们意识到有好几个漂流层在其中，浅一些的冰层对应的是更近一些的冰河时代。在这些年代更久远的冰层里有永冻土，其中包含了温带植物的化石。这些土壤表明冰川在温带来临之后就消失了。到了二十世纪初期，科学家们相信更新世中有四个不同的冰川代影响着北美和欧洲。

 

当地质学家和海洋学家研究海洋沉积发现海洋变暖和变冷的化石证据时，这种观点得到了修正。海洋沉积呈现出一种比更新世大型冰川沉积更完整的地质记录。这些沉积在更新世的化石和更早的海洋沉积物是有孔虫及分泌碳酸钙壳和方解石壳的单细胞有机物的产物。这些在有孔虫上的壳状物在方解石氧16的含量上不同，而含量决定于有机物生存环境下水的温度。不同的沉积层中的壳有不同的含量，这显示出更新代海洋温度的变化。

 

对壳的同位素分析使得地质学家能够测量一些其他的冰川影响，他们能够追踪大型冰川的萎缩和生长，即使是那些海洋中温度变化不太大的区域，比如说赤道附近。海洋中氧的同位素比率会在大量的水蒸发掉或凝结成为冰川时发生变化。在冰川作用时期轻一些的氧16比重一些的氧18更容易从海洋的表面蒸发。这样，更多的重的氧的同位素留在了海洋里并被有机物吸收。从这些海洋沉积物的分析来看，海洋地质学家比那些只从大型冰川漂移中分析冰川运动的地质学家了解到了更短更规则循环的冰川化和去冰川化过程。