Day 24

英文重现

When an ice age begins, the continental ice sheets grow, steadily reducing the amount of water evaporated from the ocean that will eventually return to it. Because heavier isotopes tend to be left behind when water evaporates from the ocean surfaces, the remaining ocean water becomes progressively enriched in oxygen 18. The degree of enrichment can be determined by analyzing ocean sediments of the period, because these sediments are composed of calcium carbonate shells of marine organisms, shells that were constructed with oxygen atoms drawn from the surrounding ocean.

——GMAT OG 2015, Reading Comprehension, Passage 15

解析

“shells that were constructed...surrounding ocean”是前面“calcium carbonate shells”的同位语；“that”引导的定语从句修饰“shells”，其中“drawn from the surrounding ocean”是过去分词短语作后置定语，修饰前面的“oxygen atoms”，意思上相当于定语从句“which were drawn from the surrounding ocean”。

词义辨析

参考翻译

（前两句的翻译见Day 23） （氧18的）丰富程度能够通过分析当时的海洋沉积物来确定，因为这些沉积物是由海洋生物的碳酸钙外壳组成的，这些外壳由从周围海洋中吸收的氧原子构造而成。

实战演练

…When an ice age begins, the continental ice sheets grow, steadily reducing the amount of water evaporated from the ocean that will eventually return to it. Because heavier isotopes tend to be left behind when water evaporates from the ocean surfaces, the remaining ocean water becomes progressively enriched in oxygen 18. The degree of enrichment can be determined by analyzing ocean sediments of the period, because these sediments are composed of calcium carbonate shells of marine organisms, shells that were constructed with oxygen atoms drawn from the surrounding ocean. The higher the ratio of oxygen 18 to oxygen 16 in a sedimentary specimen, the more land ice there was when the sediment was laid down.

It can be inferred from the passage that calcium carbonate shells

(A) are not as susceptible to deterioration as rocks.

(B) are less common in sediments formed during an ice age.

(C) are found only in areas that were once covered by land ice.

(D) contain radioactive material that can be used to determine a sediment's isotopic composition.

(E) reflect the isotopic composition of the water at the time the shells were formed.

答案：E

答案解析

由题目中“calcium carbonate shells”定位到“The degree of enrichment can be determined by analyzing ocean sediments of the period, because these sediments are composed of calcium carbonate shells of marine organisms, shells that were constructed with oxygen atoms drawn from the surrounding ocean”，关键要理解“shells that were constructed with oxygen atoms drawn from the surrounding ocean”中的“shells”是“calcium carbonate shells”的同位语，由本句话可以知道“碳酸钙外壳是由从周围海洋中吸收的氧原子构造的”，因此可以得出正确选项为E“（碳酸钙外壳）可以反映出外壳形成时水体中氧元素的构成”。其他选项在文中均未提到，意思分别为：

A.（碳酸钙外壳）不像岩石那样容易变质。

B.（碳酸钙外壳）在冰川时期形成的沉积物中不太常见。

C.（碳酸钙外壳）只有在曾经被陆地冰覆盖的地区才能被发现。

D.（碳酸钙外壳）含有放射性材料，这些放射性材料能够被用来确定沉积物的氧元素构成。