OG-9-53

baobaob

Milankovitch proposed in the early twentieth century

that the ice ages were caused by variations in the Earth’s

orbit around the Sun. For sometime this theory was

considered untestable, largely because there was no suffi-

(5) ciently precise chronology of the ice ages with which

the orbital variations could be matched.

To establish such a chronology it is necessary to

determine the relative amounts of land ice that existed

at various times in the Earth’s past. A recent discovery

(10) makes such a determination possible: relative land-ice

volume for a given period can be deduced from the ratio

of two oxygen isotopes*52 C, 16 and 18, found in ocean sedi-

ments. Almost all the oxygen in water is oxygen 16, but

a few molecules out of every thousand incorporate the

(15) heavier isotope 18. When an ice age begins, the conti-

nental 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

behid when water evaporates from the ocean surfaces,

(20) the remaining ocean water becomes progressively

enriched in oxygen 18. *53B 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

(25) constructed with oxygen atoms drawn from the sur-

rounding 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.

As an indicator of shifts in the Earth’s climate, the

(30) isotope record has two advantages. First, it is a global

record: there is remarkably little variation in isotope

ratios in sedimentary specimens taken from different

continental locations. Second, it is a more continuous

record than that taken from rocks on land. *51B Because of

(35) these advantages, sedimentary evidence can be dated

with sufficient accuracy by radiometric methods to

establish a precise chronology of the ice ages. The dated

isotope record shows that the fluctuations in global

ice volume over the past several hundred thousand years

(40) have a pattern: an ice age occurs roughly once every

100,000 years. These data have established a strong

connection between variations in the Earth’s orbit and

the periodicity of the ice ages.

However, it is important to note that other factors*50A/*50D,

(45) such as volcanic particulates or variations in the amount

of sunlight received by the Earth, could potentially have

affected the climate. The advantage of the Milankovitch

theory is that it is testable: changes in the Earth’s orbit

can be calculated and dated by applying Newton’s laws

(50) of gravity to progressively earlier configurations of the

bodies in the solar system. Yet the lack of information

about other possible factors affecting global climate does

not make them unimportant.

53. It can be inferred from the passage that precipitation formed from evaporated ocean water has

(A)   the same isotopic ratio as ocean water

(B)   less oxygen 18 than does ocean water

(C)   less oxygen 18 than has the ice contained in continental ice sheets

(D)  a different isotopic composition than has precipitation formed from water on land（B）

(E)   more oxygen 16 than has precipitation formed from fresh water

B is the best answer.

Lines 23-27 state that when water evaporates from the ocean surface, oxygen 18, a heavier isotope than oxygen 16, tends to be left behind in the remaining ocean water.

Thus, one can infer that evaporated ocean water would contain less oxygen 18 than would the

remaining ocean water.A is incorrect because it contracts information stated in lines 19-27. C is incorrect because the passage suggests that the water evaporated from the ocean contributes to the growth of continental ice sheets, which should therefore have an isotopic composition similar to that of the precipitation formed from evaporated ocean water. D and E describe information that cannot be inferred from the passage.

想了半天也没明白OG从文章哪推出的similar to？

[此贴子已经被作者于2006-3-3 1:35:22编辑过]

jonymontreal

C的意思很明确.the water evaporated from the ocean contributes to the growth of continental ice sheets  ,

从海洋里蒸发的水分是形成地表冰层的主要成分. 所以他的同位素构成应该和

降水构成是相同的.

所以C是错误的,C认为蒸发生成的降水比地面冰层中氧18的含量少.实际上海洋蒸发水是冰川时代形成地面冰层的主要成分.是同一种物质.

jonymontreal

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 behid when water evaporates from the ocean surfaces,(20) the remaining ocean water becomes progressively enriched in oxygen 18.

原文15行的内容便是, 当冰川时代开始,大陆上的冰川增多, 逐渐的海洋水蒸气减少慢慢的转变为冰层/

因为重的同位素不如轻的同位素容易挥发,所以留在海洋里的水分含有氧18的成分越来越多