OG-9-54求助

mathsgirl

    

54. 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

为什么选D不行呢？谢谢
（E）

    

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, 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. 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. 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.