Milankovitch proposed in the early twentieth century that the ice ages were caused by variations in the Earth's orbit around the Sun. For some time this theory was considered untestable, largely because there was no sufficiently 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 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 sediments. Almost all the oxygen in water is oxygen 16, but a few molecules out of every thousand incorporate the heavier isotope 18. 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.

As an indicator of shifts in the Earth's climate, the 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 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 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, 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 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.

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It can be inferred from the passage that the isotope record taken from ocean sediments would be less useful to researchers if which of the following were true?


It indicated that lighter isotopes of oxygen predominated at certain times.

It had far more gaps in its sequence than the record taken from rocks on land.

It indicated that climate shifts did not occur every 100,000 years.

It indicated that the ratios of oxygen 16 and oxygen 18 in ocean water were not consistent with those found in fresh water.

It stretched back for only a million years.

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题目分析：

文章推断题：以下哪点（如果是真的话）会让从海洋沉淀物里取出的同位素记录没那么有用了？


选项分析：

A选项：轻一点的氧同位素一度占主导地位：根据文章来看 ，轻一点的氧同位素（氧16）确实占主导地位，所以这个选项对研究没影响。

B选项：正确。比起陆地上的石头，它在顺序上有更多漏洞：原文提到，氧同位素比石头更有连贯性，这是这个方法的优点。如果现在这个优点不存在了，那这个方法就没那么有用了。

C选项：气候变化并没有每100000年发生一次：这削弱了M的理论，而不是削弱了方法的有效性。

D选项：海洋里的氧16氧18的比例和水里的不一样：水里的情况和这个方法无关。

E选项：它只往前推了一百万年：文章line 42-46提到这个方法用的时间线是一百一千年左右，所以只往前推一百万年不会影响研究。

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