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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.

【OG19-P401-483题】It can be inferred from the passage that calcium carbonate shells

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当前版本由 ya**** 更新于2015-11-04 12:04:09 感谢由 ya**** 对此题目的解答所做出的贡献。
从文章第二段可知碳酸钙壳所组成的海洋沉积岩可以反映出氧18增加的程度,而碳酸钙壳主要由周围海水中的氧原子构成
A错误,文章未作此比较
B错误,文章未作此比较
C错误,only内容文章未提及
D错误,与radioactive material无关
E正确,符合文中意思

题目讨论 (3条评论)

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    275476gao

    教主的阅读大杀器:同替
    composition= are composed of
    were formed= were constructed with
    2 0 回复 2022-04-10 23:11:35
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    847716opn

    看到determine就选了,没有仔细看整个选项的意思
    0 0 回复 2019-09-25 20:27:43
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    mzq

    定位句:because these sediments are composed of calcium carbonate shells of marine organisms, shells that were constructed with oxygen atoms drawn from the surrounding ocean. 排除ABC,在DE中选择。句中说shells本身就是用来被检测的,而D说的是shells当中含有可以被检测的物质。错误。E.composition of和 be constructed with同义替换。
    0 0 回复 2019-01-27 22:33:47
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