Milankovitch proposed in the early twentieth century that the ice ages were

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问题 Milankovitch proposed in the early twentieth century that the ice ages were caused by variations in the Earth’s orbit around the Sun. Eor sometime this the- ory 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 thou- sand 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 wator 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 oxy- gen 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 dat- ed 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. [br] According to the passage, which of the following is true of the ratios of oxygen isotopes in ocean sediments?

选项 A、They indicate that sediments found during an ice age contain more calcium carbonate than sediments formed at other times.
B、They are less reliable than the evidence from rocks on land in determining the volume of land ice.
C、They can be used to deduce the relative volume of land ice that was present when the sediment was laid down.
D、They are more unpredictable during an ice age than in other climatic conditions.
E、They can be used to determine atmospheric conditions at various times in the past.

答案 C

解析关于洋底的氧同位素比例，文中指出：A．在冰期形成的沉积物比其他时期形成的含碳酸钙多。无此比较。B．在确定冰体积上，不如陆上资料可靠。原文LA6—48，海中资料比陆上可靠。C．正确。可被用来推算形成时陆冰的体积。见原文L12—16，明确地说明了氧同位素的此种作用。D．在冰期比在其他气候条件下不易预测。无。E．可被用来测量过去的大气条件。易混，atmospheric conditions不等于climate。

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