首页
登录
职称英语
Milankovitch proposed in the early twentieth century that the ice ages were
Milankovitch proposed in the early twentieth century that the ice ages were
游客
2024-01-13
57
管理
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] The purpose of the last paragraph of the passage is to
选项
A、offer a note of caution.
B、introduce new evidence.
C、present two recent discoveries.
D、summarize material in the preceding paragraphs.
E、offer two explanations for a phenomenon.
答案
A
解析
文中最后一段的目的:A.正确。标志作者的谨慎。作者是赞赏isotope在确定年代方面作用的,但并不一味贬低其他方面。在最后一段指出,除了M的解释之外,其他原因可能也是有道理的。B.介绍新证据。无。C.给出两个新近发现。无。D.总结前几段的材料。无。E.对一个现象给两个解释。不确切。
转载请注明原文地址:https://tihaiku.com/zcyy/3359169.html
相关试题推荐
Milankovitchproposedintheearlytwentiethcenturythattheiceageswerecaus
Intheeighteenthcentury,Japan’sfeudaloverlords,fromtheshoguntothe
Intheeighteenthcentury,Japan’sfeudaloverlords,fromtheshoguntothe
Intheeighteenthcentury,Japan’sfeudaloverlords,fromtheshoguntothe
Intheeighteenthcentury,Japan’sfeudaloverlords,fromtheshoguntothe
Intheeighteenthcentury,Japan’sfeudaloverlords,fromtheshoguntothe
Attheendofthenineteenthcentury,arisinginterestinNativeAmericanc
Attheendofthenineteenthcentury,arisinginterestinNativeAmericanc
Attheendofthenineteenthcentury,arisinginterestinNativeAmericanc
Attheendofthenineteenthcentury,arisinginterestinNativeAmericanc
随机试题
Geneticallymodified(GM)foodstuffsareheretostay.That2snottosaytha
Therearefivebasicfunctionsofanewspaper:toinform,toreview,topers
[originaltext]Ourenvironmentisbeingpollutedfasterthannatureandman’spr
抗震设计时当采用剪力墙结构而底层空旷时,应有不少于()的剪力墙落地并在底层予
对骨髓有抑制作用,可引起血小板计数减少的是()A:酚磺乙胺 B:泼尼松龙
急性乙醇中毒主要是对哪个系统产生抑制作用A.运动系统 B.呼吸系统 C.心血
某果品公司急需将一批不易存放的水果从A市运到B市销售。现有四家运输公司可供选择,
2020年对中国是关键一年。一方面,中国经济调结构促改革已经取得重大进展,但离全
对于取土坑或填筑路堤土方,一般采用()进行计量。A.估价法 B.凭据法 C
女,28岁。既往月经规律,现停经45天,发现阴道流出少量咖啡色物2天,无腹痛,尿
最新回复
(
0
)