首页
外语
计算机
考研
公务员
职业资格
财经
工程
司法
医学
专升本
自考
实用职业技能
登录
外语
Milankovitch proposed in the early twentieth century that the ice ages were caused by variations in the Earth’s orbit around the
Milankovitch proposed in the early twentieth century that the ice ages were caused by variations in the Earth’s orbit around the
admin
2014-09-18
39
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.
According to the passage, which of the following is(are)true of the ice ages? I . The last ice age occurred about 25,000 years ago. II. Ice ages have lasted about 10, 000 years for at least the last several hundred thousand years. III. Ice ages have occurred about every 100,000 years for at least the last several hundred thousand years.
选项
A、I only
B、II only
C、III only
D、I and II only
E、I , II and III
答案
C
解析
哪一个关于冰期的说法是正确的?I.最近一次出现在两万五千年前。文中未提。Ⅱ.至少最近几十万年,冰期延续了一万年。文中无。Ⅲ.至少最近几十万年,每十万年出现一次。这是文中L52—55的明确说法。∴C.Ⅲonly
转载请注明原文地址:https://jikaoti.com/ti/HQzYFFFM
本试题收录于:
GMAT VERBAL题库GMAT分类
0
GMAT VERBAL
GMAT
相关试题推荐
Musiccomesinmanyforms:mostcountrieshaveastyleoftheirown.【C1】______theturnofthecenturywhenjazzwasborn,Ameri
Musiccomesinmanyforms:mostcountrieshaveastyleoftheirown.【C1】______theturnofthecenturywhenjazzwasborn,Ameri
Musiccomesinmanyforms:mostcountrieshaveastyleoftheirown.【C1】______theturnofthecenturywhenjazzwasborn,Ameri
Musiccomesinmanyforms:mostcountrieshaveastyleoftheirown.【C1】______theturnofthecenturywhenjazzwasborn,Ameri
Themeasureyouproposed______whatisexpected.
SpeakerA:CouldIgetaridewithyoutotheconcerttonight?SpeakerB:Ican’tgo,______.Ithinkshe’sleavingaround7:30
Atthebeginningoflastcentury,medicalscientistsmadeaninterestingdiscovery:wearebuiltnotjustoffleshandbloodbut
Atthebeginningoflastcentury,medicalscientistsmadeaninterestingdiscovery:wearebuiltnotjustoffleshandbloodbut
随机试题
预防中枢神经系统白血病时,常用作鞘内注射的化疗药是
A.生理需要B.安全需要C.爱与被爱的需要D.尊重的需要E.自我实现的需要发挥自己的潜能,实现自已的理想与抱负的需要是
患者,男,50岁,失眠症数年。关于其治疗与护理措施正确的是()
按照病理分类,婴幼儿最常见的肺炎是
流动资产是指预计在一个正常营业周期中变现、出售或耗用的资产。()
教师:教室正确选项为()
为保证满负荷,必须有多高的出勤率?如果该厂段优化组合掉20人,又要保护90%的出勤率,必须提高多少效率?
在FastEthernet中,为了使物理层在实现100Mbps速率时所使用的传输介质和信号编码方式的变化不会影响MAC子层,100BASE-T标准定义了______。
Readthefollowingpassageandfillintheblankswithitscontents.WritethemonyourANSWERSHEET.Asperthetermsandc
A、Byteachingwritershowtoimitate.B、Byidentifyingwriter’sstrengthandweakness.C、Bydevelopingwriter’spotential.D、By
最新回复
(
0
)