首页
登录
职称英语
Eight times within the past million years, something in the Earth’s climatic
Eight times within the past million years, something in the Earth’s climatic
游客
2025-04-28
13
管理
问题
Eight times within the past million years, something in the Earth’s climatic equation has changed, allowing snow in the mountains and the northern latitudes to accumulate from one season to the next instead of melting away. Each time, the enormous ice sheets resulting from this continual buildup lasted tens of thousands of years until the end of each particular glacial cycle brought a warmer climate. Scientists speculated that these glacial cycles were ultimately driven by astronomical factor: slow, cyclic changes in the eccentricity of the Earth’s orbit and in the tilt and orientation of its spin axis. But up until around 30 years ago, the lack of an independent record of ice-age timing made the hypothesis untestable.
Then in the early 1950’s Emiliani produced the first complete record of the waxings and wanings of first glaciations. It came from a seemingly odd place, the seafloor. Single-cell marine organisms called "foraminifera" house themselves in shells made from calcium carbonate. When the foraminifera die, sink to the bottom, and become part of seafloor sediments, the carbonate of their shells preserves certain characteristics of the seawater they inhabited. In particular, the ratio of a heavy isotope of oxygen (oxygen-18) to ordinary oxygen (oxygen-16) in the carbonate preserves the ratio of the two oxygens in water molecules.
It is now understood that the ratio of oxygen isotopes reflects the proportion of the world’s water locked up in glaciers and ice sheets. A kind of meteorological distillation accounts for the link. Water molecules containing the heavier isotope tend to condense and fall as precipitation slightly sooner than molecules containing the lighter isotope. Hence, as water vapor evaporated from warm oceans moves away from its source, its oxygen- 18 returns more quickly to the oceans than does its oxygen-16. What falls as snow on distant ice sheets and mountain glaciers is relatively depleted of oxygen-18. As the oxygen-18-poor ice builds up, the oceans become relatively enriched in the isotope. The larger the ice sheets grow, the higher the proportion of oxygen-18 becomes in seawater--and hence in the sediments
Analyzing cores drilled from seafloor sediments, Emiliani found that the isotopic ratio rose and fell in rough accord with the Earth’s astronomical cycles. Since that pioneering observation, oxygenisotope measurements have been made on hundreds of cores. The combined record enables scientists to show that the re-cord contains the very periodicities as the orbital processes. Over the past 800, 000 years, the global ice volume peaked every 100,000 years, matching the period of the orbital eccentricity variation. In addition, "wrinkles" superposed on each cycle--small decreases or surges in ice volume--have come at intervals of roughly 23,000 and 41,000 years, in keeping with the precession and tilt frequencies of the Earth’s spin axis. [br] In opening paragraph, the author introduces his topic by ______
选项
A、unfolding a phenomenon.
B、posing a contrast.
C、refuting a speculation.
D、testifying a hypothesis.
答案
A
解析
题干问:“在文章开头作者通过什么方式引入话题?”正确选项为A“揭示一种自然现象”。此题定位于第1自然段,作者先给出一种自然现象的变化性,随后提出一种假设。而选项B“提出一种对比”,选项C“驳斥某种假设”和选项D“证明某个假说”这些都不是作者在第1自然段所使用的主要的逻辑引入方式。
转载请注明原文地址:https://tihaiku.com/zcyy/4056803.html
相关试题推荐
Hewas_____inthestreetsoftheMexicancapitalbymorethanamillionpeople,
Eachyear,millionsofpeopleinBangladeshdrinkgroundwaterthathasbeen
Eachyear,millionsofpeopleinBangladeshdrinkgroundwaterthathasbeen
Eachyear,millionsofpeopleinBangladeshdrinkgroundwaterthathasbeen
Eighttimeswithinthepastmillionyears,somethingintheEarth’sclimatic
Eighttimeswithinthepastmillionyears,somethingintheEarth’sclimatic
IwasvisitingNewYorklastweekandnoticedsomethingI’dneverthoughtI’
IwasvisitingNewYorklastweekandnoticedsomethingI’dneverthoughtI’
IwasvisitingNewYorklastweekandnoticedsomethingI’dneverthoughtI’
IwasvisitingNewYorklastweekandnoticedsomethingI’dneverthoughtI’
随机试题
Somepeoplethinkthatwiththedevelopmentoftechnology,ourtraditionalc
[originaltext]F:Excuseme,professor,sinceclassiscanceledonWednesday,wh
人们抱怨当地政府在处理污染问题上力度不够。Peoplecomplainedthatthelocalgovernmentdidn’ttryits
孕妇,28岁,GP,妊娠30周。近1周腹部迅速增大,腹胀痛、气促、心悸、不能平卧
下列不属于资本市场的有()。 ①大额可转让存单市场②中长期国债市场 ③股
Thechangeinthatvillagewasmiraculou
A.30分钟 B.1天 C.2天 D.1周 E.3周血小板浓缩悬液在22
乙公司为甲公司的全资子公司,且甲公司无其他子公司,乙公司2017年实现净利润50
A.甲氧苄啶 B.甲硝唑 C.氯霉素 D.磺胺嘧啶 E.环丙沙星抑制细菌
男,38岁。间断活动后尿色加深1周,既往反复痛风发作2年。查体:BP120/8
最新回复
(
0
)