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.
In opening paragraph, the author introduces his topic by ______
A.unfolding a phenomenon.
B.posing a contrast.
C.refuting a speculation.
D.testifying a hypothesis.
第1题
船舶在港内失火,船舶消防的总指挥是:
A.港口管理部门;
B.港务监督部门;
C.船方;
D.公安消防部门。
第2题
装载有粉尘货物的船舶,在港内冲冼甲板,必须得到_______批准方可。
A.船长
B.港口部门
C.港务监督
D.环境保护局
第3题
港内火灾,下列说法不正确的是:
A.港务监督部门负责总体指挥
B.船长对船舶灭火负有主要责任
C.随时与港务监督部门、港口公安保持联系
D.其他人员没有必要参与灭火
第4题
船舶在港内失火,岸上消防队已到达现场,在灭火过程中,船方错误的做法是_____。
A.与港务监督部门、港口公安保持随时联系
B.由于路上消防队经验丰富,装备精良,当进行破拆、灌舱等作业时,船长应毫不犹豫的予以支持
C.查清所有在船人员
第6题
根据交通部(1995)137号文,船舶在港内失火总体指挥是_____。
A.海事部门
B.船长
C.公安消防队
第7题
停泊在港内或锚地的船舶失火,船方与港方的合作包括:
A.在自力灭火的同时,呼叫港口消防队,通知港务监督部门
B.在港务监督部门的认可下,船长应继续负责全面指挥灭火工作,由港口消防队负责灭火,双方密切配合
C.在威胁港口安全或港方要求下,做好随时离港准备
D.以上均对
第8题
根据交通部(1995)137号文,船舶在港内失火时,负责总体指挥的是:
A.港监部门
B.船长
C.公安消防队
D.都可以
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