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Line Milankovitch proposed in the early twentieth
century that the ice ages were caused by variations
in the Earth's orbit around the Sun. For some
time this theory was considered untestable,
(5) 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
(10) 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
(15) all the oxygen in water is oxygen 16, but a few
molecules out of every thousand 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
(20) will eventually return to it. Because heavier isotopes
tend to be left behind when water evaporates
from the ocean surfaces, the remaining ocean
water becomes progressively enriched in oxygen
18. The degree of enrichment can be determined
(25) by analyzing ocean sediments of the period,
because these sediments are composed of calcium
carbonate shells of marine organisms, shells that
were constructed with oxygen atoms drawn from
the surrounding ocean. The higher the ratio of
(30) 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
(35) 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,
(40) sedimentary evidence can be dated 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
(45) 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.
(50) 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
(55) 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
(60) global climate does not make them unimportant.
为什么我一直把这句看做是“为了学术严谨，作者对M理论不足做补充。”呢？求分析，求LZ那个分析的出处。
文章对M理论的态度应该是“大正小负”-"M理论优势：可测量；缺点：对其他潜在因素缺乏研究。
"？其中这个粉粉的部分就是“小负”的体现啊~难道一直理解错了？  |
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发表于 2012-3-20 22:05:53
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