The primary method previously used by paleontologists to estimate climatic changes that occurred during Pleistocene glacial cycles was the determination ofl80/l60 ratios in calcareous fossils. However, because this ratio is influenced by a number of factors, the absolute magnitude of the temperature difference between Pleistocene glacial and interglacial cycles could not be unequivocally ascertained.. For example, both temperature fluctuations and isotopic. Changes in seawater affect the 180/160 ratio And, since both factors influence the ratio in the same direction, the contribution of each to the 180/160 cannot be determined.
Fortunately, recent studies indicate that the racemization reaction of amino acids can be used to determine more accurately temperatures that occurred during Pleistocene glacial cycles. Only L-amino acids are usually found in the proteins of living organisms, but over long periods of geological time these acids undergo racenuzation, producing D-amino acids, which are not found in proteins. This reaction depends on both time and temperature; thus, if one variable is known, the reaction can be used to calculate the other.
The passage suggests that the 180/160 ratio could be used more successfully as a means of measurement if scientists were able to
A.Determine the 180/160 ratio in living animals as well as in fossil remains
B.Locate a greater number of calcareous fossils from the Pleistocene glacial and interglacial cycles
C.locate the factors other than temperature fluctuations and isotopic changes in seawater that affect the 180/160 ratio
D.Arrive at more exact determinations of which amino acids are found in the proteins of living organisms
E.isolate the relative effects of temperature fluctuations and isotopic changes in seawater on 180/160 ratios
The information in the passage can be used to answer which of the following questions?
A.Do temperature variations and isotopic changes in seawater cause the 180/160 ratio to shift in the same direction?
B.What are the methods used to determine the 180/160 ratio?
C.Is the study of racemization reactions useful in estimating climatic changes that occurred during Pleistocene glacial cycles if only one of the two important variables is known?
According to the passage, before the recent experiments described in the passage were completed, scientists could
A.Determine temperatures only for Pleistocene seas
B.Detennine temperatures that occurred during Pleistocene glacial cycles only by examining fossil remains
C.measure changes in temperatures that occurred during Pleistocene glacial cycles with only questionable accuracy
D.Only partially identify factors tending to lower Pleistocene temperatures
E.accurately determine temperatures only for land masses affected by glaciations
正确答案:E AC C
5天搞定800组GRE近/反义词 活动日期 : 2018.10.19-2018.10.23
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