The primary method previously used by
paleontologists to estimate climatic changes
that occurred during Pleistocene glacial cycles
was the determination of 18O/16O ratios in calcar-
(5) eous fossils. However, because this ratio is
influenced by a number of factors, the absolute
magnitude of the temperature difference be-
tween Pleistocene glacial and interglacial
cycles could not be unequivocally ascertained. For
(10)example, both temperature fluctuations and
isotopic changes in seawater affect the 18O/16O
ratio. And. since both factors influence the
ratio in the same direction, the contribution
of each to the 18O/16O ratio cannot be determined.
(15) 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 Lamino acids are usually found in the
(20)proteins of living organisms, but over long
periods of geological time these acids undergo
racemization, producing D-amino acids, which
are not found in proteins. This reaction depends
on both time and temperature, thus, if one
(25)variable is known, the reaction can be used
to calculate the other.
24. It can be inferred from the passage that determination of the temperatures mentioned in line 17 through 18O/16O ratios and determination through racemization reactions both require which of the following?
(A) Calcium deposits known to be from Pleistocene seas
(B) Proteins containing both L-amino acids and D-amino acids
(C) Glacial debris from both before and after the Pleistocene period
(D) Fossil material from organisms living during the Pleistocene period
(E) Proteins containing both amino acids and 18O
25. The passage suggests that the 18O/ 16O/ ratio could be used more successfully as a means of measurement if scientists were able to
(A) determine the 18O/ 16 O/ 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 18O/16O/ 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 18O/16O/ ratios
26. The information in the passage can be used to answer which of the following questions?
Ⅰ Do temperature variations and isotopic changes in seawater cause the 18O/16O ratio to shift in the same direction?
Ⅱ What are the methods used to determine the18 O/16O ratio?
Ⅲ Is the study of raceminzation reactions useful in estimating climatic changes that occurred during Pleistocene glacial cycles if only one of the two important variables is known?
(A) Ⅰonly
(B) Ⅰand Ⅱ only
(C) Ⅰand Ⅲ only
(D) Ⅱand Ⅲ only
(E) Ⅰ,Ⅱ,and Ⅲ
27. According to the passage, before the recent experiments described in the passage were completed, scientists could
(A) determine temperatures only for Pleistocene seas
(B) determine 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 glaciation |
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