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 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 18O/16O ratio. And, since both factors influence the ratio in the same direction, the contribution of each to the 18O/16O 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 racemization, 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.
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/16O 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 the 18O/16O ratio?
III. 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?
(A) I only
(B) I and II only
(C) I and III only
(D) II and 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