The primary method previously used by paleontologists to estimate
climatic changes that occurred during Pleistocene glacial cycles was the
determination of O18/O16 ratios in calcareous fossils. However, because
this ratio is influenced by a number of factors, the absolute magnitude of
5 the temperature difference between Pleistocene glacial and interglacial
cycles could not be unequivocally ascertained. For example, both
temperature fluctuations and isotopic changes in sea water affect the
O18/O16 ratio. And, since both factors influence the ratio in the same
direction, the contribution of each to the O18/O16 ratio cannot be
10 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
15 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.
1. It can be inferred from the passage that determination of the temperatures mentioned in line 12
through O18/O16 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 O
2. The passage suggests that O18/O16 ratio could be used more successfully as a means of measurement
if scientists were able to
(A) determine the O18/O16 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 sea water that
affect the O18/O16 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 sea water on
O18/O16 ratios