GRE Reading Comprehension
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 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?
Correct Answer: AC