GRE Reading Comprehension

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Source: NO3

The primary method previously used by paleontologists to estimate climatic changes that occurred during Pleistocene glacial cycles was the determination of 18/O16/O 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 ratio 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.

Question List: 1 2 3 4

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

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