GRE Reading Comprehension

Home > GRE Test > GRE Reading Comprehension Questions

Next steps

Source: BOOST

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.

Question List: 1 2 3

The passage suggests that the 180/160 ratio could be used more successfully as a means of measurement if scientists were able to

  • A Determine the 180/160 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 180/160 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 180/160 ratios

Show Answer

Previous       Next