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发布时间：2006-06-22 13:27     点击：

    (D) Derisive

    (E) Defensive

    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 calcar- (5) eous fossils. However, because this ratio is influenced by a number of factors, the absolute magnitude of the temperature difference be- tween Pleistocene glacial and interglacial cycles could not be unequivocally ascertained. For (10)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. (15)   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 (20)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 (25)variable is known, the reaction can be used to calculate the other.

    24. It can be inferred from the passage that deter- mination 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

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